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Fracture toughness of pipe line steels Maiti, Ranen 1978-12-31

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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 t h i s thesis as conforming to the required standard  The  University of B r i t i s h Columbia, December, 1978 0  RANEN MAITI, 1978  In p r e s e n t i n g  t h i s thesis i n p a r t i a l f u l f i l m e n t o f the requirements f o r  an advanced d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e 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 s t u d y . I f u r t h e r agree that permission  for extensive  copying o f this thesis  f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e 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 understood that copying 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 n o t be a l l o w e d w i t h o u t my written  permission.  Department  Of  Metallurgical  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 V a n c o u v e r , Canada V6T 1W5 Date  DE-6  BP  75-51 1 E  January 18th,  1979  ABSTRACT  The  fracture toughness of two  HSLA pipeline steels was  acicular f e r r i t e ,  investigated u t i l i s i n g the  l i n e a r e l a s t i c fracture mechanics analysis  (K  according to ASTM Standard E399-74) as well as e l a s t i c - p l a s t i c fracture mechanics analysis and crack opening displacement COD  The rate of 10  testing  T  the  (J-Integral  methods).  tests were conducted at a s t a t i c s t r a i n /sec, K = 10 ksi^/in/sec with H inch  thick compact tension specimens.  A resistance curve  t e s t technique developed by Landes and Begley employed to obtain the J  l c  fracture toughness; whereas  the B r i t i s h Standard for COD measuring the 6  C  was  testing was  followed for  fracture toughness.  The anisotropy i n fracture toughness and t e n s i l e properties of the two x-70  the  steels were  measured and explained i n terms of sulphur content and rare earth  additions.  An attempt was  made to correlate the l i n e a r  e l a s t i c fracture toughness K  L C  or KQ values with the  e l a s t i c - p l a s t i c fracture toughness, J , a n d COD data lc  for both steels for tests i n each of three notch orientations i . e . 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 (T-L); p a r a l l e l to the pipe axis; transverse to the r o l l i n g d i r e c t i o n (L-T).  Tests were performed at  temperatures throughout the t r a n s i t i o n range i . e . o from RT down to - 196 C.  F i n a l l y the s t a t i c fracture toughness data as generated i n t h i s study, was compared with the dynamic fracture toughness as obtained from IIT test for both s t e e l s .  iv  TABLE OF CONTENTS Page ABSTRACT . .  i i  TABLE OF CONTENTS  iv  LIST OF FIGURES  viii  LIST OF TABLES  ,  .. x i i  LIST OF SYMBOLS  xiii XV  ACKNOWLEDGEMENTS  1.  INTRODUCTION  1  2.  PIPE LINE MATERIALS  4  2.1  Introduction  4  2.2  Metallurgy of Acicular F e r r i t e Steels  5  2.2.1  E f f e c t of A l l o y Additions  5  2.2.2  M i l l Production Parameters  7  2.3 3.  Pipe Fabrication and Strength of Skelp  9  THE BASIS OF DESIGN FOR PIPELINES  11  3.1  Strength Considerations  11  3.2  Fracture Control Design  13  3.2.1  Design C r i t e r i a for Preventing Failure  Brittle  3.2.2  Design C r i t e r i a for Ductile Fracture I n i t i t a t i o n Control  15  3.2.3  Design C r i t e r i a for Ductile Fracture Propagation and Arrest  17  14  Page  4.  3.3  Review of the Design for Fracture Control  3.4  Instrumental  3.5  Project Summary  THEORY AND 4.1  4.2  ..  Impact Test Approach  20 23  TEST PROCEDURES  25  Linear E l a s t i c Fracture Mechanics 4.1.1  Plane Strain Fracture Toughness  4.1.2  Specimen Size Requirements  25 ....  27 29  E l a s t i c - P l a s t i c Fracture Mechanics  31  4.2.1  34  4.2.2  The J-Integral Approach 4.2.1.1  Experimental  Technique  4.2.1.2  Validity Criteria  .. ..  37 39  The Crack Opening Displacement Method  39  4.2.2.1  COD  40  4.2.2.2  Dugdale's Model  4.2.2.3  Experimental COD  as an Extension to LEFM  42  Determination  4.2.2.3.1 Determination «c  of  46  of 4 8  4.2.2.3.2 Evaluation of an Equivalent K from COD lc  5.  19  50  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  5.5  5.6  5.7  Fatigue Precracking  58  5.4.1  ASTM Standards for Precracking . . . .  59  5.4.2  Precracking Stress Intensity  60  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  ....  COD Test Details  70  5.6.1  Assessment of Test Data  70  5.6.2  Calculation of 6  71  5.6.3  Calculation of Equivalent K  C  lc  ....  72  J-Integral Test Details  73  5.7.1  73  Testing Parameters 5.7.2.1  Measurement of Specimen Dimension and Crack Growth (Aa)  74  5.7.2.2  5.7.3  Measurement of Area (A) under P-A record 76 o Calculation of J for RT and - 40 C Tests 76  5.7.4  Determination of J j Value  77  5.7.5  Calculation of Equivalent K  5.7.6  V e r i f i c a t i o n of V a l i d i t y C r i t e r i o n  c  lc  ....  78 79  vii  5.8 6.  Tensile Test Details  79  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  6.3 6.4  110  Comparison of Fracture Toughness Properties from K J-Integral and COD Tests 114 L  7.  Page  C  Comparison of S t a t i c and Dynamic Fracture Toughness  130  CONCLUSIONS  138  7.1  Conclusions  138  7.2  Suggestions f o r Future Work  140  REFERENCES  142  APPENDIX - I  147  APPENDIX - II  148  viii  LIST OF FIGURES Figure No.  Page No.  4.1  Distribution of P r i n c i p a l stresses at the crack t i p .. ..  26  4.2  Schematic load (P) vs load-point displacement (A) curves for (a) perfectly 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) Ductile 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  33  4.3  Dugdale's S t r i p Y i e l d Model  43  5.1  % inch thick compact tension specimen  5.2  (a)  ..  55  (b)  orientation of CT specimens with respect to Rolling Direction of the plate Dimensions of the t e n s i l e specimen ..  57 57  5.3  (a) (b)  Dimensions of the Brass tubes.. .... Photograph of the experimental set-up  64 64  5.4  (a)  Actual P-A test record for AF-1 at - 60° C Actual P-A test record for AF-1 at - 150° C  (b) 5.5  (a) (b)  6.1  6.2  steel steel  J-Integral test record of AF-2 s t e e l with crack transverse to r o l l i n g d i r e c t i o n at - 40° C J-Integral test record of AF-2 s t e e l 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 - 130° C  68 68  75 75  Temperature dependence of y i e l d stress and flow stress of AF-1 s t e e l with specimen transverse to r o l l i n g d i r e c t i o n  84  Temperature dependence of y i e l d stress and flow stress of AF-1 s t e e l with specimen transverse to pipe axis  84  ix  Figure No, 6.3  6.4  6.5  6.6  6.7  6.8  6.9.1  6.9.2  Page No. Temperature dependence of y i e l d stress and flow stress of AF-1 s t e e l with specimen p a r a l l e l to pipe axis  85  Temperature dependence of y i e l d stress and flow stress of AF-1 s t e e l with specimen 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  85  Temperature dependence of y i e l d stress and flow stress of AF-2 s t e e l with specimen transverse to r o l l i n g d i r e c t i o n  86  Temperature dependence of y i e l d stress and flow stress of AF-2 s t e e l with specimen transverse to pipe axis  86  Temperature dependence of y i e l d stress and flow stress of AF-2 s t e e l with specimen p a r a l l e l to pipe axis  87  Temperature dependence of y i e l d stress and flow stress of AF-2 s t e e l with specimen 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  87  Temperature dependence of KQ^ K J of AF-1 and AF-2 steels with crack p a r a l l e l to r o l l i n g direction  89  Temperature dependence of KQ^ K J of AF-1 and AF-2 steels with crack p a r a l l e l to the pipe axis  89  c  c  6.9.3  Temperature dependence of Kg, K of AF-1 and AF-2 steels with crack transverse to the rolling direction 89  6.10.1  Fracture surfaces of K specimens of AF-2 s t e e l with crack p a r a l l e l to the pipe axis at various temperatures  6.11.1  I c  l c  92  J-resistance curve f o r AF-1 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 97  X  Figure No. 6.11.2 6.11.3  Page No. J - r e s i s t a n c e curve f o r AF-1 s t e e l w i t h c r a c k p a r a l l e l t o t h e p i p e a x i s a t RT and - 40° C  98  J - r e s i s t a n c e curve f o r AF-1 s t e e l w i t h c r a c k 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 a t RT, - 40° C and - 80° C .„ .. .. . „ ..  99  6.12.1  J - r e s i s t a n c e curve f o r AF-2 s t e e l w i t h c r a c k 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 a t RT and - 40° C 100  6.12.2  J - r e s i s t a n c e curve f o r AF-2 s t e e l w i t h c r a c k p a r a l l e l t o t h e p i p e a x i s a t RT and - 40° C 101  6.12.3  J - r e s i s t a n c e curve f o r AF-2 s t e e l w i t h c r a c k 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 a t RT and - 40° C • 102  6.13.1  F r a c t u r e s u r f a c e s o f 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 the r o l l i n g d i r e c t i o n t e s t e d a t RT arranged i n o r d e r o f i n c r e a s i n g crack extension 103  6.14.1  Temperature dependence o f J y - o f AF-1 and AF-2 s t e e l s w i t h crack p a r a l l e l t o t h e rolling direction  105  Temperature dependence o f J y o f AF-1 and AF-2 s t e e l s w i t h crack p a r a l l e l t o t h e p i p e axis . . . . . . ..  105  Temperature dependence o f J o f AF-1 and AF-2 s t e e l s w i t h crack t r a n s v e r s e t o t h e rolling direction ..  105  Temperature dependence o f J K and Kj o f AF-1 and AF-2 s t e e l s w i t h c r a c k p a r a l l e l t o the r o l l i n g d i r e c t i o n  106  6.14.2  6.14.3  6.15.1  6.16.1  c  I  l  c  c  Q  Temperature dependence o f - COD o f AF-1 and AF-2 s t e e l s w i t h c r a c k 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 No.  Page No.  6.16.2  Temperature dependence of ^m - COD of AF-1 and AF-2 steels with crack p a r a l l e l 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 r o l l i n g d i r e c t i o n I l l  6.17.1  Temperature dependence of 6Q - 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 direction 112  6.17.2  Temperature dependence of 6 - COD of AF-1 and AF-2 steels with crack p a r a l l e l 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 r o l l i n g d i r e c t i o n  112  Temperature dependence of fracture toughness of AF-1 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  127  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  127  Temperature dependence of fracture toughness of AF-1 s t e e l along crack transverse to the r o l l i n g d i r e c t i o n  128  Temperature dependence of fracture toughness of AF-2 s t e e l along 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  128  Temperature dependence of fracture toughness of AF-2 s t e e l along crack p a r a l l e l to the pipe axis  129  Temperature dependence of fracture toughness of AF-2 steel along crack transverse to the r o l l i n g d i r e c t i o n  129  6.18.1  6.18.2  6.18.3  6.19.1  6.19.2  6.19.3  Q  xii  LIST OF TABLES Table No.  Page No.  5.1  Compositions of AF-1 and AF-2 steels  6.1  Fracture toughness data for AF-1 s t e e l with crack p a r a l l e l to the Rolling Direction  6.2 6.3  6.4  6.5 6.6  6.7  Fracture toughness data f o r AF-1 s t e e l with crack p a r a l l e l to the pipe axis  52  121 ..  122  Fracture toughness data f o r AF-1 s t e e l with crack transverse to the r o l l i n g direction  123  Fracture toughness data f o r 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 direction  124  Fracture toughness data f o r AF-2 s t e e l with crack p a r a l l e l to the pipe axis  ..  125  Fracture toughness data f o r 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 ..  126  Comparative J values of AF-1 to AF-2 steels 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 (T-L) and crack p a r a l l e l to pipe axis  108  l  c  X1X1  LIST OF SYMBOLS  e  Strain Rate  £y  S  K  Y i e l d Strain Stress Intensity Factor  •  K  Stress Intensity Rate  K  C  Plane Stress Fracture Toughness  K  L C  K  I D  Plane Strain Fracture Toughness under S t a t i c Loading Plane Strain Fracture Toughness under Dynamic Loading  KQ J  L  Calculated Fracture Toughness C  C r i t i c a l J - I n t e g r a l Plane Strain Fracture Toughness  JQ  Calculated J-value  G  Crack Extension Force under Plane Strain Condition  l c  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  max  Maximum Load  P  Load  A  Displacement  Y  Austenite  D  Inside Diameter o f Pipe  t  Pipe Wall Thickness  p  o  Hoop Stress  H  o  Tensile Stress  Oy  S  Y i e l d Stress  of  Flow Stress  E  Young's Modulus  v  Poisson's Ratio  a  Crack Length  B  Specimen Thickness  w  Depth of Specimen  A  1  c  Area o f Charpy Specimen Ligament  r^  P l a s t i c Zone Radius  Aa  Crack Extension  Vg  C l i p Gauge Displacement  r  Rotational Factor  z  Knife Edge Thickness  V  c  C r i t i c a l C l i p Gauge Displacement  V  m  C l i p Gauge Displacement at P  VQ  C l i p Gauge Displacement at P  m a x  0  XV  ACKNOWLEDGEMENTS I wish to thank my fellow graduate students and the members of the f a c u l t y , i n p a r t i c u l a r Dr. S.R. Bala and Asst. Prof. R.G. Butters for t h e i r assistance and h e l p f u l discussions during the research work.  I  very much appreciate the assistance of the technical s t a f f , i n p a r t i c u l a r , J . Walker and H. Tump throughout the experimental  programme and R. Bennett, P. Musil and  M. Bennett during the preparation of the t h e s i s . Financial assistance i n the form of a 'Commonwealth Scholarship* provided by Canadian Commonwealth Scholarship and Fellowship Committee, Ottawa, i s gratef u l l y acknowledged. I am indebted to my supervisors Prof. E.B. Hawbolt and Prof. J.S. Nadeau for t h e i r  continuous  advice, stimulating discussions, h e l p f u l suggestions and immense encouragement throughout the project. Thanks are also extended to Prof. E. Teghtsoonian, Head, Dept. of MET. ENGG, for providing necessary  facilities  to carry out the project. F i n a l l y , I thank my wife Biva and daughter Munmun for t h e i r patience.  1  1.  In Mechanics (EPFM) two  (LEFM)  thesis,  Linear  and E l a s t i c  are used t o analyse  x-70  x-70  this  INTRODUCTION  established  fracture  that  analogus  the  standard  present,  control  steel.  Full  v  and d u c t i l e  of the  the energy fracture  The possesses  these  tests  to protect failure tests  required toughness  i s well material  and i s  Instrumented  the potential  equally  considerations.  Tear Test and  test  are used as of  have e s t a b l i s h e d against  the minimum  catastrophic  of the pipelines. do n o t p r o v i d e  for fracture of  the  gas Alaska  t o assess the toughness  scale burst  Unfortunately,  i s a  t h e Drop Weight  tests  a n d BDWTT v a l u e s  brittle  design  Charpy v - n o t c h impact  quality  C  toughness  of  steels;  It  to the y i e l d strength  for pipeline  At  i n 1980.  Mechanics  toughness  a r e t o be used f o r t h e n a t u r a l t o be b u i l t  important  P l a s t i c Fracture  HSLA p i p e l i n e  Highway p i p e l i n e  property  Fracture  the fracture  acicular ferrite,  steels  Elastic  any  measure  initiation i . e .  the material.  Impact T e s t i n g  to solve  this  (IIT)  problem.  method I t has  2  established  its  superiority over  Charpy  for  the  test i)  initiation  It  fracture  It  Therefore,  and t h e  the  for  fracture  be more fracture  e s s e n t i a l to by  testing.  This  utilizes  rate and  a measure o f  toughness  the  energy. dynamic Charpy  conservative  d e t e r m i n e d by a n d may b e  to  obtain greater  investigate  the  using conventional  a  insight into failure,  static  i t  objectives of  1)  To  further  toughness  the  the  present  the  x-70  is  rate in  test  study  characterise the of  the  fracture  low s t r a i n  wall thickness for  The  fracture  more  approach a l s o has an advantage full  this  design.  processes occurring during  toughness  it  fracture  crack propagation  provides  However,  also  the  employed.  method w i l l  fracture  reasons:  i n i t i a t i o n e v e n t when a p r e c r a c k e d  specimen i s  useful  conventional  d i s t i n g u i s h e s between  energy  ii)  following  the  that  specimens.  were:  low  strain  steels using  LEFM  EPFM. 2)  toughness w i t h  To  compare t h e  dynamic  low s t r a i n r a t e  fracture  toughness  fracture  data  to  3  determine for  the  value  of  assessing fracture  The plastic linear study  the  rapid inexpensive  f r a c t u r e mechanics and the  with  the  various  the  study  of  the  elastic  l i m i t a t i o n s of  f r a c t u r e mechanics are o u t l i n e d i n  a view  test  toughness.  t h e o r e t i c a l background of  elastic  IIT  to  examining  the  fracture  toughness  testing techniques  HSLA s t r u c t u r a l  applicability  steels.  the this of to  4  2.  2.1  MATERIALS  Introduction:  Pipelines ing  PIPE LINE  energy  located  a r e economical, r e l i a b l e systems  resources from d i s t a n t  i n t h e most  such as S i b e r i a ,  severe  fields,  is  necessary  o f Canada o r A l a s k a , (1) markets  f o r t h e p r o d u c t i o n o f a new c l a s s  i t  of steels  with  combination o f q u a l i t i e s .  Strength  -  for walls rugged  2.  environments,  t h e c h a l l e n g e s e t by t h e whims o f n a t u r e ,  an unprecedented 1.  those  parts  and 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  To m e e t  transport-  particularly  a r c t i c and submarine  i n f r i g i d northern  for  Toughness  -  that  are thinner,  y e t more  i n performance.  for resistance to fracture  at  sub-zero  temperatures. 3.  Field weldability with  4. The  answer  low a l l o y  - with  little  resistance to cracking  o r no  preheat.  A l l at the lowest possible cost per unit (2) t o these problems (HSLA),  i s the evolution  Acicular Ferrite  (AF)  of  of a high  s t e e l having  strength. strength,  a yield  s t r e n g t h o f f r o m 70 t o 80 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 (155 J o u l e ) a n d a FATT o f o -60 C.  5  2  ,  Metallurgy  2  Acicular non-equiaxed diffusion  ferrite  phase  range.  