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Fracture toughness of pipe line steels 1978

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FRACTURE TOUGHNESS OF PIPE LINE STEELS by RANEN MAITI B.E. (Met), R E College, Durgapur (1967) D.I.I.T. (Foundry Engg.), I.I.T. Kharagpur (1969) M. Tech (Phy. Met), I.I.T. Kharagpur (1971) A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE STUDIES Department of METALLURGICAL ENGINEERING We accept this thesis as conforming to the required standard The University of British Columbia, December, 1978 0 RANEN MAITI, 1978 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the re q u i r e m e n t s f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and st u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y purposes may be gr a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . I t i s underst o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department Of M e t a l l u r g i c a l Engineering The U n i v e r s i t y o f B r i t i s h Columbia 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 Date January 18th, 1979 DE-6 BP 75-51 1 E ABSTRACT The fracture toughness of two acicular f e r r i t e , HSLA pipeline steels was investigated u t i l i s i n g the linear elastic fracture mechanics analysis (KT testing according to ASTM Standard E399-74) as well as the elastic-plastic fracture mechanics analysis (J-Integral and crack opening displacement COD methods). The tests were conducted at a static strain rate of 10 /sec, K = 10 ksi^/in/sec with H inch thick compact tension specimens. A resistance curve test technique developed by Landes and Begley was employed to obtain the J l c fracture toughness; whereas the British Standard for COD testing was followed for measuring the 6 C fracture toughness. The anisotropy in fracture toughness and the tensile properties of the two x-70 steels were measured and explained in terms of sulphur content and rare earth additions. An attempt was made to correlate the linear elastic fracture toughness K L C or KQ values with the elastic-plastic fracture toughness, J l c,and COD data for both steels for tests in each of three notch orientations i.e. parallel to the rolling direction (T-L); parallel to the pipe axis; transverse to the rolling direction (L-T). Tests were performed at temperatures throughout the transition range i.e. o from RT down to - 196 C. Finally the static fracture toughness data as generated in this study, was compared with the dynamic fracture toughness as obtained from IIT test for both steels. i v TABLE OF CONTENTS Page ABSTRACT . . i i TABLE OF CONTENTS iv LIST OF FIGURES v i i i LIST OF TABLES , .. x i i LIST OF SYMBOLS x i i i ACKNOWLEDGEMENTS X V 1 . INTRODUCTION 1 2. PIPE LINE MATERIALS 4 2.1 Introduction 4 2.2 Metallurgy of Acicular Ferrite Steels 5 2.2.1 Effect of Alloy Additions 5 2.2.2 M i l l Production Parameters 7 2.3 Pipe Fabrication and Strength of Skelp 9 3. THE BASIS OF DESIGN FOR PIPELINES 11 3.1 Strength Considerations 11 3.2 Fracture Control Design 13 3.2.1 Design Criteria for Preventing B r i t t l e Failure 14 3.2.2 Design Criteria for Ductile Fracture Inititation Control 15 3.2.3 Design Criteria for Ductile Fracture Propagation and Arrest 17 Page 3.3 Review of the Design for Fracture Control .. 19 3.4 Instrumental Impact Test Approach 20 3.5 Project Summary 23 4. THEORY AND TEST PROCEDURES 25 4.1 Linear Elastic Fracture Mechanics 25 4.1.1 Plane Strain Fracture Toughness .... 27 4.1.2 Specimen Size Requirements 29 4.2 Elastic-Plastic Fracture Mechanics 31 4.2.1 The J-Integral Approach 34 4.2.1.1 Experimental Technique .. .. 37 4.2.1.2 Validity Criteria 39 4.2.2 The Crack Opening Displacement Method 39 4.2.2.1 COD as an Extension to LEFM 40 4.2.2.2 Dugdale's Model 42 4.2.2.3 Experimental Determination of COD 46 4.2.2.3.1 Determination of «c 4 8 4.2.2.3.2 Evaluation of an Equivalent K l c from COD 50 5. EXPERIMENTAL 52 5.1 Test Materials 52 5.2 Specimen Preparation 53 5.3 Specimen Configuration and Dimensions .... 53 5.3.1 Compact Tension Specimen 53 5.3.2 Tensile Specimen 56 Page 5.4 Fatigue Precracking 58 5.4.1 ASTM Standards for Precracking .... 59 5.4.2 Precracking Stress Intensity 60 5.5 Kjc Test Procedure 61 5.5.1 Test Fixtures and Displacement Gauge 62 5.5.2 Test Details 63 5.5.3 Low Temperature Tests 65 5.5.4 Test Records 66 5.5.5 Measurements of Test Piece Dimensions and Crack Length 67 5.5.6 Analysis of Experimental Data .... 67 5.6 COD Test Details 70 5.6.1 Assessment of Test Data 70 5.6.2 Calculation of 6 C 71 5.6.3 Calculation of Equivalent K l c .... 72 5.7 J-Integral Test Details 73 5.7.1 Testing Parameters 73 5.7.2.1 Measurement of Specimen Dimension and Crack Growth (Aa) 74 5.7.2.2 Measurement of Area (A) under P-A record 76 o 5.7.3 Calculation of J for RT and - 40 C Tests 76 5.7.4 Determination of J j c Value 77 5.7.5 Calculation of Equivalent K l c .... 78 5.7.6 Verification of Validity Criterion 79 v i i Page Tensile 5.8 Test Details 79 6. RESULTS AND DISCUSSION 80 6.1 Tensile Properties 80 6.2 Fracture Toughness 88 6.2.1 KQ Test Results 88 6.2.2 J-Integral Test Results 96 6.2.3 COD Test Results 110 6.3 Comparison of Fracture Toughness Properties from K L C J-Integral and COD Tests 114 6.4 Comparison of Static and Dynamic Fracture Toughness 130 7. CONCLUSIONS 138 7.1 Conclusions 138 7.2 Suggestions for Future Work 140 REFERENCES 142 APPENDIX - I 147 APPENDIX - II 148 v i i i LIST OF FIGURES Figure Page No. No. 4.1 Distribution of Principal stresses at the crack tip .. .. 26 4.2 Schematic load (P) vs load-point displace- ment (A) curves for (a) perfectly elastic material (b) elastic material with pop-in behaviour (c) elastic then plastic behaviour (d) Ductile material with extensive plasticity prior to failure 33 4.3 Dugdale's Strip Yield Model 4 3 5.1 % inch thick compact tension specimen .. 55 5.2 (a) orientation of CT specimens with respect to Rolling Direction of the plate 57 (b) Dimensions of the tensile specimen .. 57 5.3 (a) Dimensions of the Brass tubes.. .... 64 (b) Photograph of the experimental set-up 64 5.4 (a) Actual P-A test record for AF-1 steel at - 60° C 68 (b) Actual P-A test record for AF-1 steel at - 150° C 68 5.5 (a) J-Integral test record of AF-2 steel with crack transverse to rolling direction at - 40° C 75 (b) J-Integral test record of AF-2 steel with crack parallel to rolling .. direction at - 130° C 75 6.1 Temperature dependence of yield stress and flow stress of AF-1 steel with specimen transverse to rolling direction 84 6.2 Temperature dependence of yield stress and flow stress of AF-1 steel with specimen transverse to pipe axis 84 ix Figure Page No, No. 6.3 Temperature dependence of yield stress and flow stress of AF-1 steel with specimen parallel to pipe axis 85 6.4 Temperature dependence of yield stress and flow stress of AF-1 steel with specimen parallel to rolling direction 85 6.5 Temperature dependence of yield stress and flow stress of AF-2 steel with specimen transverse to rolling direction 86 6.6 Temperature dependence of yield stress and flow stress of AF-2 steel with specimen transverse to pipe axis 86 6.7 Temperature dependence of yield stress and flow stress of AF-2 steel with specimen parallel to pipe axis 87 87 6.8 Temperature dependence of yield stress and flow stress of AF-2 steel with specimen parallel to rolling direction 6.9.1 Temperature dependence of KQ^ K J c of AF-1 and AF-2 steels with crack parallel to rolli n g direction 89 6.9.2 Temperature dependence of KQ^ K J c of AF-1 and AF-2 steels with crack parallel to the pipe axis 89 6.9.3 Temperature dependence of Kg, K I c of AF-1 and AF-2 steels with crack transverse to the rolling direction 89 6.10.1 Fracture surfaces of K l c specimens of AF-2 steel with crack parallel to the pipe axis at various temperatures 92 6.11.1 J-resistance curve for AF-1 steel with crack parallel to the roll i n g direction at RT and - 40° C 97 X Figure Page No. No. 6.11.2 J-resistance curve for AF-1 s t e e l with crack p a r a l l e l to the pipe axis at RT and - 40° C 98 6.11.3 J-resistance curve for AF-1 s t e e l with crack transverse to the r o l l i n g d i r e c t i o n at RT, - 40° C and - 80° C .„ .. .. . „ .. 99 6.12.1 J-resistance curve for AF-2 s t e e l with crack p a r a l l e l to the r o l l i n g d i r e c t i o n at RT and - 40° C 100 6.12.2 J-resistance curve for AF-2 s t e e l with crack p a r a l l e l to the pipe axis at RT and - 40° C 101 6.12.3 J-resistance curve for AF-2 s t e e l with crack transverse to the r o l l i n g d i r e c t i o n at RT and - 40° C • 102 6.13.1 Fracture surfaces of AF-2 s t e e l specimens with crack p a r a l l e l to the r o l l i n g d i r e c t i o n tested at RT arranged i n order of increasing crack extension 103 6.14.1 Temperature dependence of Jy- of AF-1 and AF-2 steels with crack p a r a l l e l to the r o l l i n g d i r e c t i o n 105 6.14.2 Temperature dependence of J y c of AF-1 and AF-2 steels with crack p a r a l l e l to the pipe axis . ... . . .. 105 6.14.3 Temperature dependence of J I c of AF-1 and AF-2 steels with crack transverse to the r o l l i n g d i r e c t i o n .. 105 6.15.1 Temperature dependence of J l c K Q and Kj of AF-1 and AF-2 steels with crack p a r a l l e l to the r o l l i n g d i r e c t i o n 106 6.16.1 Temperature dependence of - COD of AF-1 and AF-2 steels with crack p a r a l l e l to the r o l l i n g d i r e c t i o n . . . I l l xi Figure Page No. No. 6.16.2 Temperature dependence of ̂ m - COD of AF-1 and AF-2 steels with crack parallel to the pipe axis I l l 6.16.3 Temperature dependence of *m - COD of AF-1 and AF-2 steels with crack transverse to the rolling direction I l l 6.17.1 Temperature dependence of 6Q - COD of AF-1 and AF-2 steels with crack parallel to the rolling direction 112 6.17.2 Temperature dependence of 6 Q - COD of AF-1 and AF-2 steels with crack parallel to the pipe axis 112 6.17.3 Temperature dependence of 6Q - COD of AF-1 and AF-2 steels with crack transverse to the rolling direction 112 6.18.1 Temperature dependence of fracture tough- ness of AF-1 steel along crack parallel to ro l l i n g direction 127 6.18.2 Temperature dependence of fracture tough- ness of AF-1 steel along crack parallel to pipe axis 127 6.18.3 Temperature dependence of fracture tough- ness of AF-1 steel along crack transverse to the rolling direction 128 6.19.1 Temperature dependence of fracture tough- ness of AF-2 steel along crack parallel to the r o l l i n g direction 128 6.19.2 Temperature dependence of fracture tough- ness of AF-2 steel along crack parallel to the pipe axis 129 6.19.3 Temperature dependence of fracture tough- ness of AF-2 steel along crack transverse to the rolling direction 129 x i i LIST OF TABLES Table Page No. No. 5.1 Compositions of AF-1 and AF-2 steels 52 6.1 Fracture toughness data for AF-1 steel with crack parallel to the Rolling Direction 121 6.2 Fracture toughness data for AF-1 steel with crack parallel to the pipe axis .. 122 6.3 Fracture toughness data for AF-1 steel with crack transverse to the rolling direction 123 6.4 Fracture toughness data for AF-2 steel with crack parallel to the rol l i n g direction 124 6.5 Fracture toughness data for AF-2 steel with crack parallel to the pipe axis .. 125 6.6 Fracture toughness data for AF-2 steel with crack transverse to the rolling direction .. 126 6.7 Comparative J l c values of AF-1 to AF-2 steels along crack parallel to rolling direction (T-L) and crack parallel to pipe axis 108 X 1 X 1 LIST OF SYMBOLS e Strain Rate £y S Yield Strain K Stress Intensity Factor • K Stress Intensity Rate K C Plane Stress Fracture Toughness K L C Plane Strain Fracture Toughness under Static Loading K I D Plane Strain Fracture Toughness under Dynamic Loading KQ Calculated Fracture Toughness J L C C r i t i c a l J-Integral Plane Strain Fracture Toughness JQ Calculated J-value G l c Crack Extension Force under Plane Strain Condition G Crack Extension Force TIT Instrumented Impact Testing COD Crack Opening Displacement 6 Crack Opening Displacement 6 C C r i t i c a l COD 6 m C r i t i c a l COD corresponding to Maximum Load 6Q C r i t i c a l COD corresponding to PQ Load corresponding to 5% Offset xiv pmax Maximum Load 1 P Load A Displacement Y Austenite D Inside Diameter of Pipe t Pipe Wall Thickness o H Hoop Stress o Tensile Stress OyS Yield Stress of Flow Stress E Young's Modulus v Poisson's Ratio a Crack Length B Specimen Thickness w Depth of Specimen A c Area of Charpy Specimen Ligament r^ Plastic Zone Radius A a Crack Extension Vg Clip Gauge Displacement r Rotational Factor z Knife Edge Thickness V c C r i t i c a l Clip Gauge Displacement V m Clip Gauge Displacement at P m a x VQ Clip Gauge Displacement at P 0 X V ACKNOWLEDGEMENTS I wish to thank my fellow graduate students and the members of the faculty, in particular Dr. S.R. Bala and Asst. Prof. R.G. Butters for their assistance and helpful discussions during the research work. I very much appreciate the assistance of the technical staff, in particular, J. Walker and H. Tump throughout the experimental programme and R. Bennett, P. Musil and M. Bennett during the preparation of the thesis. Financial assistance in the form of a 'Common- wealth Scholarship* provided by Canadian Commonwealth Scholarship and Fellowship Committee, Ottawa, is grate- fully acknowledged. I am indebted to my supervisors Prof. E.B. Hawbolt and Prof. J.S. Nadeau for their continuous advice, stimulating discussions, helpful suggestions and immense encouragement throughout the project. Thanks are also extended to Prof. E. Teghtsoonian, Head, Dept. of MET. ENGG, for providing necessary f a c i l i t i e s to carry out the project. Finally, I thank my wife Biva and daughter Munmun for their patience. 1 1 . I N T R O D U C T I O N I n t h i s t h e s i s , L i n e a r E l a s t i c F r a c t u r e M e c h a n i c s ( L E F M ) a n d E l a s t i c P l a s t i c F r a c t u r e M e c h a n i c s ( E P F M ) a r e u s e d t o a n a l y s e t h e f r a c t u r e t o u g h n e s s o f t w o x - 7 0 a c i c u l a r f e r r i t e , H S L A p i p e l i n e s t e e l s ; t h e x - 7 0 s t e e l s a r e t o b e u s e d f o r t h e n a t u r a l g a s A l a s k a H i g h w a y p i p e l i n e t o b e b u i l t i n 1 9 8 0 . I t i s w e l l e s t a b l i s h e d t h a t f r a c t u r e t o u g h n e s s i s a m a t e r i a l p r o p e r t y a n a l o g u s t o t h e y i e l d s t r e n g t h a n d i s e q u a l l y i m p o r t a n t f o r p i p e l i n e d e s i g n c o n s i d e r a t i o n s . A t p r e s e n t , t h e D r o p W e i g h t T e a r T e s t a n d t h e s t a n d a r d C h a r p y v - n o t c h i m p a c t t e s t a r e u s e d a s q u a l i t y c o n t r o l t e s t s t o a s s e s s t h e t o u g h n e s s o f t h e s t e e l . F u l l s c a l e b u r s t t e s t s h a v e e s t a b l i s h e d m i n i m u m C v a n d BDWTT v a l u e s t o p r o t e c t a g a i n s t c a t a s t r o p h i c b r i t t l e a n d d u c t i l e f a i l u r e o f t h e p i p e l i n e s . U n f o r t u n a t e l y , t h e s e t e s t s d o n o t p r o v i d e a n y m e a s u r e o f t h e e n e r g y r e q u i r e d f o r f r a c t u r e i n i t i a t i o n i . e . t h e f r a c t u r e t o u g h n e s s o f t h e m a t e r i a l . T h e I n s t r u m e n t e d I m p a c t T e s t i n g ( I I T ) m e t h o d p o s s e s s e s t h e p o t e n t i a l t o s o l v e t h i s p r o b l e m . I t h a s 2 e s t a b l i s h e d i t s s u p e r i o r i t y o v e r t h e c o n v e n t i o n a l C h a r p y t e s t f o r t h e f o l l o w i n g r e a s o n s : i ) I t d i s t i n g u i s h e s b e t w e e n t h e f r a c t u r e i n i t i a t i o n e n e r g y a n d t h e c r a c k p r o p a g a t i o n e n e r g y . i i ) I t p r o v i d e s a m e a s u r e o f t h e d y n a m i c f r a c t u r e i n i t i a t i o n e v e n t w h e n a p r e c r a c k e d C h a r p y s p e c i m e n i s e m p l o y e d . T h e r e f o r e , t h e f r a c t u r e t o u g h n e s s d e t e r m i n e d b y t h i s m e t h o d w i l l b e m o r e c o n s e r v a t i v e a n d may b e m o r e u s e f u l f o r f r a c t u r e d e s i g n . H o w e v e r , t o o b t a i n g r e a t e r i n s i g h t i n t o t h e f r a c t u r e p r o c e s s e s o c c u r r i n g d u r i n g f a i l u r e , i t i s a l s o e s s e n t i a l t o i n v e s t i g a t e t h e s t a t i c f r a c t u r e t o u g h n e s s b y u s i n g c o n v e n t i o n a l l o w s t r a i n r a t e t e s t i n g . T h i s a p p r o a c h a l s o h a s a n a d v a n t a g e i n t h a t i t u t i l i z e s a f u l l w a l l t h i c k n e s s f o r t h e t e s t s p e c i m e n s . T h e o b j e c t i v e s o f t h e p r e s e n t s t u d y w e r e : 1) T o f u r t h e r c h a r a c t e r i s e t h e l o w s t r a i n r a t e f r a c t u r e t o u g h n e s s o f t h e x - 7 0 s t e e l s u s i n g L E F M a n d E P F M . 2) T o c o m p a r e t h e l o w s t r a i n r a t e f r a c t u r e t o u g h n e s s w i t h d y n a m i c f r a c t u r e t o u g h n e s s d a t a t o 3 d e t e r m i n e t h e v a l u e o f t h e r a p i d i n e x p e n s i v e I I T t e s t f o r a s s e s s i n g f r a c t u r e t o u g h n e s s . T h e t h e o r e t i c a l b a c k g r o u n d o f t h e e l a s t i c p l a s t i c f r a c t u r e m e c h a n i c s a n d t h e l i m i t a t i o n s o f t h e l i n e a r e l a s t i c f r a c t u r e m e c h a n i c s a r e o u t l i n e d i n t h i s s t u d y w i t h a v i e w t o e x a m i n i n g t h e a p p l i c a b i l i t y o f t h e v a r i o u s f r a c t u r e t o u g h n e s s t e s t i n g t e c h n i q u e s t o t h e s t u d y o f H S L A s t r u c t u r a l s t e e l s . 4 2 . P I P E L I N E M A T E R I A L S 2 . 1 I n t r o d u c t i o n : P i p e l i n e s a r e e c o n o m i c a l , r e l i a b l e s y s t e m s f o r t r a n s p o r t - i n g e n e r g y r e s o u r c e s f r o m d i s t a n t f i e l d s , p a r t i c u l a r l y t h o s e l o c a t e d i n t h e m o s t s e v e r e a r c t i c a n d s u b m a r i n e e n v i r o n m e n t s , s u c h a s S i b e r i a , i n f r i g i d n o r t h e r n p a r t s o f C a n a d a o r A l a s k a , (1) a n d t h e N o r t h S e a , t o t h e p o p u l a t e d m a r k e t s T o m e e t t h e c h a l l e n g e s e t b y t h e w h i m s o f n a t u r e , i t i s n e c e s s a r y f o r t h e p r o d u c t i o n o f a n e w c l a s s o f s t e e l s w i t h a n u n p r e c e d e n t e d c o m b i n a t i o n o f q u a l i t i e s . 1 . S t r e n g t h - f o r w a l l s t h a t a r e t h i n n e r , y e t m o r e r u g g e d i n p e r f o r m a n c e . 2 . T o u g h n e s s - f o r r e s i s t a n c e t o f r a c t u r e a t s u b - z e r o t e m p e r a t u r e s . 3 . F i e l d w e l d a b i l i t y - w i t h r e s i s t a n c e t o c r a c k i n g w i t h l i t t l e o r n o p r e h e a t . 4 . A l l a t t h e l o w e s t p o s s i b l e c o s t p e r u n i t o f s t r e n g t h . ( 2 ) T h e a n s w e r t o t h e s e p r o b l e m s i s t h e e v o l u t i o n o f a h i g h s t r e n g t h , l o w a l l o y ( H S L A ) , A c i c u l a r F e r r i t e ( A F ) s t e e l h a v i n g a y i e l d s t r e n g t h o f f r o m 70 t o 8 0 k s i ( 4 8 0 - 5 5 0 M P a ) , a C h a r p y u p p e r s h e l f e n e r g y o f w e l l o v e r 1 1 5 f t - l b ( 1 5 5 J o u l e ) a n d a F A T T o f o - 6 0 C . 5 2 , 2 M e t a l l u r g y o f A c i c u l a r F e r r i t e S t e e l s A c i c u l a r f e r r i t e i s d e f i n e d a s a h i g h l y s u b s t r u c t u r e d , n o n - e q u i a x e d p h a s e t h a t f o r m s o n c o n t i n u o u s c o o l i n g b y a m i x e d d i f f u s i o n a n d s h e a r m o d e . T h e t r a n s f o r m a t i o n b e g i n s a t a t e m p e r a t u r e s l i g h t l y h i g h e r t h a n t h e u p p e r b a i n i t e t r a n s f o r m - a t i o n r a n g e . A c i c u l a r f e r r i t e i s d i f f e r e n t f r o m b a i n i t i c f e r r i t e ; t h e p r i o r y - g r a i n b o u n d a r y n e t w o r k i s r e t a i n e d i n b a i n i t i c s t r u c t u r e s , b u t n o t i n a c i c u l a r f e r r i t e s t r u c t u r e s . T h i s f a c t o r c o n t r i b u t e s t o w a r d s h i g h e r i m p a c t t o u g h n e s s s i n c e t h e r e a r e n o s t r a i g h t , h i g h a n g l e b o u n d a r i e s t o b e c o m e e m - ( 3 ) b r i t t l e d b y p r e c i p i t a t e s o r s e g r e g a t e d i m p u r i t i e s T h e h i g h s t r e n g t h o f t h e m a t e r i a l c a n b e a t t r i b u t e d t o t h r e e s t r e n g t h e n i n g m e c h a n i s m s . 1 . G r a i n R e f i n e m e n t 2 . D i s l o c a t i o n S u b s t r u c t u r e 3 . P r e c i p i t a t i o n s t r e n g t h e n i n g b y N b ( C , N ) N i o b i u m C a r b o n i t r i d e . 2 . 2 . 1 E f f e c t o f A l l o y A d d i t i o n s ; C a r b o n ; New s t e e l m a k i n g p r a c t i s e s h a v e b e e n a d o p t e d t o l i m i t t h e c a r b o n c o n t e n t t o a p p r o x i m a t e l y 0 . 0 5 % ; i n o r d e r t o a c h i e v e i m p r o v e d w e l d a b i l i t y a n d f o r m a b i l i t y . I n c r e a s i n g 6 the carbon, though i t increases the yi e l d strength, impairs the toughness by increasing the transition temperature and lowering the upper shelf energies. This i s due to the formation of an increasing amount of cementite. Since about 0.01 to 0.02 % C is adequate to f a c i l i t a t e precipitation strengthening, a higher carbon level i s undesirable in acicular ferrite steels. Molybdenum and Manganese: The combination of molybdenum (0.25 - 0.50 wt % ) , manganese (1.50 - 2.25 wt %) and to a lesser extent niobium ( 0.05 wt %) suppresses the transform- o ation temperature of y+a to below 700 C. The fine grain a structure i s ensured by having a fine y grain as well. The result i s a fine grained (ASTM No. 13 to 14) acicular ferrite microstructure. This exceptionally fine grain size, provides the basic building block for both the high strength and the (4) excellent thoughness Further, molybdenum decreases the rate of Nb(C,N) pre- cipitation in austenite and thereby allows a greater amount of Nb(C,N) in fe r r i t e . This precipitation process results in a higher strength product. Manganese plays a role similar to that of molybdenum. Niobium; Niobium i s a potent micro alloy that improves the steel through three mechanisms. 7 i ) I t r e f i n e s t h e a u s t e n i t e a n d u l t i m a t e l y t h e f e r r i t e g r a i n s t r u c t u r e d u r i n g r o l l i n g b y i n h i b i t i n g r e - c r y s t a l l i z a t i o n a n d g r a i n g r o w t h , i i ) I t s u p p r e s s e s t h e n u c l e a t i o n o f p o l y g o n a l f e r r i t e i i i ) F i n a l l y , i t i n c r e a s e s s t r e n g t h b y p r e c i p i t a t i o n o f u l t r a f i n e p a r t i c l e s o f n i o b i u m c a r b o n i t r i d e s o r N b - C - N c l u s t e r s d u r i n g c o o l i n g f r o m t h e f i n i s h r o l l i n g t e m p e r a t u r e . S u l p h u r ; A v e r y l o w s u l p h u r , o f t h e o r d e r o f l e s s t h a n 0 . 0 0 5 w t % , i s d e s i r a b l e i n a c i c u l a r f e r r i t e s t e e l s . H i g h e r s u l p h u r c o n t e n t s i m p a i r t h e i m p a c t s t r e n g t h , p a r t i c u l a r l y i n t h e r o l l i n g d i r e c t i o n ( T - L ) t h r o u g h t h e f o r m a t i o n o f MnS s t r i n g e r s . T h e d e t r i m e n t a l e f f e c t o f s u l p h i d e i n c l u s i o n s c a n b e m i n i m i z e d t h r o u g h t h e a d d i t i o n s o f r a r e e a r t h e l e m e n t s w h i c h c h a n g e t h e m o r p h o l o g y o f t h e s u l p h i d e s f r o m e l o n g a t e d r i b b o n s ( 5 ) t o g l o b u l a r e g u i a x e d s p h e r o i d s . T h e p r e s e n c e o f e g u i a x e d s u l p h i d e s e n s u r e s a d e q u a t e t o u g h n e s s a n d r e d u c e s t h e a n i s o t r o p y i n t o u g h n e s s o f t h e p r o c e s s e d s t e e l . T h e s e s t e e l s a r e o f t e n k i l l e d w i t h A l a n d a r e d u c e d a m o u n t o f S i ( 0 . 0 5 t o 0 . 1 7 w t %) (6) a s s i l i c o n g r e a t e r t h a n 0 . 1 7 w t % r e d u c e s t h e i m p a c t r e s i s t a n c e 2 . 2 . 2 M i l l P r o d u c t i o n P a r a m e t e r s : T h e p r o c e s s c o n t r o l a p p l i e d d u r i n g h o t - r o l l i n g i s 8 e x t r e m e l y i m p o r t a n t t o a c h i e v e a f a v o u r a b l e b a l a n c e b e t w e e n t h e s t r e n g t h a n d t h e t o u g h n e s s . T h e r o l l i n g p a r a m e t e r s f o u n d t o b e m o s t i n f l u e n t i a l a r e t h e s l a b r e h e a t t e m p e r a t u r e a n d t h e t o t a l o r e d u c t i o n b e l o w 9 0 0 C a n d t o s o m e e x t e n t , t h e f i n i s h r o l l i n g t e m p e r a t u r e . T h e s l a b r e h e a t t e m p e r a t u r e i s i m p o r t a n t f o r t w o r e a s o n s : 1 . I t d e t e r m i n e s t h e d e g r e e o f s o l u t i o n i s i n g o f N b ( C , N ) i n y, 2 . I t a l s o d e t e r m i n e s t h e y - g r a i n s i z e a t t h e b e g i n n i n g o f r o l l i n g . o A l o w s l a b - p r e h e a t t e m p e r a t u r e o f a p p r o x i m a t e l y 1 1 5 0 C g i v e s r i s e t o a s m a l l e r y - g r a i n s i z e a n d s o m e u n d i s s o l v e d N b ( C , N ) p a r t i c l e s . D u r i n g h o t r o l l i n g y r e c r y s t a l l i z e s , b u t t h e s u b s e q u e n t g r a i n g r o w t h i s i n h i b i t e d b y t h e u n d i s s o l v e d N b ( C , N ) p a r t i c l e s w h i c h r e s t r i c t g r a i n g r o w t h . A s a r e s u l t , a f t e r s e v e r a l c y c l e s o f r e c r y s t a l l i z a t i o n , t h e y - g r a i n s t r u c t u r e b e c o m e s e x t r e m e l y f i n e . A d e c r e a s e o f t h e F r a c t u r e A p p e a r a n c e o o T r a n s i t i o n T e m p e r a t u r e ( F A T T ) f r o m 5 t o - 9 0 C h a s b e e n o r e p o r t e d a s t h e r e h e a t t e m p e r a t u r e i s l o w e r e d f r o m 1 1 4 0 C t o o 1 0 3 0 C . A s t h e n i o b i u m c o n t a i n i n g s t e e l s d o n o t r e c r y s t a l l i z e o b e l o w a b o u t 9 8 0 C d u r i n g h o t r o l l i n g t h e r e i s a n e v e r i n c r e a s i n g 9 a c c u m u l a t i o n o f s t r a i n i n t h e y a s t h e r o l l i n g i s c o n t i n u e d i n t h e l o w e r t e m p e r a t u r e r a n g e . T h i s i n t u r n r e s u l t s i n h e a v y d e f o r m a t i o n o f f i n e g r a i n e d y i n t r o d u c i n g a h e a v i l y d i s l o c a t e d s t r u c t u r e . T h i s s t r u c t u r e p r o v i d e s m o r e s i t e s f o r t h e s u b s e q u e n t n u c l e a t i o n a n d g r o w t h o f a f i n e g r a i n e d f e r r i t e . T h e h e a v y d e f o r m a t i o n o f t h e y - p h a s e a l s o s u p p r e s s e s t h e y - a t r a n s - f o r m a t i o n t e m p e r a t u r e . T h e m i n i m u m r o l l i n g t e m p e r a t u r e i s c o n t r o l l e d t o e n s u r e t h a t n o d e f o r m a t i o n o f t h e f e r r i t e p h a s e o c c u r s a s t h i s w o u l d i m p a i r t h e t o u g h n e s s o f t h e f i n a l p r o d u c t . I n g e n e r a l , d e c r e a s i n g t h e s l a b r e h e a t t e m p e r a t u r e t o l i m i t g r a i n g r o w t h , d e c r e a s i n g t h e f i n i s h r o l l i n g t e m p e r a t u r e t o l i m i t y r e c r y s t a l l i z a t i o n a n d i n c r e a s i n g t h e p e r c e n t a g e o f r e d u c t i o n i n t h e l a t e r o l l i n g s t a g e s ( t o e n h a n c e s u b s t r u c t u r e s t r e n g t h e n i n g ) r e s u l t s i n a m o r e r e f i n e d f e r r i t e g r a i n s t r u c t u r e , t h e r e b y i m p r o v i n g t h e s t r e n g t h a n d t o u g h n e s s o f t h e s t e e l . 2 . 3 P i p e F a b r i c a t i o n a n d S t r e n g t h o f S k e l p I n t e s t i n g t h e s t r e n g t h a n d t o u g h n e s s o f t h e p i p e , t h e t e s t c o n d i t i o n s a r e i m p o r t a n t . T h e u s u a l m e t h o d i s t o c u t a p i e c e f r o m t h e c u r v e d p i p e w a l l . T h i s p i e c e i s f l a t t e n e d p r i o r t o s p e c i m e n f a b r i c a t i o n a n d t e s t i n g . T h e f l a t t e n i n g i n t r o d u c e s a B a i i c h i n g e r e f f e c t t h a t c a u s e s t h e m e a s u r e d y i e l d s t r e n g t h t o 10 b e l o w e r t h a n t h e t r u e y i e l d s t r e n g t h o f t h e p i p e . T h e d i f f e r e n c e b e t w e e n t h e m e a s u r e d y i e l d s t r e n g t h o f t h e p l a t e a n d t h e f a b r i c a t e d p i p e i s e q u a l t o t h e s t r e n g t h i n c r e a s e d u e t o t h e w o r k h a r d e n i n g r e s u l t i n g f r o m t h e s p i r a l f o r m i n g (3) o f t h e p i p e m i n u s t h e B a u c h i n g e r e f f e c t O n e o f t h e m o s t i m p o r t a n t a d v a n t a g e s o f t h e a c i c u l a r f e r r i t e M n - M o - N b s t e e l s o v e r t h e c o n v e n t i o n a l f e r r i t e - p e a r l i t e p i p e s t e e l s i s t h a t t h e y i e l d s t r e n g t h o f t h e a c i c u l a r f e r r i t e i n c r e a s e s c o n t i n u o u s l y w i t h a d d i t i o n a l p i p e p r o c e s s i n g p a r t i c u l a r l y d u r i n g c o l d e x p a n s i o n o f t h e p i p e . T h i s m e a n s t h a t r a p i d w o r k h a r d e n i n g t a k e s p l a c e d u r i n g t h e f a b r i c a t i o n o f t h e p l a t e i n t o t h e p i p e . A s t h e a m o u n t o f w o r k h a r d e n i n g i s v e r y l a r g e i n t h e a c i c u l a r f e r r i t e s t e e l s a n d v e r y s m a l l o r n i l i n t h e c o n v e n t i o n a l f e r r i t e - p e a r l i t e s t e e l s , t h e n e t e f f e c t o f c o n v e r t i n g s k e l p i n t o p i p e i s a n i n c r e a s e i n t h e s t r e n g t h o f t h e a c i c u l a r f e r r i t e s t e e l s a n d a d e c r e a s e i n t h e s t r e n g t h o f t h e f e r r i t e - p e a r l i t e s t e e l s . 3 . THE B A S I S OF D E S I G N F O R P I P E L I N E S 3 . 1 S t r e n g t h C o n s i d e r a t i o n s : E c o n o m y i n e x t r a c t i n g e n e r g y r e s o u r c e s r e q u i r e s t h e u s e o f l a r g e r d i a m e t e r p i p e l i n e s w h i c h c a n o p e r a t e a t h i g h e r p r e s s u r e s . T h i s w i l l m a x i m i s e t h e o u t p u t a n d r e d u c e t h e o p e r a t i n g c o s t s o v e r t h e ( 7 ) l i f e o f t h e l i n e - p i p e s . T h e h i g h e r o p e r a t i n g p r e s s u r e s a n d t h e l a r g e r d i a m e t e r s n e c e s s i t a t e t h e u s e o f t h i c k e r p i p e w a l l s o r h i g h e r s t r e n g t h m a t e r i a l ; t h i s i s e v i d e n t f r o m t h e f o l l o w i n g H o o p S t r e s s , ( 8 ) r e l a t i o n s h i p W h e r e P = O p e r a t i n g p r e s s u r e D = I n s i d e d i a m e t e r o f t h e p i p e t = P i p e w a l l t h i c k n e s s T h e r e a r e l i m i t a t i o n s t o t h e p i p e w a l l t h i c k n e s s d u e t o : 1) R e s t r i c t i o n s i m p o s e d b y m i l l f a c i l i t i e s . 2) T h e t o u g h n e s s r e q u i r e m e n t o f a p i p e l i n e , 3) D i f f i c u l t i e s i n r e t a i n i n g h i g h s t r e n g t h a n d t o u g h n e s s i n v e r y t h i c k p l a t e . 4 ) A d d i t i o n a l p r o b l e m s i n w e l d i n g a n d f i e l d i n s p e c t i o n . T h e m o d e r n t r e n d i n l i n e - p i p e p r o j e c t s i s t o u s e h i g h e r s t r e n g t h s t e e l p i p e s . T h e i d e a b e h i n d t h i s i s e s s e n t i a l l y e c o n o m i c , a n d i s r e l a t e d t o t h e m a t e r i a l s s a v i n g s r e a l i s e d b y u s i n g a r e d u c e d p i p e w a l l t h i c k n e s s a n d t h e i n c r e a s e d c a p a c i t y o b t a i n e d t h r o u g h u s i n g l a r g e r d i a m e t e r s a n d h i g h e r p r e s s u r e s . T h e c u r r e n t n a t u r a l g a s p i p e l i n e p r o j e c t s a r e c o m m i t t e d t o x - 7 0 p i p e b e c a u s e i t i s a v a i l a b l e a s a p r o v e n p r o d u c t . T h e f u t u r e g e n e r a t i o n o f f r o n t i e r p r o j e c t s a i m t o w a r d s u s i n g h i g h e r s t r e n g t h s t e e l t h a n x - 7 0 t o i n c r e a s e t h e o u t p u t a n d r e d u c e ( 9 ) t h e c o s t o f t h e l i n e - p i p e H o w e v e r , t h e i m p r o v e d p i p e y i e l d s t r e n g t h a n d o r h i g h e r w a l l t h i c k n e s s o n l y e n s u r e t h a t t h e l i n e c a n o p e r a t e a t a p a r t i c u l a r p r e s s u r e . T h e y i e l d s t r e n g t h d o e s n o t g u a r a n t e e t h e i n t e g r i t y o f t h e l i n e w i t h r e s p e c t t o a r r e s t i n g a p r o p a g a t i n g ( 7 ) f r a c t u r e 3.2 Fracture Control Design; The modern trend in line-pipe specifications i s concerned with increasing the fracture resistance. Extensive fracture research has been conducted by many segments of the industry to prevent the catastrophic (10) b r i t t l e failure of gas transmission pipelines or (9) ductile tearing of o i l pipelines . Such failures cause a loss of production and at the same time a significant amount of money i s required for damage repair. The social and economic implications of these failures have motivated an increased awareness for the development of fracture toughness parameters for pipelines. The basic fracture control philosophy (2, 7, 9 - 12) considers the following three factors : 1. To prevent b r i t t l e fracture propagation by assuring that the pipelines operate above the ductile-to-brittle transition temperature of the material. 2• To prevent ductile fracture i n i t i a t i o n by specifying a minimum toughness for a pipe operating at a specific stress level. 14 3 . T o c o n t r o l d u c t i l e c r a c k p r o p a g a t i o n b y s p e c i f y i n g s o m e a v e r a g e t o u g h n e s s t h a t w i l l a s s u r e s e l f - a r r e s t . T h e s e c r i t e r i a s h o u l d b e f u l f i l l e d f o r t h e m i n i m u m d e s i g n t e m p e r a t u r e . C o n t i n u i n g r e s e a r c h i n t h i s a r e a i s b e i n g c o n d u c t e d a t t h e B a t t e l l e C o l u m b u s l a b o r a t o r i e s u n d e r t h e s p o n s o r s h i p o f t h e A m e r i c a n G a s A s s o c i a t i o n . W o r k e r s a t t h e B a t t e l l e l a b o r a t o r i e s h a v e e v o l v e d f r a c t u r e c o n t r o l g u i d e l i n e s f o r p i p e w h i c h h a v e b e e n a d o p t e d b y v i r t u a l l y a l l o f t h e p i p e l i n e i n d u s t r i e s i n t h e w o r l d . 3 . 2 . 1 D e s i g n C r i t e r i a f o r P r e v e n t i n g B r i t t l e F r a c t u r e ; T w e n t y y e a r s a g o , n o c o n s i d e r a t i o n w a s g i v e n t o t h e f r a c t u r e r e s i s t a n c e o r n o t c h t o u g h n e s s o f p i p e w i t h t h e r e s u l t t h a t p i p e c o m m o n l y o p e r a t e d b e l o w i t s n i l - d u c t i l i t y t e m p e r a t u r e ( N D T ) . T h e o c c u r r e n c e o f v e r y l o n g b r i t t l e f r a c t u r e s ( o n e o f u p ( 1 1 ) t o 1 3 Km ) l e d t o t h e r e a l i z a t i o n t h a t b r i t t l e f r a c t u r e t r a v e l l e d f a s t e r t h a n t h e d e c o m p r e s s i o n v e l o c i t y o f t h e g a s i n t h e g a s t r a n s m i t t i n g l i n e . T h e B a t t e l l e D r o p W e i g h t T e a r T e s t (BDWTT) i s u s e d t o e s t a b l i s h t h e r e s i s t a n c e o f t h e p i p e s t e e l ( 2 , 1 3 ) t o b r i t t l e f r a c t u r e . T h e t e s t s p e c i f i e s t h a t a t t h e l o w e s t d e s i g n o p e r a t i n g t e m p e r a t u r e , t h e p i p e - l i n e s h o u l d e x h i b i t 8 5 % s h e a r t o e n s u r e t h a t d u c t i l e f r a c t u r e m e c h a n i s m s a r e o p e r a t i v e . H o w e v e r , f o r a n y i n d i v i d u a l t e s t 6 0 % s h e a r i s a c c e p t a b l e . 3 . 2 . 2 . D e s i g n C r i t e r i a f o r D u c t i l e F r a c t u r e I n i t i a t i o n C o n t r o l : V a r i o u s f r a c t u r e m e c h a n i c s c o n c e p t s a n d t e s t s h a v e b e e n d e v e l o p e d f o r a s s e s s i n g d u c t i l e f r a c t u r e i n i t i a t i o n . T h e g e n e r a l o b j e c t i v e i s t o d e t e r m i n e t h e c r i t i c a l s t r e s s i n t e n s i t y f a c t o r ( K c ) , w h i c h i s a m e a s u r e o f t h e m a t e r i a l t o u g h n e s s a n d w h i c h c a n b e r e l a t e d t o t h e c r i t i c a l d e f e c t s i z e t h a t c a u s e s d u c t i l e f r a c t u r e i n i t i a t i o n . T h e B a t t e l l e S t a t i c l o a d i n g f u l l s c a l e b u r s t s t u d i e s g e n e r a t e d a n e m p i r i c a l f o r m u l a e w h i c h r e l a t e s 16 a c r i t i c a l crack s i z e with the Charpy upper s h e l f ( 1 1 - 1 4 ) energies K c2 n 8a o £2 Where K, as given below: = In Sec 12EC V \ 2 o H H 2a o f MH d t OH Fracture Toughness parameter Impact t e s t absorbed energy ( f t - l b ) , J) Specimen fracture area (inch , mm ) C r i t i c a l through wall defect length (inch, mm) Flow stress = Y.S. ••+ 10,000 p s i (68.95 MPa) F o l i a s Correction factor, a function of Pipe diameter, (inch, mm) Wall thickness, (inch, mm) F a i l u r e Hoop Stress = E£ ( p s i , MPa) 2t E = E l a s t i c Modulus ( p s i , MPa) This equation i s widely used i n the pipeline industry to p r e d i c t the allowable defect size for arresting d u c t i l e crack i n i t i a t i o n . However, i t should be noted that t h i s equation i s applicable only over the temp- erature range of the Charpy upper s h e l f . Furthermore 1 7 i t i s f o r m u l a t e d f o r s t a t i c l o a d i n g c o n d i t i o n s a n d i s b a s e d u p o n c r a c k i n i t i a t i o n a l o n g t h e p i p e a x i s . I n p r a c t i c e , i n i t i a t i o n m a y o c c u r u n d e r d y n a m i c l o a d i n g c o n d i t i o n s a r i s i n g f r o m i m p a c t s d u e t o m e c h a n i c a l d a m a g e . F o r d e s i g n p u r p o s e s , p i p e l i n e c o m p a n i e s c a l c u l a t e t h e c r i t i c a l c r a c k s i z e t h a t w o u l d b e d e t e c t e d a s a l e a k d u r i n g h y d r a u l i c p r o o f t e s t i n g o r f r o m NDT t e c h n i q u e a n d t h e n d e t e r m i n e f r o m e q u a t i o n ( 3 . 2 ) t h e m i n i m u m C h a r p y u p p e r s h e l f e n e r g y n e c e s s a r y t o p r e v e n t t h e i n i t i a t i o n o f s u c h a c r a c k . 3 . 2 . 3 D e s i g n C r i t e r i a f o r D u c t i l e F r a c t u r e P r o p a g a t i o n a n d A r r e s t ; I t w a s o r i g i n a l l y b e l i e v e d t h a t u n s t a b l e p r o p a g a t i o n o f a s h e a r f r a c t u r e w o u l d n o t o c c u r a s m o s t s h e a r f r a c t u r e s w e r e a r r e s t e d o v e r s h o r t d i s t a n c e s ( 9 , 1 2 ) T h i s may b e t r u e b e c a u s e t h e s p e e d o f f r a c t u r e p r o p a g a t i o n r e d u c e s a s t h e g a s i n t h e p i p e l i n e e s c a p e s t h e r e b y l o w e r i n g t h e s t r e s s a t t h e t i p o f t h e f r a c t u r e . H o w e v e r , r e c e n t p i p e l i n e f a i l u r e s i n d i c a t e t h e o c c u r r - e n c e o f a t l e a s t e i g h t s h e a r f r a c t u r e s o f 1 0 0 m o r more in length in the U.S.A. and Canada. Therefore, to prevent such a failure i t was necessary to determine the toughness level sufficient to arrest a propagating ductile fracture. The Battelle research group suggested that the Charpy v-notch energy was adequate for specify- (2, 11, 1 ing a material's resistance to ductile failure From f u l l scale crack propagation studies, a relation- ship was established to show the minimum Charpy energy required to provide fracture arrest in large diameter pipelines. C v m 0.0873 o H 2 ( R t ) 1 / 3 A c (ft - lb, J) (3.3) Where o H = Operating stress level = 0.8 o v s specimen minimum yield stress SMYS (psi, MPa) R = Pipe Radius (inch, m) t = Pipe wall thickness (inch, m) A c = Area of Charpy specimen ligament (inch2, m2) An energy level of 80 f t - lb (108 J) is often used in pipeline steel specifications as an a l l heat a v e r a g e t o u g h n e s s v a l u e t o p r e v e n t d u c t i l e f r a c t u r e . U n f o r t u n a t e l y , t h e a b o v e r e l a t i o n s h i p h a s n o t a l w a y s c o r r e l a t e d w e l l w i t h r e s u l t s f r o m l a r g e r d i a m e t e r ( o v e r 4 2 i n c h , 1 0 7 cm) h i g h e r s t r e n g t h ( o v e r - 6 5 ( 1 1 - 1 3 ) g r a d e ) a n d h e a v i e r w a l l p i p e . F o r A F s t e e l s , i t i s i m p o s s i b l e t o a c c u r a t e l y s p e c i f y t h e ( 1 3 ) t o u g h n e s s r e q u i r e m e n t s f o r d u c t i l e f a i l u r e a r r e s t 3 . 