Acicular  bainitic  structures,  factor are  brittled  three  prior  as a h i g h l y  The  than  the  upper  but  not  in  towards  strength  of  from  network  is  acicular ferrite higher  impact  boundaries  segregated  the  begins  bainite  is different  high angle  by p r e c i p i t a t e s o r  substructured,  c o o l i n g by  transformation  g r a i n boundary  contributes  high  Steels  on c o n t i n u o u s  ferrite  y -  no s t r a i g h t ,  The to  forms  s l i g h t l y higher  the  there  i s defined  that  ferrite;  This  Acicular Ferrite  and s h e a r mode.  temperature ation  of  a  at  a  transform-  bainitic retained  in  structures.  toughness  since  become (3) impurities  material  mixed  to  em-  can be  attributed  strengtheningmechanisms.  1.  Grain  Refinement  2.  Dislocation  3.  Precipitation strengthening  Substructure by Nb(C,N)  Niobium  Carbonitride.  2.2.1  Effect  Carbon; to  l i m i t the  to  achieve  New  of  Alloy  Additions;  s t e e l making p r a c t i s e s have been  carbon content  to  improved w e l d a b i l i t y  approximately and  0.05%;  formability.  adopted in  order  Increasing  6  the carbon, though i t increases the y i e l d strength, impairs the toughness by increasing the t r a n s i t i o n temperature and lowering the upper shelf energies.  This i s due to the formation of an  increasing amount o f cementite.  Since about 0.01  to 0.02  % C  i s adequate to f a c i l i t a t e p r e c i p i t a t i o n strengthening, a higher carbon l e v e l i s undesirable i n a c i c u l a r f e r r i t e s t e e l s .  Molybdenum and Manganese: The combination of molybdenum (0.25  - 0.50  wt % ) , manganese (1.50  lesser extent niobium  ( 0.05  - 2.25  wt %) and to a  wt %) suppresses the transformo  ation temperature of y+a to below 700 C.  The fine grain  structure i s ensured by having a fine y grain as w e l l . r e s u l t i s a fine grained (ASTM No. 13 to 14) microstructure.  a  The  acicular f e r r i t e  This exceptionally fine grain s i z e , provides  the basic building block f o r both the high strength and the (4) excellent thoughness Further, molybdenum decreases the rate of Nb(C,N) prec i p i t a t i o n i n austenite and thereby allows a greater amount of Nb(C,N) i n f e r r i t e .  This p r e c i p i t a t i o n process results i n  a higher strength product.  Manganese plays a role s i m i l a r to  that of molybdenum. Niobium;  Niobium i s a potent micro a l l o y that improves  the s t e e l through three mechanisms.  7  i)  It  refines  grain  the  austenite  structure  crystallization ii)  It  iii)  during  i t  ultrafine  r o l l i n g by  and g r a i n  suppresses the  Finally,  increases strength  p a r t i c l e s of  Sulphur; 0.005 wt sulphur the  %,  is  The  be m i n i m i z e d change to  the  sulphides in  impair  killed  through  eguiaxed  ensures  toughness  of  with Al  the  (T-L)  polygonal by  re-  ferrite  precipitation  c o o l i n g from  the  of  or  finish  the of  the  order  of  less  steels.  impact  particularly  strength, the  formation  than  Higher  of  in  MnS  effect  of  sulphide i n c l u s i o n s can  additions  of  rare  the  sulphides (5)  spheroids  adequate  the  of  acicular ferrite  through  detrimental  morphology  globular  inhibiting  niobium carbonitrides  low s u l p h u r ,  desirable in  rolling direction  stringers.  ferrite  temperature.  A very  contents  the  growth,  n u c l e a t i o n of  Nb-C-N c l u s t e r s during rolling  and u l t i m a t e l y  .  toughness  earth  from elongated  The  presence of  and reduces  processed s t e e l .  and a r e d u c e d amount o f  elements  ribbons  eguiaxed  the  anisotropy  These s t e e l s are Si  (0.05  to  which  often  0.17  wt  %)  (6) as  silicon  greater  than  0.17  wt  % reduces  the  impact  2 . 2p r o c M T2 h. e e si sl l c Po rnot dr uo cl t iaopnp l iPeadr admuer ti en rgs : h o t - r o l l i n g  resistance  is  8  extremely  important  to achieve  strength and the toughness. most  a favourable  The r o l l i n g p a r a m e t e r s  i n f l u e n t i a l are the slab reheat o  reduction  below  b a l a n c e between  temperature  900 C a n d t o some e x t e n t ,  the  found  t o be  and the  total  the f i n i s h  rolling  temperature. The  slab  1.  It in  2.  It of  reheat  temperature  determines  i s important  the degree  of  f o r two  reasons:  solutionising of  Nb(C,N)  y, a l s o determines  the y -  grain size  at the  beginning  rolling. o  A low slab-preheat rise  temperature  to a smaller y -  particles. subsequent particles  During  of  grain size  approximately  a n d some u n d i s s o l v e d  hot r o l l i n g y recrystallizes,  g r a i n growth  1150 C  gives Nb(C,N)  but the  i s i n h i b i t e d by the u n d i s s o l v e d  which r e s t r i c t grain growth.  As a r e s u l t ,  Nb(C,N)  after  several cycles of becomes e x t r e m e l y  r e c r y s t a l l i z a t i o n , the y - grain structure fine. A decrease of the Fracture Appearance o o T r a n s i t i o n Temperature (FATT) f r o m 5 t o - 90 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 1140 C t o o 1030 C .  As  below  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 do n o t r e c r y s t a l l i z e o a b o u t 980 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  accumulation of the  lower  strain  temperature  deformation  of  structure.  This  nucleation  to  the  to  ensure  general,  y  reduction  2.3  in  the  In test piece to  no  impair  grained  sites  for  ferrite.  deformation the  of  toughness  the  of  the  i n a more strength  Fabrication  testing  conditions from the  the  are  slab reheat  subsequent  The  heavy  y -  a  trans-  enhance  refined  ferrite  of  The  specimen f a b r i c a t i o n and t e s t i n g . causes the  of  of  u s u a l method  This  phase product.  to  temperature percentage  of  substructure grain the  structure,  steel.  Skelp  and toughness  important.  final  is  temperature  (to  and S t r e n g t h  strength  that  the  finish rolling  and toughness  curved pipe w a l l .  a Baiichinger e f f e c t  dislocated  ferrite  the  in  heavy  minimum r o l l i n g t e m p e r a t u r e  r o l l i n g stages  results  in  a heavily  a l s o suppresses the  decreasing the  late  improving  Pipe  The  more  continued  results  r e c r y s t a l l i z a t i o n and i n c r e a s i n g the  strengthening) thereby  a fine  that  in turn  provides  phase  rolling is  introducing  y  decreasing the  grain growth,  limit  of  y -  as t h i s would  In limit  structure  as the  y  This  grained  temperature.  controlled occurs  of  the  range.  and growth  deformation formation  fine  in  piece The  is  the is  pipe, to  cut  flattened  flattening  the a prior  introduces  measured y i e l d s t r e n g t h  to  10  be  lower  than  difference and the due of  to the  between  y i e l d strength  the  p i p e minus  the  One  most  of  the  i s equal to  ferrite  steels  Bauchinger  important  is  during  that  the  is  very  or  n i l in  effect  plate  into  large  of  strength  the  in  the the  of  strength  pipe.  the of  the  the  into  ferrite  -  pipe. the  amount o f  -  pipe  forming  acicular  ferrite of  additional pipe of  the  pearlite  This  work  means  hardening  steels,  an i n c r e a s e i n  steels.  acicular  fabrication  s t e e l s and a decrease  pearlite  -  processing  s t e e l s and very  is  plate  increase  the  strength  place during  ferrite  acicular ferrite the  of  yield  As  the  spiral  the  acicular ferrite  skelp  the  advantages  with  of  The  (3)  conventional  takes  conventional  converting  pipe.  strength  from  effect  c o l d expansion  r a p i d work hardening  of  the  the  increases continuously  particularly  the  resulting  Mn-Mo-Nb s t e e l s o v e r pipe  of  measured y i e l d s t r e n g t h  the work hardening  pearlite  the  true  fabricated pipe  ferrite  that  the  small the the in  net  3.  THE B A S I S O F D E S I G N F O R  3.1  Strength  Considerations:  Economy i n e x t r a c t i n g requires  the use of larger  can operate the life  output  at higher and reduce  energy  diameter  pressures.  .  and the l a r g e r  will  costs  The h i g h e r  diameters  resources  pipelines  This  the operating (7)  of the line-pipes  pressures  PIPELINES  which  maximise  over the  operating  necessitate  the  use o f t h i c k e r pipe w a l l s o r higher 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 Hoop S t r e s s , (8) relationship  Where P = O p e r a t i n g D = Inside t  = Pipe  There a r e l i m i t a t i o n s  pressure  diameter  wall  of the  pipe  thickness  to the pipe wall  t h i c k n e s s due  to: 1)  Restrictions  2)  The toughness  3)  Difficulties  and toughness  i n very  thick  imposed by m i l l requirement i n retaining plate.  facilities.  of a pipeline high  strength  ,  4) field The  trend  use h i g h e r  the  problems  in welding  and  inspection.  modern  this  Additional  in  line-pipe projects  strength  steel pipes.  i s e s s e n t i a l l y economic, materials  pipe wall obtained  savings  thickness through  The  and i s  r e a l i s e d by  and the  to  idea  behind  related  using a  increased  using larger  is  to  reduced  capacity  diameters  and  higher  pressures.  The are as  committed a proven  frontier steel the  current  natural  to  pipe  x-70  product.  projects  than  x-70  cost of  line  of  higher wall  the  strength  at  does  line with (7) fracture  the  is  available  using higher  increase the (9) line-pipe  the  i t  improved  output  and  pipe y i e l d  thickness only  ensure  respect  guarantee to  the  arresting  of  strength reduce  strength  that  a particular pressure. not  projects  future generation  to  can operate  yield  because  aim towards  However, and o r  The  gas p i p e l i n e  the The  integrity a  propagating  3.2  Fracture Control Design;  The modern trend i n line-pipe s p e c i f i c a t i o n s 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 f a i l u r e of gas transmission pipelines or (9) d u c t i l e tearing of o i l pipelines  . Such f a i l u r e s  cause a loss of production and at the same time a s i g n i f i c a n t amount of money i s required f o r damage repair.  The s o c i a l and economic implications of these  f a i l u r e s have motivated an increased awareness f o r the development of fracture toughness parameters f o r pipelines.  considers  The basic fracture control philosophy (2, 7, 9 - 12) the following three factors : 1.  To prevent b r i t t l e fracture propagation  by assuring that the pipelines operate above the d u c t i l e - t o - b r i t t l e t r a n s i t i o n temperature of the material. 2•  To prevent d u c t i l e fracture i n i t i a t i o n  by specifying a minimum toughness for a pipe at a s p e c i f i c stress l e v e l .  operating  14  3. by  To  specifying  control  ductile  some a v e r a g e  crack  toughness  propagation  that w i l l  assure  self-arrest.  These c r i t e r i a minimum  design  the  the  sponsorship  Workers  at  fracture adopted the  at  the  research in  Battelle of  the  the  a l l of  area  is  being  laboratories  Gas  under  Association.  laboratories  guidelines  virtually  this  Columbus  American  Battelle  control by  f u l f i l l e d for  temperature.  Continuing conducted  s h o u l d be  have  evolved  for  pipe which have  the  pipeline  been  industries  world.  3.2.1  Design  Criteria  for  Preventing  Brittle  Fracture;  Twenty y e a r s given of  in  to  the  pipe with  below  its  fracture the  ago,  no  consideration  resistance or  result  that  pipe  commonly  n i l - d u c t i l i t y temperature  occurrence of (11) t o 1 3 Km )  very  long b r i t t l e  led to  the  notch  (NDT).  fractures  realization  that  was  toughness operated The (one  of  brittle  up  fracture of  the  travelled  gas  in  The  the  faster gas  Battelle  than  the  decompression  transmitting  line.  Drop Weight  Tear Test  (BDWTT)  is  used to  e s t a b l i s h the r e s i s t a n c e of (2, 13)  the  to  brittle  fracture  specifies  at  the  line  lowest  mechanisms  individual  test  85% s h e a r  Design  fracture  have been developed  fracture  initiation.  determine which i s can be ductile  the  a measure of  related  to  fracture  The studies  critical  the  to  ensure  The  steel  the  that  that pipe-  ductile  However,  for  any  acceptable.  for  Ductile  Fracture  Control:  mechanics c o n c e p t s and for  assessing ductile  general objective  stress intensity the m a t e r i a l critical  is  to  factor  toughness  (Kc),  and  defect  size  that  loading  full  scale  which causes  initiation.  Battelle  generated  is  pipe  temperature,  Criteria  Initiation  Various  test  are operative.  60% s h e a r  3.2.2.  tests  The  design operating  should exhibit  fracture  .  velocity  Static  an e m p i r i c a l f o r m u l a e w h i c h  burst  relates  16  a c r i t i c a l c r a c k s i z e w i t h the Charpy upper s h e l f (11-14) as g i v e n below: energies n 8a o £2  K 2 c  = In Sec  12EC  Where K,  \  H  2 o  H  V  F r a c t u r e Toughness parameter Impact t e s t absorbed  energy  Specimen f r a c t u r e area 2a  , mm  )  Critical  through w a l l d e f e c t l e n g t h  (inch,  mm)  Flow s t r e s s = Y.S. ••+ 10,000 p s i  of  (68.95 M  (inch  (ft-lb),  MPa)  Folias Correction factor, a function  H  of  d  P i p e diameter,  t  Wall thickness, (inch, F a i l u r e Hoop S t r e s s =  OH E  (inch,  =  mm) mm) E£ 2t  ( p s i , MPa)  E l a s t i c Modulus ( p s i , MPa)  T h i s e q u a t i o n i s w i d e l y used i n the p i p e l i n e i n d u s t r y to p r e d i c t the a l l o w a b l e d e f e c t s i z e f o r a r r e s t i n g d u c t i l e crack i n i t i a t i o n .  However, i t should be  noted  t h a t t h i s e q u a t i o n i s a p p l i c a b l e o n l y over the tempe r a t u r e range o f the Charpy upper s h e l f .  Furthermore  J)  17  it  is  formulated  is  b a s e d upon c r a c k  In  practice,  loading  for  calculate  loading conditions  i n i t i a t i o n along  the  a r i s i n g from  impacts  due  For  design purposes,  the  critical  pipeline  crack size  companies  that would  during hydraulic proof  f r o m NDT  technique  and then  to  minimum C h a r p y  prevent  the  3.2.3  i n i t i a t i o n of  Design  It propagation most shear (9, 12)  of  This  However, ence of  such a  Criteria  a shear  and  reduces  recent  the  fracture  were  may b e  lowering  at  shelf  for  as  the  stress at  pipeline  least eight  crack.  Ductile  would not  in the  failures  shear  necessary  Fracture  Arrest;  because gas  or  equation  energy  arrested over  true  be  testing  from  was o r i g i n a l l y b e l i e v e d t h a t  fractures  propagation  determine  upper  Propagation  thereby  to  damage.  as a leak  the  axis.  dynamic  detected  (3.2)  and  pipe  i n i t i a t i o n may o c c u r u n d e r  conditions  mechanical  static  the the tip  occur  short  pipeline of  of  the the 100  as  distances  speed of  indicate  fractures  unstable  fracture escapes fracture. occurr-  m or  more i n length i n the U.S.A. and Canada.  Therefore, to prevent such a f a i l u r e i t was necessary to determine the toughness l e v e l s u f f i c i e n t to arrest a propagating d u c t i l e fracture.  The B a t t e l l e research group suggested that the Charpy v-notch energy was adequate f o r specify(2, 11, 1 ing a material's  resistance to d u c t i l e f a i l u r e  From f u l l scale crack propagation studies, a r e l a t i o n ship was established to show the minimum Charpy energy required to provide fracture arrest i n large diameter pipelines. C  v  m  0.0873 o  2 H  (Rt)  1 / 3  A  c  ( f t - l b , J) (3.3)  Where  o  H  =  Operating stress l e v e l = 0.8 o  v s  specimen minimum y i e l d stress SMYS (psi, MPa) R  =  Pipe Radius  t  =  Pipe wall thickness (inch, m)  =  Area of Charpy specimen ligament  A  c  (inch2,  (inch, m)  m) 2  An energy l e v e l of 80 f t - l b (108 J) i s often used i n pipeline s t e e l s p e c i f i c a t i o n s as an a l l heat  average  toughness  Unfortunately,  value  the  42  grade)  inch,  107  i t  is  results  cm)  and h e a v i e r  steels,  prevent  ductile  fracture.  above r e l a t i o n s h i p has n o t  correlated well with (over  to  from  higher  larger  strength (11 13)  wall pipe  impossible to  always  diameter  (over .  accurately  For  65  AF  specify  the (13)  toughness  requirements  3.3  Review  To  of  meet  for  the  the  their this  Charpy  Design  carry  v-notch  stress  done  because  and t h i s  direction.  But  the  out  peak  the  .  toughness minimum, that  Hence  for  properties  that  may b e o b t a i n e d  p a r a l l e l to  would be e x p e c t e d  the to  the  stress is up t h e  pipe lie  for  less  having  pipe the  along  axis;  Hoop  not  this  properly  may e x h i b i t e s p e c i a l l y tough-  than  minimum the  specified  directions other  axis.  the  crack along  welded pipe  are  the  BDWTT a n d  s t e e l s which are  spiral  Control:  specimens  desulphurised or rare earth treated anisotropy i n mechanical properties US) ness  arrest  Fracture  p a r a l l e l to  s t r e s s opens pipe  failure  requirements  tests with  crack orientations is  for  toughness  p i p e l i n e manufacturers standard  ductile  The  lowest  than  toughness  a direction parallel  to the r o l l i n g d i r e c t i o n i . e . (T - L) o r i e n t a t i o n . Hence the measurement of fracture toughness i s required i n various orientations to e s t a b l i s h the weakest crack i n i t i a t i o n condition; that combination of toughness and stress that results i n a minimum i n i t i a t i o n  energy  should be used to decide the necessary s p e c i f i c a t i o n condition.  Materials s p e c i f i c a t i o n i s based on maintaining BDWTT of 85% shear and a minimum C  v  of 50 f t - lbs  (67.8 J) at the lowest operating temperature.  The  shear s p e c i f i c a t i o n ensures d u c t i l e fracture i n i t i a t i o n and the 50 f t - lbs (67.8J)c toughness ensures d u c t i l e v  fracture propagation.  However, the standard Charpy  v-notch test does not provide crack i n i t i a t i o n information.  