3 R e v i e w o f t h e D e s i g n f o r F r a c t u r e C o n t r o l : T o m e e t t h e t o u g h n e s s r e q u i r e m e n t s t h e p i p e l i n e m a n u f a c t u r e r s c a r r y o u t t h e BDWTT a n d t h e s t a n d a r d C h a r p y v - n o t c h t e s t s w i t h s p e c i m e n s h a v i n g t h e i r c r a c k o r i e n t a t i o n s p a r a l l e l t o t h e p i p e a x i s ; t h i s i s d o n e b e c a u s e t h e p e a k s t r e s s i s t h e H o o p s t r e s s a n d t h i s s t r e s s o p e n s u p t h e c r a c k a l o n g t h i s d i r e c t i o n . B u t p i p e s t e e l s w h i c h a r e n o t p r o p e r l y d e s u l p h u r i s e d o r r a r e e a r t h t r e a t e d may e x h i b i t a n i s o t r o p y i n m e c h a n i c a l p r o p e r t i e s e s p e c i a l l y t o u g h - U S ) n e s s . H e n c e f o r s p i r a l w e l d e d p i p e m i n i m u m t o u g h n e s s p r o p e r t i e s t h a t a r e l e s s t h a n t h e s p e c i f i e d m i n i m u m , may b e o b t a i n e d f o r d i r e c t i o n s o t h e r t h a n t h a t p a r a l l e l t o t h e p i p e a x i s . T h e l o w e s t t o u g h n e s s w o u l d b e e x p e c t e d t o l i e a l o n g a d i r e c t i o n p a r a l l e l to the rolling direction i.e. (T - L) orientation. Hence the measurement of fracture toughness i s required in various orientations to establish the weakest crack i n i t i a t i o n condition; that combination of toughness and stress that results in a minimum in i t i a t i o n energy should be used to decide the necessary specification condition. Materials specification i s based on maintain- ing BDWTT of 85% shear and a minimum C v of 50 f t - lbs (67.8 J) at the lowest operating temperature. The shear specification ensures ductile fracture i n i t i a t i o n and the 50 f t - lbs (67.8J)c v toughness ensures ductile fracture propagation. However, the standard Charpy v-notch test does not provide crack i n i t i a t i o n inform- ation. The standard Charpy v-notch sample also uses a blunt notch which in no way represents a sharp crack condition as could be realized in service. 3 . 4 Instrumented Impact Test Approach; (15) Paul McConnell , in his M.A.Sc thesis, made an extensive study of the impact strength and fracture toughness of two acicular f e r r i t e , HSLA steels. His data w i l l be used for comparison with the results of the present investigation. In evaluation of the fracture toughness of the material, the IIT technique has proven to be rapid and inexpensive. It remains to assess i t s v a l i d i t y with respect to other fracture toughness test procedures. Fracture toughness data is obtained from an IIT using a Charpy specimen which has been, fatigue pre- cracked prior to fracture. The IIT test techniques provides a measure of the crack i n i t i a t i o n energy and the crack propagation energy under impact loading. Since the ligament size i s small, the measured propagation energy may not be a valid measure of extensive propagation; the combined i n i t i a t i o n and propagation energy i s equal to the C v energy for a fatigue precracked specimen. Since this method determines the dynamic properties of the material, the fracture toughness data w i l l be more conservative in the case of strain rate sensitive materials. McConnell also points out that the present pipeline toughness specifications which require a minimum toughness in the longitudinal axis may be inadequate for fracture control i f very low i n i t i a t i o n energies or toughness values less than the specified minimum, are present for a crack parallel to the roll i n g direction. For this reason he suggests that pipeline toughness specifications are necessary in a l l directions in the pipe especially in the weakest direction. The material toughness acceptance c r i t e r i a should be based on the magnitude of the i n i t i a t i o n energy from a precracked Charpy specimen as this simulates a sharp crack and therefore a peak stress intensity at the tip of the crack. Although the fracture toughness calculations in the IIT method are based upon the i n i t i a t i o n energy, the theoretical analysis of the process does not define clearly the i n i t i a t i o n event. Fracture i n i t i a t i o n i s assumed to occur at the point of maximum load; this may or may not be a valid assumption. The IIT method gives only the dynamic fracture toughness values K I d > Since the IIT method uses a 10mm x 10 mm square standard Charpy specimen, the stress intensity factor at the specimen crack tip may be different from that experienced at the tip of a defect in the thicker walled pipe. This i s true in that no valid plane strain fracture toughness data can be produced at the minimum operating temperature of the pipe; that i s at o - 18 C, due to the pipe wall thickness limitations. 3.5 Project Summary: A summary of the project proposal i s given below: 1. The study is to measure the fracture toughness values of two acicular f e r r i t e , HSLA pipe steels under static loading conditions; No fracture toughness values for these steels have been reported in the literature. This may be due to the fact that the reduced pipe wall thickness 0.54 inch (13.7 mm) makes i t impossible to obtain valid plane strain fracture toughness, K l c data for the range of Operating temperatures. The fracture toughness data is to be obtained for the fracture path a) parallel to rolling direction i.e. the T-L orientation, b) transverse to the r o l l i n g direction i.e. the L-T orientation, and c) parallel to the pipe axis. 2. Both the linear elastic fracture mechanics (LEFM) K I c and the elastic-plastic fracture mechanics (EPFM), J-Integral and COD approach w i l l be u t i l i s e d to determine the fracture toughness values throughout the complete temperature range extending from lower shelf through the trans- ition to the upper shelf energies. 3. The comparative study of the fracture toughness transition behaviour of both steels in each of the three test directions by the three test methods w i l l provide complete information on the anisotropy of the toughness and transition behaviour. 4. The fracture toughness data obtained w i l l be compared with reported IIT data for these steels to compare the static and the dynamic fracture toughness. It i s hoped that the analysis of the KjCf J l c and COD experimental data may contribute to a more fundamental basis for fracture control of the pipeline steels. 4.1 4. THEORY AND TEST PROCEDURES Linear Elastic Fracture Mechanics The fundamental principle of fracture mechanics is that the stress f i e l d at the tip of a crack in a structural component can be characterised by a single parameter, K, The Stress Intensity Factor. K i s related to the magnitude of the applied nominal stress, 'o' and the square root of the crack length ' a'« In general, the stress intensity factor is (16) given by K = f (g) a V * t 4- 1) Where f (g) * a parameter that depends upon the specimen and the crack geometry. For example, f (g) - , v / l l for an in f i n i t e plate containing a t h r o u g h thickness crack of length 2a and subjected to a (17) uniform tensile stress o . In this case the stress intensity factor reduces to For mode I deformation, the crack surfaces are displaced perpendicular to each other in opposite direction. The corresponding stress intensity factor i s represented by K In a thin sheet of metal under tensile loading, the stress at the crack tip in the thickness (18, 19) direction (033 = 0) tends to zero . A schematic distribution of the principle stresses at the crack tip i s shown in Fig. 4.1. A biaxial state of stress exists which i s commonly referred to as the Plane Stress Condition. As the thickness of the sheet is increased the crack tip is subjected to a t r i a x i a l state of (18, 19) stress which severely restricts straining or plastic deformation through the thickness. Such a state of stress i s known as Plane Strain. Figure 4.1. D i s t r i b u t i o n of p r i n c i p a l stresses at the cra c k - t i p . When the stress-intensity factor at the crack tip reaches a c r i t i c a l value, K_ unstable crack propagation, that i s , fracture, occurs. The c r i t i c a l stress intensity factor for static loading under plane stress conditions is designated as K c whereas K l c i s the c r i t i c a l stress intensity factor for static loading conditions under mode I deform- ation and a plane strain. K I (j represents the c r i t i c a l stress intensity factor for dynamic (impact) loading and plane strain. These c r i t i c a l values are described as the Fracture Toughness and represent a basic property of the material. From a knowledge of the Kj value for a structural component at service conditions (temperature and strain rate), a design engineer can estimate the flaw size that can be tolerated under a particular stress level (as equation 4.1). 4.1.1 Plane Strain Fracture Toughness: The linear elastic fracture mechanics (LEFM) analysis can be used for determining K j c only for the cases where the crack-tip plastic zone is small in relation to the specimen dimensions. For steels this occurs under the following conditions. i) Relatively b r i t t l e material i i ) Testing at a low temperature, normally below the service temperature i i i ) High rates of loading iv) Thick structural component. To determine a valid K I c^ the specimen should f a i l under completely elastic p l a n e strain conditions. With thinner sections, the c r i t i c a l combinations of load and crack length at ins t a b i l i t y gives K c > This K c value decreases with an increase in thickness; a constant minimum value, K I C f i s reached when plane strain conditions are attained. Therefore, K l c values are reproducible and are the minimum stress intensity factors. Hence i t i s termed the Plane-Strain Fracture Toughness property of a material and considered to be a material property analogus to the yield strength. The K I c value refers to quasi-static test conditions, that i s i t is determined at strain rates of approximately -5 10 / sec ; this corresponds to a stress intensity rate, K, of approximately 10 ksi < v / i n per second where K * ^ic Time to fracture For strain rate sensitive materials increasing the loading rate to that corresponding to an impact test, that i s approximately 10/sec (K = 10 ksi ^/in/sec) at a constant temperature causes a decrease in the plane strain fracture toughness to a minimum value known as the Dynamic Fracture Toughness 'Kid'. For o example, HY-80 steel at a test temperature of - 184 C ( - 300 F) exhibits a K I c « 67 ksi v^in at e - 5 x ~ 5 v /— (16) . 10 / sec and *Id = 43 ksi ̂ /in at e = 20 / sec. 4.1.2 Specimen Size Requirements: Some b r i t t l e materials exhibit plastic deformation at the crack t i p before unstable crack propagation takes place. This i s shown by the non- linearity of the load-displacement test records. The question arises, what is the size of the plastic zone that can be allowed while s t i l l satisfying the elastic plain strain requirement. The size of the plastic zone ahead of a crack 30 can be estimated from the equation for the elastic stress-field distribution at the crack tip (Fig. 4.1) at ( r,8) position in y - direction. K a y = — — COS I ( l + Sin -| Sin <4-3> For 9 =s o, along the x-axis o y = ( 4 . 4 ) Considering o y = OyS = yield strength of the material at the test temperature and strain rate employed, the extent of yielding at the crack tip i s At i n s t a b i l i t y Kj = KCf and therefore the plastic zone size under plane-stress conditions i s r . _ L 2 ( 4 . 6 ) r y 2lT \ o y s ) Under plane strain conditions the plastic zone radius at the center of a plate where the maximum constraintis realized, is equal to 1/3 of this value ( 2 0 ) , that i s 1 (4.7) 31 The major dimensions of the plate specimens for K I c testing are: a • crack length B = thickness and w-a = uncracked ligament ( w « overall depth) (21) After considerable experimental work , the following minimum specimen size requirements have been established to ensure elastic plane-strain behaviour: ( K * c \ a, B, w - a > 2.5 \ ays J Thus, i t i s observed that specimens satisfying the above requirements w i l l have a thickness « 50 times the radius of the plastic zone size. 4.2 Elastic - Plastic Fracture Mechanics In the previous section, i t has been shown that LEFM is applicable only to those situations where crack propagation i s accompanied by l i t t l e or no plastic deformation. Quantitatively this means that the extent of crack tip plasticity should be at least f i f t y times smaller than the dimensions of the structure including the crack length. Almost a l l low to medium strength and HSLA structural steels that are used for large complex structures such as bridges, ships, pressure vessels etc. are of insufficient thickness to maintain the plane strain conditions at the temperature and strain rate of the service conditions. Hence in such applications insufficient constraint i s available to maintain plane strain conditions and a large plastic zone forms. For pipe steels, neither the specimen nor the structure (the pipe) i s amenable to LEFM analysis. This i s shown clearly in Fig. 4.2 in which are shown typical schematic load-deflection curves for small specimens of various materials. Fig. 4.2 (a) depicts fu l l y linear behaviour which is easily handled by LEFM; Fig. 4.2 (b) shows a "Pop-in" behaviour which characterises the i n i t i a l crack growth, for a given material, regardless of the specimen thickness. Here, LEFM can also be used to calculate K I c by the offset procedure as described in the ASTM standard E-399-74;Fig. 42 (c) shows considerable non-linear behaviour in the load-deflection curve prior to sudden failure; while Fig. 4.2 (d) shows the behaviour of a ductile material where sudden failure never occurs. These non-linearities can arise from two sources, plastic deformation at the crack tip and (22) stable crack extension . Therefore, the test behaviour described in Fig. 4.2 (c) and (d) are the subject matters of Elastic-Plastic Fracture Mechanics (16, 22, (EPFM). In recent years considerable work 24, 28, 30 - 35) has been reported on the development of EPFM analyses as an extension of LEFM analyses. A — A — A - (a) (b) (c) P A - (d) Figure 4.2 Schematic Load (P) vs. Load-point displacement ( A ) curves for (a) p e r f e c t l y e l a s t i c material. (b) e l a s t i c material with pop-in behaviour. (c) e l a s t i c then p l a s t i c behaviour. (d) d u c t i l e material with extensive p l a s t i c i t y p r i o r to f a i l u r e . The two most promising and widely accepted techniques for analysing elastic-plastic fracture are 1. The J-Integral Method 2. The crack opening Displacement (COD) Method. 4.2.1 The J-Integral Approach; (23) The J-Integral, as proposed by Rice , is a way of characterising the stress-strain f i e l d ahead of a crack tip by an integration path such that J l at a distant f i e l d = J2 at the tip of the crack where J = fwdy - T i ds (4.8) r = a contour travelling counterclock- wise around the crack tip Ti = the tension vector perpendicular to r in an outward direction Ui » displacement in x - direction ds = an element of r w = / o^j e^j (Strain Energy density for elastic materials) Therefore, even i f considerable yielding occurs near the crack t i p , the region away from the crack t i p can be analysed and the condition in the crack t i p region can be derived. (24) Later, Hutchinson and Rice and Rosen- (25) gren described a stress-strain distribution around a crack tip surrounded by a plastic strain f i e l d . They developed a model known as the HRR crack tip model which establishes that J i s the amplitude of the near f i e l d singularity at the crack t i p . (26) McClintock has also shown that by combining J with the HRR crack tip model, the near tip values of stress and strain can be expressed as a function of J. This is directly analogus to the stress f i e l d equation of LEFM. (27) Rice has also shown that the J-Integral may be interpreted as the difference in potential energy between two identically loaded bodies with differing crack lengths. This is stated mathematic- ally as J = - £° (4.9) da where U = the potential energy where a = crack length For the linear elastic behaviour and also for small scale yielding, J i s therefore equal to G, the strain energy release rate per unit crack extension, i.e. the crack driving force. In cases where the deform- ation i s not reversible that i s the general elastic- plastic problem, J loses i t s physical significance as ( 2 8 ) a crack driving force. Begley and Landes suggest that J i s s t i l l equal to - a n d t n e physical significance of J for elastic-plastic materials i s that J i s a measure of the characteristic crack tip elastic-plastic f i e l d similar to K in LEFM. They made the additional suggestion that the J l c fracture criterion applies to crack i n i t i a t i o n rather than propagation and i s limited to the case of plane strain which i s denoted by the subscript I in J i c . ( 2 9 ) Later Rice et a l developed a simple, single specimen technique for measuring J I using the expression j T - 2A (4.10) J l B (w-a) where A » Area under the load vs load-point displacement curve B - Specimen thickness a = Crack length In this technique, a bending load i s applied to a bar or compact tension specimen containing a deep notch a ^ crack ^ i 0.6 and Jj i s determined as a function of the load-point displacement. The c r i t i c a l value of Jj i s then J l c which refers to that value of J j at which crack i n i t i a t i o n takes place. Once the J l c value i s determined, the corresponding K l c values can be computed from the relationship between the elastic-plastic and the (28, 30, 31) LEFM parameters J I C « G I C - 1 - v 2 K I c2 (4.11) Where v = Poisson's Ratio E - Young's Modulus 4.2.1.1 Experimental Technique; Several experimental techniques have (30 -33) been reported for determining the point of crack i n i t i a t i o n in a static J-Integral test. These 1. Heat Tinting (J-Resistance curve) 2. Compliance 3. Ultrasonic 4 . E l e c t r i c a l Potential 5. Resonance Frequency The heat tinting method i s simple and requires less sensitive electronic equipment in comparison with the other test methods. This i s the reason why this method has been selected for the present investigation. This method i s also known as the Resistance curve technique. It has been developed by Landes and (30) Begley . Briefly the testing procedure involves: a) Loading each specimen to a different displacement value b) Unload each specimen, mark the crack extension by heat tinting the crack. Heat tinting o of steels i s done by heating the specimen at 320 C for 10-20 minutes. c) Pull the specimen apart and measure the crack extension d) Construct a resistance curve by plotting J for each specimen vs i t s corresponding crack extension. In order to find out the Jjc value from the resistance curve Landes and Begley suggested the use of a best f i t line to the J vs crack size curve. The point of intersection of this curve with the line J = 2 O f i o w Aa gives the value of J l c at the test temperature and loading conditions. This is the most widely used method. The only disadvantage of this method is that i t requires several specimens usually 4 to 6 to draw the resistance curve. Currently, ASTM is preparing a standard for J-Integral testing. where ° f l o w oyield stress + oUTS .... (4.12) 2 4.2.1.2 Validity C r i t e r i a : (30, 34) Landes and Begley have proposed a size requirement which must be satisfied by an elastic-plastic fracture toughness test specimen to obtain valid J j c data. This size requirement is stated analytically as a, B, w-a > 25 J q (4.13) ' °flow If this condition i s satisfied, then J Q becomes Jjc. 4.2.2 The Crack-opening Displacement Method: For low to medium strength steels extensive plastic flow takes place before the in i t i a t i o n of the fracture. Under an externally applied load, the two faces of the crack tip move apart without an increase (36) in the length of the crack . The relative move- ment of the two faces at the crack tip has been termed the Crack-opening Displacement (COD) and has been (37) designated as '6' The consequence of yielding at the crack tip giving rise to physical displacement of the crack surfaces was f i r s t applied as a possible fracture (38) criterion by Wells 4.2.2.1 COD as an Extension to LEFM; For a material that exhibits appreciable crack tip pl a s t i c i t y , i t i s possible to develop a relationship between the stress intensity factor K and 6 near the tip for the crack tip opening dis- placement. The size of the plane stress plastic zone may be approximated by the relation *y ' n <4-l4> Where r v = the extent of the plastic zone along the crack plane O y S = yield strength of the material in a uniaxial tensile test. With this corrected crack border model, the y direction displacement, ' n', within the crack at any distance 'r' from i t s tip may be evaluated using Westergaard's (38) expression n = 2K / | r (4.15) E V E Now the displacement at the elastic-plastic interface corresponds to the displacement at the tip of the crack. Therefore, the crack opening displacement near the crack tip i s . « - *n - ^ - / ^ <«•"> Combining the equation for ry (4.14), the plastic zone K2 size and the relationship = G, (4.16) gives rise to £ * - 4 G (4.17) Y (38) This relationship was developed by Wells . He inferred that under local plastic conditions, COD gives a measure of the crack extension force G. Thus the COD can be related to the plane-strain fracture toughness. This also indicates that i f the COD is large for a specified value of yield stress such that oyS<5 exceeds the c r i t i c a l crack extension force G, then (39) fracture follows . Hence the COD measurement in the presence of extensive plastic deformation ahead of the crack tip for elastic-plastic and f u l l y plastic behaviour, is an index of the fracture toughness and is a direct extension of LEPM into yielding materials. 4.2.2.2 Dugdale's Model; (40) Dugdale proposed a strip yield model as shown in Fig. 4.3. This model describes the extent of yielding ahead of a crack as a function of the external load. A thin sheet containing a straight cut of length 2a i s loaded in a direction perpendicular to the cut. It is assumed that y i e l d i n g occurs ahead of the cut by an extent a^ - a and i s confined to a narrow band lying along the line of the cut. This model also suggests that the stresses in the yielded zone may be considered to be a continuous distribution of point loads °ys.dt per unit thickness which act to restrain the crack from opening. An expression for the restraining stress intensity factor may be obtained by integrating the appropriate Westergaard Stress function for point Figure 4.3. Dugdale's S t r i p Y i e l d Model. loads in cracks from a to a i which gives 2 'y&f C o s _ 1 ( s i ) < 4 - 1 8 ' Where the stress intensity factor for the opening of the crack under the applied stress o and the total crack length aj i s K = o .yiiai (4.19) Therefore, the extent of yielding may be given by — = Cos J £ — (4.20) a l 2 °ys Dugdale's analysis also suggests that the displacement at the original crack t i p , the COD - $, increases as the crack length increases or as the applied loading increases. Hence an extension of Dugdale's analysis results in the following relation- ship between COD, the crack length a, and the applied stress = ( 3 7' 4 1 ) Using a standard method of series expansion due to (16) McLaurin this expression gives: 45 (4.22) ] For ° / 0 v s « 1# taking only the f i r s t term of the series « • « . » ) ys Since Kj « â /na", the above expression can be written as K j 2 * 6 E o y s (4.24) ° r ' - i - = ( ^ M * (4.25) eys v °ys ' as E - !2! At the onset of crack instability where Kj reaches K I c , t* i e C 0 D reaches a c r i t i c a l value, 6Cf 2 6 c _ / * i c Cy S ^^"ys ( o ^ - ) ( 4 - 2 6 ) This expression shows that 6c is a measure of the eys c r i t i c a l crack size in a structure exhibiting elastic- plastic behaviour. Therefore, the crack opening dis- placement, <*c, i s a material property like K l c and i s a function of the test temperature and loading rate. The advantages of using the COD approach are: 46 i) The COD values can be measured through- out the entire span of the plane strain, the ela s t i c - plastic and the fu l l y plastic regions. i i ) A much smaller size test specimen i s required. The K j c values can be measured only under plane strain conditions and often require the use of a prohibitively large size specimen. 4.2.2.3 Experimental Determination of COD; (36, 37* 39, 42 - 44) Several authors have described different techniques for the experiment- al determination of COD. However, the British Standards (45) Institution Draft for Development 19:1972 i s the only document which gives the details of a recommended procedure for COD testing. The DD19 test method i s very similar to the ASTM E399-74 test method for K l c Similar specimen preparation, fatigue precracking procedures, and instrumentation and test procedures are followed. The displacement gauge i s similar to the one used in K l c testing and a continuous load- displacement record i s obtained during the test. (39) Egan's evaluation of the fracture toughness of materials using the COD technique shows that a single specimen test procedure may be used to determine both K_ and 6_. 1 From the load-displacement curve, the c r i t i c a l value of displacement i s recorded at the point where a specified amount of crack growth has occurred. In the British Standard Test Procedure, the crack opening displacement i s usually calculated by assuming that plastic extension has occurred at the crack tip up to the point of maximum load. This assumes that crack extension initiates at the maximum load. The c r i t i c a l displacement at the tip of the crack 6 C - COD i s determined from the c r i t i c a l value of the c l i p gauge displacement, Vg^ as obtained from the P-A record. DD19 suggests several methods for obtaining « c. A l l of these methods assume that deformation occurs by a hinge mechanism about a center of rotation at a depth of r(w-a) below the crack t i p . The relationship between the c l i p gauge displacement V_ and <5C i s : Where Z = Knife edge thickness i.e. the distance above the test piece surface at which point the measurement is made a j= crack length w = test piece width r = rotational factor (46) T. Ingham et. a l suggested that on the basis of tests on a wide range of materials and geometries that 1 a fixed value of r = 5 can be used to obtain conservative 44 values of COD. Several works (Robinson and Tetelman , 47 R.R.Barret et. a l ) confirm this viewpoint. 4.2.2.3.1 Determination of 6C. The British Standard method of using the maximum load point for calculating the (48) c r i t i c a l C0D-6C^ has been c r i t i c i s e d . This may be due to the fact that 6 C i s defined as the value of COD which should correspond to the onset of stable crack growth - similar to J l c in the J-Integral approach. It should be noted that in K l c testing, the crack i n i t i a t i o n load PQ is taken to be the load corresponding to 2% beyond the yield point. S e v e r a l m e t h o d s h a v e b e e n s u g g e s t e d f o r d e t e c t i n g t h e 6_ v a l u e a s s o c i a t e d w i t h t h e o n s e t o f C ( 4 9 ) s t a b l e c r a c k g r o w t h . S m i t h a n d K n o t t ' s t e c h n i q u e i n v o l v e s l o a d i n g s e v e r a l s p e c i m e n s t o v a r i o u s s t a g e s o n t h e l o a d - d e f l e c t i o n c u r v e f o l l o w e d b y u n l o a d i n g . T h e e x t e n t o f c r a c k g r o w t h i n e a c h s p e c i m e n i s t h e n m a r k e d b y h e a t t i n t i n g . T h e c r a c k g r o w t h v a l u e i s p l o t t e d a g a i n s t c r a c k o p e n i n g d i s p l a c e m e n t . T h e d i s p l a c e m e n t a s s o c i a t e d w i t h t h e o n s e t o f c r a c k i n g £ c i s o b t a i n e d b y e x t r a p o l a t i n g t h e c r a c k e x t e n s i o n b a c k t o z e r o . T h i s m e t h o d i s t h e s a m e a s t h a t u s e d i n ( 4 8 ) d e t e r m i n i n g J l c D i e s b e r g h a s c a l c u l a t e d « c o n t h e b a s i s o f a m a x i m u m s t r e t c h z o n e . H e h a s d e f i n e d t h i s COD a s t h e c r a c k - o p e n i n g s t r e t c h v a l u e , C O S . w i t h t h e h e l p o f t h e r e l a t i o n s 8 ° v s a , / no \ « c » COS ' • j f i g — l o g 6 S e c ( ) . . . ( 4 . 2 8 ) ys a ? 7 « r C ° S " B ^ ( 4 - 2 9 ( a n d J _ » 2 O £ l o w A a c . . . . . . ( 4 . 3 0 ) a n d e x p e r i m e n t a l v a l u e s o f J I c h e h a s d e t e r m i n e d t h e COS v a l u e . H i s COD v a l u e s w e r e m u c h l o w e r t h a n t h o s e o b t a i n e d b y a s s u m i n g t h a t t h e c r a c k i n i t i a t e d a t t h e point of maximum load. 4.2.2.3.2 Evaluation of an Equivalent K l c from COD: From linear elastic fracture mechanics K l c2 (1 - v 2) 6 = _i£_ V i 1 (4.31 E and from Well's treatment for the c r i t i c a l value of COD «c = f < 4« 1 7> ys Equation 4.17 can be written as G = X 6 C 0 y S (4.32) Where X - a constant.Several theoretical analyses predict different values of X e.g. 1, n/4, 1.27, 1.48, 2 and 2.4. This variation in X values may be due to the differences in the definition of the COD values. (37) However, calculations by Burdekin and Stone , (41) CIST" Bilby et. a l , Rice and Rosengren a l l yielded results identical with experiment for X = 1. Therefore, using A = 1 and equating G from equations (4.31) and (4.32), the following expression results K 2 -1°- (1 - v2) . fic 0 y s (4.33) This in turn gives rise to an equivalent K I c from the c r i t i c a l COD values 52 5 . E X P E R I M E N T A L 5 * 1 T e s t M a t e r i a l s : S e c t i o n s o f s p i r a l w e l d e d p i p e f r o m p r o d u c t i o n h e a t s w e r e s u p p l i e d b y t w o C a n a d i a n S t e e l M a n u f a c t u r e r s f o r t h e t e s t p r o g r a m . B o t h o f t h e p i p e p r o d u c t s w e r e 4 2 i n c h ( 1 0 7 cm) o u t s i d e d i a m e t e r , w i t h a 0 . 5 4 - i n c h ( 1 3 . 7 mm) w a l l t h i c k n e s s a n d w e r e r a t e d a s a n x - 7 0 g r a d e s t e e l ( m i n i m u m y i e l d s t r e n g t h o f 70 k s i ) . T h e c h e m i c a l c o m p o s i t i o n s o f t h e s e s t e e l s a r e g i v e n i n t a b l e . T a b l e N o . 5 . 1 S t e e l C o m p o s i t i o n s C Mn MO N b S i A l S P C U N i C r S n T i C e A F - 1 0-0 5 1-9 3 0-2 6 •063 0-0 3 — •023 •0 12 0-2 <• 010 0-0 H 0-0 2 — — A F - 2 0-0 6 1-8 2 <H»5 0-0 5 0-2 6 •0M5 •006 •0 0 6 •0 3 7 •0 2 7 •0 6 8 •0 0 5 •0 0 2 •0 3>» T h e A F - 2 s t e e l c o m p o s i t i o n i n d i c a t e s t h a t i t i s f u l l y k i l l e d a n d r a r e e a r t h t r e a t e d . T h e A F - 1 s t e e l c o n t a i n s 53 c o n s i d e r a b l y m o r e S a n d i s a s e m i k i l l e d s t e e l . 5 . 2 S p e c i m e n P r e p a r a t i o n ; S a m p l e s o f t h e A F - 1 s t e e l w e r e c u t f r o m s e c t i o n s o f t h e p i p e . S a m p l e s o f t h e A F - 2 s t e e l w e r e c u t f r o m s m a l l e r s e c t i o n s o f t h e p i p e o b t a i n e d b y a c e t y l e n e c u t t i n g . I n b o t h c a s e s s p e c i m e n s w e r e c u t u s i n g a n a u t o m a t i c h a c k s a w ; n o s p e c i m e n s w e r e c u t a n y c l o s e r t h a n 5 0 mm f r o m a f l a m e c u t e d g e . T h e t e s t s a m p l e s w e r e c u t s o t h a t t h e t h r o u g h - t h i c k n e s s m a c h i n e d n o t c h c o u l d h a v e o n e o f t h r e e d i f f e r e n t o r i e n t a t i o n s . 1) P a r a l l e l t o t h e p i p e a x i s . 2 ) P a r a l l e l t o t h e r o l l i n g d i r e c t i o n . 3) T r a n s v e r s e t o t h e r o l l i n g d i r e c t i o n . o T h e r o l l i n g d i r e c t i o n w a s a t a n a n g l e o f 6 3 t o t h e o p i p e a x i s f o r t h e A F - 1 p i p e a n d 4 5 t o t h e p i p e a x i s f o r t h e A F - 2 p i p e , , 5 . 3 S p e c i m e n C o n f i g u r a t i o n a n d D i m e n s i o n s : 5 . 3 . 1 C o m p a c t T e n s i o n S p e c i m e n : C o m p a c t t e n s i o n s p e c i m e n s w e r e u s e d f o r a l l o f t h e f r a c t u r e t o u g h n e s s t e s t s , t h e K I C t e s t , t h e J - I n t e g r a l t e s t a s w e l l a s t h e COD t e s t . S i n c e t h e p i p e w a l l t h i c k n e s s w a s 0 . 5 4 i n c h ( 1 3 . 7 m m ) , t h e m a x i m u m f a b r i c a t e d s p e c i m e n t h i c k n e s s w a s 0 . 5 0 i n c h . F i g . 5 . 1 i l l u s t r a t e s t h e s i z e o f t h e c o m p a c t t e n s i o n s p e c i m e n u s e d i n t h i s s t u d y . T h e s p e c i m e n d i m e n s i o n s c o n f o r m t o t h e s p e c i m e n r e q u i r e m e n t s d e s c r i b e d i n t h e A S T M E 3 9 9 - 7 4 s t a n d a r d f o r p l a n e s t r a i n f r a c t u r e t o u g h n e s s t e s t i n g . T h e t h r e e t e s t d i r e c t i o n s w e r e e x a m i n e d ( 1 5 ) f o r t h e f o l l o w i n g r e a s o n s i ) C r a c k p a r a l l e l t o p i p e a x i s ; T h i s i s c o n s i d e r e d t o b e t h e m o s t i m p o r t a n t o r i e n t a t i o n b e c a u s e f u l l s c a l e b u r s t t e s t s i n d i c a t e t h a t f a i l u r e s ( 5 0 , 5 1 ) d o p r o p a g a t e a l o n g t h e p i p e a x i s . T h e m a x i m u m o p e r a t i n g s t r e s s i n p i p e l i n e s i s t h e h o o p s t r e s s w h i c h t e n d s t o o p e n c r a c k s p a r a l l e l t o t h e a x i s o f t h e p i p e . i i ) C r a c k p a r a l l e l t o R o l l i n g D i r e c t i o n ? A l t h o u g h t o u g h n e s s s p e c i f i c a t i o n s r e q u i r e t e s t i n g o n l y i n t h e p i p e a x i s o r i e n t a t i o n , i t i s i m p o r t a n t t o d e t e r m i n e t h e e f f e c t o f t h e d i s t r i b u t i o n a n d s p a c i n g 55 FIG. 5.1. 0-5 inch Thick Compact Tension Specimen. of non-metallic inclusions etc. For this reason samples were cut with the crack running parallel to the rolling direction; these were designated as T-L orientation and i t i s expected to be the weakest direction of a material after r o l l i n g . i i i ) Crack transverse to roll i n g direction: This orientation i s designated as the L-T orientation. The pipe i s known to possess a maximum upper shelf (52) energy along this orientation . This direction was included to determine the maximum toughness attain- able in the structural component. 5.3.2 Tensile Specimens: In order to assess the fracture toughness validity c r i t e r i a and the equivalent Kjg data from COD measurements at the test temperature and strain rate conditions for each specimen orientation, i t i s necessary to know the yield strength of the material. The orientation of the tensile specimen with respect to the axis of the compact tension specimen and the specimen dimensions are presented in Fig. 5.2. The flow stress of both the steels for a l l 57 (b) Figure 5.2. (a) Orientation of CT specimens with respect to Rolling Direction of the plate. (b) Dimensions of the tensile specimen. orientation and test conditions are also required to calculate the J l c values. Therefore, appropriate tensile tests were conducted along with fracture toughness tests. Substandard sized tensile specimens having dimensions proportional to the standard were used to enable testing at sub-zero temperatures; i t was necessary to immerse the specimen and the testing fixtures into a temperature controlled bath and to complete the test in this environment. 5.4 Fatigue Precracking; The compact tension specimens were fatigue precracked before testing. This was necessary for the following reasons: 1) The validity of the K I c^ « c and J l c values are dependent upon the establishment of a sharp crack at the tip of the machined notch. 2) The fatigue crack simulates a sharp internal crack which might exist inside the material as a result of processing and would remain undetected by standard NDT techniques„ T h e n o t c h e d s p e c i m e n s w e r e c l e a n e d a n d d e g r e a s e d . T h e s u r f a c e o f e a c h s p e c i m e n w a s p o l i s h e d t o a 6 0 0 g r i t e m e r y p a p e r ; t h e 6 0 0 g r i t p o l i s h i n g l i n e s r u n n i n g t r a n s v e r s e t o t h e n o t c h . T h e m a x i m u m e x t e n s i o n o f t h e f a t i g u e c r a c k d u r i n g t h e p r e - c r a c k i n g o p e r a t i o n w a s r e a d i l y v i s i b l e o n e i t h e r s u r f a c e . F a t i g u e p r e - c r a c k i n g w a s p e r f o r m e d u s i n g a S o n n t a g F a t i g u e T e s t i n g M a c h i n e , M o d e l S F - l - U , o p e r a t e d w i t h a m a n u a l p r e l o a d . T h i s e q u i p m e n t i n t r o d u c e s a c y c l i c l o a d w h i c h i s s y m m e t r i c a l w i t h r e l a t i o n t o t h e n o t c h . 5 . 4 . 1 A S T M S t a n d a r d f o r P r e c r a c k i n g ; T h e A S T M s t a n d a r d f o r d e t e r m i n i n g t h e p l a n e s t r a i n f r a c t u r e t o u g h n e s s o f m e t a l l i c m a t e r i a l s s t i p - u l a t e s c e r t a i n i m p o r t a n t p r e r e q u i s i t e s f o r f a t i g u e p r e c r a c k i n g w h i c h a r e l i s t e d b e l o w . . i ) T h e f a t i g u e c r a c k i s t o b e e x t e n d e d f r o m t h e n o t c h a t l e a s t 0 . 0 5 i n c h ( 1 . 3 m m ) . i i ) T h e r a t i o o f t h e m a x i m u m s t r e s s i n t e n s i t y o f t h e f a t i g u e c y c l e t o t h e Y o u n g ' s M o d u l u s K f ( m a x ) / E s h a l l n o t e x c e e d 0 . 0 0 2 i n a ( 0 . 0 0 3 2 m*i) . i i i ) K f ( m a x ) m u s t n o t e x c e e d 6 0 % o f t h e KQ v a l u e . i v ) W h e n f a t i g u e c r a c k i n g i s c o n d u c t e d a t a t e m p e r a t u r e a n d t e s t i n g a t a t e m p e r a t u r e o f T 2 K f ( m a x ) m u s t n o t e x c e e d 0 # 6 ° y s l — KQ ° y s 2 W h e r e o _ a n d o v s a r e t h e y i e l d s t r e n g t h s o f t h e y 1 y 2 m a t e r i a l a t t h e r e s p e c t i v e t e m p e r a t u r e s . 5.4 . 2 P r e c r a c k i n g S t r e s s I n t e n s i t y K f d a a x ) . T h e p r e v i o u s f r a c t u r e t o u g h n e s s d a t a o n ( 1 5 ) t h e s e s t e e l s f r o m I I T t e s t s i n d i c a t e d t h a t t h e KQ v a l u e s o b t a i n e d a t a m b i e n t t e m p e r a t u r e m a y b e g r e a t e r t h a n 8 8 k s i V i n ( 9 6 . 7 1 M P a V m T . I n t h i s i n v e s t i g a t i o n , f o r p r e c r a c k i n g w a s a s s u m e d t o b e 1 0 0 k s i ^ n ( 1 1 0 M P a ^ ) . C o n s i d e r i n g | » 0 . 