The standard Charpy v-notch sample also uses  a blunt notch which i n no way  represents a sharp crack  condition as could be r e a l i z e d i n service.  3.4  Instrumented  Impact Test Approach; (15)  Paul McConnell  , i n his M.A.Sc thesis,  made an extensive study of the impact strength and fracture toughness of two a c i c u l a r f e r r i t e , HSLA  steels.  His data w i l l be used for comparison with  the r e s u l t s of the present i n v e s t i g a t i o n .  In  evaluation of the fracture toughness of the m a t e r i a l , the IIT technique has proven to be rapid and inexpensive.  I t remains to assess i t s v a l i d i t y w i t h  respect to other fracture toughness test procedures. Fracture toughness data i s obtained from an IIT using a Charpy specimen which has been, fatigue precracked p r i o r 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 s i z e i s small, the measured propagation energy may not be a v a l i d measure of extensive propagation; the combined i n i t i a t i o n propagation energy i s equal to the C fatigue precracked specimen. determines  v  and  energy f o r a  Since t h i s method  the dynamic properties of the m a t e r i a l , the  fracture toughness data w i l l be more conservative i n the case of s t r a i n rate s e n s i t i v e materials.  McConnell also points out that the present p i p e l i n e toughness s p e c i f i c a t i o n s which require a minimum toughness i n the longitudinal axis may inadequate f o r fracture control i f very low  be initiation  energies or toughness values less than the s p e c i f i e d minimum, are present f o r a crack p a r a l l e l to the rolling direction.  For t h i s reason he suggests that  pipeline toughness s p e c i f i c a t i o n s are necessary i n a l l directions i n the pipe e s p e c i a l l y i n 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 t h i s simulates a sharp crack and therefore a peak stress intensity at the t i p of the crack.  Although the fracture toughness calculations i n 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 c l e a r l y 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; t h i s may or may not be a v a l i d 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 t i p may be d i f f e r e n t from that experienced at the t i p of a defect i n the thicker walled pipe.  This i s true i n that no v a l i d plane  s t r a i n fracture toughness data can be produced at the  minimum operating temperature of the pipe; that i s at o - 18 C, due  3.5  to the pipe wall thickness l i m i t a t i o n s .  Project Summary:  A summary of the project proposal i s given below: 1.  The study i s to measure the  fracture  toughness values of two a c i c u l a r f e r r i t e , HSLA pipe steels under s t a t i c loading conditions;  No  fracture  toughness values for these steels have been reported i n the l i t e r a t u r e .  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 v a l i d plane s t r a i n fracture toughness, K  lc  Operating temperatures.  data for the range of The  fracture toughness data  i s to be obtained for the fracture path  a) 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 i . e . the T-L o r i e n t a t i o n , transverse to the r o l l i n g d i r e c t i o n i . e . the orientation, and 2.  b)  L-T  c) p a r a l l e l to the pipe axis.  Both the l i n e a r e l a s t i c fracture  mechanics (LEFM) K  Ic  and the e l a s t i c - p l a s t i c  fracture mechanics (EPFM), J-Integral and approach w i l l be u t i l i s e d to determine the  COD fracture  toughness values throughout the complete temperature range extending from lower shelf through the transi t i o n to the upper shelf energies. 3.  The comparative study of the fracture  toughness t r a n s i t i o n behaviour of both steels i n each of the three t e s t directions by the three t e s t methods w i l l provide complete information on the anisotropy of the toughness and t r a n s i t i o n behaviour. 4.  The fracture toughness data obtained  w i l l be compared with reported IIT data f o r these steels to compare the s t a t i c and the dynamic fracture toughness.  I t i s hoped that the analysis of the  Kj  J l c and COD experimental data may contribute to a more fundamental basis for fracture control of the pipeline s t e e l s .  Cf  4.  4.1  THEORY AND TEST PROCEDURES  Linear E l a s t i c Fracture Mechanics  The fundamental p r i n c i p l e of fracture mechanics i s that the stress f i e l d at the t i p of a crack i n a s t r u c t u r a l 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 ' '«  In general, the stress i n t e n s i t y factor i s (16) given by a  K Where  =  f (g)  f (g) *  V*  a  t - ) 4  1  a parameter that depends upon the specimen and the crack geometry.  -  , /ll v  For example, f (g)  for an i n 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 t e n s i l e stress o  .  In this case the  stress intensity factor reduces to For mode I deformation,  the crack surfaces are  displaced perpendicular to each other i n opposite direction.  The corresponding  factor i s represented by K  stress intensity  In a thin sheet of metal under t e n s i l e loading, the stress at the crack t i p i n the thickness (18, 19) d i r e c t i o n (033  = 0) tends to zero  . A  schematic d i s t r i b u t i o n of the p r i n c i p l e stresses at the crack t i p i s shown i n F i g . 4.1.  A b i a x i a l state  of stress exists which i s commonly referred to as the Plane Stress  Condition.  As the thickness o f the sheet i s increased the crack t i p i s subjected to a t r i a x i a l state o f (18, 19) stress  which severely r e s t r i c t s s t r a i n i n g  or p l a s t i c deformation through the thickness. a state of stress i s known as Plane S t r a i n .  F i g u r e 4.1.  D i s t r i b u t i o n of p r i n c i p a l at the c r a c k - t i p .  stresses  Such  When the s t r e s s - i n t e n s i t y factor at the crack t i p reaches a c r i t i c a l value, K_  unstable  crack propagation, that i s , fracture, occurs. c r i t i c a l stress i n t e n s i t y factor for s t a t i c under plane stress conditions whereas K  l c  The  loading  i s designated as K  c  i s the c r i t i c a l stress i n t e n s i t y factor  for s t a t i c loading conditions ation and a plane s t r a i n .  under mode I deform-  K j represents the c r i t i c a l I(  stress i n t e n s i t y factor for dynamic (impact) loading and plane s t r a i n .  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  s t r u c t u r a l component at service conditions  (temperature  and s t r a i n r a t e ) , a design engineer can estimate the flaw size that can be tolerated under a p a r t i c u l a r stress l e v e l (as equation  4.1.1  4.1).  Plane Strain Fracture  Toughness:  The l i n e a r e l a s t i c fracture mechanics (LEFM) analysis can be used for determining K j  c  only for the  cases where the crack-tip p l a s t i c zone i s small i n  r e l a t i o n to the specimen dimensions.  For steels t h i s  occurs under the following conditions. i)  Relatively b r i t t l e material  ii)  Testing at a low temperature, normally  below the service temperature iii)  High rates of loading  iv)  Thick s t r u c t u r a l component.  To determine a v a l i d K ^ the specimen should Ic  f a i l under completely e l a s t i c p l a n e s t r a i n conditions.  With thinner sections, the c r i t i c a l combinations of load and crack length at i n s t a b i l i t y gives K  c>  This K  c  value decreases with an increase  i n thickness; a constant minimum value, K  I C f  is  reached when plane s t r a i n conditions are attained. Therefore, K  l c  values are reproducible and are the  minimum stress i n t e n s i t y 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 y i e l d strength.  value  The K  I c  refers to q u a s i - s t a t i c t e s t conditions, that i s i t i s determined at s t r a i n rates of approximately -5 10 / sec ; t h i s corresponds to a stress i n t e n s i t y  rate, K, of approximately 10 k s i / i n per second where < v  K  *  ^ic  Time to fracture For s t r a i n rate sensitive materials increasing the loading rate to that corresponding to an impact t e s t , that i s approximately 10/sec (K at a constant temperature  = 10  ksi  ^/in/sec)  causes a decrease i n the  plane s t r a i n fracture toughness to a minimum value known as the Dynamic Fracture Toughness 'Kid'.  For  o example, HY-80 s t e e l at a t e s t temperature of - 184 C ( - 300 F) exhibits a I c « 67 k s i v^in at e - 5 x ~ /— (16) . 10 / sec and *Id = 43 k s i ^/in at e = 20 / sec. K  5  v  4.1.2  Specimen Size Requirements:  Some b r i t t l e materials exhibit p l a s t i c deformation at the crack t i p before unstable crack propagation takes place.  This i s shown by the non-  l i n e a r i t y of the load-displacement t e s t records. The question a r i s e s , what i s the size of the p l a s t i c zone that can be allowed while s t i l l s a t i s f y i n g the elastic plain strain  requirement.  The s i z e of the p l a s t i c zone ahead of a crack  30  can be estimated  from the equation for the e l a s t i c  s t r e s s - f i e l d d i s t r i b u t i o n at the crack t i p (Fig. 4.1) at ( r,8) position i n y - d i r e c t i o n .  a  =  y  K — —  COS  I  ( l + Sin - | Sin  <-> 4  For 9 =s o, along the x-axis o  y  Considering o  (4.4)  =  = Oy  y  = y i e l d strength of the material  S  at the t e s t temperature and s t r a i n rate employed, the extent of y i e l d i n g at the crack t i p i s  At i n s t a b i l i t y Kj = K  Cf  and therefore the p l a s t i c  zone s i z e under plane-stress conditions i s .  r r  y  _L  (4.6)  2  2lT  \o ) y s  Under plane s t r a i n conditions the p l a s t i c zone radius at the center of a plate where the maximum constraintis r e a l i z e d , i s equal to 1/3 of t h i s value (20)  , that i s 1  (4.7)  3  31  The major dimensions of the plate specimens f o r K  I c  testing are:  and  a  •  crack length  B  =  thickness  w-a  =  uncracked ligament ( w « o v e r a l l depth) (21)  A f t e r considerable experimental work  , the following  minimum specimen s i z e requirements have been established to ensure e l a s t i c plane-strain behaviour: ( *c\ K  a , B, w - a  >  2.5 \ ays J  Thus, i t i s observed that specimens s a t i s f y i n g the above requirements w i l l have a thickness « 50 times the radius of the p l a s t i c zone s i z e . 4.2  E l a s t i c - P l a s t i c Fracture Mechanics  In the previous section, i t has been shown that LEFM i s applicable only to those situations where crack propagation i s accompanied by l i t t l e or no p l a s t i c deformation.  Quantitatively t h i s means that  the extent o f crack t i p p l a s t i c i t y 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 s t r u c t u r a l steels that are used f o r large complex structures such as bridges, ships, pressure  vessels e t c . are of i n s u f f i c i e n t thickness to maintain the plane s t r a i n conditions at the temperature and s t r a i n rate of the service conditions.  Hence i n such  applications i n s u f f i c i e n t constraint i s available to maintain plane s t r a i n conditions and a large p l a s t i c zone forms.  For pipe s t e e l s , neither the specimen nor the structure (the pipe) i s amenable to LEFM analysis. This i s shown c l e a r l y i n F i g . 4.2 i n which are shown t y p i c a l schematic load-deflection curves f o r small specimens of various materials.  F i g . 4.2 (a) depicts  f u l l y l i n e a r behaviour which i s e a s i l y handled by LEFM; F i g . 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  o f f s e t procedure as described i n the ASTM standard E-399-74;Fig. 42 (c) shows considerable non-linear behaviour i n the load-deflection curve p r i o r to sudden f a i l u r e ; while F i g . 4.2 (d) shows the behaviour of a d u c t i l e material where sudden f a i l u r e never occurs.  These n o n - l i n e a r i t i e s can arise from two  sources, p l a s t i c deformation at the crack t i p and  stable crack extension  (22)  .  Therefore, the test  behaviour described i n F i g . 4.2 (c) and (d) are the subject matters of E l a s t i c - P l a s t i c 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)  F i g u r e 4.2  Schematic Load (a) (b) (c) (d)  (P) v s . Load-point  displacement(A)  perfectly elastic material. e l a s t i c m a t e r i a l w i t h pop-in b e h a v i o u r . e l a s t i c then p l a s t i c behaviour. d u c t i l e m a t e r i a l 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 .  curves f o r  The two most promising and widely accepted techniques for analysing e l a s t i c - p l a s t i c 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  i s a way of characterising the s t r e s s - s t r a i n  , field  ahead of a crack t i p by an integration path such that J l at a distant f i e l d = 2 at the t i p of the crack J  where  J = fwdy - T i r = a  ds  (4.8)  contour t r a v e l l i n g counterclockwise around the crack t i p  Ti  =  the tension vector perpendicular to r  i n an outward d i r e c t i o n  Ui  »  displacement i n x - d i r e c t i o n  ds  =  an element of r  w  =  /  o^j e^j  (Strain Energy  density for e l a s t i c materials) Therefore, even i f considerable y i e l d i n g occurs near the crack t i p , the region away from the crack t i p can  be analysed and the condition i n the crack t i p region can be derived. (24) Later, Hutchinson and Rice and Rosen(25) gren described a s t r e s s - s t r a i n d i s t r i b u t i o n around a crack t i p surrounded by a p l a s t i c s t r a i n field.  They developed a model known as the HRR crack  t i p model which establishes that J i s the amplitude of the near f i e l d s i n g u l a r i t y at the crack t i p . (26) McClintock  has also shown that by combining J  with the HRR crack t i p model, the near t i p values o f stress and s t r a i n can be expressed as a function of J.  This i s d i r e c t l y analogus to the stress  field  equation of LEFM. (27) Rice  has also shown that the J-Integral  may be interpreted as the difference i n potential energy between two i d e n t i c a l l y loaded bodies with d i f f e r i n g crack lengths. a l l y as J  =  -  This i s stated mathematic-  £° da  (4.9)  where U = the potential energy where a = crack length  For the l i n e a r e l a s t i c behaviour and also f o r small scale y i e l d i n g , J i s therefore equal to G, the s t r a i n 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 e l a s t i c p l a s t i c problem, J loses i t s physical significance as (28)  a crack driving force.  Begley and Landes  that J i s s t i l l equal to -  a n d  suggest  physical  t n e  significance of J for e l a s t i c - p l a s t i c materials i s that J i s a measure of the c h a r a c t e r i s t i c crack t i p e l a s t i c - p l a s t i c f i e l d s i m i l a r to K i n LEFM. the additional suggestion that the J  l  c  They made  fracture  c r i t e r i o n applies to crack i n i t i a t i o n rather than propagation and i s l i m i t e d to the case of plane s t r a i n which i s denoted by the subscript I i n J i  c  .  (29)  Later Rice et a l  developed a simple,  single specimen technique f o r measuring I using J  the expression j  -  T  B  J l  where  2A  A  »  (4.10) (w-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. J j i s then J  l  c  The c r i t i c a l value of  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 corresponding K  l c  l  c  value i s determined, the  values can be computed from the  relationship between the e l a s t i c - p l a s t i c and the (28, 30, 31) LEFM parameters J  I C  « G  I C  -  1  -  v  2  K 2 Ic  (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 i n a s t a t i c J-Integral t e s t .  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 t i n t i n g method i s simple and requires less sensitive e l e c t r o n i c equipment i n comparison with the other t e s t methods.  This i s the reason why  t h i s method  has been selected f o r the present investigation.  This method i s also known as the Resistance curve technique. I t has been developed by Landes and (30) Begley . B r i e f l y the testing procedure involves: a)  Loading each specimen to a d i f f e r e n t  displacement value b)  Unload each specimen, mark the crack  extension by heat t i n t i n g the crack.  Heat t i n t i n g o  of steels i s done by heating the specimen at for 10-20  320 C  minutes. c)  P u l l the specimen apart and measure  the crack extension d)  Construct a resistance curve by p l o t t i n g  J f o r each specimen vs i t s corresponding crack extension. In order to find out the J j c value from the resistance curve Landes and Begley suggested the use  of a best f i t l i n e to the J vs crack s i z e curve.  The  point of i n t e r s e c t i o n of t h i s curve with the l i n e J  =  2  O f i  o  w  Aa  gives the value of  where °  f l o w  J  l  c  o y i e l d stress + oUTS .... 2  at the t e s t temperature and loading conditions. i s the most widely used method.  (4.12) This  The only disadvantage  of t h i s method i s that i t requires several specimens usually 4 to 6 to draw the resistance curve. ASTM i s preparing a standard  4.2.1.2  Currently,  f o r J-Integral t e s t i n g .  Validity Criteria:  Landes and Begley  (30,  34)  have proposed  a s i z e requirement which must be s a t i s f i e d by an e l a s t i c - p l a s t i c fracture toughness t e s t specimen to obtain v a l i d J j  c  data.  This s i z e requirement  i s stated a n a l y t i c a l l y as a, B, w-a  > 25  J q  (4.13) '  °flow I f t h i s condition i s s a t i s f i e d , then J Q becomes J j c .  4.2.2  The Crack-opening Displacement Method:  For low to medium strength s t e e l s extensive  p l a s t i c flow takes place before the i n i t i a t i o n of the fracture.  Under an externally applied load, the two  faces of the crack t i p move apart without an increase (36) i n the length of the crack  .  The r e l a t i v e move-  ment of the two faces at the crack t i p has been termed the Crack-opening  Displacement (37)  (COD) and has been  designated as '6' The consequence of y i e l d i n g at the crack t i p giving r i s e to physical displacement of the crack surfaces was f i r s t applied as a possible fracture (38) c r i t e r i o n by Wells 4.2.2.1  COD as an Extension to LEFM;  For a material that exhibits appreciable crack t i p p l a s t i c i t y , i t i s possible to develop a relationship between the stress i n t e n s i t y factor K and 6 near the t i p for the crack t i p opening placement.  