5 0 a n d K f ( m a x ) = m t o *Q, t * i e s t r e s s c y c l e t o b e e m p l o y e d w a s c a l c u l a t e d . A s a m p l e c a l c u l a t i o n a n d p f ( m a x ) ( i n l b s ) f o r c o r r e s p o n d i n g K f ( m a x ) v a l u e s a r e s h o w n i n A p p e n d i x - I . I n t h i s w o r k , a v a l u e o f K f ( m a x ) * 1 5 t o 18% KQ w a s f o u n d t o b e a d e q u a t e t o g e n e r a t e f a t i g u e p r e c r a c k i n g i n t h e s p e c i m e n s . T h i s l o v e r v a l u e o f t h e s t r e s s i n t e n s i t y f a c t o r w a s u s e d t o m i n i m i z e t h e p l a s t i c d e f o r m a t i o n a h e a d o f t h e c r a c k t i p . F o r t h e K g a n d COD t e s t s , f a t i g u e p r e - c r a c k i n g w a s d o n e s u c h t h a t 0 . 4 5 < a < 0 . 5 5 w F o r t h e J - I n t e g r a l t e s t s , t h e s p e c i m e n s a w e r e d e e p l y n o t c h e d t o a t t a i n w > 0 . 6 . N o r m a l l y , 0 . 6 < a < 0 . 7 w a s o b t a i n e d , w T h e t i m e r e q u i r e d f o r p r e c r a c k i n g t h e KQ a n d COD s p e c i m e n s v a r i e d f r o m a p p r o x i m a t e l y 1 5 t o 2 0 m i n u t e s ; i n t h e c a s e o f t h e J - I n t e g r a l s p e c i m e n s , a p p r o x i m a t e l y d o u b l e t h e t i m e w a s n e e d e d . T h e d e t a i l s o f t h e p r e c r a c k i n g a r e i n d i c a t e d i n A p p e n d i x - I I . 5 . 5 K i c T e s t P r o c e d u r e : a t : K I c f r a c t u r e t o u g h n e s s t e s t i n g w a s a i m e d a) Obtaining a load (P) vs. load-point displacement ( A)curve. b) Establishing the notch-toughness behaviour of the materials in the transition temp- erature range. c) Obtaining an accurate measurement of the load-point displacement so as to obtain the c r i t i c a l value of the crack-opening displacement. Therefore, one test technique was used to obtain the K I C and the COD data. The c r i t i c a l value of the load (P) from the P-A record was uti l i s e d to calculate the KQ or K I C values and the c r i t i c a l value of displacement (A) was used to calculate the COD values. 5.5.1 Test Fixtures and Displacement Gauge; To measure the load-point displacement at room temperature and also at sub-zero temperatures o down to the liquid nitrogen temperature (-196 C), the following fixture was designed and constructed. Two concentric, closely f i t t i n g brass tubes, were attached to the loading pins in such a way that when the pins m o v e d a p a r t , t h e i n n e r t u b e m o v e d w i t h r e s p e c t t o t h e o u t e r o n e . F o r d i s p l a c e m e n t m e a s u r e m e n t s , a 5 0 % - 1 i n c h e x t e n s o m e t e r w a s c l a m p e d t o t h e s e t u b e s . E c c e n t r i c i t y o f l o a d i n g d u e t o m i s a l i g n m e n t o f t h e l o a d i n g p i n s w i t h t h e h o l d i n g f i x t u r e w a s a v e r t e d b y a p p l y i n g a m i n o r l o a d o f a p p r o x i m a t e l y 1 0 - 2 0 l b s p r i o r t o e x p e r i m e n t a t i o n . T h e d e s i g n a n d d i m e n s i o n s o f t h e t u b e s a n d t h e e x p e r i m e n t a l s e t - u p a r e s h o w n i n F i g . 5 . 3 . 5 . 5 . 2 T e s t D e t a i l s : S t a t i c K j £ t e s t s w e r e c a r r i e d o u t u s i n g t h e I N S T R O N m a c h i n e . A l m o s t a l l o f t h e t e s t s w e r e c o n d u c t e d a t a f u l l s c a l e l o a d o f 5 0 0 0 l b s ( 2 2 . 2 5 K N ) , w i t h t h e s t r a i n g a u g e p r e a m p l i f i e r s e t t i n g a t t h e 2 x r a n g e , a c h a r t s p e e d o f 1 i n c h p e r m i n u t e a n d a c r o s s - h e a d s p e e d o f 0 . 0 5 i n c h p e r m i n u t e . U s i n g t h e s e s e t t i n g s 1 0 i n c h e s o f c h a r t i s e q u i v a l e n t t o a d i s p l a c e m e n t o f 0 . 0 5 i n c h . T h e r e f o r e , 1 - i n c h o f c h a r t g i v e s a m e a s u r e m e n t o f 0 . 0 0 5 i n c h a n d 1 d i v i s i o n , 0 . 0 0 0 5 i n c h . T o e s t a b l i s h t h e n o t c h t o u g h n e s s t r a n s i t i o n b e h a v i o u r o f t h e m a t e r i a l s , t e s t s w e r e c o n d u c t e d o v e r 64 Figure 5.3(a) Dimensions of Brass Tubes. (b) Photograph of the Experimental Set-up. 6 5 o o a r a n g e o f t e m p e r a t u r e s e x t e n d i n g f r o m 2 0 t o 2 5 C (RT) o o o o o o t o - 4 0 C , - 6 0 C , - 8 0 C , - 1 0 0 C , - 1 1 0 C , - 1 3 0 C , o o - 1 5 0 C a n d - 1 9 6 C . I n i t i a l l y , i t w a s t h o u g h t t h a t o t e s t s t o - 1 0 0 C w o u l d e s t a b l i s h t h e t r a n s i t i o n b e h a v i o u r , o b u t t h e r e s u l t s d o w n - 8 0 C d i d n o t d e v i a t e a p p r e c i a b l y f r o m t h e u p p e r s h e l f t o u g h n e s s l e v e l . T h e r e f o r e , t o g e t t o u g h n e s s v a l u e s i n t h e t r a n s i t i o n r a n g e , e x p e r i - o o m e n t s w e r e c a r r i e d o u t b e l o w - 1 0 0 C a n d t e s t s a t - 6 0 C w e r e d i s c o n t i n u e d . 5 . 5 . 3 L o w T e m p e r a t u r e T e s t s : L o w t e m p e r a t u r e t e s t s w e r e c o n d u c t e d u s i n g d i f f e r e n t b a t h s d e p e n d i n g u p o n t h e t e m p e r a t u r e d e s i r e d . T h e t e s t s w e r e c a r r i e d o u t k e e p i n g t h e s p e c i m e n , t h e s p e c i m e n h o l d i n g f i x t u r e , t h e l o a d i n g p i n s a n d t h e b r a s s t u b e s i m m e r s e d i n s i d e t h e c o n s t a n t t e m p e r a t u r e b a t h a s s h o w n i n F i g . 5 . 3 . T h e s t r a i n g a u g e w a s k e p t a b o v e t h e l e v e l o f t h e b a t h i n a l l e x p e r i m e n t s . o T e s t s d o w n t o - 8 0 C w e r e d o n e u s i n g a b a t h o f e i t h e r d e n a t u r e d a l c o h o l o r a 6 0 - 4 0 e t h a n o 1 - m e t h a n o 1 m i x t u r e . T e s t s a t - 1 0 0 C , - 1 1 0 C a n d s o m e t e s t s a t o - 1 3 0 C w e r e a l s o c a r r i e d o u t u s i n g t h e 6 0 - 4 0 m i x t u r e . o T h e t e s t s a t - 1 5 0 C w e r e c a r r i e d o u t i n a n i s o p e n t a n e o ( D i m e t h y l B u t a n e , F r e e z i n g P o i n t * - 1 6 0 C ) b a t h . A l l l o w t e m p e r a t u r e e x p e r i m e n t s w e r e d o n e k e e p i n g t h e s p e c i m e n i n s i d e t h e b a t h f o r a m i n i m u m t i m e o f 2 0 m i n u t e s . T h e t e m p e r a t u r e o f t h e s p e c i m e n w a s m e a s u r e d b y p l a c i n g a c h r o m e l - a l u m e l t h e r m o c o u p l e j u n c t i o n a d j a c e n t t o t h e s a m p l e . T h e b a t h t e m p e r a t u r e + ° w a s c o n t r o l l e d t o a n a c c u r a c y o f — 1 C . I n a l l o f t h e e x p e r i m e n t s (RT a s w e l l a s l o w t e m p e r a t u r e ) , t h e s t r a i n g a u g e w a s m o n i t o r e d u n t i l t h e m a x i m u m l o a d w a s a t t a i n e d . T h e g a u g e w a s t h e n d i s c o n n e c t e d a n d t h e t e s t w a s c o n t i n u e d u n t i l t h e s p e c i m e n w a s b r o k e n i n t o t w o h a l v e s f o r m e a s u r e - m e n t p u r p o s e s a n d t o a l l o w e x a m i n a t i o n o f t h e f r a c t u r e o s u r f a c e s . F o r t h e t e s t s a t - 1 3 0 C a n d b e l o w , t h e s p e c i m e n s b e c a m e s o b r i t t l e t h a t t h e f r a c t u r e w a s a l w a y s u n s t a b l e a n d t h e s p e c i m e n s a l w a y s b r o k e i n t o t w o p i e c e s . 5 . 5 . 4 T e s t R e c o r d s : T e s t r e c o r d s o f a n a u t o g r a p h i c p l o t o f t h e o u t p u t o f t h e l o a d - s e n s i n g t r a n s d u c e r v s . t h e o u t p u t o f t h e d i s p l a c e m e n t g a u g e w e r e o b t a i n e d . T y p i c a l t e s t r e c o r d s a r e s h o w n i n F i g . 5 . 4 ( a ) a n d ( b ) . 5 . 5 . 5 M e a s u r e m e n t s o f t h e T e s t P i e c e D i m e n s i o n s a n d C r a c k L e n g t h ; B e f o r e c a r r y i n g o u t t h e e x p e r i m e n t s B , W, L , o f t h e s p e c i m e n s w e r e m e a s u r e d t o t h e n e a r e s t 0 . 0 0 1 i n c h ( . 0 2 5 m m ) . L w a s t h e t o t a l l e n g t h o f t h e s p e c i m e n . A f t e r f r a c t u r e , t h e c r a c k l e n g t h w a s m e a s u r e d a t 2 5 , 5 0 a n d 75% B ( a i , a 2 , a 3 , ) « a i + a 2 + a 3 T h e r e f o r e , a = ^ I n a l l s p e c i m e n s , t h e f a t i g u e c r a c k t i p r e m a i n e d i n a s i n g l e p l a n e . I n a l l c a s e s t h e a / w r a t i o w a s f o u n d t o b e 0 . 4 5 < a < 0 . 5 5 . w 5 . 5 . 6 A n a l y s i s o f the E x p e r i m e n t a l D a t a : o T h e RT a n d - 4 0 C P - A r e c o r d ( F i g . 5 . 4 ( a ) ) Fig. 5.4(b) Actual P - A Test Record for AF-1 Steel at - 150°C. were typical of ductile behaviour indicating a high level of notch toughness, whereas the P-A test records o at temperatures - 130 C and below were characteristic of b r i t t l e plane strain behaviour. The test records resembled type - III as given in the ASTM E399-74 standard as shown in Fig. 5.4 (b). In a l l cases, the 5% secant offset procedure was adopted to measure the PQ load value. The Pmax ratio was observed to be greater than 1.10 for o o RT, - 40 and - 60 C tests and less than 1.10 for tests conducted at lower temperatures. Through the linear elastic portion of the P-A curve, a best f i t t i n g straight line was drawn cutting the A-axis. A 5% offset line was then drawn through this point on the A-axis. The point where the offset cuts the curve was taken as the PQ value. From the a/w ratio, f(a/w) was calculated using the table for compact tension specimens. KQ in psi (MPa/m.) was then calculated with the help of the relation. KQ = _!Q f (a/w) (5.1) Finally the validity c r i t e r i o r for K l c testing was calculated using the relation 2 Where B >2.5 'ys KQ (5.2) 'ys 0.2% offset yield strength at the respective test temperature and strain rate. 5.6 COD Test Details: Since COD tests and K I c tests are essentially the same, a single test was conducted to obtain both types of data. At each test temperature a minimum of two compact tension specimens were tested. In certain cases, where COD results or K j c results showed a wide scatter, up to 4 specimens were tested. 5.6.1 Assessment of Test Data: Some judgment was necessary to determine the c r i t i c a l value of displacement associated with the onset of unstable fracture. For the smooth curves obtained at RT and - 40 C, the c r i t i c a l displacement value was taken as that occurring at the maximum load, including elastic-plastic components. This value of displacement i s termed V M and indicates the displacement of the loading pins. For comparison purposes and also to understand the basic mechanism of crack i n i t i a t i o n a displacement value corresponding to P Q was measured and reported. This displacement reading was termed VQ. In low temp- erature tests, VQ > V M ; hence the measurement VQ was discontinued. Since 1 inch of chart corresponded to 0.005 inch displacement, VQ and V M values were obtained by multiplying chart readings by 0.005 inch. 5.6.2 Calculation of 6C: Raving obtained the c r i t i c a l values of strain gauge displacements ( V M and VQ), i t was necessary to convert these to the true c r i t i c a l COD(6c - «m or 6Q) at the crack t i p . V C values were converted to the correspond- ing 6 C value using the generalised relationship Where r r o t a t i o n a l factor 0.33 z = knife edge thickness In the present experimental technique adopted z - o. Therefore, 6C = V ° ( v - a)' w + 2a correspondingly, 6 m = V n > < w ' a > and *Q VQ ( w - w + 2a w 2 a 5.6.3 Ca l c u l a t i o n of Equivalent K I C : — -.- Once the c r i t i c a l value of COD at the crack t i p i s obtained, an equivalent value of K j c was calculated using the r e l a t i o n s h i p Gic " ^ d- v 2) " °ys «ic (44,54) which gives f " 'S. 6 I c K I C - v -—ir-^T The value of the equivalent Kic i s obtained i n k s i ^ / i n and i s converted to MPâ ŷ n unit multiply- ing by 1.099 73 5.7 J-Integral Test Details 5.7.1 Testing Parameters; The heat tinting technique which i s the same as the Resistance curve test technique developed by Landes and Begley was adopted in this investigation. CT specimens 0.5 inch thick having crack lengths 0.6<a/w o < 0.7 were used. For RT and - 40 c temperature tests a f u l l resistance curve was developed. For tests at - o 80 C and below, a f u l l resistance curve was not necessary as the slope of the J - Aa curve tends to become zero due to the ductile-brittle transition. Therefore, for RT and o - 40 C tests 5 to 6, CT specimens were used and for other temperatures a minimum of two specimens were used. Each specimen was loaded, with increasing dis- o placement, then unloaded and heat tinted at 316 C for 15 - 20 minutes to t i n t the cracked area. Then the specimens were pulled to failure in the Instron. As the rate of oxidation at the fatigue precracking segment was different from that of the crack growth zone, the crack growth zone was easily detectable by visual inspection. T y p i c a l P - A r e c o r d s f o r J - I n t e g r a l t e s t s o o f o r A F - 2 s t e e l a t - 4 0 C a n d - 1 3 0 C a r e s h o w n i n F i g . 5 . 5 ( a ) a n d ( b ) r e s p e c t i v e l y . 5 . 7 . 2 . 1 . M e a s u r e m e n t o f S p e c i m e n D i m e n s i o n s a n d C r a c k G r o w t h ( A a ) t U s i n g t e c h n i q u e s s i m i l a r t o t h o s e e m p l o y e d i n t h e K i c o r COD t e s t s , B , L , W w e r e m e a s u r e d t o t h e n e a r e s t 0 . 0 0 1 i n c h p r i o r t o e x p e r i - m e n t a t i o n . F r o m t h e f r a c t u r e d s p e c i m e n s a f t e r h e a t t i n t i n g , t h e c r a c k p a r a m e t e r s w e r e m e a s u r e d a t t h r e e p o s i t i o n s a n d a n a v e r a g e o f t h e t h r e e r e a d i n g s w a s u s e d a s t h e c r a c k l e n g t h ' a ' . A s p r e v i o u s l y m e n t i o n e d , a l l s p e c i m e n s h a d a a / w r a t i o b e t w e e n 0 . 6 5 t o 0 . 7 0 . T h e c r a c k g r o w t h o r c r a c k e x t e n s i o n , A a , a h e a d o f t h e f a t i g u e p r e c r a c k w a s a l s o m e a s u r e d t o a n a c c u r a c y o f 0 . 0 0 1 i n c h . T h e m i n i m u m v a l u e o f c r a c k g r o w t h m e a s u r e d w a s 0 . 0 0 4 i n c h a n d t h e m a x i m u m v a l u e , 0 . 1 6 i n c h . © © © @ © I hied (75 > A F i g . 5.5(a) J-Integral Test Record of AF-2 Steel with Crack Transverse to Rolling Direction. Test Temp. - 40°C. t mi s- FIG. 5.5(b) J-lnttgrol T#»t R.cord of AF-2 Stoel with Crock Porolol lo Roling Direction. Tort Ttmp. -130 #C. Spoclmtn No. 25-21 5 , 7 . 2 . 2 M e a s u r e m e n t o f t h e a r e a (A) u n d e r t h e P - A R e c o r d ; F o r e a c h s p e c i m e n t h e a r e a u n d e r t h e P - A r e c o r d c o n s i s t i n g o f b o t h t h e e l a s t i c a n d p l a s t i c d e f o r m a t i o n w a s m e a s u r e d w i t h t h e h e l p o f a c o m p e n s a t i n g t y p e p o l a r p l a n i m e t e r ( 3 3 6 6 1 M a d e i n J a p a n ) . K e e p i n g t h e p o s i t i o n o f t h e v e r n i e r o n t h e t r a c e r b a r a t 1 4 3 . 8 , i n d i v i d u a l a r e a s w e r e m e a s u r e d . A n a v e r a g e o f 3 t o 4 r e a d i n g s o f e a c h a r e a w a s t a k e n . T h e m e a n v a l u e o f t h e r e a d i n g s w a s m u l t i p l i e d b y 0 . 0 1 5 t o g i v e t h e a r e a i n s q u a r e i n c h e s . O n e s q u a r e i n c h i n t h e r e c o r d d e n o t e s a n e q u i v a l e n t e n e r g y o f 2 . 5 i n - l b s i n c e Y - a x i s , l o a d » 5 0 l b s / i n c h o f c h a r t X - a x i s , A = 0 . 0 0 5 i n c h / i n c h o f c h a r t . T h e r e f o r e f o r e a c h s p e c i m e n , t h e a m o u n t o f e n e r g y e x p e n d e d f o r c r a c k e x t e n s i o n w a s m e a s u r e d b y t h e a r e a A u n d e r t h e P - A r e c o r d i n i n - l b . o 5 . 7 . 3 C a l c u l a t i o n o f J f o r RT a n d - 4 0 C t e s t s o J - I n t e g r a l t e s t s a t RT a n d - 4 0 C r e s u l t e d i n d i f f e r e n t v a l u e s o f a r e a (A) w i t h a c o r r e s p o n d i n g A a f o r e a c h i n d i v i d u a l s p e c i m e n . J f o r e a c h s p e c i m e n (29 - 33, 35) was then calculated using the relationship J = B (w2- a) i n " l b / i n 2 Therefore, for a single specimen A.a and a correspond- ing J value were obtained. Since 5 to 6 specimens were used at a particular temperature, 5 to 6 sets of J vs Aa values were obtained. As each specimen differed from each other by an increasing amount of Aa J values also increased in magnitude. Hence, 5 to 6 sets of increasing order J and Aa values were plotted to construct a curve. The best f i t t i n g line was drawn through these points to generate the resistance curve. 5.7.4 Determination of Jj; c Value: It was pointed out in Section 4.2.11 that the c r i t i c a l value of J l c should be obtained by extra- polating the resistance curve backward to the point of zero crack extension due to actual material separation. Hence a line J = 2 <Jfi o w A a was constructed on the resistance curve. The ©flow value of each orientation for each steel at each testing temperature was calculated from the yield strength and the UTS values. The J j c value was determined where 78 the line J = 2 o f l o w Aa cuts the resistance curve. o For tests at - 80 C and below a "single specimen" test technique could be used to obtain a J j c value. This was verified by a minimum of 2 specimens in each case. It was also verified in the case of the AF-2 steel with specimens having cracks running parallel to o the rolling direction at the - 80 C test temperature; specimen No. 17 extended 0.001 inch less than specimen No. 16 and did not indicate any crack growth as revealed by the heat tinting technique. Therefore J calculated for specimen No. 16 i s the J l c value. 5.7.5 Calculation of Equivalent K I c. From the J j c value corresponding K j c values were calculated from the relationship between elastic- plastic and linear elastic fracture mechanics para- (30, 31, 54) meters J l c " G I c = — j ? — which gives K /E. Jj. c 4 Where v = Poisson's ratio • 0.3 E • Young'8 Modulus - 30xl0 6 psi 5.7.6 Verification of Validity Criterion; For valid J L C results, the following size requirement must be met by an elastic-plastic fracture (28, 30 - 32, 34, 35) toughness specimen a, B, w - a * 25 JQ of low It was verified that for a l l test results the c r i t e r i a were satisfied. Therefore, a l l JQ values obtained were valid J L C values. 