dis-  The s i z e of the plane stress p l a s t i c zone  may be approximated by the r e l a t i o n  *y ' n Where r  v  <- > 4  l4  = the extent of the p l a s t i c zone along the crack plane  OyS  =  y i e l d strength of the material i n a uniaxial t e n s i l e t e s t .  With t h i s corrected crack border model, the y d i r e c t i o n displacement,  ' n', within the crack at any distance ' r '  from i t s t i p may be evaluated using Westergaard's (38) expression n  =  2K E  Now  /|r V  (4.15)  E  the displacement at the e l a s t i c - p l a s t i c interface  corresponds to the displacement at the t i p of the crack. Therefore, the crack opening displacement near the crack tip is «  . ^ - / ^  - *n -  <«•">  Combining the equation for ry (4.14), the p l a s t i c zone size and the relationship * -  4 G  K2  £  = G, (4.16) gives r i s e to (4.17)  Y  This relationship was developed by Wells  (38)  .  He  i n f e r r e d that under l o c a l p l a s t i c 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 i s  large for a s p e c i f i e d value of y i e l d stress such that oy <5 S  exceeds the c r i t i c a l crack extension force G, then  fracture follows  (39)  .  Hence the COD measurement i n  the presence of extensive p l a s t i c deformation ahead of the crack t i p f o r e l a s t i c - p l a s t i c and f u l l y p l a s t i c behaviour, i s an index of the fracture toughness and i s a d i r e c t extension of LEPM into y i e l d i n g materials.  4.2.2.2  Dugdale's Model; (40)  Dugdale model as shown i n F i g . 4.3.  proposed a s t r i p y i e l d This model describes the  extent of y i e l d i n g ahead of a crack as a function of the external load.  A thin sheet containing a s t r a i g h t  cut of length 2a i s loaded i n a d i r e c t i o n perpendicular to the cut.  I t i s 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 l y i n g along the l i n e of the cut.  This  model also suggests that the stresses i n the yielded zone may  be considered to be a continuous  distribution  of point loads °ys.dt per unit thickness which act to r e s t r a i n the crack from opening. An expression for the restraining stress intensity factor may be obtained by integrating the appropriate Westergaard Stress function for point  F i g u r e 4.3.  Dugdale's S t r i p Y i e l d  Model.  loads i n cracks from a to a i which gives  2  'y&f  C o s  _ 1  (si)  < - ' 4  18  Where the stress intensity factor f o r the opening of the crack under the applied stress o and the t o t a l crack length a j i s K = o .yiiai  (4.19)  Therefore, the extent of y i e l d i n g may be given by — l  = Cos  a  2  J £ — °ys  (4.20)  Dugdale's analysis also suggests that the displacement at the o r i g i n a l 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 i n the following r e l a t i o n ship between COD, stress  =  ( 3 7  '  the crack length a, and the applied  4 1 )  Using a standard method of series expansion due to (16) McLaurin t h i s expression gives:  45  For ° /  «  0 v s  1# taking only  «  ] the f i r s t  •  (4.22) term of the series  « . » ) ys  Since Kj « a^/na", the above expression can be written as Kj ° '  2  * 6 E  - i ys  r  =  e  o  (4.24)  y s  (^M* °ys '  (4.25)  v  as  E-  !2!  At the onset of crack i n s t a b i l i t y where Kj reaches K  Ic,  t  *  ie  C  0  reaches a c r i t i c a l value,  D  c Cy 6  S  _ / *ic (^^"ys o^-)  6  Cf  2 ( 4  This expression shows that c eys 6  -  2 6 )  i s a measure of the  c r i t i c a l crack size i n a structure exhibiting e l a s t i c p l a s t i c behaviour.  Therefore, the crack opening  placement, <*c, i s a material property l i k e K  lc  dis-  and i s  a function of the t e s t 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 s t r a i n , the e l a s t i c p l a s t i c and the f u l l y p l a s t i c regions. ii)  A much smaller size t e s t specimen i s  required. The K  j c  values can be measured only under  plane s t r a i n conditions and often require the use of a p r o h i b i t i v e l y large s i z e specimen.  4.2.2.3  Experimental  Determination  of COD;  (36, 37* 39, 42 - 44) Several authors have described d i f f e r e n t techniques a l determination of COD.  for the experiment-  However, the B r i t i s h Standards (45)  I n s t i t u t i o n Draft f o r Development 19:1972  i s the  only document which gives the d e t a i l s of a recommended procedure for COD t e s t i n g .  The DD19 t e s t method i s  very s i m i l a r to the ASTM E399-74 t e s t method for K Similar specimen preparation, fatigue procedures,  precracking  and instrumentation and t e s t procedures  are followed.  The displacement  the one used i n K displacement  l c  l c  gauge i s s i m i l a r to  t e s t i n g and a continuous  load-  record i s obtained during the t e s t . (39) Egan's evaluation o f the fracture  toughness of materials using the COD technique shows that a single specimen t e s t 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 s p e c i f i e d amount of crack growth has occurred.  In the B r i t i s h Standard Test Procedure,  the crack opening displacement i s usually calculated by assuming that p l a s t i c extension has occurred at the crack t i p up to the point of maximum load.  This  assumes that crack extension i n i t i a t e s a t the maximum load.  The c r i t i c a l displacement at the t i p 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 f o r  obtaining « . A l l o f these methods assume that c  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 <5 i s : C  Where Z = Knife edge thickness i . e . the distance above the t e s t piece surface at which point the measurement i s made a j= crack length w = t e s t piece width r = r o t a t i o n a l factor (46) T. Ingham et. a l suggested that on the basis o f 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 e t . a l ) confirm t h i s viewpoint.  4.2.2.3.1  Determination  of 6 . C  The B r i t i s h Standard method of using the maximum load point for calculating the (48) c r i t i c a l C0D-6 ^ has been c r i t i c i s e d  . This may  C  be due to the f a c t that 6  C  i s defined as the value  of COD which should correspond crack growth - s i m i l a r to J approach.  l  c  to the onset of stable i n the J-Integral  I t should be noted that i n K  l c  testing,  the crack i n i t i a t i o n load PQ i s taken to be the load corresponding  to 2% beyond the y i e l d point.  Several detecting  the  methods value  6_  have  crack growth.  involves on  the  The  loading  marked by plotted  of  several  is  obtained  to  zero.  curve  tinting.  against  displacement  specimens  crack growth  heat  This  to  various  followed  by  stages  unloading.  i n each specimen i s  The  crack growth  crack opening  associated with by  for  the onset of (49) and K n o t t ' s technique  Smith  load-deflection  extent  suggested  associated with  C  stable  been  then  value  displacement.  The  the  onset  extrapolating  the  crack extension  method  same a s  is  the  of  is  cracking  that  used  £c  back  in  (48) determining the  J  basis of  Diesberg  l c  a maximum  this  COD  as  the  with  the  help  of  «c »  COS ' •  the  J_  and e x p e r i m e n t a l COS  value.  obtained  by  His  stretch  crack-opening  8 °vs  zone.  C  assuming  °  2 O£  values COD  He  stretch  ,  a  jfig—log  »  calculated « has  value,  on  c  defined COS.  relations  a ? 7 « r and  has  of  values that  6  Sec  / no \ )...(4.28) ys  (  " B ^  S  Aac  l o w  J  . . . .  he has  I c  were the  ( 4  crack  2 9 (  ..(4.30)  determined  much l o w e r  -  than  i n i t i a t e d at  the  those the  point of maximum load.  4.2.2.3.2  Evaluation of an Equivalent K  l c  from COD:  From l i n e a r e l a s t i c fracture mechanics K 2 (1 - v ) _i£_ E 2  6 =  lc  V i  1  (4.31  and from Well's treatment f o r the c r i t i c a l value of COD  «c  =  f  < « > 4  17  ys Equation 4.17 can be written as G = X 6  C  0 y  (4.32)  S  Where X - a constant.Several t h e o r e t i c a l analyses predict d i f f e r e n t values of X e.g. 1, n/4, 1.27, 1.48, 2 and 2.4. This v a r i a t i o n i n X values may be due to the differences i n the d e f i n i t i o n of the COD values. (37) However, calculations by Burdekin and Stone , (41) CIST" Bilby e t . a l , Rice and Rosengren a l l yielded results i d e n t i c a l with experiment  f o r 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)  . fi  c  0  y  This i n turn gives r i s e to an equivalent K c r i t i c a l COD values  (4.33)  s  Ic  from the  52  5.  5  *  Test  1  EXPERIMENTAL  Materials:  Sections of production  heats were  Manufacturers  a 0.54-inch  rated of  steels  program.  42 i n c h  (13.7  as an x-70 grade  70 k s i ) .  welded pipe  (107  mm) w a l l steel  in  MO  t h i c k n e s s and were  Nb  Si  Al  0-0 3  —  0-0 5 1-9 3 0-2 6 •063  AF-2  0-0 6 1-8 2 <H»5 0-0 5 0-2 6  and rare  these  5.1  •0M5  S  P  CU  earth  Ni  •023 •0 12 0-2 <• 010  Cr  Sn  0-0 H 0-0 2  Ti  Ce  —  —  •006 •0 0 6 •0 3 7 •0 2 7 •0 6 8 •0 0 5 •0 0 2 •0 3>»  AF-2 s t e e l composition i n d i c a t e s that  killed  strength  Compositions  AF-1  The  diameter,  table.  Steel  Mn  cm) o u t s i d e  (minimum y i e l d  Table No.  C  Steel  Both of the  The c h e m i c a l c o m p o s i t i o n s o f  are given  from  s u p p l i e d by two Canadian  for the test  p i p e products were with  spiral  treated.  The A F - 1  i t i s steel  fully contains  53  considerably  5.2  more  S and i s  Specimen  Samples  of  the  cut  smaller sections of  using any  pipe.  AF-1  of  acetylene  cutting.  an a u t o m a t i c  c l o s e r than  The  through-thickness three  The pipe for  different  In hack  s t e e l were  Samples  both  of  the  cut  the AF-2 pipe  from  steel  obtained  were  by  cases specimens were  saw;  50 mm f r o m  test  steel.  Preparation;  sections from  the  a semi k i l l e d  no s p e c i m e n s were  a flame  samples were  cut  cut  machined notch  cut  cut  edge.  so t h a t  the  c o u l d have one  of  orientations.  1)  P a r a l l e l to  the  pipe  2)  P a r a l l e l to  the  rolling direction.  3)  Transverse  to  the  r o l l i n g d i r e c t i o n was  at  an angle o  axis the  for  AF-2  5.3  the  AF-1  pipe  rolling  a n d 45  of  to  direction. o 63 to the  the  pipe  pipe,,  Specimen C o n f i g u r a t i o n  5.3.1  axis.  Compact  Tension  and  Dimensions:  Specimen:  axis  Compact of  the  fracture  Integral wall  test  toughness  as w e l l  t h i c k n e s s was  fabricated  used i n the  as  0.54  specimens were  used for  tests,  test,  the  COD  inch  the  this  size  of  study.  the  The  standard  KIC  test.  Since  mm), 0.50  compact  for  plane  three  test  the the  Jpipe  maximum  inch.  Fig.  tension  described i n  strain  a l l  the  5.1  specimen  specimen dimensions  specimen requirements  E399-74  the  (13.7  s p e c i m e n t h i c k n e s s was  illustrates  to  tension  conform  the  fracture  ASTM  toughness  testing.  The for  the  following i)  is  Crack  considered to  because  full  do p r o p a g a t e  reasons  be  maximum o p e r a t i n g stress of  the  which  the  most  tends  to  This  orientation  indicate that (50, 51)  axis  failures  .  in pipelines  open  axis;  important  tests  pipe  stress  Crack  Although toughness the  the  pipe  examined  is  The  the  hoop  cracks p a r a l l e l to  the  axis  pipe. ii)  in  p a r a l l e l to  scale burst along  d i r e c t i o n s were (15)  pipe  axis  p a r a l l e l to  Direction?  specifications require  orientation,  determine the e f f e c t  Rolling  of  the  i t  is  testing  important  distribution  and  only  to  spacing  55  FIG. 5.1.  0-5 inch Thick Compact Tension Specimen.  of non-metallic inclusions etc.  For t h i s reason  samples were cut with the crack running 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 ; these were designated as T-L orientation and i t i s expected to be the weakest d i r e c t i o n of a material a f t e r r o l l i n g . iii)  Crack transverse to r o l l i n g d i r e c t i o n :  This orientation i s designated as the L-T o r i e n t a t i o n . The pipe i s known to possess a maximum upper shelf (52) energy along this orientation was  . This d i r e c t i o n  included to determine the maximum toughness a t t a i n -  able i n the s t r u c t u r a l component. 5.3.2  Tensile Specimens:  In order to assess the fracture toughness v a l i d i t y c r i t e r i a and the equivalent  Kjg data from  COD measurements at the t e s t temperature and s t r a i n rate conditions  for each specimen o r i e n t a t i o n , i t  i s necessary to know the y i e l d strength of the The orientation of the t e n s i l e specimen with  material.  respect  to the axis of the compact tension specimen and the specimen dimensions are presented i n F i g . 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 R o l l i n g Direction of the plate. (b) Dimensions of the t e n s i l e specimen.  orientation and t e s t conditions are also required to calculate the J  l  c  values.  Therefore, appropriate  t e n s i l e tests were conducted along with fracture toughness t e s t s .  Substandard  sized t e n s i l e 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 t e s t i n t h i s environment.  5.4  Fatigue Precracking;  The compact tension specimens were fatigue precracked before t e s t i n g .  This was necessary f o r  the following reasons: 1)  The v a l i d i t y of the K ^ Ic  «  c  and J  l  c  values are dependent upon the establishment of a sharp crack at the t i p of the machined notch. 2)  The fatigue crack simulates a sharp  internal crack which might e x i s t inside the material as a r e s u l t of processing and would remain undetected by standard NDT  techniques„  The degreased. to  notched  The  a 600 g r i t  lines  of  operation  was  surface  of  transverse the  readily  the  to  fatigue  with  a manual  cyclic  the  preload. is  notch.  and  crack during  the  maximum  pre-cracking  surface.  was p e r f o r m e d  Machine, This  polished  polishing The  v i s i b l e on e i t h e r  Testing  load which  cleaned  600 g r i t  Fatigue pre-cracking Sonntag Fatigue  were  e a c h s p e c i m e n was  emery p a p e r ;  running  extension  specimens  Model  SF-l-U,  equipment  symmetrical  with  using  a  operated  introduces relation  a  to  the  notch.  5.4.1  ASTM S t a n d a r d  The  ASTM s t a n d a r d  strain  fracture  ulates  certain  toughness important  precracking which i) from  the  The  notch  at  ii)  The  intensity  of  Kf(  shall  m a x  )/E  are  the  of  ratio  fatigue  Precracking;  determining  the  plane  metallic materials  stip-  prerequisites  fatigue  not  for  listed  least  for  of  is  inch the  cycle  exceed  fatigue  below..  crack  0.05  for  to  be  (1.3  mm).  maximum to  0.002  the in  a  extended  stress  Young's (0.0032  Modulus m*i) .  iii)  K  f(max)  st  m u  not  exceed  60% o f  the  KQ  conducted  at  value. iv)  When  a temperature K  f(max)  m  u  s  fatigue  cracking i s  and t e s t i n g  t  n  o  exceed  t  at °  0 # 6  y s  °ys Where  o _  and o 1 at  y  material  previous from  K  than  k s i ^ n  f(max)  e m p l o y e d was p  f(max)  shown  (in  t  of  the  temperatures.  *Q,  for -  temperature  . t  *  was  may In  Considering | i e  A  on  s  t  r  e  s  s  sample  the  be  this  assumed t o »  0.50  cycle to  m a x  be and  be  calculation  corresponding Kf(  and  ) values  are  I.  t h i s work, to  data  (96.71 MPaVmT.  MPa^)  Kfdaax).  indicated that  ambient  calculated.  found  Intensity  precracking  o  i n Appendix  KQ w a s  Q  y i e l d strengths  tests  at  for  lbs)  In 18%  T2  2  f r a c t u r e toughness (15)  88 k s i V i n  (110  m  =  IIT  obtained  investigation, 100  K  Precracking Stress  steels  greater  2 respective  the  The  KQ v a l u e s  the  l —  of  y  5.4.2  these  are  v s  a temperature  a value  of  K  f(  be adequate  to  generate  m a x  )  *  15  to  fatigue  precracking the  i n the  specimens.  stress intensity  plastic  deformation  For c r a c k i n g was  the  were  deeply  0.6  < a w  minutes;  the  notched to  <  0.7  was  time  i n the  of  the  crack  tests, 0.45  value  of  minimize  the  tip.  fatigue  < a w  Integral a attain w  <  pre-  0.55  tests,  the  > 0.6.  specimens  Normally,  obtained,  required for  case of  double  lover  used to  J-  specimens v a r i e d  approximately details  ahead of  done s u c h t h a t  The a n d COD  f a c t o r was  K g a n d COD  For  This  from approximately the  the  precracking the  J-Integral  to  The  indicated in  Appendix-  II.  5.5  K i  K  at:  c  Ic  Test  Procedure:  fracture  toughness  20  specimens,  t i m e was n e e d e d .  the precracking are  15  KQ  t e s t i n g was  aimed  a) displacement b)  Obtaining a load (P) vs. load-point ( )curve. A  Establishing the notch-toughness  behaviour o f the materials i n the t r a n s i t i o n temperature 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 t e s t technique was used to obtain the K  I C  and the COD data.  value of the load (P) from the P-A u t i l i s e d to calculate the KQ or K c r i t i c a l value of displacement  I C  The c r i t i c a l record was values and the  (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 l i q u i d nitrogen temperature (-196 C), the following f i x t u r e was designed and constructed.  Two  concentric, closely f i t t i n g brass tubes, were attached to the loading pins i n such a way that when the pins  moved a p a r t , outer  one.  the For  inner  of  prior of  a minor  tubes  due  to  holding  load of  respect  these  The  10  set-up  of  the  averted -  d e s i g n and  experimental  1  tubes.  was  approximately  the  a 50% -  misalignment fixture  to  20  by  lbs  dimensions are  shown  in  5.3. Test  Static  conducted the  at  Almost  a full  a chart  settings  displacement  0.0005  gives  10  of  were  a l l of  scale  the  0.05  1  tests  setting  inch per  chart  a measurement  is  of  0.005  (22.25KN), at  the and  a  Using  equivalent  Therefore,  the  were  minute  inch per minute.  inch.  using  5000 l b s  preamplifier  inches of  0.05  c a r r i e d out  load of  speed of  cross-head speed of  chart  tests  s t r a i n gauge  range,  these  Details:  Kj£  INSTRON m a c h i n e .  2x  the  and the  5.5.2  with  clamped to  to experimentation.  the  Fig.  was  loading  loading pins with applying  moved w i t h  displacement measurements,  inch extensometer Eccentricity  tube  1-inch  i n c h and 1  to  a  of division,  inch.  behaviour  To  e s t a b l i s h the  of  the  materials,  notch tests  toughness were  transition  conducted  over  64  F i g u r e 5.3(a) Dimensions of Brass Tubes. (b) Photograph of the E x p e r i m e n t a l Set-up.  65  o o 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 20 t o 25 C (RT) o o o o o o t o - 40 C , - 60 C , - 80 C , - 1 0 0 C , - 110 C , - 1 3 0 C , o o - 150 C a n d - 196 C . I n i t i a l l y , i t was t h o u g h t t h a t o t e s t s t o - 100 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 down - 80 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 a  from the get  upper  toughness  ments were  shelf values  toughness  level.  Therefore,  to  i n the  t r a n s i t i o n range, experio o b e l o w - 100 C a n d t e s t s a t - 6 0 C  c a r r i e d out  were d i s c o n t i n u e d .  5.5.3  Low  Low different The  Temperature  temperature  baths  specimen h o l d i n g f i x t u r e ,  The the  bath  the  using  temperature  desired.  specimen,  loading pins constant  and  the  the  temperature  5.3.  s t r a i n gauge was k e p t  in a l l  conducted  keeping the  immersed i n s i d e the  b a t h as shown i n F i g .  of  t e s t s were  d e p e n d i n g upon t h e  t e s t s were c a r r i e d out  brass tubes  Tests:  above  the  level  experiments. o  Tests of  down t o  - 8 0 C were done u s i n g a  e i t h e r denatured alcohol or  a 60-40  bath  ethano1-methano1  mixture. T e s t s a t - 1 0 0 C , - 110 C a n d some t e s t s a t o - 130 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 The 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 an i s o p e n t a n e o (Dimethyl Butane, Freezing Point * - 160 C) b a t h .  All keeping the of  low temperature  experiments  specimen i n s i d e the  20 m i n u t e s .  The  temperature  bath of  for  the  measured by p l a c i n g a c h r o m e l - a l u m e l junction  adjacent  was c o n t r o l l e d t o  In  the  sample.  an a c c u r a c y of  a l l of  low temperature), until  to  the  the  the  surfaces.  and t o  For  the  into  +  °  (RT  tests  at  always  unstable  and the  Test  that  gauge  continued for of  Records:  was  measure-  the  C and below, the  as  until  fracture  specimens always broke  pieces. 5.5.4  130  as w e l l monitored  two h a l v e s  -  temperature  C.  The  allow examination o  s p e c i m e n s became s o b r i t t l e  two  bath  t h e maximum l o a d w a s a t t a i n e d .  ment p u r p o s e s  s p e c i m e n was  s t r a i n gauge was  s p e c i m e n was b r o k e n  time  thermocouple  — 1  t e s t was  done  a minimum  The  experiments  then d i s c o n n e c t e d and the  were  fracture the was into  Test  an a u t o g r a p h i c  output  of  of  d i s p l a c e m e n t gauge were  the  test  the  records of  records  load-sensing transducer  are  5.5.5  shown  and Crack  Before of  the  inch  measured a t  L  was  the  25,  a l l specimens,  a single plane. to  be  the  50 a n d  the  In  0.45  75%  B  the  output  Typical  and  (b).  Test Piece  Dimensions  ai  nearest  length  crack length  was  a i ,  a2, +  a  a  a/w  3,)«  remained ratio  in  was  0.55.  w  5.5.6  Analysis  The  RT  and -  of  o 40 C  the E x p e r i m e n t a l  P-A  record  (Fig.  L,  specimen.  3  crack t i p the  W,  0.001  the  + a2 ^  a l l cases <  the  B,  of  (  fatigue  < a  the  experiments  to  total  the  a =  the  of  Length;  fracture,  Therefore,  found  of  carrying out  mm).  After  In  5.4(a)  specimens were measured  (.025  vs.  obtained.  in Fig.  Measurements  plot  Data:  5.4(a))  F i g . 5.4(b) Actual P - A Test Record for AF-1 Steel at - 150°C.  were t y p i c a l of d u c t i l e behaviour indicating a high l e v e l of notch toughness, whereas the P-A t e s t records o at temperatures - 130 C and below were c h a r a c t e r i s t i c of b r i t t l e plane s t r a i n behaviour.  The t e s t records  resembled type - III as given i n the ASTM E 3 9 9 - 7 4 standard as shown i n F i g . 5.4 (b). In a l l cases, the 5% secant o f f s e t procedure was adopted to measure the PQ load value.  The Pmax  r a t i o was observed to be greater than 1.10 f o r o o RT, - 40  and - 60 C tests and less than 1.10 f o r  tests conducted at lower temperatures. Through the l i n e a r e l a s t i c portion o f the P-A curve, a best f i t t i n g straight l i n e was drawn cutting the A-axis.  A 5% o f f s e t l i n e was then drawn  through t h i s point on the A-axis.  The point where  the o f f s e t 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 i n p s i  (MPa/m.) was then  with the help of the r e l a t i o n . KQ  =  _!Q  f (a/w)  (5.1)  calculated  F i n a l l y the v a l i d i t y c r i t e r i o r for K  lc  testing  was  calculated using the r e l a t i o n B Where  >2.5  2  KQ  'ys 0.2%  'ys  (5.2)  o f f s e t y i e l d strength at the  respective  t e s t temperature and  s t r a i n rate.  5.6  COD  Test D e t a i l s :  Since COD  tests and K  the same, a single t e s t was types of data. two  Ic  tests are e s s e n t i a l l y  conducted to obtain both  At each t e s t temperature a minimum of  compact tension specimens were tested.  cases, where COD  results or K j  c  In certain  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 components.  elastic-plastic  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 t e s t s , VQ > V ; hence the measurement VQ was M  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 6 : C  Raving obtained the c r i t i c a l values of s t r a i n gauge displacements ( V and V Q ) , i t was necessary to M  convert these to the true c r i t i c a l COD(6  c  - «m or 6Q)  at the crack t i p .  V ing 6  C  C  values were converted to the correspond-  value using the generalised relationship  Where r  rotational factor  0.33  z = k n i f e edge t h i c k n e s s In the p r e s e n t e x p e r i m e n t a l  technique  adopted z - o.  Therefore,  6  =  C  V  ° ( v - a)' w + 2a  correspondingly,  6  5.6.3  —  -.-  =  m  V n >  <  and *Q  ' > w + 2a  w  a  V  Q  (w w  C a l c u l a t i o n of Equivalent  K  I C :  Once the c r i t i c a l v a l u e o f COD a t the crack  t i p i s o b t a i n e d , an e q u i v a l e n t value o f K j  c  was  c a l c u l a t e d u s i n g the r e l a t i o n s h i p  Gic  "  ^  d- ) v 2  "  °ys «ic  (44,54) which g i v e s  f  " 'S. I c 6  K  IC -  v  -—ir-^T  The v a l u e o f the e q u i v a l e n t K i c i s o b t a i n e d i n k s i ^ / i n and i s converted t o MPa^y^n u n i t m u l t i p l y ing  by 1.099  2a  73  5.7  J-Integral Test Details  5.7.1  Testing Parameters;  The heat t i n t i n g technique which i s the same as the Resistance curve t e s t technique developed by Landes and Begley was adopted i n this i n v e s t i g a t i o n .  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 d u c t i l e - b r i t t l e t r a n s i t i o n . o - 40  Therefore, f o r RT and  C tests 5 to 6, CT specimens were used and f o r  other temperatures a minimum of two specimens were used. Each specimen was loaded, with increasing d i s o placement, then unloaded and heat t i n t e d at 316 15 - 20 minutes to t i n t the cracked area.  C for  Then the  specimens were pulled to f a i l u r e i n the Instron. As the rate of oxidation at the fatigue precracking segment was d i f f e r e n t from that of the crack growth zone, the crack growth zone was e a s i l y detectable by v i s u a l inspection.  Typical for  AF-2  5.5  (a)  P -  steel at and  (b)  -  A records o  40 C a n d -  for J-Integral o 130  C are  tests  shown i n  Fig.  respectively.  5.7.2.1.  Measurement Dimensions  of  Specimen  and Crack  Growth  (Aa) t  Using employed  in  the K i  o r COD  c  measured to  the  mentation.  From the  tinting, positions  the  nearest  length  s p e c i m e n s h a d a a/w  The Aa, to  ahead of  the  an a c c u r a c y o f  crack  growth  value,  0.16  of  0.001  W were to  experi-  measured a t  three As  or  0.004  The  0.65  crack  p r e c r a c k was inch.  readings  previously  between  crack growth  measured was inch.  were  ' a ' .  fatigue  L,  those  specimens a f t e r  the  ratio  B,  inch prior  fractured  crack parameters  crack  s i m i l a r to  tests,  0.001  and an average  used as the all  techniques  to  heat three was  mentioned, 0.70.  extension,  also  measured  minimum v a l u e  i n c h and the  of  maximum  ©  ©  ©  @  ©  I hied (75  > A  F i g . 5.5(a) J - I n t e g r a l T e s t Record o f AF-2 S t e e l with Crack Transverse to R o l l i n g D i r e c t i o n . T e s t 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  Measurement the  For P  -  deformation  compensating Japan).  tracer bar  at  143.8,  An  of  3 to  to the  mean v a l u e give  the  of  area  individual  i n square  load »  X -  axis,  A  expended A under  for the  5.7.3  elastic the  of  and  ( 3 3 6 6 1 Made  the  vernier  a  in  on  the  measured.  e a c h a r e a was  inches.  One  energy  50 l b s / i n c h o f  = 0.005  inch /  each specimen,  taken.  the  A record in  in  C a l c u l a t i o n of  -  J  0.015  square  inch  of  in-lb  2.5  in  chart  inch of  amount o f  chart. energy  c r a c k e x t e n s i o n was measured by P -  the  help of  areas were  an e q u i v a l e n t  axis,  for  under  r e a d i n g s was m u l t i p l i e d by  -  Therefore  the  p o s i t i o n of  the  record denotes  since Y  both  4 readings  (A)  area under  was m e a s u r e d w i t h  the  area  Record;  type polar planimeter  Keeping  average  A  the  each specimen the  A record consisting of  plastic  The  P -  of  the  area  lb.  for  RT  and -  o 40 C  tests  o J-Integral in  different  Aa  for  values  tests of  each i n d i v i d u a l  area  at  RT  and -  (A) w i t h  specimen.  J  a for  40 C  resulted  corresponding each  specimen  was then calculated using the relationship  J  =  B (w - a) 2  i  n  "  l  b  /  i  n  (29 - 33, 35)  2  Therefore, f o r a single specimen A.a and a corresponding J value were obtained.  Since 5 to 6 specimens were  used at a p a r t i c u l a r temperature, Aa values were obtained.  5 to 6 sets of J vs  As each specimen d i f f e r e d  from each other by an increasing amount of Aa also increased i n magnitude.  J values  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 l i n e was drawn  through these points to generate the resistance curve.  5.7.4  Determination of J j ; Value: c  It was pointed out i n 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 l i n e J = 2 < J f i  on the resistance curve.  ow  A a was constructed  The ©flow value of each  orientation for each s t e e l at each testing temperature was calculated from the y i e l d strength and the UTS values.  The J j value was determined where c  78  the l i n e J = 2 o f  Aa cuts the resistance curve.  l o w  o For tests at - 80 C and below a "single specimen" t e s t technique could be used to obtain a J j value.  c  This was v e r i f i e d by a minimum of 2 specimens  i n each case. I t was also v e r i f i e d i n the case of the  AF-2  s t e e l with specimens having cracks running p a r a l l e l to o the r o l l i n g d i r e c t i o n at the - 80 C t e s t specimen No. 17 extended 0.001  temperature;  inch less than specimen  No. 16 and d i d not indicate any crack growth as revealed by the heat t i n t i n g technique. for specimen No. 16 i s the J 5.7.5  l  c  Therefore J calculated value.  Calculation of Equivalent  K . Ic  From the J j value corresponding K j c  c  values  were calculated from the relationship between e l a s t i c p l a s t i c and l i n e a r e l a s t i c fracture mechanics para(30, 31, 54) meters  J  which gives  4  K  l c " Ic G  =  —j?—  /E. J j .  c  Where  v = Poisson's r a t i o • 0.3 E • Young'8 Modulus - 30xl0 p s i 6  5.7.6  V e r i f i c a t i o n of V a l i d i t y C r i t e r i o n ;  For v a l i d J  L  r e s u l t s , the following size  C  requirement must be met by an e l a s t i c - p l a s t i c fracture (28, 30 - 32, 34, 35) toughness specimen a, B, w - a  *  25  Q of low J  I t was v e r i f i e d that for a l l t e s t r e s u l t s the c r i t e r i a were s a t i s f i e d . obtained were v a l i d J  5.8  L  C  Therefore, a l l J Q values values.  Tensile Test D e t a i l s :  Tensile tests were conducted f o r the purposes already mentioned i n section 5.3.2.  At each t e s t temperature, a minimum of two specimens were used.  Y i e i d strength and UTS values  were calculated and the average values were The  0 f i  o  w  stress was calculated as  reported.  6.  6.1  RESULTS AND  DISCUSSIONS  Tensile Properties;  The increase i n the y i e l d strength as well as the flow strength with decreasing temperature are presented i n Figures 6.1 and i n Figures 6.5 to 6.8  to 6.4  for the AF-1  for the AF-2  steel  steel.  The  increase i n the flow strength and y i e l d 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 y i e l d strength and the flow strength o increase markedly; at -150 C an increase of approxo imately 80% i s observed while at -196 C the y i e l d strength and the flow strength are  approximately  double the values measured at room temperature.  It  i s well established that the t e n s i l e properties of most metals are governed by the thermally activated motion of d i s l o c a t i o n s .  Where t h i s i s the predominant  mechanism, the y i e l d stress and flow stress decrease (55) with increasing temperature For the AF-1  s t e e l , the RT y i e l d strength  varied from 71,000 p s i (489 MPa)  along the axis  transverse to the pipe axis to 80,000 p s i (551 MPa) along the axis transverse to the r o l l i n g d i r e c t i o n . Surprisingly, the RT y i e l d strength was the same 80,000 p s i (551 MPa) on the r o l l i n g d i r e c t i o n and transverse to the r o l l i n g d i r e c t i o n . o  For the tests  conducted at -196 C, the y i e l d strength varied from a minimum o f 144,000 p s i (993 MPa) transverse to the pipe axis to a maximum of 157,000 p s i (1082 MPa) transverse to the r o l l i n g d i r e c t i o n .  The y i e l d  strengths along the pipe axis and the r o l l i n g d i r e c t i o n were comparable throughout the temperature range examined. In terms of the y i e l d strength, the AF-1 plate i s strongest along an axis transverse to the r o l l i n g d i r e c t i o n , weakest along an axis transverse to the pipe axis and exhibits an intermediate strength along the pipe axis and the r o l l i n g d i r e c t i o n .  The y i e l d strength of the AF-2 s t e e l at RT varied from 73,000 p s i (503 MPa) to 88,000 p s i (606 MPa),  being a minimum along the axis transverse to  the r o l l i n g d i r e c t i o n and a maximum along the axis transverse to the pipe axis.  In contrast, the y i e l d  strength to  at  160,000  direction the  -196 psi  (1103  MPa)  rolling direction.  exhibits and the  transverse  to  the  pipe  an i n t e r m e d i a t e  strength  In  contrast,  along  the  an a x i s  shows  pipe In  to  stronger  properties,  the  pipe  the  steels, than  the  those  result of  rare  earth  axis, axis  rolling  the  plate  is  rolling along  an  weakest  rolling direction.  i s observed  that  of  the  the  AF-2  steel.  anisotropic  of  e.g.  and more i s o t r o p i c b e h a v i o u r  is  pipe  tensile properties i t  to  lowest  the  and i s  the AF-1  tensile properties  C, to  the  properties  anisotropic than  its  strength  axis  steels exhibit  but  -150  as  the  to the o  transverse  as w e l l  comparing  and the AF-2 is  below  an i n t e r m e d i a t e  axis  Both  the  along  transverse  rolling  transverse  a l o n g an a x i s  transverse  better  axis o  plate  direction;  steel  the  the  a l o n g an a x i s  MPa)  a minimum a l o n g  C,  strongest  AF-1  being  (1034  Above -150  direction.  along  150,000 P S i  r o l l i n g d i r e c t i o n and e x h i b i t s  strength  axis  from  a n d a maximum a l o n g  strongest  more  C varied  of  of  the  strength  AF-1  the AF-2  steel  steel.  higher y i e l d the  additions  AF-2 and o f  are The  strength  s t e e l may the  lower  be  (5) sulphur content.  However, Lyckx e t . a l  suggested  that rare earth additions d i d not have any e f f e c t on the t e n s i l e properties of the s t e e l .  The AF-1  steel  possesses i n f e r i o r t e n s i l e properties, that i s , a lower y i e l d strength and greater anisotropy. may  This  be due to the higher sulphur content of t h i s  steel.  The higher sulphur content results i n the  presence of more second phase p a r t i c l e s i n the form of sulphides.  The y i e l d strength of a material  decreases as the volume of the second phase p a r t i c l e s increases.  This i s because the second phase p a r t i c l e s  enhance the process of void nucleation and growth. The decrease i n y i e l d strength and greater anisotropy of the AF-1  s t e e l can be attributed to the deleterious  e f f e c t of the sulphur content on the true s t r a i n to (61) fracture as reported by W.G.  Wilson and G.S.  Klems  84  A F - 1 STEEL SPECIMEN TRANSVERSE TO ROLLING 160 CO  CO CO UJ  »-  DIRECTION 1200  A - o r FLOW STRESS o - c r Y I E L D STRESS  140  1000  120 800 100  o a-  O^A, *A-  CO  A  80  600  •0  60 -200H60-I20  -80  -40  0  J420  40  TEMPERATURE FIG. 6.1 A F - 1 STEEL SPECIMEN TRANSVERSE TO PIPE A-crFLOW CO  AXIS 1200  STRESS  o-c Y I E L D STRESS  CO CO UJ  1000  800  tr hco  600  b - 2 0 0 - 1 6 0 -120 - 8 0 - 4 0  0  40  420  TEMPERATURE °c FIG. 6.2  S.  AF-1  STEEL  SPECIMEN PARALLEL I60h *  140  " co  120 h  CO  5 £  b  100 h  TO PIPE  A - tr FLOW STRESS o -  o - Y I E L D STRESS  800  o-  80h X  -200-160-120  -80  O  X  X  X  -40  0  40  AF-1 SPECIMEN A -  p  c  STEEL  PARALLEL  TO ROLLING  DIRECTION 1200  a FLOW STRESS 1000  140 120  8 0 0 7> Q.  CO CO  W 100 o.  