5.8 Tensile Test Details: Tensile tests were conducted for the purposes already mentioned in section 5.3.2. At each test temperature, a minimum of two specimens were used. Yieid strength and UTS values were calculated and the average values were reported. The 0 f i o w stress was calculated as 6. RESULTS AND DISCUSSIONS 6.1 Tensile Properties; The increase in the yield strength as well as the flow strength with decreasing temperature are presented in Figures 6.1 to 6.4 for the AF-1 steel and in Figures 6.5 to 6.8 for the AF-2 steel. The increase in the flow strength and yield strength on o decreasing the temperature from RT to -100 C i s less than 30% of the RT value. However, for temperatures o below -100 C, the yield strength and the flow strength o increase markedly; at -150 C an increase of approx- o imately 80% i s observed while at -196 C the yield strength and the flow strength are approximately double the values measured at room temperature. It i s well established that the tensile properties of most metals are governed by the thermally activated motion of dislocations. Where this i s the predominant mechanism, the yield stress and flow stress decrease (55) with increasing temperature For the AF-1 steel, the RT yield strength varied from 71,000 psi (489 MPa) along the axis transverse to the pipe axis to 80,000 psi (551 MPa) along the axis transverse to the roll i n g direction. Surprisingly, the RT yield strength was the same 80,000 psi (551 MPa) on the roll i n g direction and transverse to the rolling direction. For the tests o conducted at -196 C, the yield strength varied from a minimum of 144,000 psi (993 MPa) transverse to the pipe axis to a maximum of 157,000 psi (1082 MPa) transverse to the rolling direction. The yield strengths along the pipe axis and the roll i n g direction were comparable throughout the temperature range examined. In terms of the yield strength, the AF-1 plate i s strongest along an axis transverse to the rolling direction, weakest along an axis transverse to the pipe axis and exhibits an intermediate strength along the pipe axis and the roll i n g direction. The yield strength of the AF-2 steel at RT varied from 73,000 psi (503 MPa) to 88,000 psi (606 MPa), being a minimum along the axis transverse to the rolling direction and a maximum along the axis transverse to the pipe axis. In contrast, the yield s t r e n g t h a t - 1 9 6 C v a r i e d f r o m 1 5 0 , 0 0 0 P S i ( 1 0 3 4 M P a ) t o 1 6 0 , 0 0 0 p s i ( 1 1 0 3 M P a ) b e i n g a m i n i m u m a l o n g r o l l i n g d i r e c t i o n a n d a m a x i m u m a l o n g t h e a x i s t r a n s v e r s e t o o t h e r o l l i n g d i r e c t i o n . A b o v e - 1 5 0 C , t h e p l a t e i s s t r o n g e s t a l o n g a n a x i s t r a n s v e r s e t o t h e p i p e a x i s , e x h i b i t s a n i n t e r m e d i a t e s t r e n g t h a l o n g t h e p i p e a x i s a n d t h e r o l l i n g d i r e c t i o n a n d e x h i b i t s i t s l o w e s t s t r e n g t h a l o n g a n a x i s t r a n s v e r s e t o t h e r o l l i n g o d i r e c t i o n . I n c o n t r a s t , b e l o w - 1 5 0 C , t h e p l a t e i s s t r o n g e s t a l o n g a n a x i s t r a n s v e r s e t o t h e r o l l i n g d i r e c t i o n ; s h o w s a n i n t e r m e d i a t e s t r e n g t h a l o n g a n a x i s t r a n s v e r s e t o t h e p i p e a x i s a n d i s w e a k e s t a l o n g t h e p i p e a x i s a s w e l l a s t h e r o l l i n g d i r e c t i o n . I n c o m p a r i n g t h e t e n s i l e p r o p e r t i e s o f t h e A F - 1 a n d t h e A F - 2 s t e e l s , i t i s o b s e r v e d t h a t t h e A F - 2 s t e e l i s s t r o n g e r t h a n t h e A F - 1 s t e e l . B o t h s t e e l s e x h i b i t a n i s o t r o p i c s t r e n g t h p r o p e r t i e s , b u t t h e p r o p e r t i e s o f t h e A F - 1 s t e e l a r e m o r e a n i s o t r o p i c t h a n t h o s e o f t h e A F - 2 s t e e l . T h e b e t t e r t e n s i l e p r o p e r t i e s e . g . h i g h e r y i e l d s t r e n g t h a n d m o r e i s o t r o p i c b e h a v i o u r o f t h e A F - 2 s t e e l may b e t h e r e s u l t o f r a r e e a r t h a d d i t i o n s a n d o f t h e l o w e r (5) sulphur content. However, Lyckx et. al suggested that rare earth additions did not have any effect on the tensile properties of the steel. The AF-1 steel possesses inferior tensile properties, that i s , a lower yield strength and greater anisotropy. This may be due to the higher sulphur content of this steel. The higher sulphur content results in the presence of more second phase particles in the form of sulphides. The yield strength of a material decreases as the volume of the second phase particles increases. This i s because the second phase particles enhance the process of void nucleation and growth. The decrease in yield strength and greater anisotropy of the AF-1 steel can be attributed to the deleterious effect of the sulphur content on the true strain to (61) fracture as reported by W.G. Wilson and G.S. Klems CO CO CO UJ »- CO AF-1 STEEL SPECIMEN TRANSVERSE TO ROLLING DIRECTION 1200 84 160 140 120 100 8 0 6 0 A - o r FLOW STRESS o - c r Y I E L D STRESS O^A, *A- A •0 - 2 0 0 H 6 0 - I 2 0 - 8 0 - 4 0 0 TEMPERATURE 4 0 1000 o 8 0 0 a- 6 0 0 J 4 2 0 FIG. 6.1 CO CO CO UJ tr h-co b AF-1 STEEL SPECIMEN TRANSVERSE TO PIPE AXIS 1200 A - c rFLOW STRESS o-c Y I E L D STRESS - 2 0 0 - 1 6 0 -120 - 8 0 - 4 0 0 4 0 TEMPERATURE °c 1000 8 0 0 S. 6 0 0 4 2 0 FIG. 6.2 A F - 1 STEEL SPECIMEN PARALLEL TO PIPE AXIS 1200 I 6 0 h * 1 4 0 " 120 h co CO £ 100 h 5 8 0 h b A - tr FLOW STRESS o - o -Y IELD STRESS V X o- X A O X X 6 0 - 2 0 0 - 1 6 0 - 1 2 0 - 8 0 - 4 0 0 4 0 TEMPERATURE p c 1000 8 0 0 £ H600 4 2 0 FIG.6.3 A F - 1 STEEL SPECIMEN PARALLEL TO ROLLING DIRECTION 1200 ^ 160 CO * 140 120 CO CO W 100 CO 8 0 6 0 A - a FLOW STRESS x o. X •A. X © X X 1000 8 0 0 7> Q. 6 0 0 4 2 0 - 2 0 0 - 1 6 0 -120 - 8 0 - 4 0 0 4 0 TEMPERATURE FIG. 6.4 A F - 2 STEEL 86 CO CO CO 111 t r I- co b SPECIMEN TRANSVERSE TO ROLLING DIRECTION 1200 A -o-FLOW STRESS o - c r Y I E L D STRESS H 6 0 - 2 0 0 -160 -120 - 8 0 - 4 0 0 4 0 TEMPERATURE °c 1000 8 0 0 6 0 0 4 2 0 o Q_ FIG. 6.5 A F - 2 STEEL CO CO UJ tr fr- co SPECIMEN TRANSVERSE TO PIPE AXIS 1200 1 6 0 1 2 140 X   120 100 8 0 6 0 A - o-FLOW STRESS - ° - <rYIELD STRESS ° 4 ^ A , JL -L JL ± 1000 8 0 0 £. 6 0 0 4 2 0 - 2 0 0 H 6 0 - I 2 0 - 8 0 - 4 0 0 4 0 TEMPERATURE °c FIG. 6.6 A F - 2 STEEL 87 CO CO CO UJ or CO SPECIMEN PARALLEL TO PIPE AXIS 1200 A-o-FLOW STRESS o-cr Y I E L D STRESS H I 0 0 0 - 8 0 0 2 : 6 0 0 J 4 2 0 - 2 0 0 - 1 6 0 - 1 2 0 - 8 0 - 4 0 0 4 0 TEMPERATURE °c FIG. 6 . 7 2 STEEL CO CO CO UJ tr I - co b SPECIMEN PARALLEL TO ROLLING DIRECTION 1200 A-o-FLOW STRESS o-cr YIELD STRESS 6 0 - 2 0 0 -160 - 1 2 0 - TEMPE x 0 40 1000 8 0 0 600 4 2 0 o CL FIG. 6 . 8 88 6.2 Fracture Toughness; 6.2.1 K N Test Results: The fracture toughness values (KQ or K L C ) for the AF-1 steel as obtained by the K L C tests, are reported in Tables 6.1, 6.2 and 6.3 for cracks parallel to the roll i n g direction, cracks parallel to the pipe axis and cracks transverse to the roll i n g direction respectively. The comparable results for the AF-2 steel are shown in Tables 6.4, 6.5 and 6.6. In general the KQ values for both steels decrease in each test direction with decreasing temperatures as can be seen in Figures 6.9.1, 6.9.2, 6.9.3. This decrease in fracture toughness with decreasing temperature i s expected in strain rate sensitive materials* and i s related to the increase in yield stress with decreasing temperature. The increase in yield strength allows a higher level of tensile stress to be present in the plastic zone ahead of the crack and ensures crack tip t r i a x i a l i t y . Cleavage failure occurs when the stress attained over a distance of one or two grain diameters ahead of * Strain rate sensitive materials exhibit a marked var- iation in yield strength with variations in the test temperature and strain rate (56). The low carbon structural steels are more strain rate sensitive than high strength aluminum and high strength alloy steels. ( 5 6» 5 7) 89 to CO if) U l z X o O < OS u. in CO 8 X o 1 0 0 8 0 6 0 4 0 2 0 C R A C K P A R A L L E L T O R O L L I N G D I R E C T I O N 8 8 A V A L I D K I C o - 1 0 0 o - 8 0 - 6 0 4 0 2 0 JL i i o a 5 - 2 0 0 - 1 6 0 - 1 2 0 - 8 0 - 4 0 0 4 0 T E M P E R A T U R E °C F I G . 6 . 9 . 1 . ui rr < on if CO to to HI z X o o U l tc < u. 1 0 0 8 0 6 0 4 0 2 0 C R A C K P A R A L L E L T O P I P E A X I S - . § 8 - e S 8 » o IT".' 1 \ o I N V A L I D K | C 1 .1. i i i 1 0 0 8 0 6 0 If a a. - 2 0 0 - 1 6 0 - 1 2 0 - 8 0 - 4 0 0 4 0 T E M P E R A T U R E ° ' c 1 0 0 F I G . 6 . 9 . 2 . C R A C K T R A N S V E R S E TO R O L L I N G D I R E C T I O N 1 0 0 - 2 0 0 - 1 6 0 - 1 2 0 - 8 0 - 4 0 T E M P E R A T U R E 0 4 0 A - A F I o - A F 2 A - A F I 0 - A F 2 A - A F I o - A F 2 F I G . 6 . 9 . 3 . (58) the material . As the temperature increases the yield stress decreases until the cleavage stress i s (54) no longer achieved and the material tears instead The tearing mode of failure occurs predominantly by (55) a process of void i n i t i a t i o n and coalescence It should be noted that there are two possible transitions in fracture behaviour with temperature, namely: 1) A K l c transition (Plane Strain transition) 2) A Plane-strain to Plane-stress transition commonly called the 'Elastic-Plastic Transition'. (59) Although Wessel suggested that the failure mechanism in K l c tests was cleavage over the whole temperature range, Barsom and Rolfe's fractographic (60) analyses established that the K I c transition i s associated with the onset of a change in the micro- scopic fracture mode. At low temperatures, the fracture i s 100% cleavage or guasi-cleavage whereas at the transition or intermediate temperatures, the fracture surface exhibits a combination of guasi- cleavage and tear dimples. At higher temperatures above the transition region, the fracture surface consists of 100% tear dimples. The test results as obtained using the h inch thick compact tension specimens indicate that o valid K L C values are obtained at - 130 C and below for both steels in a l l test directions; in the rolling direction the AF-2 steel i s the exception. The AF-2 steel exhibits a lower yield strength in this direction and therefore valid K L C data i s o not obtained until the temperature i s below - 130 C. o o Thus, the test values obtained at - 130 C, - 150 C, o and - 196 C l i e on the lower shelf of the fracture toughness transition curve. The data obtained at o - 40 C and RT l i e on the upper shelf and are KQ values o o o The results for - 80 C, - 100 C and - 110 C l i e in the transition zone and are also KQ values. Fig. 6.10.1 shows the fracture surface of o o the AF-2 steel after testing at RT, - 40 , - 80 , o o o - 110 , - 130 and - 196 C with the crack parallel to the pipe axis. The fracture surfaces obtained o at RT and - 40 C are gray and fibrous, typical of ductile failure. In fact, the RT specimen exhibited o a fracture extending at 45 to the specimen axis. In -40°C -80°C -130°C -196°C Fracture surfaces of K l c specimens of Af-2 steel with crack parallel to pipe axis at various temperatures. c o n t r a s t t h e f r a c t u r e s u r f a c e s o n s p e c i m e n t e s t e d a t o - 1 3 0 C a n d b e l o w a r e b r i g h t a n d g r a n u l a r , i n d i c a t i v e o f b r i t t l e f a i l u r e . T h e f r a c t u r e s u r f a c e s o b t a i n e d o o a t - 8 0 a n d - 1 1 0 C c o n s i s t o f b o t h f i b r o u s a n d g r a n u l a r s t r u c t u r e ; 40% f i b r o u s a n d 6 0 % g r a n u l a r a t o o - 8 0 C , 2 0 % f i b r o u s a n d 80% g r a n u l a r a t - 1 1 0 C . T h e t e s t r e s u l t s i n d i c a t e t h a t t h e p l a n e o s t r a i n t o p l a n e s t r e s s t r a n s i t i o n o c c u r s a b o v e - 1 3 0 C w i t h i n c r e a s i n g t e m p e r a t u r e . T h i s i s d u e t o t h e f a c t t h a t a s t h e t e m p e r a t u r e i n c r e a s e s , t h e y i e l d s t r e n g t h o f t h e m a t e r i a l d e c r e a s e s . T h i s i n t u r n r e d u c e s t h e t h r o u g h t h i c k n e s s c o n s t r a i n t a t t h e c r a c k t i p a l t h o u g h a t r i a x i a l s t a t e o f s t r e s s e x i s t s . A s a c o n s e q u e n c e t h e p l a n e s t r a i n c o n d i t i o n n o l o n g e r p r e v a i l s . T h e f r a c t u r e s u r f a c e s t u d y e s t a b l i s h e d t h a t e v e n i n t h e c a s e o f s a m p l e s t e s t e d i n t h e e l a s t i c - p l a s t i c t r a n s i t i o n , t h e m i c r o - m o d e o f f r a c t u r e r e m a i n s t h e s a m e a s w i t h P l a n e S t r a i n t r a n s i t i o n , t h a t i s , 1 0 0 % c l e a v a g e a t l o w t e m p e r a t u r e , 100% d u c t i l e t e a r a t h i g h e r t e m p e r a t u r e a n d a c o m b i n a t i o n o f b o t h i n t h e t r a n s i t i o n t e m p e r a t u r e r a n g e . Considering the directionality of the toughness behaviour of the AF-2 steel, the test results as shown by the transition curves in Fig. 6.9.1, 6.9.2, 6.9.3 do not indicate much deviation from one orientation to another orientation. In other words, the AF-2 steel exhibits essentially isotropic fracture toughness behaviour. The same is not true for the AF-1 steel; for tests with the crack parallel to the rolling direction a lower toughness value i s obtained in the upper shelf region in comparison to the other two test directions. In comparing the KQ and K L C results of the AF«L and the AF 2 steel the following observations can be made. 1. In the upper shelf region, for a crack parallel to the roll i n g direction, the AF-1 steel possesses a significantly lower toughness than the AF-2 steel. The KQ values of both steels are comparable in the other two test directions. 2. In the transition region, both the AF-1 and AF-2 steels show steeper transition behaviour for the crack parallel to the rolling direction; but the transition temperature range remains the same for each steel in a l l three test directions. 3. Both steels exhibit comparable fracture toughness in a l l test directions in the valid K l c range. The more isotropic nature of the fracture toughness behaviour of the AF-2 steel and i t s higher fracture toughness values along the transverse direction ( T-- L ) in comparison to the AF-1 (containing more sulphur) clearly reveals the beneficial effects of rare earth addition during the steel making process. 96 6.2.2 J-Integral Test Results; The J-Integral test results for the AF-1 and the AF-2 steel are shown as J-resistance curves in Figure: 6.11.1 - for the crack parallel to the ro l l i n g direction (T-L) 6.11.2 - for the crack parallel to the pipe axis 6.11.3 - for the crack transverse to the roll i n g direction (L-T) 6.12.1 - for the crack parallel tothe rol l i n g direction (T-L) 6.12.2 - for the crack parallel to the pipe axis 6.12.3 - for the crack transverse to the roll i n g direction (L-T) respectively. The J-resistance curve for the AF-1 o steel tested at - 80 C with the crack transverse to the rolli n g direction as shown in Fig. 6.11.3 confirms the fact that the slope of the J-Aa curve approaches zero with decreasing test temperature and the J i c value remains constant independent of the crack growth (Aa) at these low temperatures. Therefore, this curve o verifies that for tests done at temperatures < - 80 C, a f u l l resistance curve i s not obtainable or necessary. To indicate how J increases with crack growth, Fig. 6.13.1 shows the fracture surfaces of the AF-2 steel specimens which were tested at RT and were used c 750 JQ • 500 c 250 (mm.) 10 2 0 3 0 40 5 0 T T J = 2o" Flow A a OO—O* 97 •\160~ E 120 ® Temp. = R.T J , c s 300 in-lb/in (52-50 kJ/m 2 ) —i i • 80 o O 40 w 04 08 12 16 -20 Act , Crack Extension (inch) (m.m.) 0-5 10 1-5 2 0 2-5 30 3-5 40 T T JsZcrFlow A a J Temp, s — 40 °c J , c =275 in-lb/ in 2 (4818 kJ/m ) ii . i • \ o -> 80 to 40 •02 04 06 08 10 12 14 16 A a , Crack Extension (inch) FIG. 6.11.1. J - resistance curve for AF-1 steel with crack parallel rolling direction at R.T © - 4 0 °c to (m.m.) Act , Crack Extension (inch) (m.m.) •02 04 06 08 10 A a , Crack Extension (inch) Figure 6.11.2. J-Resistance curve for AF-1 steel with crack parallel to Pipe Axis at RT & -40 (m.m.) 0-5 10 1-5 2 0 2 5 •£ 1500 jO ]L 1000 -> 500 „J = 2 CT F l o w A a Temp.= R.T J , c = 4 2 5 In-lb/lrT- ( 7 4 - 4 6 K J / m 2 ) 320 c 240 t <D 160 3 O ~3 M O 80 £ 750 •>•* J_ 500 -> 250 •02 -04 06 08 10 A a , Crack Extension (inch) (m.m.) 0-5 10 1-5 2 0 2-5 — i — j 1 1 1 - J = 2o~ Flow Aa -O- Temp. = - 4 0 c J , c = 3 9 0 i n - l b / i n 8 ( 6 8 - 3 3 K J / m 8 ) i i • 160 120 80 40 E a> o 10 O •02 04 06 08 10 A a o Crack Extension (inch) a c 0-5 10 1-5 (m.m.) 2 0 2-5 3 0 3-5 4 0 750 • i • J = 2 c r F l o w A a i i i i i T6mp. = - 8 0 ° c J , c s 2 5 0 I n - l b / i n * 500 (43-8 K J / m 8 ) 250 o. o ... i i i i i i i • 160 120 80 40 •02 04 06 08 10 12 14 16 -18 A a , Crack Extension (inch) FI6.6.IL3. J-resistance curve for steel AF-1 with crack transverse to rolling direction at R.T. 8-40°c 8 - 8 0 ° c Temp. = R.T. J , c = 7 8 5 i n - l b / i n " ( 1 3 7 - 5 3 k J / m 2 ) •01 0 2 0 3 0 4 0 5 0 6 0 7 0 8 Aa , C r a c k E x t e n s i o n ( i n c h ) 2 4 0 200*1 160 ' I 1 2 0 8 0 4 0 1200 1000 eoo 600 400 200 05 — I — (ram.) 10 1-5 J = 2 o " F l o w A a Temp. • — 4 0 ° c J I C * 6 0 0 I n - l b / i n 8 ( 1 0 5 1 2 k J / m 2 ) 2 0 — r - = 210 180 150 E to 120 3 Q 90 n O . 60 30 •01 02 03 04 05 06 07 08 Aa , C r a c k E x t e n s i o n ( i n c h ) Figure 6.12.1. J-Resistance curve for Af-2 Steel - with crack p a r a l l e l to R o l l i n g D i r e c t i o n at RT and -40"C. 2000 1500 1000 500 0-5 (m.m.) 10 I -5 T J = 2 CT Flow A a - R.T. J i e - 800 in-lb/in2 (14016 kJ/m 2) i _ i _ -01 02 03 04 05 06 A a , Crack Extension (inch) 0-5 -T— J * 20" Flow A Q (m.m.) 10 1-8 Temp, s - 4 0 ° c J | C * 640 in-lb/in 2 (II2I2 kJ/m 2 ) - 320 ej - 240 6 HI60 o •180 2 320 ^ 240 160 o -5 80 — •01 02 03 04 05 06 A a , Crack Extension (inch) Figure 6.12.2. J-Resistance curves for AF-2 s t e e l with crack p a r a l l e l to Pipe Axis at RT and -40°C. (mm.) •01 02 03 04 05 06 A a , Crack Extension (inch) (m.m.) •01 02 03 04 05 06 A a , Crack Extension (inch) Figure 6.12.3. J-Resistance curves for AF-2 steel with crack Transverse to Rolling Direction at RT & -40°C. 103 F i g u r e 6.13.1 F r a c t u r e s u r f a c e s of AF-2 s t e e l specimens w i t h c r a c k p a r a l l e l t o R o l l i n g d i r e c t i o n , t e s t e d a t R.T. A r r a n g e d i n o r d e r o f i n c r e a s i n g c r a c k e x t e n s i o n . to construct the J-resistance curve in Fig. 6.12.1. The specimens in Fig. 6.13.1 are kept in the order of increasing crack extension. Heat-tinting clearly distinguishes the crack extension experienced by each specimen. The fracture toughness value, J J C *-he c r i t i c a l value of J obtained for each of these curves o for temperatures - 40 C and RT and the direct values from the tests conducted at other temperatures are reported in Tables 6.1, 6.2, 6.3 for the AF-1 steel and in Tables 6.4, 6.5, 6.6 for the AF-2 steel. The J-Integral tests confirmed that with increasing temperature, the fracture toughness of a material increases. This i s demonstrated for both steels in Figs. 6.14.1, 6.14.2 and 6.14.3. The test results show a similar fracture toughness transitional behaviour as was observed in the K I C tests. Fig. 6.15.1 compares the KQ and J data for both steels and shows that for the crack o parallel to the rolling direction the results at - 130 105 CRACK PARALLEL TO ROLLING DIRECTION 800 400 4: 200 o - 0 O 0. o O A A - _ A© A A A 8 • l I • 1 1 150 _ CM 120 v UJ 90 ^ o » g 30 ~ A - AF I O- AF 2 60 •200-160 -120 -80 - 4 0 0 40 TEMPERATURE "c CRACK PARALLEL TO PIPE AXIS FIG. 6.14.1 u -3 800 O 600 O 400 o A A 200 A A ._! A o § O 1 A O 1 • i i - 150 - E 120 ^ ui 90 g 60 „ A - AF I , o - A F 2 -200 -160 -120 -80 -40 40 TEMPERATURE c FIG. 6. 14.2. CRACK TRANSVERSE TO ROLLING DIRECTION M C 800 © 150 JO c 600 O 120 - 90 • o 400 8 A A - 60 -9 200 B • i i i A - 30 E v . U l _J Z3 o ~3 A — AF I O - A F 2 -200 -160 -120 -80 -40 0 40 TEMPERATURE FIG. 6.14.3 106 150 V CM \ 120 9> -2 90 O ~ 60 u 30 1000 800 ~_ 600 400 200 CRACK PARALLEL TO ROLLING DIRECTION JLZ\ @ • <-} o . ' A - - A e A O - m A° A A A I • • • i -200 -160 -120 -80 -40 40 A-AF I J , c Value A-AF I KQ.KlcValue ©-AF2 J , c Value &-AF2 KQi<,c Value 100 80 60 <o 4 0 o 20 o 100 80 60 o a. 40 20 FIG. 6. 15.1. 107 o o o - 150 , and - 196 C l i e in the lower shelf, - 40 C and o o RT in the upper shelf region and - 80 , - 100 C and o - 110 C l i e in the transitional range. This i s true for both of the steels for a l l three test directions. It should be noted that the J l c values are valid over the whole temperature range - and that this fracture toughness transitional behaviour depicts the J I c transitional pattern; whereas in the KQ - K L C testing, the plane strain to plane stress transition ( K I C to KQ) was observed and not a K I C transition. For the AF-1 steel, the J l c transitional curves do show anisotropic behaviour; the highest toughness i s realised in specimens in which the crack propagates transverse to the rolling direction. This i s quite usual as along this longitudinal (L-T) direction a material would be expected to possess i t s highest toughness. The minimum upper shelf tough- ness i s realized for the crack growing parallel to the rolling direction (T-L). This indicates that the material possesses i t s minimum toughness along the transverse (T-L) direction. However, the lower shelf Jjc values are similar in magnitude in a l l three test directions. 108 In the case of the AF-2 steel, the J j c transition curves do not show any marked anisotropy in fracture toughness behaviour. These J j c curves clearly show that the AF-2 steel i s much tougher than the AF-1 steel in the transition and upper shelf temperature range. At RT, the AF-2 steel tested with the crack parallel to the pipe axis exhibits a J l c value of 800 i n - l b / i n 2 (140.16 KJ/m2), whereas for the same test direction, the AF-1 steel shows approx- imately 400 i n - l b / i n 2 (70.08 KJ/m2). This i s shown in Table 6.7. Table; 6.7 Comparative J I C values of AF-1 and AF-2 Steels along crack parallel to rolling direction (T-L) and crack parallel to pipe axis Test Temp- Crack P a r a l l e l to Rolling Direction Crack P a r a l l e l to Pipe • Axis erature o c AF- 1 AF-2 AF-1 AF-2 J I C Jlc. J l c JIC J l c J l c J l c J l c i n - l b / i n 2 KJ/m2 i n - l b / i n 2 KJ/ m2 i n - l b / i n 2 KJ/m2 i n - l b / i n 2 KJ/m2 RT 300 52.56 785 137.53 400 70.08 800 140.16 -40 275 48.18 600 105.12 325 56.94 640 112.12 -80 250 43.80 418 73.23 240 42.05 379 66.40 -100 _ 264 46.25 - - - --110 237 41.52 _ - 284 49.75 176 30.83 -130 _ • 142 24.87 - - 124 21.72 -150 127 22.25 - - 161 28.20 - --196 82 14.36 118 20.67 54 9.46 80 14.01 109 o Fbr the tests conducted at - 40 C along the same orientations, the AF-2 steel exhibits a J l c toughness of 640 i n - l b / i n 2 (112.12 KJ/m2) whereas the AF-1 steel gives only 325 i n - l b / i n 2 (56.94 KJ/m2). The greatest difference in toughness between the AF-1 and the AF-2 steels i s observed for specimens tested with the crack parallel to the roll i n g direction (T-L) (Table 6.7). For example, at RT, the AF-2 steel exhibits a J j c toughness of 785 i n - l b / i n 2 (137.53 KJ/m2) 2 whereas the AF-1 steel gives only 300 in-lb/in (52.56 KJ/m ). The data shown in Table 6.7 also suggests that in the design consideration for pipelines, the fracture toughness of a pipe should be considered not only along the pipe axis.but also parallel to the roll i n g direction, the (T-L) direction of the plate since the material is weakest along this orientation. It i s the combination of the stress and the toughness that w i l l influence failure. Even though the Hoop stress i s the maximum operating stress, high residual welding stresses can exist parallel to the lower toughness rolling direction (T-L). The high toughness J I c values reported for 110 the AF-2 steel are thought to be due to the beneficial effect of the low sulphur content of the steel and (5) the sulphide shape control . The rare earth sul- phides that are present in the AF-2 steel are spherical in shape, have a high melting point and do not elongate to form stringers in the direction parallel to the rolling direction as i s the case for the AF-1 steel. Hence, the lower non-metallic inclusion content and (61) the beneficial effect of the rare earth additions have resulted in the high toughness for the AF-2 steel. 6.2.3 COD Test Results; The c r i t i c a l COD values (e>m and <5Q) are shown in Tables 6.1, 6.2, 6.3 for the AF-1 steel and Tables 6.4, 6.5, 6.6 for the AF-2 steel. The variation of 6 C in terms of 6m and *Q with temperature for a l l of the test specimens is also shown in Figures 6.16.1, 6.16.2, 6.16.3 for the 6 m values and in Figures 6.17.1, 6.17.2 and 6.17.3 for the 6Q values. The general trends and characteristics of I l l 20 ? 16 c 8 CRACK PARALLEL TO ROLLIN6 DIRECTION o •4 © © - © G •3 © % A & •2 a 6 I & S A •1 7 1 1 1 1 1 1 A - AF I D - A F 2 -200 -160-120 -80 -40 0 40 TEMPERATURE °c CRACK PARALLEL TO PIPE AXIS FIG. 6.16.1. A — AF I © - A F 2 -200-160 -120 -80 -40 0 TEMPERATURE °( FIG. 6.16.2 CRACK TRANSVERSE TO ROLLING DIRECTION A - A F I © - A F 2 -200-160 -120 -80 -40 TEMPERATURE FIG. 6.16.3 112 CRACK PARALLEL TO ROLLING DIRECTION o 10 '2 12 8 o A o o A o £ - ,C O D  4 - i i i • i i_ - 2 0 0 - 1 6 0 -120 - 8 0 - 4 0 TEMPERATURE 0 40 • 3 • 2 ^ •o A - AF I O - AF 2 FIG. 6.171 CRACK PARALLEL TO PIPE AXIS u 'o 8 Q 4 O CO - o g i a o 8 A - i o A 1 • < ...1 L- - - 2 0 0 -160 -120 - 8 0 - 4 0 TEMPERATURE 0 4 0 • 3 © +- to 2 £ 10 A - AF I O - AF 2 FIG. 6.17. 2 CRACK TRANSVERSE TO ROLLING DIRECTION jC u O O A - AF I O - AF 2 - 2 0 0 -160 -120 - 8 0 - 4 0 TEMPERATURE FIG. 6.17. 3 the COD transition curves remain similar to those of the J L C or KQ - K L C curves. But the COD data exhibits much more scatter than the data obtained from the other two methods. From the 6 m vs. T data (Fig. 6.16.1 to 6.16.3) i t i s obvious that the 6 m values of both steels are comparable at - 8 0 ° C and below, this being the lower temperature portion of the transition temperature range and the lower shelf condition. In the upper shelf region, the AF-2 steel possesses a much higher 6 m value than does the AF-1 steel for the crack running parallel to the rolling direction (T-L orient- ation) . The same i s found to be true i f the 6 Q value of both steels are considered (refer to Fig. 6.17.1). In contrast, the 6 and the 6n values of both steels m do not differ significantly in the other two test directions. The 6 m vs T transition curves clearly show that the COD properties of the AF-2 steel are more isotropic than those of the AF-1 steel. Therefore, the COD test results also verify that the AF-1 steel which contains more sulphur than the AF-2 steel possesses a minimum upper shelf toughness for the crack running parallel to the rol l i n g direction (the T-L orientation). Similar observations have been reported in various works (5, 49, 61) 6.3 Comparison of Fracture Toughness from Kj J-Integral and COD Tests: The fracture toughness values, KQ and K L C from the K L C tests and the equivalent K L C values from the J l c and COD tests are compared in the transition curves shown in Figures 6.18.1, 6.18.2, 6.18.3 for the AF-1 steel and in Figures 6.19.1, 6.19.2, 6.19.3 for the AF-2 steel. The respective data are summarised in Tables 6.1 to 6.6. It should be noted that KQ values at o temperatures - 130 C and below are valid and hence represent the linear elastic fracture toughness K L C values. In general, the three approaches to determine the temperature dependence of fracture toughness reveal 1) that the fracture toughness of the two acicular ferrite steels increases with increasing temperature 2) that the COD elastic-plastic fracture toughness values l i e above the J l c elastic-plastic fracture toughness and that the J l c values l i e above the KQ values at higher temperatures and above the K I c v a i u e s at lower temperatures 3) that 6 m - K L C data obtained from the COD tests are much higher in magnitude than the K I C values predicted by the other test results; the values are approximately twice the magnitude of the J-Integral values over the entire temperature range of the tests. 4) that the K J C data obtained from the J- Integral test i s larger in magnitude than the K^ or Kjc data; the magnitude of the difference between two sets of data generally increases with increasing temperature. The difference i s most pronounced for the AF-2 steel at the higher temperatures for the three test directions examined 5) that in one case for the AF-1 steel with the crack propagating parallel to the pipe axis, very good agreement was obtained between the linear elastic, K I C f and the elastic-plastic, J i c , fracture toughness results. At higher temperatures, the COD and the J I c v a l u e s indicate a large increase in toughness which i s not reflected in the K N values. This has (39) w been explained by Egan as the inab i l i t y of the K-type analyses to take account of the increase in the size of the plastic zone; the larger plastic zone would have required more work than i f the same load value (PQ) had been reached by linear elastic loading. Therefore, at higher temperatures, where KQ tests do not give valid K l c results, a more meaningful and representative toughness level i s indicated by the COD and J I c test results. A wide difference between the valid K l c data and the equivalent K l c from « m - COD and J I C tests i s observed at lower shelf temperatures. The equivalent Kic data from 6 m - COD at low temperatures indicate that the fracture toughness of both steels increased o at - 130 C and below. This i s misleading in that i f o the experimental COD data at - 130 C and lower temperature for both steels i s considered (refer Fig. 6.16.1 to 6.16.3). The equivalent K I e from 6 - COD was obtained using the following relationship K Ic equivalent = J E 6 m a ys Where ays ~ yield strength of the material at test temperature and strain rate E = Young's Modulus v as Poisson's Ratio At higher temperatures, the conversion of fim - COD to K L C gives reasonable agreement, whereas at low temp- era tes, - 130 C and below, the conversion results in an increase in fracture toughness with decreasing temperature. This apparent increase in equivalent K l c from * m - COD may be due to the high yie l d strength of the material at low temperature. Therefore, the conversion relationship for 6 m - COD to equivalent K l c does not seem to be applicable at the lower temperature range. The difference between the two estimates of the elastic-plastic fracture toughness as indicated by the equivalent K L C data obtained from &m or 6"Q values and J l c values, particularly at upper shelf temperatures, o 118 can be explained as follows. The linear elastic fracture toughness values, that i s , the K j c or KQ values are based on a 2% effective crack growth which includes the effect of plastic zone formation. There- fore, the c r i t i c a l COD- 6Q value represents a 2% effective crack growth condition. Hence in this case, 6Q represents a 0.01" (0.2 x 0.5 « 0.01 inch) inch crack growth on a 0.5 inch ligament on a 0.5 inch thick compact tension specimen. In contrast, the 6 m - COD value represents a much higher crack growth condition as this dis- placement i s obtained for the maximum load. In comparing the COD values with the J-Integral values, i t should be recalled that the corresponding J l c value i s based on crack i n i t i a t i o n only, that i s , zero crack growth due to actual material separation. Consider the following example in which i s examined the difference between the J l c value and the o 6Q value at - 40 C for the crack parallel to the pipe axis for the AF-2 steel (Fig. 6.19.2 or Table 6.5): J l c - 145 ksi/sjl.n (159.35 NPaJm) whereas 6Q Average = 162 k s i J i n (178.03 MPaAyrm) . Using - 40°C the J-resistance curve data for the AF-2 steel with the 119 crack parallel to the pipe axis (Fig. 6.12.2),Aa = .01 inch gives J • 775 i n - l b / i n 2 , (135.78 KJ/m2). The correspond- ing value of K I c i s 158.84 ksi^Jln (175.66 MPâ /m) . This equivalent K l c value agrees with the 6Q value of 162 k s i y l n (178.03 MPâ Jm) shown in the Fig. 6.19.2. Therefore, i t i s apparent that in the upper shelf temperature region, the difference between the equi- valent K I c as obtained from the J i c data and the K l c obtained from the $Q - COD data i s insignificant provided an appropriate correction for crack growth is taken into account. The large deviation between the equivalent K l c from « m COD data and the equivalent K l c from Jjc data in the upper shelf temperature region, where K I c = * m i s approximately twice the K l c - J i c * i s not surprising. The simplest explanation is that the K l c - J J c stands for NIL CRACK GROWTH whereas Klc - «m Stands for EXTENSIVE CRACK GROWTH CORRESPONDING TO MAXIMUM LOAD. In the transition temperature region, the large difference between the K l c - J I c and K l c sm or $Q can be attributed to two effects: i) the definitions of K i c and 6 m fracture toughness are based upon different crack growth c r i t e r i a i i ) the increase in yield strength at low temperatures. The tendency of equivalent K I c values from J_ data to be larger in magnitude than the K j c values even at the low temperatures where both the test procedures involve h inch thick compact tension specimens and both the test samples experience 100% cleavage fracture is thought to be due to the smaller energy expended in fracturing J l c specimens in comparison to that required to fracture K l c specimens. The ligament length in the K l c specimen is approximately .45 to .50 W, whereas the ligament length in the J l c specimen is approximately .35 to .30 Wj where W i s the specimen depth. TABLE - 6 . 1 AF-1 Steel crack Parallel to Rolling Direction (T-L Orientation) c COD Equivalent K i c Temp. ' (m COD -«» C 0 D - i , l ° C k s i ^ / C n i n - l b / i n 2 KJ/m2 i n c h nm i n c h k s i ^ / ^ n RT 6 8 . 8 8 66.00 75 .70 72.53 3 0 0 . 0 0 • 5 2 . 5 6 0 . 0 0 7 8 0 . 0 0 7 6 0 . 1 9 8 1 0 . 1 9 3 0 0 . 0 0 7 8 C . 0 0 7 4 0 . 1 9 8 1 0 , 1 8 7 9 9 9 . 4 4 1 0 9 . 2 8 144.00 141.69 1 5 8 . 2 5 1 5 5 . 7 1 1 4 4 . 0 0 1 3 9 . 8 0 15S.25 1 5 3 . 6 4 - 4 0 7 2 . 2 5 7 2 . 3 0 7 9 . 4 0 7 9 . 4 5 2 7 5 . 0 0 4 8 . 1 8 0 . 0 0 7 5 0 . 0 0 7 5 0 . 1 9 0 5 0 . 1 9 0 5 0 . 0 0 7 0 0 . 0 0 7 0 0 . 1 7 7 8 0 . 1 7 7 8 9 5 . 2 1 1 0 4 . 6 3 1 4 3 . 1 8 1 4 3 . 1 8 1 5 7 . 3 5 1 5 7 . 3 5 1 3 8 . 3 2 1 3 8 . 3 2 1 5 2 . 0 1 1 5 2 . 0 1 - 6 0 7 1 . 9 9 7 4 . 9 9 7 9 . 1 1 8 2 . 4 1 2 2 0 . 0 0 3 8 . 5 4 0 . 0 0 8 1 0 . 0 0 8 1 0 . 2 0 5 7 0 . 2 0 5 7 0 . 0 0 7 2 0 . 0 0 7 9 0 . 1 8 2 8 0 . 2 0 0 6 8 5 . 1 6 9 3 . 5 9 1 5 2 . 9 1 1 5 2 . 7 3 1 6 8 . 0 4 1 6 7 . 8 5 1 4 3 . 9 5 1 5 0 . 3 7 1 5 8 . 2 0 1 6 5 . 2 5 - 8 0 7 0 . 7 3 6 2 . 1 8 7 7 . 7 3 6 8 . 3 3 2 5 0 . 0 0 4 3 . 8 0 0 . 0 0 8 2 0 . 0 0 9 1 0 . 2 0 8 2 0 . 2 3 1 1 0 . 0 0 8 2 0 . 0 0 9 1 0 . 2 0 8 2 0 . 2 3 1 1 9 0 . 7 8 9 9 . 7 6 1 5 3 . 9 6 1 6 2 . 1 9 1 5 3 . 9 6 1 6 2 . 1 9 1 6 9 . 2 0 1 7 8 . 2 4 - 1 1 0 4 4 . 3 3 4 8 . 7 1 2 3 6 . 8 6 4 1 . 4 9 0 . 0 0 4 6 0 . 1 1 6 8 8 8 . 3 6 9 7 . 1 0 1 2 3 . 9 9 - 1 5 0 3 9 . 7 4 4 2 . 0 5 4 3 . 6 7 4 6 . 2 1 1 2 7 . 1 2 2 2 . 2 7 0 . 0 0 4 6 0 . 0 0 4 1 0 . 1 1 6 8 0 . 1 0 4 1 6 4 . 7 3 7 1 . 1 3 1 3 5 . 2 4 1 2 7 . 2 5 - 1 9 6 3 0 . 2 5 3 1 . 2 8 3 3 . 2 4 3 4 . 3 7 8 3 . 5 3 8 0 . 9 4 1 4 . 6 3 1 4 . 1 8 0 . 0 0 2 8 0 . 0 0 2 9 0 . 0 7 1 1 0 . 0 7 3 6 5 2 . 4 7 5 1 . 6 5 5 7 . 6 6 5 6 . 7 6 1 2 2 . 3 9 1 2 2 . 9 1 TABLE - 6.2 AF-1 Steel crack Parallel to Pine Axis Temp. 0 C J l c COD Eauivalent Ki-le ksi ^ / T n MPâ /m in-lb/in 2 KJ/m2 6 m Ic C0D-«m COD-iQ inch mm inch mm k s i ^ / i n MPa^n ksi^^n ksi ^ / T n RT 73.05 76.70 80.28 84.29 400.00 70.08 0.0115 0.0115 0.2921 0.2921 0.0084 0.0068 0.2133 0.1727 .114.83 126.19 171.00 171.00 187.92 187.92 146.00 132.00 160.45 145.06 - 40 76.20 70.70 70.70 83.74 77.69 77.69 325.00 56.94 0.0134 0.0113 0.0150 0.3403 0.2870 0.3810 0.0087 0.0066 0.0092 0.2209 0.1676 0.2336 103.50 113.74 177.24 204.34 194.78 224.56 136.52 160.55 150.03 176.44 - 80 59.00 64.84 240.00 42.04 0.0061 0.1549 88.94 97.74 133.14 146.32 - 110 55.83 54.18 61.35 59.54 328.94 239.33 57.63 41.93 0.0042 0.0051 0.1066 0.1295 104.13 88.82 114.43 97.61 115.12 126.30 126.51 138.80 - 150 41.60 40.39 45.71 44.38 211.20 111.28 37.00 19.49 0.0036 0.0037 0.0914 0.0939 83.44 60.57 91.70 66.56 113.99 116.03 125.2-7 127.51 - 196 32.89 34.07 36.14 37.44 50.68 57.97 8.87 10.15 0.0032 0.0044 0.0812 0.1117 40.87 43.72 44.91 48.04 124.49 146.30 136.81 160.78 TABLE 6 . 3 AF-1 Steel crack Transverse to Rolling Direction (L-T orientation) J l c " C O D Equivalent K j c Terap. ° c ksi^/n KPâ /m" in-lb/in 2 KJ/m2 F 0 J l c C 0 D - « M C 0 D - 6 Q inch nun inch mm ksi.yTn MPa.̂ /fi" ksl v4n MPâ /S" ksi^/Tn MPa,̂ /̂ " RT 7 1 . 9 3 7 7 . 9 1 7 9 . 0 5 8 5 . 6 2 4 2 5 . 0 0 7 4 . 4 6 0 . 0 1 3 0 0 . 0 1 5 3 0 . 3 3 0 2 0 . 3 8 8 6 0 . 0 0 8 4 0 . 0 1 0 5 0 . 2 1 3 3 0 . 2 6 6 7 1 1 8 . 3 6 1 3 0 . 0 7 1 8 5 . 3 1 2 0 1 . 0 4 2 0 3 . 6 5 2 2 0 . 9 4 1 4 8 . 9 6 1 6 6 . 8 6 1 6 3 . 7 0 1 8 3 . 3 7 - 4 0 7 3 . 2 4 8 0 . 6 3 8 0 . 4 9 8 8 . 6 1 3 9 0 . 0 0 6 8 . 3 2 0 . 0 1 2 6 0 . 0 1 4 8 0 . 3 2 0 0 0 . 3 7 5 9 0 . 0 0 7 2 . 0 . 0 1 1 5 0 . 1 8 2 8 0 . 2 9 2 1 1 1 3 . 3 8 1 2 4 . 6 0 1 9 1 . 8 7 2 0 7 . 2 9 2 1 0 . 8 6 2 2 7 . 8 1 1 4 4 . 5 8 1 8 3 . 4 3 1 5 8 . 8 9 2 0 1 . 5 8 - 8 0 5 5 . 9 0 6 4 . 7 1 6 1 . 4 3 7 1 . 1 1 2 5 0 . 0 0 4 3 . 8 0 0 . 0 0 5 0 0 . 0 0 6 1 0 . 1 2 7 0 0 . 1 5 4 9 0 . 0 0 4 8 0 . 0 0 6 0 0 . 1 2 1 9 0 . 1 5 2 4 9 0 . 7 8 9 9 . 7 6 1 2 7 . 3 7 1 4 0 . 7 0 1 3 9 . 9 7 1 5 4 . 6 2 1 2 5 . 8 4 1 3 9 . 4 7 1 3 8 . 2 9 1 5 3 . 2 7 - 1 1 0 4 2 . 1 2 5 5 . 6 8 4 6 . 2 8 6 1 . 1 9 2 2 2 . 5 6 2 4 3 . 9 0 3 8 . 9 9 4 2 . 7 3 0 . 0 0 3 4 0 . 0 0 6 6 0 . 0 8 6 3 0 . 1 6 7 6 8 5 . 6 5 8 9 . 6 7 9 4 . 1 3 9 8 . 5 4 1 0 6 . 0 1 1 4 6 . 2 6 1 1 6 . 5 0 1 6 1 . 8 3 - 1 5 0 3 3 . 7 7 4 4 . 6 9 3 7 . 1 1 4 9 . 1 1 1 4 1 . 1 3 1 9 0 . 7 2 2 4 . 7 2 3 3 . 4 1 0 . 0 0 2 7 0 . 0 0 3 2 0 . 0 6 8 5 0 . 0 8 1 2 6 8 . 2 1 7 9 . 2 9 7 4 . 9 6 8 7 . 1 4 1 0 3 . 2 3 1 1 1 . 5 0 1 1 3 . 4 4 1 2 2 . 5 3 - 1 9 6 3 3 . 3 9 3 2 . 3 9 3 6 . 6 9 3 5 . 5 9 8 3 . 7 7 1 0 7 . 1 9 1 4 . 6 7 1 8 . 7 7 0 . 0 0 3 7 0 . 0 0 2 1 0 . 0 9 3 9 0 . 0 5 3 3 5 2 . 5 5 5 9 . 4 4 5 7 . 7 5 6 5 . 3 2 1 3 5 . 4 2 1 0 1 . 8 1 1 4 8 . 8 2 1 1 1 . 8 8 TABLE - 6 . 4 AF-2 Steel crack Parallel to Rolling Direction (T-L orientation) Temp. ° C K Q JIc COD Equivalent KT(, ksi^/in MPâ /T in-lb/in 2 KJ /m2 «n 60 C 0 D - 5 M C0D-6Q inch mm inch ran HPa^/T k s i y f n MPâ ym" ksi ̂ /d MPâ /5" RT 7 8 . 1 2 8 5 . 7 2 8 5 . 8 5 9 4 . 2 0 7 8 5 . 0 0 1 3 7 . 5 3 0 . 0 1 1 9 0 . 0 1 4 2 0 . 3 0 2 2 0 . 3 6 0 6 0 . 0 0 8 4 0 . 0 1 1 4 0 . 2 1 3 3 0 . 2 8 9 5 1 6 0 . 8 6 1 7 6 . 7 8 1 6 8 . 9 1 1 8 4 . 7 3 1 8 5 . 6 3 2 0 3 . 0 1 1 4 2 . 6 2 1 6 5 . 3 3 1 5 6 . 7 3 1 8 1 . 6 9 - 40 7 9 . 5 9 8 3 . 4 0 8 7 . 4 6 9 1 . 6 5 6 0 0 . 0 0 1 0 5 . 1 2 0 . 0 1 0 9 0 . 0 1 6 6 0 . 2 7 6 8 0 . 4 2 1 6 0 . 0 0 8 9 0 . 0 1 0 7 0 . 2 2 6 0 0 . 2 7 1 7 1 4 0 . 6 4 1 5 4 . 5 6 1 6 8 . 0 3 2 0 7 . 6 0 1 8 4 . 6 6 2 2 8 . 1 5 1 5 1 . 8 4 1 6 6 . 8 2 1 6 6 . 8 7 1 8 3 . 3 3 - 60 8 0 . 9 0 8 4 . 6 4 8 8 . 9 0 9 3 . 0 1 0 . 0 1 2 5 0 . 0 1 5 0 0 . 3 1 7 5 0 . 3 8 1 0 0 . 0 1 0 6 0 . 0 0 8 7 0 . 2 6 9 2 0 . 2 2 0 9 1 8 2 . 6 3 1 9 9 . 9 6 2 0 0 . 7 1 2 1 9 . 7 5 1 6 8 . 2 0 1 5 2 . 7 1 1 8 4 . 8 5 1 6 7 . 7 8 - 80 7 1 . 6 5 7 6 . 5 3 7 8 . 6 7 7 8 . 7 4 8 4 . 1 0 8 6 . 4 5 4 4 6 . 2 4 4 9 2 . 5 1 3 1 7 . 8 3 7 8 . 1 8 8 6 . 2 8 5 5 . 6 8 0 . 0 0 9 8 0 . 0 0 8 7 0 . 0 0 8 5 0 . 2 4 8 9 0 . 2 2 0 9 0 . 2 1 5 9 0 . 0 0 9 1 0 . 2 3 1 1 1 2 1 . 0 0 1 2 7 . 4 2 1 0 2 . 3 6 1 3 2 . 9 7 1 4 0 . 0 3 1 1 2 . 4 9 1 6 3 . 4 7 1 5 4 . 1 4 1 5 2 . 1 9 1 7 9 . 6 5 1 6 9 . 3 9 1 6 7 . 3 6 1 5 7 . 1 4 1 7 2 . 6 9 - 100 5 5 . 0 0 4 9 . 9 0 6 0 . 4 4 5 4 . 8 4 2 3 6 . 2 1 3 2 3 . 6 7 2 3 3 . 0 0 4 1 . 3 8 5 6 . 7 0 4 0 . 8 2 0 . 0 0 5 8 0 . 0 0 5 7 0 . 1 4 7 3 0 . 1 4 4 7 8 8 . 2 4 1 0 3 . 2 9 8 7 . 6 4 9 6 . 9 7 1 1 3 . 5 1 9 6 . 3 1 1 3 0 . 7 6 1 3 0 . 2 8 1 4 3 . 7 0 1 4 3 . 1 7 - 130 4 6 . 5 2 4 8 . 1 8 . 4 0 . 3 2 5 1 . 1 2 5 2 . 9 4 4 4 . 3 1 1 3 5 . 6 2 1 4 9 . 5 0 2 3 . 7 6 2 6 . 1 9 0 . 0 0 5 4 0 . 0 0 5 0 0 . 0 0 4 8 0 . 1 3 7 1 0 . 1 2 7 0 0 . 1 2 1 9 6 6 . 8 6 7 0 . 2 0 1 3 2 . 1 3 7 3 . 4 7 7 7 . 1 5 1 4 5 . 2 1 1 4 0 . 7 5 1 3 5 . 0 7 1 5 4 . 6 8 • 1 4 8 . 4 4 - 196 3 0 . 7 6 2 5 . 8 6 3 3 . 8 0 2 8 . 4 2 1 2 4 . 5 0 1 1 3 . 0 2 2 1 . 8 1 1 9 . 8 0 0 . 0 0 2 6 0 . 0 0 3 6 0 . 0 6 6 0 0 . 0 9 1 4 6 4 . 0 6 6 1 . 0 4 7 0 . 4 0 6 7 . 0 8 118.61 1 3 7 . 9 2 1 3 0 . 3 6 ' " 1 5 1 . 5 7 TABLE - 6 . 5 AF-2 Steel crack Parallel to Pine Axis Temp. ° c KQ J l c COD Equivalent K, Ic ksiy'in MPav/o" in-lb/in 2 KJ/m2 <5 n JIc COD-6 m C0D-6Q inch mm inch urn • ksi^/in Wa^/m* . k s l ^ i i Ml'a.ym" ksLy/iri MPa^XT P.T 74.46 74.98 77.43 81.83 82.40 85.09 800.00 140.16 0.0148 0.0142 0.0152 0.3759 0.3706 0.3860 0.0103 0.0087 0.0103 0.2616 0.2209 0.2616 162.39 178.46 207.66 203.34 • 210.69 228.21 223.47 231.54 173.29 159.34 173.47 190.44 175.11 190.64 - 40 76.96 69.76 71.98 84.57 76.66 79.10 640.00 112.12 0.0104 .0.0107 0.0)09 0.2641 0.2717 0.2768 0.0092 0.0070 0.0089 0.2336 0.1778 0.2260 145.25 159.62 181.40 184.27 185.62 199.35 202.51 203.99 171.09 149.47 167.86 188.02 164.26 184.47 - 80 57.72 52.15 66.83 63.43 57.31 73.44 382.52 193.95 376.50 67.01 33.98 65.96 0.0072 0.0051 0.0066 0.1828 0.1295 0.1676 112.29 79.96 111.41 123.40 87.87 122.43 155.69 131.02 148.86 171.10 143.99 163.59 - 110 55.14 61.04 60.59 67.08 188.77 163.00 33.07 28.55 0.0054 0.0071 0.1371 0.1803 78.88 73.30 86.68 80.55 138.63 158.91 152.35 174.64 -130 55.53 34.11 57.92 61.02 37.48 63.65 165.31 83.35 28.96 14.60 0.0067 0.0037 0.005 3 0.1701 0.0939 0.1346 73.82 52.42 81.12 57.60 160.56 119.09 142.19 176.45 130.87 156.26 - 196 26.09 26.39 28.67 29.00 93.97 67.27 16.46 11.78 0.0022 0.0027 0.0558 0.0685 55.66 47.09 61.17 51.75 106.47 118.61 117.01 130.35 - TABLE - 6.6 AF-2 Steel crack Transversa to Rolling Direction (L-T orientation) Temp. 0 C KQ JIc COD Equivalent Ktc Jcsi,/in V in-lb/in 2 KJ/m2 5m «n JIc C0D-6„ m C0D-6n inch mm inch mm ksi^/n MPâ /n" ksi^/iri ksi^/^n MPâ /G" RT 73.58 76.53 80.86 34.10 820.00 143.66 0.0149 0.0140 0.3784 0.3556 0.0064 0.0071 0.1625 0.1803 164.41 180.68 192.35 186.48 211.39 204.94 126.07 133.47 136.55 146.68 - 40 81.07 74.23 89.09 81.57 660.00 115.63 0.0182 0.0113 0.4622 0.2870 0.0103 0.0069 0.2616 0.1752 147.50 162.10 224.80 177.08 246.05 194.61 169.22 138.99 185.97 152.75 - 80 77.03 69.55 76.48 84.65 76.43 84.05 364.86 379. 39 63.92 66.47 0.0077 0.0082 0.0078 0.1955 0.2082 0.1981 0.0069 0.1752 109.67 111.83 120.52 122.90 150.00 154.32 150.76 164.85 169.59 165.68 142.22 156.29 - 110 49.85 63.37 60.10 54.78 69.64 66.05 168.56 194.07 29.53 34.00 0.0046 0.0049 0.0059 0.1168 0.1244 0.1498 74.54 79.98 81.91 87.89 121.66 125.12 137.72 133.70 137.50 151.35 - 130 51.82 52.81 56.95 58.03 173.56 146.35 30.40 25.64 0.0050 0.0054 0.1270 0.1371 75.64 69.46 83.12 76.33 132.60 137.36 145.72 150.95 - 196 24.97 26.97 27.44 29.64 73.03 85.99 12.79 15.06 0.0022 0.0029 0.0558 0.0736 49.07 53.24 53.92 58.51 105.62 120.93 116.07 132.90 127 > 200 CO Si « 160 to CO UJ z 120 a: o r> TO  80 TU  40 o < (E U. - + * * * * A A A O i A 8 i •a • © • © * 1 i i -200 -160 -120 -80 -40 0 40 200 160 120 o a. s TEMPERATURE °c Figure 6.18.1. Temperature dependence of fracture toughness of AF-1 s t e e l along crack p a r a l l e l to Ro l l i n g D i r e c t i o n . >200 CO co |60 CO UJ z © '20 O r- or I- O < 60 40 + + - X + X + + + I X * + A A . •a A A A • S A © S A 0 -O A • i t i i • -200 -160 -120 -80 -40 0 40 200 160 _ 120 > a 80 E •(40 O K Q - ^ - T C T A K | C - C a l c . From J.c-iTCT + K , C — C o l e . F r o m S m . ^ T C T X K , C — C a l c . F r o m SQ.^TCT • K J IIT K i d IIT KC0D I IT TEMPERATURE °c Figure 6.18.2. Temperature dependence of fracture toughness of AF-1 s t e e l along crack p a r a l l e l to Pipe Axis. 128 •200 -160 -120 -80 -40 TEMPERATURE °c Figure 6.18.3. Temnerature Dependence of Fracture toughness of AF-1 s t e e l along crack transverse to R o l l i n g D i r e c t i o n . - • • + * I ' • + + + A A A . O 8 @ a O 0 • o 8 I 1 I -200 -160 -120 -80 - 4 0 0 40 2 4 0 2 0 0 I60C| 120 | 8 0 4 0 © A K Q - £ T C T K , c — C a l c . From J i c i T C T + K i c — C a l c . F r o m S m . j T C T X K ( e - C o l e . F r o m S q - i T C T A K J IIT K i d IIT K C 0 D IIT TEMPERATURE °c Figure 6.19.1. Temperature Dependence of Fracture Toughness of AF-2 s t e e l along crack p a r a l l e l to R o l l i n g D i r e c t i o n . 129 - 2 0 0 - 1 6 0 - 1 2 0 - 8 0 - 4 0 0 4 0 TEMPERATURE °c Figure 6.19.2. Temperature Dependence of Fracture Toughness of AF-2 s t e e l along crack p a r a l l e l to Pipe Axis. KS  2 0 0 * CO CO 160 lit z X o 1 2 0 r» o »- k l 8 0 or r> C T 4 0 < or + • © *• J A • + A X X x • + + + & o o f t I A 8 1 • 3 • J 1 1 i . I I— - 2 0 0 - 1 6 0 - 1 2 0 - 8 0 - 4 0 0 TEMPERATURE °c 4 0 2 4 0 2 0 0 1 2 0 2 S 8 0 4 0 © K Q - £ T C T A K I C — C o l o . F r o m "ic- l T C T + K | C — C a l o . F r o m S m . - ^ T C T X K , c — C a l c F r o m 8Q. ± T C T • *<J IIT * * i d l l T T K C 0 D IIT Figure 6.19.3. Temperature dependence of fracture toughness of AF-2 s t e e l along crack transverse to R o l l i n g D i r e c t i o n . 130 6.4 Comparison of Static and Dynamic Fracture Toughness; In this section, an attempt i s made to compare the static fracture toughness data of the AF-1 and the AF-2 steel as obtained in the present investigation (K I c^ Jjc, COD) with dynamic, equi- valent fracture toughness values obtained from the Instrumented Impact Tests conducted by Paul McConnell (15) • Static and Dynamic fracture toughness data are superimposed on the following figures: For the AF-1 steel a) Fig. 6.18.1 shows fracture toughness data for the crack parallel to the roll i n g direction (the T-L orientation); b) Figure 6.18.2 shows fracture toughness data for the crack parallel to the pipe axis; and c) Fig. 6.18.3 shows fracture toughness data for the crack transverse to the roll i n g direction (the L-T orientation). A similar comparison for the AF-2 steel i s shown in Fig. 6.19.1, 6.19.2 and 6.19.3. The average dynamic fracture toughness data were calculated and are shown in the figures*, dynamic data o were available only for temperatures down to - 100 C. 131 The data for the AF-1 steel with the crack running parallel to the rol l i n g direction (Fig. 6.18.1) indicates that a very good correlation exists between the KQ static data, the Kj U T and the Kia H T data for o test temperatures from RT down to - 100 C. The mag- nitude of the Kj ^ T and Kj^ results i s much smaller than the results obtained for the J T„ 6„ i c , m, 6Q, COD static data. If the static J l c and 6 m 6Q COD are considered representative fracture toughness properties of the o steel for temperatures above - 130 C, where K l c becomes invalid, then i t i s obvious that dynamic fracture toughness is less than the static fracture toughness for the same test temperature. It i s clear that an equivalent K j c obtained from 6 m or 6Q COD data i s a poor indication of the fracture toughness over the whole temperature range. This i s due to the fact that 6 m corresponds to a COD for the maximum load where extensive crack extension has occurred. For samples of the AF-2 steel having the same crack orientation, the static and dynamic fracture toughness data are shown in Fig. 6.19.1. This diagram 132 o o shows that at - 100 and - 80 C, static KQ^ Kj IIT, and K I ( j are comparable; the dynamic data have a much smaller magnitude than the respective static o J I c , 6m a n d fiQ C 0 D v a l u e s « At - 40 C the K I d 1 1 T value i s comparable to the KQ. but the Kj ̂ T value is comparable to the J J c value, J l c being much higher than the K n value. At RT the magnitude of the dynamic Kj ^ T and K C O D]_^fri results are comparable to the J l c 6 m and 6Q COD value. In the case of the AF-1 steel with the crack running parallel to the pipe axis (Fig. 6.18.2), at o o - 60 C and - 80 C, the Kj a n <^ K i d 11T r e s u l t s o are comparable to the KQ data. At - 40 C, the K Q 0 D 2.1T and Kj H T results are comparable to the *Q - COD and are higher than the J j c value. At RT the K C 0 D value is comparable to the * m - COD value. For a similar specimen orientation of the AF-2 steel, Fig. 6.19.2 indicates that a good correlation exists between the KQ data and Kj U T and Kja H T results obtained o o at - 80 C and below. For - 40 C and higher temper- atures, the K C 0 D -j_2.T d a t a a r e comparable to J i c , fim o and *Q COD values. However, at - 40 C, the K I (j value is comparable to the KQ value, Kj n-j* l i e s much above the KQ values but below the J L C data. For the AF-1 s t e e l with the crack running transverse to the r o l l i n g d i r e c t i o n (Fig. 6.18.3) o i t can be seen that at - 40 C and below there i s a good c o r r e l a t i o n between the KQ data and the Kj l l T o and K I d 1 1 T r e s u l t s . At temperatures of - 40 C and above, a good c o r r e l a t i o n exists between KQQQ H T and 5m, &Q C 0 D values. For the AF-2 s t e e l with the same specimen orient a t i o n (Fig. 6.19.3) the KQ data i s o comparable to the K I d H T and Kj 11T results at - 80 C o and below. At - 40 C and RT the K C Q D ^ resu l t s are comparable to 6Q - COD and J I C values; whereas at o - 4 0 C, the Kj I I T data i s comparable to 6 m - COD value. In summary, i t may be stated that: i) the K C 0 D I I T value i s comparable to the 6m and the 6Q COD s t a t i c data i n the upper shelf tem- erature range. o i i ) At - 60 C and below, the Kj H T and K I (j •Q t values are comparable to the KQ data. o i i i ) At temperatures > - 60 C, the Kj I I T data i s larger i n magnitude than the KQ data and i s less than the J l c and the 6^ - COD static data; however, K J d 1 1 T i s comparable to KQ static data. From the above observations, i t i s apparent that in the upper shelf region there is a close correlation between the static ( 6 m > 6Q) and the dynamic K C 0 D l l T fracture toughness values. However, with the exception of the two cases (refer Fig. 6.18.2 o and 6.19.3 at - 40 C) one for the AF-1 steel with crack running parallel to the pipe axis and one for the AF-2 steel with crack running transverse to the rolling direction, the dynamic Kj 1 1 T i s less than the J I c static value and the dynamic Kj^ n»p i s comparable to the KQ data, in the upper shelf temperature region. The «Q F 6 m static and the dynamic COD fracture toughness data give a poor indication of the fracture toughness of both steels at upper-shelf temperatures; this i s due to the fact that the calculation of 6ro and K^Q D corresponds to a COD for a maximum load where extensive crack extension has occurred. There- fore, in the upper shelf region, i f only the Kj ^XT K I d 11T a n <* static J i c and 6Q values are considered, i t i s obvious that static fracture toughness J I C i s higher than the dynamic fracture toughness (Kj H T ) , Kid 11T) . In the transition temperature region the available static and the dynamic data indicated that K I C > K I d . (62) Shoemaker and Rolfe have established that for structural steels which are strain rate sensitive, dynamic fracture toughness values are more conservative than the static fracture toughness values. Barsom and (63) (64) Rolfe and Barsom advanced the observation -5 that the effect of a slow loading rate (e * 10 /sec) • as compared to an impact loading rate (e «• 10/sec) in steels of various yield strengths is to sh i f t the equivalent fracture-toughness behaviour to a lower temperature, the magnitude of the change being given by the following relation T deg Shift = 119 - 0.12 o y s for 250 MPa < o y s < 965 MPa T Shift = o for o v S< 965 MPa 136 They have also shown that for steels having 965 MPa, K I c = K I {j throughout the transition range. Therefore, the present relationship between the static and the dynamic fracture toughness data of the AF-1 and the AF-2 steels ( oyield strength = 480MPa) is one in which K I tj < K l c for the entire temperature o range down to - 100 C. This behaviour of the static and the dynamic fracture toughness of the AF-1 and AF-2 steels i s in good agreement with the observations of (62) (63) Shoemaker and Rolfe , Barsom and Rolfe and (64) (65) Barsom . In contrast, G.R. Irwin reported that for structural steels the variations of loading speed from slow to fast did not change K j c when the fracture was mainly by cleavage and the testing (55) temperature was sufficiently low. A.H. Priest also made a similar observation, that when fracture occurs by cleavage, K l c values are relatively i n - dependent of the tensile properties and the K I C values do not vary with strain rate. In the present investigation static data i s available in the cleavage range but dynamic data from 11T tests o is available only down to the - 100 C transition range. Therefore, no comparison i s possible in the 137 cleavage range. But the test results of this study did o indicate that up to - 100 C, the fracture toughness of the AF-1 and the AF-2 steels are strain rate sensitive. 7. CONCLUSIONS i 7.1 Conclusions: 1. The tensile studies established that with a decrease in the test temperature, the yield strength and flow strength of both the AF-1 and the AF-2 steel increased. The AF-1 steel, containing more sulphur, possesses inferior yield strength and exhibits higher anisotropy than the AF-2 steel. The isotropy of AF-2 steel may be also due to rare earth addition. 2. K I C as well as J l c and COD test methods established that with increasing temperature, the fracture toughness of both the AF-1 and AF-2 steels increased. 3. A l l three test methods showed similar fracture toughness transition behaviour for both o o o steels; - 130 , - 150 , - 196 C toughness data o constituted the lower shelf, - 40 C and RT toughness o o data constituted the upper shelf and - 80 C, - 100 C, o - 110 C toughness data, the transition region. 4. The KQ test established that both steels o possessed valid K L C up to a temperature of - 130 C and the fracture toughness transition was from a plane-strain to plane-stress testing condition. 5. A l l of the test methods established that the AF-1 steel is anisotropic, possessing i t s highest toughness in specimens having the crack transverse to the rolling direction (L-T) and i t s minimum toughness for the crack parallel to the rolling (T-L) direction. 6. A l l three test methods confirmed that the AF-2 steel i s more isotropic and tougher than the AF-1 steel in the upper shelf region. The J-Integral test method indicated that the AF-2 steel is also tougher than the AF-1 steel in the transition region. The AF-2 steel possesses twice the toughness o of the AF-1 steel at RT and - 40 C for a crack running parallel to the rolling (T-L) direction. 7. The lowest toughness of the AF-1 steel was realized for samples having a crack parallel to the 140 rol l i n g (T-L) direction as described in McConnell's 11T study. Therefore, both static and dynamic fracture toughness data reveal that the current pipeline toughness specification for the crack propagating parallel to the pipe axis may be inadequate for ensuring fracture control; minimum toughness properties (dynamic and static) are realized for the T-L orient- ation i.e. crack running parallel to the roll i n g direction. 8. A comparison of static and dynamic fracture toughness data revealed that for the complete temperature range of testing, K i c > K^a, which indicates both AF-1 and AF-2 steel are strain rate sensitive. 7.2 Suggestions for Future Work; i) To further analyse the strain rate sensitive character of both AF-1 and AF-2 steels, the fracture toughness measurements under dynamic strain rate 5 / - conditions ( e = 10 - 20/sec, K =10 ksi/in/sec) should be carried out for the same range of temperatures by the ASTM standard E - 399 - 74 method and compared with K I c fracture toughness. To confirm the validity of the 11T data on these steels, direct measurement of the dynamic fracture toughness data (Kid) w i l l be useful. i i ) A fractographic analysis of the specimens tested in the present work would reveal a clear picture of the micromode mechanism of failure in entire transition temperature range. i i i ) More correlation between K I C or K with Drop Weight Tear Test w i l l be useful for developing an effective fracture control plan. iv) 11T, K l c > K I d and Drop Weight test data for each steel should be stored to make an effective comparison. This may contribute towards toughness design considerations of pipeline steels. 142 REFERENCES 1. Molybdenum Mosaic - J. of Molybdenum Metallurgy, climax Molybdenum, v-1, No. 3, Spring 1976, P. 14. 2. M.P. Boussel, K. Miyano, J.A. Straatmann - Tech. Report M-316, " x-70 Mo-Nb. A new family of HSLA steels designated for Arctic and offshore pipelines", climax Molybdenum, 1978. 3. A.O. Coldren and J.L. 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Tiffany and J.N. Masters - "Fracture Toughness Testing", ASTM STP - 381, P. 249. 52. Metals Handbook - "Failure Analysis and Prevention", vol. 10, P. 54. 53. ASTM Standards E 399 - 74, in anual Standards, 1975, P.955. 54. J.D.G. Sumpter - Metal Science, October, 1976, P. 354. 146 55. A.H. Priest - Dynamic Fracture Toughness, WI and ASM, I n t l . Conf. v-1, London, 1976, P. 95. 56. G.D. Fearnehough - 'Physical basis of Yield and Fracture' - Conf. Proc. Oxford, September, 1966. P. 88. 57. G.R. Irwin - Trans ASME, J. of Engg for Power, Ser A, Oct 1964, P. 444. 58. R.O. Ritchie, J.F. Knott and J.R. Rice - J. Mech. Phys. Solids, vol. 21, 1973, P. 395. 59. E.T. Wessel: LEFM for thick walled welded steel pressure vessels: Material property considerations. 'Practical Fracture Mechanics for Structural Steel' - 1969, UKAEA, Risley. 60. J.M. Barsom and S.T. Rolfe: Engg. Fracture Mechanics, vol. 2, 1971, P. 341. 61. W.G. Wilson, Q.J. Klems - Industrial Heating, vol. 41, Oct. 1974, P. 12. 62. A.K. Shoemaker and S.T. Rolfe - Engg. Fracture Mechanics, vol. 2, 1971, PP 319. 63. J.M. Barsom and S.T. Rolfe - ASTM STP - 466, 1970, P. 281. 64. J.M. Barsom - in ref. 55, P. 113. 65. G.R. Irwin - in ref. 55. P.l. 147 APPENDIX - I Calculation of Pf(max) We know for CT specimen (16,53) BWls f (a/w) considering KQ = 100 ksi^yTn, K f ( m a x j = 15% K^ a = 0.50 inch w = 1.00 inch a/w = 0.50 f(a/w) = 9.60 from Standard Table Kf(nax) « P£$%X) f ^/w) Pf (max)1 15,000 x .50 x 1 9~7~5Tj TABLE = 781 lbs, Settings for Precracking m/c Kf(max) (ksî /In) Pf (max) (lbs) p f (max) lbs * f ( m a x ) -= xx.001" 2x4.05 10% K 0 11% KQ 12% K, 13% K 14% 15% K 16% K 17% Ki 18% K 19% KQ 20% KQ K Q :Q Q :Q :Q 520 573 625 677 729 781 833 885 937 989 1041 260.00 64.30 286.50 70.70 312.50 77.10 338.50 83.59 364.50 90.00 390.50 96.45 416.50 102.90 442.50 109.30 468.50 115.76 494.50 122.17 520.50 128.60 148 APPENDIX - II AF-2 Steel crack parallel to Rolling Direction Sp. NO, Kf(max) No. of Stress Cycles Sp. for Time taken (Min.) 1 15% 2 15% 3 15% 4 15% 5 15% 35 15% 36 15% 37 15% 38 15% 39 15% 30,000 32,000 41,000 29.000 31,000 58.000 50.000 64,000 50,000 61,000 38 % KQ J - l J - l J - l J - l J - l 17 18 25 16 17 33 29 35 28 36 AF-1 Steel crack parallel to Rolling Direction 10 12 20 26 29 21 22 23 24 25 15% KQ 15% KQ 15% KQ 15% KQ 15% KQ 15% KQ 15% KQ 15% KQ 15% KQ 15% KQ 31,000 35,000 35,000 38,000 37,000 60.000 72,000 55,000 65,000 60,000 17 19 19 21 20 34 39 31 38 33 PUBLICATIONS 1. R.HAITI,P.DUTTA, & Y.G.ANDREEV, A study on the mechanical behaviour ii of low carbon martensite , Metallurgical Engineers,Department of Metallurgical Engineering,1.1.T.. Kharagpur, 1972. II 2. Y.G.ANDREEV,& R.MAITI, Experimental techniques for studying microplasti i i city of metals , Sixteenth Congress Of Indian Society of Theoritical and Applied Mechanics, Allahabad,India,1972, II 3. R.MAITI & Dr.M.K.MUKHERJEE, Effect of thermal cycling on the hardening II behaviour of wrought AZ- 61 Mg-Alloy, I.R.S. Symposium, Trivandrum, India, September,1973. II 4. S.K.DUTTA,Dr.M.K.MUKHERJEE & R.MAITI, Experimental.and theoretical if studies on the problem of shielding in welding of Mg-Alloy ., I.I.W. Symposium, Durgapur, November, 1974, 5. S.K.DUTTA,R.MAITI & Dr.M.K.MUKHERJEE, Development of a procedure for II surfacing welded Mg-Alloy pressure vessels , I.I.U. Symposium,Tiruchira- paily, India,December, 1975. it 6. R.MAITI,E.R.GHOSH et a l l , C r i t i c a l heat treatment parameters for ii AZ-92 Mg-Alloy casting for s a t e l l i t e application , I.I.F. Symposium, I.I*T. Madras,January,1976. 7. P. P. SINHA,R.MAITI,Dr.K.V.NAGRAJAN, Dr.M.K.MUKKERJEE, Kinetics of eleva- ii ted temperature reactions in maraging steel ,I.I.M. Symposium, Suratkal, March,1976.

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