CO  420  FIG.6.3  CO  *  •A.  80  600  ©  60 x X - 2 0 0 - 1 6 0 -120 - 8 0  £  H600  A  TEMPERATURE  160  1000  V  60  ^  AXIS 1200  X  X  X  - 40  0  40  TEMPERATURE  420  FIG. 6.4  A F - 2 STEEL SPECIMEN  TRANSVERSE  86  TO ROLLING DIRECTION 1200  A - o - F L O W STRESS o-crYIELD  CO  STRESS  H1000 o Q 800 _  CO CO  111 tr Ico b  600  60  420  - 2 0 0 -160 -120 - 8 0 - 4 0  0  40  TEMPERATURE  °c  FIG. 6.5  A F - 2 STEEL SPECIMEN TRANSVERSE 160 1 6 0  2  140 140  X  TO PIPE  A - o-FLOW STRESS  1  ° -  < r Y I E L D STRESS  120 CO CO UJ  100  fr-  80  tr  co  60  AXIS 1200 1000  800 ° 4 ^ A , 600  JL  -L  -200H60-I20-80  -40  TEMPERATURE  JL  ±  0  40  °c  420  FIG. 6.6  £.  AF-2 SPECIMEN  87  STEEL  P A R A L L E L TO PIPE  AXIS 1200  A-o-FLOW STRESS o-cr Y I E L D STRESS  CO  HI000  CO CO UJ  - 800  CO  600  2:  or  J420 -200-160-120 -80  -40  0  TEMPERATURE  40  °c FIG.  6.7  2 STEEL SPECIMEN  PARALLEL TO ROLLING  DIRECTION 1200  A-o-FLOW STRESS CO  o-cr Y I E L D STRESS  1000 o  CO  800  CO UJ  tr Ico b  600 60 - 2 0 0 -160 - 1 2 0 -  TEMPE  0  x  40  FIG.  420  6.8  CL  88  6.2  Fracture Toughness;  6.2.1  K  N  Test Results:  The fracture toughness values (KQ or K the AF-1 s t e e l as obtained by the K  L C  L C  ) for  t e s t s , are  reported i n Tables 6.1, 6.2 and 6.3 f o r cracks 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 , cracks p a r a l l e l to the pipe axis and cracks transverse to the r o l l i n g d i r e c t i o n respectively.  The comparable results f o r the AF-2  s t e e l are shown i n Tables 6.4, 6.5 and 6.6.  In general  the KQ values for both steels decrease i n each test d i r e c t i o n with decreasing temperatures  as can be seen i n  Figures 6.9.1, 6.9.2, 6.9.3.  This decrease i n fracture toughness with decreasing temperature i s expected i n s t r a i n rate sensitive materials* and i s related to the increase in y i e l d stress with decreasing temperature.  The  increase i n y i e l d strength allows a higher l e v e l of t e n s i l e stress to be present i n the p l a s t i c zone ahead of the crack and ensures crack t i p t r i a x i a l i t y . Cleavage f a i l u r e occurs when the stress attained over a distance of one or two grain diameters ahead of * Strain rate s e n s i t i v e materials exhibit a marked vari a t i o n i n y i e l d strength with variations i n the test temperature and s t r a i n rate (56). The low carbon s t r u c t u r a l steels are more s t r a i n rate s e n s i t i v e than high strength aluminum and high strength a l l o y s t e e l s . (  5 6  »  5 7  )  89  to CO if) Ul  z  X o  100  C R A C K P A R A L L E L TO R O L L I N G  A  60 40  O < OS  u.  o o  8 8  80  VALID  KIC  JL  -160 - 1 2 0  -80  CRACK PARALLEL  in  80  8  X o  ui rr  60 40  100  -  80  -  60  A-AFI o - A F 2  ii o a 5  20  -200  100  -  40  20  -40  0  TEMPERATURE  CO  DIRECTION  -  \  e S »  .  FIG.  °C  TO P I P E  6.9.1.  AXIS  - 100  §  8  60  o  INVALID  K  A-AFI 0 - A F 2  80  8  IT".'1 o  40  | C  If a a.  20  <  on  1  .1.  -200-160-120  i  i  -80 -40  i 0  TEMPERATURE  if  CO 1 0 0  CRACK  40 FIG.  °'c  T R A N S V E R S E TO R O L L I N G  6.9.2.  DIRECTION 100  to to  A-AF I o - A F 2  HI z X o o Ul  tc  <  u.  - 2 0 0 -160 -120  -80 - 4 0  TEMPERATURE  0  40 FIG.  6.9.3.  (58) the material  . As the temperature increases the  y i e l d stress decreases u n t i l the cleavage stress i s (54) no longer achieved and the material tears instead The tearing mode of f a i l u r e occurs predominantly by (55) a process of void i n i t i a t i o n and coalescence  I t should be noted that there are two possible transitions i n fracture behaviour with temperature, namely: 1)  A K  2)  A Plane-strain to Plane-stress t r a n s i t i o n  l c  t r a n s i t i o n (Plane Strain transition)  commonly c a l l e d the ' E l a s t i c - P l a s t i c Transition'. (59) Although Wessel suggested that the f a i l u r e mechanism i n K  l c  tests was cleavage over the whole  temperature range, Barsom and Rolfe's (60)  fractographic  analyses  transition i s  established that the K  I c  associated with the onset of a change i n the microscopic fracture mode.  At low temperatures, the  fracture i s 100% cleavage or guasi-cleavage whereas at the t r a n s i t i o n or intermediate temperatures, the fracture surface exhibits a combination of guasicleavage and tear dimples.  At higher temperatures  above the t r a n s i t i o n region, the fracture surface  consists of 100% tear dimples.  The t e s t results as obtained using the h inch thick compact tension specimens indicate o valid K  L C  that  values are obtained at - 130 C and below  for both steels i n a l l t e s t directions; i n the r o l l i n g d i r e c t i o n the AF-2 s t e e l i s the exception. The AF-2  s t e e l exhibits a lower y i e l d strength  i n t h i s d i r e c t i o n and therefore v a l i d K  data i s o not obtained u n t i l the temperature i s below - 130 C. o o Thus, the t e s t values obtained at - 130 C, - 150 C, o and - 196 C l i e on the lower shelf of the fracture toughness t r a n s i t i o n 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 i n L C  the t r a n s i t i o n zone and are also KQ values.  Fig.  shows the fracture surface of o o the AF-2 s t e e l a f t e r testing at RT, - 40 , - 80 , o o o - 110 , - 130 and - 196 C with the crack p a r a l l e l 6.10.1  to the pipe axis. The fracture surfaces obtained o at RT and - 40 C are gray and fibrous, t y p i c a l of ductile failure.  In f a c t , 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 specimens of Af-2 steel with crack parallel to pipe axis at various temperatures. lc  c o n t r a s t the f r a c t u r e s u r f a c e s on specimen t e s t e d a t o - 130 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 , indicative of at  brittle failure. The 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 - 80 a n d - 110 C c o n s i s t o f b o t h f i b r o u s a n d  granular structure; o - 8 0 C , 20% f i b r o u s  The  test  40% f i b r o u s  a n d 60% g r a n u l a r o a n d 80% g r a n u l a r a t - 1 1 0 C.  results  indicate  that  the  at  plane o  strain  to  plane  s t r e s s t r a n s i t i o n o c c u r s above -  with  increasing temperature.  that  as  of  the  through  the  temperature  material  plane  in  plastic the  at  the  case of  transition,  cleavage  higher  transition  at  at  the  in the  surface  samples the  Plane low  temperature  As  study  the  a  tip  the  although  prevails.  established  in  the  micro-mode  of  fracture  transition, 100%  that  elastic-  that  ductile  and a combination o f range.  fact  consequence  tested  Strain  C  strength  reduces  crack  longer  temperature,  temperature  to  yield  turn  stress exists.  fracture  same a s w i t h  100%  This  s t r a i n c o n d i t i o n no  The even  decreases.  of  i s due  increases,  thickness constraint  a t r i a x i a l state the  This  130  both  remains i s , tear in  the  Considering the d i r e c t i o n a l i t y of the toughness behaviour of the AF-2 s t e e l , the test results as shown by the t r a n s i t i o n curves i n F i g . 6.9.1, 6.9.2, 6.9.3 do not indicate much deviation from one orientation  to another orientation.  other words, the AF-2 s t e e l exhibits  In  essentially  i s o t r o p i c fracture toughness behaviour.  The same  i s not true f o r the AF-1 s t e e l ; for tests with the 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 a lower toughness value  i s obtained i n the upper shelf  region i n comparison to the other two test  In comparing the KQ and K AF«L and the AF  2  L  C  directions.  results o f the  s t e e l the following observations can  be made. 1.  In the upper shelf region, f o r a 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 , the AF-1 s t e e l possesses a s i g n i f i c a n t l y lower toughness than the AF-2 s t e e l .  The KQ values of both steels are comparable  i n the other two test 2.  directions.  In the t r a n s i t i o n region, both the AF-1  and AF-2 steels show steeper t r a n s i t i o n behaviour f o r the 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 ; but the t r a n s i t i o n temperature range remains the same f o r each  s t e e l i n a l l three t e s t d i r e c t i o n s . 3.  Both steels e x h i b i t comparable fracture  toughness i n a l l t e s t directions i n the v a l i d K  l c  range.  The more i s o t r o p i c nature of the fracture toughness behaviour of the AF-2 s t e e l and i t s higher fracture toughness values along the transverse d i r e c t i o n ( T-- L ) i n comparison to the AF-1 (containing more sulphur) c l e a r l y reveals the b e n e f i c i a l e f f e c t s o f rare earth addition during the s t e e l making process.  96  6.2.2  J-Integral Test Results;  The J-Integral t e s t results for the AF-1 and the AF-2 s t e e l are shown as J-resistance curves i n Figure: 6.11.1 - for the 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 (T-L) 6.11.2 - for the crack p a r a l l e l to the pipe axis 6.11.3 - f o r the crack transverse to the r o l l i n g d i r e c t i o n (L-T) 6.12.1 - for the crack p a r a l l e l tothe r o l l i n g d i r e c t i o n (T-L) 6.12.2 - for the crack p a r a l l e l to the pipe axis 6.12.3 - for the crack transverse to the r o l l i n g d i r e c t i o n (L-T) respectively.  The J-resistance curve for the AF-1 o  s t e e l tested at - 80 C with the crack transverse to the r o l l i n g d i r e c t i o n as shown i n F i g . 6.11.3 confirms the fact that the slope o f the J-Aa curve approaches zero with decreasing t e s t temperature and the J i value c  remains constant independent of the crack growth (Aa) at these low temperatures.  Therefore, t h i s curve o  v e r i f i e s 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, F i g . 6.13.1 shows the fracture surfaces of the AF-2 s t e e l specimens which were tested at RT and were used  97 10  (mm.) 20 30 T  c  750  40  50  T  •\160~  E  J = 2o" Flow A a  120 ®  JQ  • c  500  o  80 OO—O*  250  Temp. = R.T J , c 300 in-lb/in (52-50 k J / m )  40  s  —i 04  08  12  i  •  16  -20  O w  2  Act , Crack Extension (inch)  0-5  10  1-5  (m.m.) 2 0 2-5  30  3-5  40  T  T  JsZcrFlow A a J  o -> 80 to  Temp, s — 40 °c J , =275 in-lb/in (4818 kJ/m )  40  2  c  •02  04  06  i.  08  i  10  •  12  \  14  16  A a , Crack Extension (inch) FIG. 6.11.1. J - resistance curve for A F - 1 steel with crack parallel to rolling direction at R.T © - 4 0 °c  (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 p a r a l l e l to Pipe Axis at RT & -40  0-5  (m.m.) 1-5 2 0  10  2 5 320  •£ 1500  „J = 2 CT F l o w  Aa  ]L 1000  t  <D 3  160 Temp.= R.T  ->  c  240  jO  500  O  ~3 M  J , c = 4 2 5 In-lb/lrT-  80  O  (74-46 KJ/m2)  •02  -04  06  08  10  A a , Crack Extension (inch) 0-5  — i —  £ 750 •>•* J_  10 j  - J = 2o~ Flow  2-5 1  160  E  Aa  a>  120  o  500  80  -O->  (m.m.) 1-5 2 0 1 1  10  O  Temp. = - 4 0 c  250  J , c = 390 in-lb/in  40  8  ( 68-33 KJ/m8 ) i i •  •02  04  06  08  10  A a o Crack Extension (inch) 0-5 • 750  10 i  1-5 •  J = 2cr F l o w A a  a 500 c  (m.m.) 2 0 2-5 i i J,c  o. ... i •02  i  04  i  3-5 i  40 i  120  s 250 In-lb/in* KJ/m8)  80 40  o i  06  160  T6mp. = - 8 0 ° c  (43-8  250  3 0  i 08  i 10  i 12  i 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. -40°c 8 - 8 0 ° c 8  240  200*1 160  '  I  120 Temp. =  R.T. 80  J , c = 7 8 5 in - l b / i n " (137-53 k J / m 2 )  •01  02 Aa  03  04  05  40  06  07  08  , Crack Extension (inch)  (ram.) 10  05 —I—  1-5  2 0  —r-=  1200  210 180  J = 2o" Flow A a  1000  150 E  eoo  to  120 3  600  Q  90 n  O  Temp. • — 4 0 ° c  400  60  J I C * 6 0 0 In-lb/in8  200  (10512  •01  02 Aa  03  04  kJ/m2)  05  06  30 07  08  , C r a c k E x t e n s i o n (inch)  F i g u r e 6.12.1. J - R e s i s t a n c e curve f o r Af-2 S t e e l - w i t h c r a c k 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 a t RT and -40"C.  .  (m.m.) 0-5 10 T J = 2 CT Flow A a  2000  I -5 - 320  1500  ej - 240 6  1000  HI60  o  •180  2  - R.T.  500  J - 800 in-lb/in  2  i e  (14016 kJ/m ) i_ i _ 03 04 05 06 2  -01  02  A a , Crack Extension (inch) (m.m.) 10  0-5  1-8  -T—  320  J * 20" Flow A Q  ^  240 160  Temp, s - 4 0 ° c J * 640 in-lb/in  2  | C  80  (II2I2 k J / m ) 2  •01  02  03  A a , Crack  F i g u r e 6.12.2.  04  05  06  Extension (inch)  J - R e s i s t a n c e curves f o r AF-2 s t e e l w i t h c r a c k p a r a l l e l to P i p e A x i s a t RT and -40°C.  o -5 —  (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 s t e e l with crack Transverse to Rolling Direction at RT & -40°C.  103  Figure  6.13.1  F r a c t u r e s u r f a c e s o f AF-2 s t e e l s p e c i m e n s w i t h 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. i n order of i n c r e a s i n g crack extension.  crack Arranged  to construct the J-resistance curve i n F i g . 6.12.1.  The specimens i n F i g . 6.13.1 are kept i n the order o f increasing crack extension. Heat-tinting c l e a r l y 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 f o r each o f these curves o for temperatures  - 40 C and RT and the d i r e c t values  from the tests conducted at other temperatures are reported i n Tables 6.1, 6.2, 6.3 for the AF-1 s t e e l and i n Tables 6.4, 6.5, 6.6 for the AF-2 s t e e l . The J-Integral tests confirmed that with increasing temperature, material increases.  the fracture toughness of a  This i s demonstrated f o r both  steels i n Figs. 6.14.1, 6.14.2 and 6.14.3.  The t e s t results show a s i m i l a r fracture toughness t r a n s i t i o n a l behaviour as was observed i n the K  I  C  tests.  F i g . 6.15.1 compares the KQ and J  data for both steels and shows that f o r the crack o 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 the results at - 130  105  CRACK PARALLEL TO ROLLING DIRECTION 800  150 _  o  -  0  UJ  90  0.  4:  200  o O A©  _  A  A  - 60  A  A  A 8 •  30 l  I  •  •200-160 -120 -80 - 4 0  O- AF 2  120 v  O  400  A - AF I  CM  1 0  TEMPERATURE  ^ o  » g ~  1 40  "c  FIG. 6.14.1  CRACK PARALLEL TO PIPE AXIS 800  150  E 120 ^  O  ui  O  600 400  o  u -3 200  A A o § A O  90  A A  ,o-AF 2  g  60 „  A  O  1 ._! 1 -200 -160 -120 -80  A - AF I  •  i  i  -40  TEMPERATURE  40 c  FIG. 6. 14.2.  CRACK TRANSVERSE TO ROLLING DIRECTION M 800 C JO c 600  120  O  • 400  o -9  150  ©  8  200  ii  B •  -  A  A  90  - 60  A  -  A  E  v. Ul  _J Z3  o  ~3  30  i  -200 -160 -120 -80  -40  TEMPERATURE  0  40 FIG. 6.14.3  —  AF I  O-AF  2  106  1000 150 V CM  \  CRACK PARALLEL TO ROLLING DIRECTION  100 800  . ' A  ~_ 600  -2 90 O ~ 60 u 30  80  @ <-}  120  9>  100  80 60 o  JLZ\ 400 200  - 60 <o  o  •  e  A  A  - 40  O  A° A  m  A  A  a.  o  40 20  o 20  I •  •  -200 -160 -120  i  •  -80  -40  40  A-AF I  J , Value  A - A F I K .K Value  ©-AF2  J , Value  &-AF2 KQi<, Value  c  c  Q  lc  c  FIG. 6. 15.1.  107  o o o - 150 , and - 196 C l i e i n the lower s h e l f , - 40 C and o o RT i n the upper shelf region and - 80 , - 100 C and o - 110 C l i e i n the t r a n s i t i o n a l range.  This i s true  for both of the steels for a l l three t e s t d i r e c t i o n s . I t should be noted that the J  l  c  values are v a l i d over  the whole temperature range - and that t h i s  fracture  toughness t r a n s i t i o n a l behaviour depicts the J t r a n s i t i o n a l pattern; whereas i n the KQ - K  L C  I C  c  testing,  the plane s t r a i n to plane stress t r a n s i t i o n ( K KQ) was observed and not a K  I  I C  to  transition.  For the AF-1 s t e e l , the J  l  c  transitional  curves do show anisotropic behaviour; the highest toughness i s r e a l i s e d i n specimens i n which the crack propagates transverse to the r o l l i n g d i r e c t i o n . This i s quite usual as along t h i s longitudinal (L-T) d i r e c t i o n a material would be expected to possess i t s highest toughness.  The minimum upper shelf tough-  ness i s r e a l i z e d for the crack growing p a r a l l e l to the rolling direction  (T-L).  This indicates that the  material possesses i t s minimum toughness along the transverse (T-L) d i r e c t i o n .  However, the lower shelf  J j c values are s i m i l a r i n magnitude i n a l l three t e s t directions.  108  In the case of the AF-2 s t e e l , the J j  c  t r a n s i t i o n curves do not show any marked anisotropy i n fracture toughness behaviour.  These J  j  c  curves  c l e a r l y show that the AF-2 s t e e l i s much tougher  than  the AF-1 s t e e l i n the t r a n s i t i o n and upper shelf temperature range.  At RT, the AF-2 s t e e l tested with  the crack p a r a l l e l to the pipe axis exhibits a J value of 800 i n - l b / i n  2  c  (140.16 KJ/m ), whereas f o r  2  2  the same test d i r e c t i o n , imately 400 i n - l b / i n  l  the AF-1 s t e e l shows approx-  (70.08 KJ/m ).  This i s shown  2  i n Table 6.7. Table; 6.7  Comparative J  I  C  values of AF-1 and AF-2  Steels 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 (T-L) and crack p a r a l l e l to pipe axis  Test Temperature oc  AF-2  AF- 1 J  Jlc.  IC  in-lb/in RT -40 -80 -100 -110 -130 -150 -196  Crack P a r a l l e l t o Pipe • Axis  Crack P a r a l l e l to Rolling Direction  2  KJ/m  Jlc 2  300 275 250  52.56 48.18 43.80  237  41.52  127 82  22.25 14.36  _  _ •  AF-1  J  IC  Jlc  AF-2 Jlc  Jlc  in-lb/in  KJ/m  in-lb/in  785 600 418 264  137.53 105.12 73.23 46.25  400 325 240  70.08 56.94 42.05  800 640 379  140.16 112.12 66.40  142  24.87  -  284  49.75  -  176 124  30.83 21.72  118  20.67  161 54  9.46  _  -  m  -  2  KJ/m  Jlc  KJ/ 2  in-lb/in  2  2  28.20  -  -80  2  -  14.01  2  109  o Fbr the tests conducted at - 40 C along the same orientations, the AF-2 s t e e l exhibits a J of 640 i n - l b / i n  2  l  c  toughness  (112.12 KJ/m ) whereas the AF-1 s t e e l 2  gives only 325 i n - l b / i n  2  (56.94 KJ/m ). 2  The greatest difference i n toughness between the AF-1 and the AF-2 steels  i s observed f o r specimens  tested with the 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 (T-L) (Table 6.7).  For example, at RT, the AF-2 s t e e l  exhibits a J j toughness of 785 i n - l b / i n c  2  (137.53 KJ/m ) 2  2  whereas the AF-1 s t e e l gives only 300 i n - l b / i n KJ/m  ).  (52.56  The data shown i n Table 6.7 also suggests that  in the design consideration for p i p e l i n e s , the fracture toughness o f a pipe should be considered not only along the pipe axis.but also 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 , the (T-L) d i r e c t i o n o f the plate since the material i s weakest along t h i s o r i e n t a t i o n .  I t 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 e x i s t p a r a l l e l to the lower toughness r o l l i n g d i r e c t i o n (T-L). The high toughness J  I  c  values reported for  110  the AF-2 s t e e l are thought to be due to the b e n e f i c i a l e f f e c t of the low sulphur content of the s t e e l and (5) the sulphide shape control  .  The rare earth s u l -  phides that are present i n the AF-2 s t e e l are spherical i n shape, have a high melting point and do not elongate to form stringers  i n the d i r e c t i o n 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 as i s the case f o r the AF-1 s t e e l . Hence, the lower non-metallic inclusion content and (61) the b e n e f i c i a l e f f e c t of the rare earth additions have resulted i n the high toughness f o r the AF-2 s t e e l . 6.2.3  COD Test Results;  The c r i t i c a l COD values (e> and <5Q) are m  shown i n Tables 6.1, 6.2, 6.3 f o r the AF-1 s t e e l and Tables 6.4, 6.5, 6.6 f o r the AF-2 s t e e l .  The variation of 6  C  i n terms of 6  m  and *Q  with temperature f o r a l l of the test specimens i s also shown i n Figures 6.16.1, 6.16.2, 6.16.3 f o r the 6  m  values and i n Figures 6.17.1, 6.17.2 and  6.17.3 f o r the 6Q values.  The general trends and c h a r a c t e r i s t i c s of  Ill  20 ? c  CRACK PARALLEL TO ROLLIN6 DIRECTION  16  ©  -  o  •4 © G  ©  A - AF I  D-AF2  •3  ©  8  % A  I  a & S  &  •2  6  •1  A  7  1 1 1 1 - 2 0 0 -160-120 -80 -40 TEMPERATURE  1 0 °c  1 40 FIG. 6.16.1.  CRACK PARALLEL TO PIPE AXIS A  —  AF I  ©-AF  -200-160 -120 -80 - 4 0 TEMPERATURE  2  0 °(  FIG. 6.16.2  CRACK TRANSVERSE TO ROLLING DIRECTION A-AF I ©-AF 2  -200-160 -120 -80  -40  TEMPERATURE  FIG. 6.16.3  112  CRACK PARALLEL TO ROLLING DIRECTION o  12  '2  8  ,COD  10  o o o A A  o  •3  •  £  - 2  ^  i i i - 2 0 0 - 1 6 0 -120 - 8 0  •  -40  i 0  O - AF 2 •o  -  4  A - AF I  i_ 40 FIG. 6.171  TEMPERATURE  CRACK PARALLEL TO PIPE AXIS  u 'o 8 Q  O  CO  4  -  o  g  o A  i 1  •  a  <  - 2 0 0 -160 -120 - 8 0  o  8  i  -40  TEMPERATURE  A  ...1  0  -  •3 © +to 2 £  -  10  A - AF I O - AF 2  L-  40 FIG. 6.17. 2  CRACK TRANSVERSE TO ROLLING DIRECTION  jC u  A - AF I O - AF 2  O O  - 2 0 0 -160 -120 - 8 0  -40  TEMPERATURE  FIG. 6.17. 3  the COD t r a n s i t i o n curves remain s i m i l a r 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 steels are comparable at -  80°C  m  values of both  and below, t h i s  being the lower temperature portion of the t r a n s i t i o n temperature range and the lower shelf condition.  In  the upper shelf region, the AF-2 s t e e l possesses a much higher 6  m  value than does the AF-1 s t e e l f o r the crack  running 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 ation) .  (T-L  The same i s found to be true i f the 6  Q  orientvalue  of both steels are considered (refer to F i g . 6.17.1). In contrast, the 6 and the 6n values of both steels m do not d i f f e r s i g n i f i c a n t l y i n the other two test directions.  The 6  m  vs T t r a n s i t i o n curves c l e a r l y show  that the COD properties of the AF-2 s t e e l are more i s o t r o p i c than those of the AF-1 s t e e l . Therefore, the COD test results also v e r i f y that the AF-1 s t e e l which contains more sulphur than the AF-2 s t e e l possesses a minimum upper  shelf  toughness f o r the crack running p a r a l l e l t o the r o l l i n g d i r e c t i o n (the T-L o r i e n t a t i o n ) .  Similar  observations have been reported i n 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 from the K the J  l  c  L  C  tests and the equivalent K  L  C  L  C  values from  and COD tests are compared i n the t r a n s i t i o n  curves shown i n Figures 6.18.1, 6.18.2, 6.18.3 f o r the AF-1 s t e e l and i n Figures 6.19.1, 6.19.2, 6.19.3 for the AF-2 s t e e l .  The respective data are summarised  i n Tables 6.1 to 6.6.  I t should be noted that KQ values at o temperatures - 130 C and below are v a l i d and hence represent the l i n e a r e l a s t i c 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  a c i c u l a r f e r r i t e steels increases with increasing temperature 2)  that the COD e l a s t i c - p l a s t i c fracture  toughness values l i e above the J  l  c  fracture toughness and that the J  l  elastic-plastic c  the KQ values at higher temperatures K  Ic  v  a  i  u  e  s  at lower 3)  values l i e above and above the  temperatures  that 6  m  - K  L C  data obtained from the  COD tests are much higher i n magnitude than the K  I C  values predicted by the other t e s t r e s u l t s ; the values are approximately twice the magnitude of the J-Integral values over the e n t i r e temperature  range  of the tests. 4)  that the K  J C  data obtained from the J -  Integral t e s t i s larger i n 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 f o r  the AF-2 s t e e l at the higher temperatures  f o r the  three t e s t directions examined 5)  that i n one case f o r the AF-1 s t e e l  with the crack propagating p a r a l l e l to the pipe axis, very good agreement was obtained between the l i n e a r elastic, K  I C f  and the e l a s t i c - p l a s t i c , J i , fracture c  toughness r e s u l t s .  At higher temperatures, the COD and the J  Ic  v  a  l  u  e  s  indicate a large increase i n toughness  which i s not r e f l e c t e d i n the K (39) been explained by Egan  N  values.  This has  w  as the i n a b i l i t y of the  K-type analyses to take account of the increase i n the size of the p l a s t i c zone; the larger p l a s t i c zone would have required more work than i f the same load value (PQ) had been reached by l i n e a r e l a s t i c loading. Therefore, at higher temperatures, where KQ tests do not give v a l i d K  r e s u l t s , a more meaningful and  lc  representative toughness l e v e l i s indicated by the COD and J  I  c  test results. A wide difference between the v a l i d K  and the equivalent K  lc  from «  m  - COD and J  observed at lower shelf temperatures. Kic data from 6  m  I  C  lc  data  tests i s  The equivalent  - COD at low temperatures indicate  that the fracture toughness of both steels increased o at - 130 C and below. This i s misleading i n that i f o the experimental COD data at - 130 C and lower temperature for both steels i s considered (refer F i g . 6.16.1 to 6.16.3).  The equivalent K  Ie  from 6  - COD  was obtained using the following r e l a t i o n s h i p  K  Where ys  Ic  ~  a  equivalent =  J  E 6  m  a ys  y i e l d strength of the material at t e s t temperature and s t r a i n rate  E =  Young's Modulus  v as  Poisson's  Ratio  At higher temperatures, the conversion of fi - COD m  K  to  gives reasonable agreement, whereas at low tempo era tes, - 130 C and below, the conversion results i n L C  an increase i n fracture toughness with decreasing temperature. K  lc  from *  m  This apparent increase i n equivalent - COD may  be due to the high y i e l d strength  of the material at low temperature.  Therefore, the  conversion r e l a t i o n s h i p f o r 6  to equivalent  m  - COD  K  lc  does not seem to be applicable at the lower temperature range.  The difference between the two estimates of the e l a s t i c - p l a s t i c fracture toughness as indicated by the equivalent K and J  l  c  L C  data obtained from &  m  or 6"Q values  values, p a r t i c u l a r l y at upper shelf temperatures,  118  can be explained as follows.  The l i n e a r e l a s t i c  fracture toughness values, that i s , the K j or KQ c  values are based on a 2% e f f e c t i v e crack growth which includes the e f f e c t of p l a s t i c zone formation.  There-  fore, the c r i t i c a l COD- 6Q value represents a 2% e f f e c t i v e crack growth condition. Hence i n t h i s 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 t h i s d i s placement i s obtained f o r the maximum load.  In comparing the COD values with the J-Integral values, i t should be r e c a l l e d 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 i n which i s examined the difference between the J o  l  c  value and the  6Q value at - 40 C f o r the crack p a r a l l e l to the pipe axis f o r the AF-2 s t e e l (Fig. 6.19.2 or Table 6.5): J  l c  - 145 ksi jl.n  = 162 k s i J i n  /s  (159.35 NPaJm) whereas 6Q Average  (178.03 MPa y m) . r  A  Using - 40°C the  J-resistance curve data f o r the AF-2 s t e e l with the  119  crack p a r a l l e l to the pipe axis (Fig. 6.12.2),Aa = .01 inch gives J • 775 i n - l b / i n ,  (135.78 KJ/m2).  2  ing value of K  i s 158.84  I c  T h i s equivalent K 162 k s i y l n  ksi^Jln  The correspond(175.66 MPa^/m) .  value agrees with the 6Q value of  l c  (178.03  MPa^Jm)  shown i n the F i g . 6.19.2.  Therefore, i t i s apparent that i n the upper shelf temperature region, the difference between the equivalent K  as obtained from the J i data and the K  I c  c  l c  obtained from the $Q - COD data i s i n s i g n i f i c a n t provided an appropriate correction for crack growth i s taken into account. the equivalent K K  lc  l c  The large deviation between  from « COD data and the equivalent m  from J j c data i n the upper shelf temperature  region, where K  I c  = *  m  i s approximately  - J i c * i s not s u r p r i s i n g . that the K  l c  - J  J c  twice the K  l c  The simplest explanation i s  stands for NIL CRACK GROWTH whereas  Klc - «m Stands for EXTENSIVE CRACK GROWTH CORRESPONDING TO MAXIMUM LOAD. In the t r a n s i t i o n temperature region, the large difference between the K  l c  - J  I  c  and K  l c  s  m  or $Q can  be attributed to two e f f e c t s : i)  the d e f i n i t i o n s of K i  c  and 6  m  fracture  toughness are based upon d i f f e r e n t crack growth c r i t e r i a ii) temperatures.  the increase i n y i e l d strength a t low  The tendency of equivalent K J_  Ic  values from  data to be larger i n magnitude than the K j  c  values  even at the low temperatures where both the t e s t procedures involve h inch thick compact tension specimens and both the t e s t samples experience 100% cleavage fracture i s thought to be due to the smaller energy expended i n fracturing J to that required to fracture K length i n the K  lc  lc  l  c  specimens i n comparison specimens.  specimen i s approximately  .50 W, whereas the ligament length i n the J i s approximately depth.  The ligament .45 to l  c  specimen  .35 to .30 Wj where W i s the specimen  TABLE - 6 . 1 AF-1 Steel crack Parallel to Rolling Direction (T-L  COD  c Temp. ° C  -  -  -  66.00  72.53  7 2 . 2 5  79.40  7 2 . 3 0 71.99  1 5 0  3 0 0 . 0 0  •  2  COD -«»  inch  nm  inch  0.0078  0 . 1 9 8 1  0 . 0 0 7 8  C0D-  i,l  ksi^/^n 0 . 1 9 8 1  5 2 . 5 6  9 9 . 4 4  144.00  1 5 8 . 2 5  1 4 4 . 0 0  15S.25  1 0 9 . 2 8  0 . 1 9 3 0  C.0074  0,1879  141.69  1 5 5 . 7 1  1 3 9 . 8 0  153.64  0.0075  0.1905  0 . 0 0 7 0  0.1778  1 4 3 . 1 8  1 5 7 . 3 5  1 3 8 . 3 2  1 5 2 . 0 1  79.45  0.0075  0 . 1 9 0 5  0 . 0 0 7 0  0.1778  1 4 3 . 1 8  1 5 7 . 3 5  1 3 8 . 3 2  1 5 2 . 0 1  79.11  0 . 0 0 8 1  0.2057  0.0072  0 . 1 8 2 8  1 5 2 . 9 1  1 6 8 . 0 4  1 4 3 . 9 5  1 5 8 . 2 0  1 6 7 . 8 5  2 7 5 . 0 0  2 2 0 . 0 0  48.18  9 5 . 2 1  3 8 . 5 4  85.16  1 0 4 . 6 3  9 3 . 5 9  7 4 . 9 9  8 2 . 4 1  0 . 0 0 8 1  0 . 2 0 5 7  0.0079  0.2006  1 5 2 . 7 3  1 5 0 . 3 7  1 6 5 . 2 5  7 0 . 7 3  7 7 . 7 3  0.0082  0 . 2 0 8 2  0 . 0 0 8 2  0.2082  1 5 3 . 9 6  1 5 3 . 9 6  1 6 9 . 2 0  1 6 2 . 1 9  1 6 2 . 1 9  1 7 8 . 2 4  8 0  -  KJ/m  c  0.0076  6 0  1 1 0  -  in-lb/in  2  75.70  4 0  -  Equivalent K i  ' (m ksi^/Cn 6 8 . 8 8  RT  Orientation)  2 5 0 . 0 0 62.18  6 8 . 3 3  4 4 . 3 3  4 8 . 7 1  3 9 . 7 4  43.67  4 3 . 8 0  2 3 6 . 8 6  4 1 . 4 9  1 2 7 . 1 2  2 2 . 2 7  9 0 . 7 8 0.0091  0 . 2 3 1 1  0.0046  0 . 1 1 6 8  0.0046  0 . 1 1 6 8  0 . 0 0 9 1  9 9 . 7 6  0 . 2 3 1 1  8 8 . 3 6  97.10  6 4 . 7 3  71.13  1 2 3 . 9 9 1 3 5 . 2 4  4 2 . 0 5  4 6 . 2 1  0 . 0 0 4 1  0 . 1 0 4 1  3 0 . 2 5  3 3 . 2 4  8 3 . 5 3  1 4 . 6 3  0 . 0 0 2 8  0 . 0 7 1 1  5 2 . 4 7  5 7 . 6 6  1 2 2 . 3 9  3 1 . 2 8  34.37  8 0 . 9 4  14.18  0.0029  0 . 0 7 3 6  5 1 . 6 5  5 6 . 7 6  1 2 2 . 9 1  1 2 7 . 2 5  1 9 6  TABLE - 6.2 AF-1 Steel crack Parallel to Pine Axis  J  COD  lc  Temp. 0  C  RT - 40 - 80 - 110 - 150 - 196  ksi^/Tn  MPa^/m  73.05  80.28  in-lb/in  2  KJ/m  2  400.00  70.08  325.00  56.94  6 m inch 0.0115  76.70 76.20 70.70 70.70  84.29 83.74 77.69 77.69  59.00  64.84  240.00  55.83  61.35  328.94  54.18 41.60  59.54 45.71  239.33 211.20  41.93 37.00  0.0051 0.0036  40.39 32.89  44.38 36.14  111.28 50.68  19.49 8.87  34.07  37.44  57.97  10.15  Eauivalent Kile C0D-«  Ic mm 0.2921  inch  mm  0.0084  0.2133  0.0068 0.0087 0.0066 0.0092  0.1727 0.2209 0.1676 0.2336  ksi^/in  MPa^n  .114.83  126.19  103.50  113.74  m  COD-iQ ksi^/Tn  ksi^^n 171.00  187.92  146.00  160.45  171.00 177.24  187.92 194.78  132.00 136.52  145.06 150.03  204.34  224.56  160.55  176.44  0.0115 0.0134 0.0113 0.0150  0.2921 0.3403 0.2870 0.3810  42.04  0.0061  0.1549  88.94  97.74  133.14  146.32  57.63  0.0042  0.1066  104.13  114.43  115.12  126.51  0.1295 0.0914  88.82 83.44  97.61 91.70  126.30 113.99  138.80 125.2-7  0.0037 0.0032  0.0939 0.0812  60.57 40.87  66.56 44.91  116.03 124.49  127.51 136.81  0.0044  0.1117  43.72  48.04  146.30  160.78  TABLE 6 . 3 AF-1 Steel crack Transverse to Rolling Direction (L-T orientation)  J  Equivalent K j  COD  lc"  Terap.  °c  F  ksi^/n 7 1 . 9 3  KPa^/m"  -  -  -  -  KJ/m  2  inch 0 . 0 1 3 0  7 9 . 0 5  RT  -  in-lb/in  2  4 2 5 . 0 0  nun 0 . 3 3 0 2  0 . 0 0 8 4  0 . 0 1 5 3  0 . 3 8 8 6  0 . 0 1 0 5  8 0 . 4 9  0 . 0 1 2 6  0 . 3 2 0 0  0 . 0 0 7 2  C0D-6Q  1 8 5 . 3 1  2 0 3 . 6 5  1 4 8 . 9 6  1 6 3 . 7 0  0 . 2 6 6 7  2 0 1 . 0 4  2 2 0 . 9 4  1 6 6 . 8 6  1 8 3 . 3 7  0 . 1 8 2 8  1 9 1 . 8 7  2 1 0 . 8 6  1 4 4 . 5 8  1 5 8 . 8 9  MPa.^/fi"  0 . 2 1 3 3  .  ksl 4n  M  MPa,^/^"  1 1 8 . 3 6  8 5 . 6 2  C 0 D - «  lc  MPa^/S" ksi^/Tn  7 4 . 4 6  7 3 . 2 4  3 9 0 . 0 0  mm ksi.yTn  inch  7 7 . 9 1  4 0  J  0  c  6 8 . 3 2  1 1 3 . 3 8  v  1 3 0 . 0 7  1 2 4 . 6 0  8 0 . 6 3  8 8 . 6 1  0 . 0 1 4 8  0 . 3 7 5 9  0 . 0 1 1 5  0 . 2 9 2 1  2 0 7 . 2 9  2 2 7 . 8 1  1 8 3 . 4 3  2 0 1 . 5 8  5 5 . 9 0  6 1 . 4 3  0 . 0 0 5 0  0 . 1 2 7 0  0 . 0 0 4 8  0 . 1 2 1 9  1 2 7 . 3 7  1 3 9 . 9 7  1 2 5 . 8 4  1 3 8 . 2 9  1 4 0 . 7 0  1 5 4 . 6 2  1 3 9 . 4 7  1 5 3 . 2 7  1 0 6 . 0 1  1 1 6 . 5 0  8 0  2 5 0 . 0 0  4 3 . 8 0  3 8 . 9 9  0 . 0 0 6 1  0 . 1 5 4 9  0 . 0 0 3 4  0 . 0 8 6 3  0 . 0 0 6 0  9 0 . 7 8  9 9 . 7 6  8 5 . 6 5  9 4 . 1 3  0 . 1 5 2 4  6 4 . 7 1  7 1 . 1 1  4 2 . 1 2  4 6 . 2 8  2 2 2 . 5 6  5 5 . 6 8  6 1 . 1 9  2 4 3 . 9 0  4 2 . 7 3  0 . 0 0 6 6  0 . 1 6 7 6  8 9 . 6 7  3 3 . 7 7  3 7 . 1 1  1 4 1 . 1 3  2 4 . 7 2  0 . 0 0 2 7  0 . 0 6 8 5  6 8 . 2 1  4 4 . 6 9  4 9 . 1 1  1 9 0 . 7 2  3 3 . 4 1  0 . 0 0 3 2  0 . 0 8 1 2  3 3 . 3 9  3 6 . 6 9  8 3 . 7 7  1 4 . 6 7  0 . 0 0 3 7  0 . 0 9 3 9  3 2 . 3 9  3 5 . 5 9  1 0 7 . 1 9  1 8 . 7 7  0 . 0 0 2 1  0 . 0 5 3 3  5 9 . 4 4  1 1 0 1 4 6 . 2 6  1 6 1 . 8 3  7 4 . 9 6  9 8 . 5 4  1 0 3 . 2 3  1 1 3 . 4 4  7 9 . 2 9  8 7 . 1 4  1 1 1 . 5 0  1 2 2 . 5 3  5 2 . 5 5  5 7 . 7 5  1 3 5 . 4 2  1 4 8 . 8 2  6 5 . 3 2  1 0 1 . 8 1  1 1 1 . 8 8  1 5 0  1 9 6  TABLE - 6 . 4 AF-2 Steel crack Parallel to Rolling Direction (T-L orientation)  K  J  Q  Temp. °  C  ksi^/in  MPa^/T  78.12  -  -  -  -  -  T(  0  6  «n in-lb/in  2  KJ /m  2  inch 0.0119  85.85  RT -  Equivalent K ,  COD  Ic  785.00  mm 0.3022  inch 0.0084  C0D-5  ran  HPa^/T  0.2133 160.86  137.53  C0D-6Q  M  ^/d  ksiyfn  MPa^ym"  ksi  168.91  185.63  142.62  MPa^/5" 156.73  176.78  85.72  94.20  0.0142  0.3606  0.0114  0.2895  184.73  203.01  165.33  181.69  79.59  87.46  0.0109  0.2768  0.0089  0.2260  168.03  184.66  151.84  166.87  83.40  91.65  0.0166  0.4216  0.0107  0.2717  207.60  228.15  166.82  183.33  80.90  88.90  0.0125  0.3175  0.0106  0.2692  182.63  200.71  168.20  184.85  84.64  93.01  0.0150  0.3810  0.0087  0.2209  71.65  78.74  446.24  78.18  0.0098  0.2489  0.0091  0.2311  76.53  84.10  492.51  86.28  0.0087  78.67  86.45  317.83  55.68  55.00  60.44  236.21  41.38  323.67  56.70  40  600.00  105.12  140.64  154.56  60  80  100  130  199.96  219.75  152.71  167.78  121.00  132.97  163.47  179.65  157.14  172.69  0.2209  127.42  140.03  154.14  169.39  0.0085  0.2159  102.36  112.49  152.19  167.36  0.0058  0.1473  88.24  96.97  130.76  143.70  103.29  113.51 143.17  49.90  54.84  233.00  40.82  0.0057  0.1447  87.64  96.31  130.28  46.52  51.12  135.62  23.76  0.0054  0.1371  66.86  73.47  140.75  154.68  52.94  149.50  26.19  0.0050  0.1270  70.20  77.15  135.07  148.44  118.61  130.36'"  48.18  .  40.32  44.31  0.0048  0.1219  132.13  145.21  30.76  33.80  124.50  21.81  0.0026  0.0660  64.06  70.40  25.86  28.42  113.02  19.80  0.0036  0.0914  61.04  67.08  196  137.92  151.57  •  TABLE - 6 . 5 AF-2 Steel crack Parallel to Pine Axis  K  Q  J  <5 n  Temp.  °c P.T - 40 - 80 - 110 -130 - 196  Equivalent K, Ic  COD  lc  ksiy'in  MPa /o" v  74.46 74.98 77.43 76.96 69.76 71.98 57.72 52.15 66.83 55.14  81.83 82.40 85.09 84.57 76.66 79.10 63.43 57.31 73.44 60.59  61.04 55.53 34.11 57.92 26.09 26.39  in-lb/in  2  KJ/m  2  J  inch  mm  inch  urn •  0.0148 0.0142 0.0152 0.0104 .0.0107 0.0)09 0.0072 0.0051 0.0066 0.0054  0.3759 0.3706 0.3860 0.2641 0.2717 0.2768 0.1828 0.1295 0.1676 0.1371  0.0103 0.0087 0.0103 0.0092 0.0070 0.0089  0.2616 0.2209 0.2616 0.2336 0.1778 0.2260  0.1803 0.1701 0.0939 0.1346 0.0558 0.0685  800.00  140.16  640.00  112.12  382.52 193.95 376.50 188.77  67.01 33.98 65.96 33.07  67.08 61.02 37.48 63.65 28.67  163.00 165.31 83.35  28.55 28.96 14.60  93.97  16.46  0.0071 0.0067 0.0037 0.005 3 0.0022  29.00  67.27  11.78  0.0027  COD-6 m  Ic  ksi^/in Wa^/m*  C0D-6Q  Ml'a.ym"  ksLy/iri  MPa^XT  207.66 203.34 • 210.69 181.40 184.27 185.62 155.69 131.02 148.86 138.63  228.21 223.47 231.54 199.35 202.51 203.99 171.10 143.99 163.59 152.35  173.29 159.34 173.47 171.09 149.47 167.86  190.44 175.11 190.64 188.02 164.26 184.47  174.64 176.45 130.87 156.26 117.01 130.35  . ksl^ii  162.39  178.46  145.25  159.62  112.29 79.96 111.41 78.88  123.40 87.87 122.43 86.68  73.30 73.82  80.55 81.12  52.42 55.66  57.60 61.17  158.91 160.56 119.09 142.19 106.47  47.09  51.75  118.61  -  TABLE - 6.6 AF-2 Steel crack Transversa to Rolling Direction (L-T orientation)  K  J  Q  COD  Ic  0  C  RT - 40 - 80 - 110 - 130 - 196  Equivalent Kt C0D-6„ m  c  Temp.  «n  m  5  Jcsi,/in  V  i n - l b / i n KJ/m 2  73.58  80.86  76.53 81.07  34.10 89.09  74.23 77.03 69.55 76.48 49.85 63.37 60.10 51.82  81.57 84.65 76.43 84.05 54.78 69.64 66.05 56.95  52.81 24.97 26.97  820.00 660.00  2  143.66 115.63  inch  J  mm  inch  mm  0.0149  0.3784  0.0064  0.1625  0.0140 0.0182  0.3556 0.4622  0.0071 0.0103  0.1803 0.2616  0.2870 0.1955 0.2082 0.1981 0.1168 0.1244 0.1498 0.1270  0.0069 0.0069  0.1752 0.1752  364.86 379. 39  63.92 66.47  168.56 194.07 173.56  29.53 34.00 30.40  0.0113 0.0077 0.0082 0.0078 0.0046 0.0049 0.0059 0.0050  58.03 27.44  146.35 73.03  25.64 12.79  0.0054 0.0022  29.64  85.99  15.06  0.0029  Ic  ksi^/n 164.41  147.50  n  ksi^/^n  MPa^/G"  211.39  126.07  136.55  186.48 224.80  204.94 246.05  133.47 169.22  146.68 185.97  194.61 164.85 169.59 165.68 133.70 137.50 151.35 145.72  138.99 142.22  152.75 156.29  MPa^/n"  ksi^/iri  180.68  192.35  162.10  C0D-6  109.67 111.83 74.54  120.52 122.90 81.91  79.98 75.64  87.89 83.12  177.08 150.00 154.32 150.76 121.66 125.12 137.72 132.60  0.1371 0.0558  69.46 49.07  76.33 53.92  137.36 105.62  150.95 116.07  0.0736  53.24  58.51  120.93  132.90  127  >200 CO Si «  to CO UJ  160 -  z 120  *  r>  A  TO TU  a: o  80 40  o  *  A  i  U.  © *  •a  8  O  *  ©  A  < (E  200  +  i  •  •  -200 -160 -120 -80 -40  *  160  A  120  o  a.  s  1 i  i  0  40  TEMPERATURE °c F i g u r e 6.18.1.  Temperature dependence o f f r a c t u r e toughness o f AF-1 s t e e l a l o n g c r a c k 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 .  O  >200  + +  CO  co CO UJ z  |60 -  © '20  O r-  or IO <  60  + +  •a  X  + + +  * A A O  40 i  200  •S  A  A  ©  SA  t  I A  X X  160 _  A  . 120 > a  0  - 80  A  E  •  •(40 i  -200 -160 -120 -80 -40  i  0  •  40  TEMPERATURE °c F i g u r e 6.18.2.  Temperature dependence o f f r a c t u r e toughness o f AF-1 s t e e l a l o n g c r a c k p a r a l l e l t o Pipe A x i s .  KQ-^-TCT  A  K | - C a l c . From  +  K , — C o l e . From  C  J.c-iTCT C  Sm.^TCT K, —Calc.From  X  C  SQ.^TCT •  KJ id  K  IIT IIT  C0D IIT  K  128  •200 -160 -120 - 8 0  -40  TEMPERATURE °c F i g u r e 6.18.3.  Temnerature Dependence o f F r a c t u r e toughness o f AF-1 s t e e l a l o n g 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 .  •  •+ •+  + +  A A A .  •  ©  K  A  K , c — C a l c . From  Q  -  £  TCT  240  8  I  O o  O  *  I  '  8  @  -200 -160 -120 - 8 0 - 4 0  |  K i c — C a l c . From Sm.jTCT  X  K  - C o l e . From S q - i T C T  40  J  IIT  A  Kid  IIT  K  0  ( e  80  40  TEMPERATURE °c F i g u r e 6.19.1.  +  K  0  1I  I60C| 120  a  JiciTCT  200  Temperature Dependence o f F r a c t u r e Toughness o f AF-2 s t e e l a l o n g c r a c k 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 .  C0D  IIT  129  -200  -160  -120  - 80  -40  0  40  TEMPERATURE °c F i g u r e 6.19.2.  Temperature Dependence o f F r a c t u r e Toughness of AF-2 s t e e l along c r a c k p a r a l l e l to Pipe A x i s .  KS  + 200  * CO CO  160  lit z X o  r»  120  o  kl or  80  CT  »-  40  r>  <  or  ©  •  + +  3 •  -200  o  oft  I  200  J  1 -120  1  -80  +  i.  120  2  S  I 0  X  K,c—CalcFrom  8. ± Q  1  •  80  I— 40  TEMPERATURE °c F i g u r e 6.19.3.  K|C — Calo. From Sm.-^TCT  x  8 -40  K I C —Colo. From l TCT  "ic-  40  -160  - £ T C T  X  X  +  &  Q  A  A  A  •  K  240 A  J  *•  +  © •  Temperature dependence o f f r a c t u r e toughness o f AF-2 s t e e l along 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 .  •  *<J  * T  IIT  *idllT K  C0D  IIT  TCT  130  6.4  Comparison of S t a t i c and Dynamic Fracture Toughness;  In this section, an attempt i s made to compare the s t a t i c fracture toughness data of the AF-1 and the AF-2 s t e e l as obtained i n the present investigation ( K ^ J j c , COD) with dynamic, equiIc  valent fracture toughness values obtained from the Instrumented (15)  Impact Tests conducted by Paul McConnell  •  S t a t i c and Dynamic fracture toughness data are superimposed on the following figures: For the AF-1 s t e e l  a) F i g . 6.18.1 shows fracture toughness  data f o r the 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 (the T-L orientation);  b) Figure 6.18.2 shows fracture  toughness data f o r the crack p a r a l l e l to the pipe axis; and  c) F i g . 6.18.3 shows fracture toughness data f o r  the crack transverse to the r o l l i n g d i r e c t i o n (the L-T o r i e n t a t i o n ) .  A s i m i l a r comparison f o r the AF-2  s t e e l i s shown i n F i g . 6.19.1, 6.19.2 and 6.19.3. The average dynamic fracture toughness data were calculated and are shown i n the figures*, dynamic data o were available only f o r temperatures down to - 100 C.  131  The data f o r the AF-1 s t e e l with the crack running 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 (Fig. 6.18.1) indicates that a very good c o r r e l a t i o n exists between the KQ s t a t i c data, the Kj U T and the Kia H T data for o test temperatures nitude of the Kj ^  from RT down to - 100 T  and K j ^  C.  The mag-  results i s much  smaller than the results obtained for the J „ 6„ ic, m, 6Q, COD s t a t i c data. T  I f the s t a t i c J  l  c  and 6  6Q  m  COD are considered  representative fracture toughness properties of the o s t e e l for temperatures  above - 130  C, where  K  lc  becomes i n v a l i d , then i t i s obvious that dynamic fracture toughness i s less than the s t a t i c fracture toughness for the same test temperature. that an equivalent K j  c  obtained from 6  m  I t i s clear or 6Q  COD  data i s a poor indication of the fracture toughness over the whole temperature fact that 6  m  range.  corresponds to a COD  This i s due to the for the maximum load  where extensive crack extension has occurred. For samples of the AF-2  s t e e l having the same  crack orientation, the s t a t i c and dynamic fracture toughness data are shown i n F i g . 6.19.1.  This diagram  132  o  o  shows that at - 100 and - 80 C, s t a t i c K ^ K j IIT, Q  and K j  are comparable; the dynamic data have  I (  a much smaller magnitude than the respective s t a t i c o J  Ic, m 6  a  n  Q  d  fi  C  0  D v a l u e s  «  At - 40 C the K  I d  value i s comparable to the KQ. but the K j ^ i s comparable to the J than the K  n  value.  J  c  value, J  l  c  1  1  T  value  T  being much higher  At RT the magnitude o f the dynamic  Kj ^ and K ]_^fri results are comparable to the J 6 and 6Q COD value. T  COD  l  c  m  In the case of the AF-1 s t e e l with the crack running p a r a l l e l to the pipe axis (Fig. 6.18.2), at o o - 60 C and - 80 C, the K j ^ i d 11T l o a n <  are comparable to the KQ data. and Kj H  T  K  At - 40  r  e  s  u  t  s  C, the K Q  0 D  2.1T  results are comparable to the *Q - COD and  are higher than the J j value. c  value i s comparable to the *  m  At RT the K - COD value.  C 0 D  For a  s i m i l a r specimen orientation o f the AF-2 s t e e l , F i g . 6.19.2 indicates that a good c o r r e l a t i o n exists between the KQ data and Kj U T and Kja H T results obtained o  o  at - 80 C and below. atures, the K  C 0 D  For - 40 C and higher temper-  -j_2.T d  and *Q COD values.  a t a  a  r  e  comparable to J i c , o  fi m  However, at - 40 C, the K j I(  value i s comparable to the KQ value, K j n-j* l i e s much  above t h e KQ v a l u e s b u t below the J  L  C  data.  For the AF-1 s t e e l w i t h the c r a c k running transverse t o the r o l l i n g d i r e c t i o n ( F i g . 6.18.3) o i t can be seen t h a t a t - 4 0 C and below t h e r e i s a good c o r r e l a t i o n between the KQ data and the K j o and K  I d  1  1  results.  T  A t temperatures  l  l  T  o f - 4 0 C and  above, a good c o r r e l a t i o n e x i s t s between KQQQ H T and m,  5  &Q  C  0  D  values.  F o r the AF-2 s t e e l w i t h the same  specimen o r i e n t a t i o n  ( F i g . 6 . 1 9 . 3 ) the KQ data i s  o comparable t o the K H T and K j 1 1 T r e s u l t s a t - 8 0 C o and below. A t - 4 0 C and RT the K ^ results are comparable t o 6Q - COD and J v a l u e s ; whereas a t o I d  C Q D  I  -40  C  C , the K j I I T data i s comparable t o 6  m  - COD  value. In summary, i t may be s t a t e d i)  the K  C 0 D  that:  I I T value i s comparable t o the 6m  and the 6Q COD s t a t i c data i n the upper s h e l f temerature  range. o  •Q  t  ii) A t - 6 0 C and below, t h e K j H T and K values a r e comparable t o the KQ d a t a . o i i i ) A t temperatures  I (  j  > - 60 C , the K j I I T  data i s l a r g e r i n magnitude than the KQ data and i s  less than the J  l  c  however,  and the 6^ - COD s t a t i c data; K  J d  1  1  T  i s comparable to KQ s t a t i c  data.  From the above observations, i t i s apparent that i n the upper shelf region there i s a close c o r r e l a t i o n between the s t a t i c ( 6 dynamic K  C 0 D  l  l  T  m>  6Q) and the  fracture toughness values.  However,  with the exception of the two cases (refer F i g . 6.18.2 o and 6.19.3 at - 40  C) one for the AF-1 s t e e l with  crack running p a r a l l e l to the pipe axis and one f o r the AF-2 s t e e l with crack running transverse to the r o l l i n g d i r e c t i o n , the dynamic Kj  1  J  1  T  i s less than the  I c s t a t i c value and the dynamic K j ^ n»p i s comparable  to the KQ data, i n the upper shelf temperature region. The « Q 6 F  m  s t a t i c and the dynamic COD fracture  toughness data give a poor i n d i c a t i o n of the fracture toughness of both steels at upper-shelf  temperatures;  t h i s i s due to the fact that the c a l c u l a t i o n of 6 and K ^ Q  D  ro  corresponds to a COD for a maximum load  where extensive crack extension has occurred.  There-  fore, i n the upper shelf region, i f only the K j ^XT K  I d 11T  a n <  * s t a t i c J i c and 6Q values are considered,  i t i s obvious that s t a t i c fracture toughness J  I C  is  higher than the dynamic fracture toughness (Kj H T ) , Kid 11T) .  In the t r a n s i t i o n temperature region the available s t a t i c and the dynamic data indicated that K  I  > C  K  Id. (62)  Shoemaker and Rolfe  have established that  for s t r u c t u r a l steels which are s t r a i n rate s e n s i t i v e , dynamic fracture toughness values are more conservative than the s t a t i c fracture toughness values. Barsom and (63) (64) Rolfe and Barsom advanced the observation -5 that the e f f e c t of a slow loading rate (e * 10 /sec) •  as compared to an impact loading rate (e «• 10/sec) i n steels of various y i e l d strengths i s to s h i f t the equivalent fracture-toughness  behaviour to a lower  temperature, the magnitude of the change being by the following r e l a t i o n T deg S h i f t = 119 - 0.12  o  y s  for 250 MPa < o T S h i f t = o for o  v S  < 965  MPa  y s  < 965  MPa  given  136  They have also shown that for steels having K  I c  965 MPa,  = K j throughout the t r a n s i t i o n range. I{  Therefore, the present relationship between the s t a t i c and the dynamic fracture toughness data of the AF-1 and the AF-2 steels ( o y i e l d strength = 480MPa) i s one i n which K j < K for the e n t i r e temperature o range down to - 100 C. This behaviour of the s t a t i c It  l c  and the dynamic fracture toughness of the AF-1 and AF-2 steels i s i n 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 s t r u c t u r a l 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 t e s t i n g (55) temperature was s u f f i c i e n t l y low. A.H. P r i e s t also made a s i m i l a r observation, that when fracture occurs by cleavage, K  l c  values are r e l a t i v e l y i n -  dependent o f the t e n s i l e properties and the K values do not vary with s t r a i n rate.  I C  In the  present investigation s t a t i c data i s available i n the cleavage range but dynamic data from 11T tests o i s available only range.  down to the - 100  C transition  Therefore, no comparison i s possible i n the  137  cleavage range.  But the t e s t results of t h i s study d i d o indicate that up to - 100 C, the fracture toughness  of the AF-1 and the AF-2 steels are s t r a i n rate s e n s i t i v e .  7.  7.1  CONCLUSIONS  i  Conclusions:  1.  The t e n s i l e studies established that with  a decrease i n the t e s t temperature, the y i e l d strength and flow strength of both the AF-1 and the AF-2 s t e e l increased.  The AF-1 s t e e l , containing more sulphur,  possesses i n f e r i o r y i e l d strength and exhibits higher anisotropy than the AF-2 s t e e l .  The isotropy of AF-2  s t e e l 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 s t e e l s increased.  3.  A l l three t e s t methods showed s i m i l a r  fracture toughness t r a n s i t i o n behaviour f o r both o o o steels; - 130 , - 150 , - 196 C toughness data o constituted the lower s h e l f , - 40 C and RT toughness o o data constituted the upper shelf and - 80 C, - 100 C, o - 110 C toughness data, the t r a n s i t i o n region.  4.  The KQ test established that both steels o possessed v a l i d K up to a temperature of - 130 C L C  and the fracture toughness t r a n s i t i o n was from a plane-strain  5.  to plane-stress testing condition.  A l l of the test methods established  that the AF-1 s t e e l i s anisotropic,  possessing i t s  highest toughness i n specimens having the crack transverse to the r o l l i n g d i r e c t i o n minimum toughness rolling  (L-T) and i t s  f o r the crack p a r a l l e l to the  (T-L) d i r e c t i o n .  6.  A l l three test methods confirmed that  the AF-2 s t e e l i s more i s o t r o p i c and tougher than the AF-1 s t e e l i n the upper shelf region. J-Integral test method indicated  The  that the AF-2 s t e e l  i s also tougher than the AF-1 s t e e l i n the t r a n s i t i o n region.  The AF-2 s t e e l possesses twice the toughness o  of the AF-1 s t e e l at RT and - 40 p a r a l l e l to the r o l l i n g 7.  C for a crack running  (T-L) d i r e c t i o n .  The lowest toughness of the AF-1 s t e e l was  r e a l i z e d for samples having a crack p a r a l l e l to the  140  rolling  (T-L) d i r e c t i o n as described i n McConnell's  11T study.  Therefore, both s t a t i c and dynamic fracture  toughness data reveal that the current p i p e l i n e toughness s p e c i f i c a t i o n for the crack propagating p a r a l l e l to the pipe axis may be inadequate f o r ensuring fracture control; minimum toughness  properties  (dynamic and s t a t i c ) are r e a l i z e d for the T-L o r i e n t ation i . e . crack running p a r a l l e l to the r o l l i n g direction.  8.  A comparison of s t a t i c and dynamic  fracture toughness data revealed that for the complete temperature range of t e s t i n g , K i > K^a, c  which indicates both AF-1 and AF-2 s t e e l are s t r a i n rate s e n s i t i v e .  7.2  Suggestions for Future Work;  i)  To further analyse the s t r a i n rate s e n s i t i v e  character of both AF-1 and AF-2 s t e e l s , the fracture toughness measurements under dynamic s t r a i n rate 5  conditions ( e  =  10 - 20/sec, K  =10  / -  ksi/in/sec)  should be carried out for the same range o f temperatures by the ASTM standard E - 399 - 74 method and compared  with K  fracture toughness.  Ic  To confirm the v a l i d i t y of the 11T data on these s t e e l s , d i r e c t measurement of the dynamic fracture toughness data (Kid) w i l l be  ii)  A fractographic  useful.  analysis of the specimens  tested i n the present work would reveal a clear picture of the micromode mechanism of f a i l u r e i n entire t r a n s i t i o n temperature range.  iii)  More c o r r e l a t i o n between K  IC  or K  with  Drop Weight Tear Test w i l l be useful for developing an e f f e c t i v e fracture control  iv)  11T,  K  l c >  K  Id  plan.  and Drop Weight t e s t data  for each s t e e l should be stored to make an e f f e c t i v e comparison.  This may  design considerations  contribute  towards toughness  of pipeline s t e e l s .  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. 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STP - 466,  147  APPENDIX - I Calculation of Pf(max)  We know f o r CT specimen (16,53) f (a/w)  BWls considering KQ  = 100 ksi^yTn,  Kf(  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  $%  P£  Kf(nax) «  Pf (max)  1  X)  f ^/w)  15,000 x .50 x 1 9~7~5Tj  = 781 lbs,  TABLE  K  f(max)  (ksi^/In) 10% 11% 12% 13% 14% 15% 16% 17% 18% 19% 20%  K  0  KQ  K, K K  Q  K K:Q KiQ K:Q  :Q KQ KQ  Pf (max) (lbs) 520 573 625 677 729 781 833 885 937 989 1041  p  Settings for Precracking m/c f (max) * -= xx.001" 2x4.05 lbs f ( m a x )  260.00 286.50 312.50 338.50 364.50 390.50 416.50 442.50 468.50 494.50 520.50  64.30 70.70 77.10 83.59 90.00 96.45 102.90 109.30 115.76 122.17 128.60  148  APPENDIX - II  AF-2  Sp. NO,  K  1 2 3 4 5 35 36 37 38 39  f(max)  No. of Stress Cycles  30,000 32,000 41,000 29.000 31,000 58.000 50.000 64,000 50,000 61,000  15% 15% 15% 15% 15% 15% 15% 15% 15% 15%  AF-1  10 12 20 26 29 21 22 23 24 25  Steel crack p a r a l l e l to Rolling Direction  Sp. f o r  38  % KQ  J-l J-l J-l J-l J-l  Time taken (Min.)  17 18 25 16 17 33 29 35 28 36  Steel crack p a r a l l e l to Rolling Direction  15% 15% 15% 15% 15% 15% 15% 15% 15% 15%  KQ KQ KQ KQ KQ KQ KQ KQ KQ 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, ii  A  study on the mechanical behaviour  of low carbon martensite , M e t a l l u r g i c a l Engineers,Department of M e t a l l u r g i c a l Engineering,1.1.T..  Kharagpur, 1972.  II  2.  Y.G.ANDREEV,& R.MAITI, Experimental techniques for studying m i c r o p l a s t i ii  c i t y of metals , Sixteenth Congress Of Indian Society of T h e o r i t i c a l 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 t h e o r e t i c a l if  studies on the problem of s h i e l d i n g i n welding of Mg-Alloy Symposium, Durgapur, 5.  November,  1974,  S.K.DUTTA,R.MAITI & Dr.M.K.MUKHERJEE,  Development of a procedure f o r II  surfacing welded Mg-Alloy pressure vessels , p a i l y , India,December, 1975. 6.  ., I.I.W.  I.I.U. Symposium,Tiruchira-  it  R.MAITI,E.R.GHOSH et a l l , C r i t i c a l heat treatment parameters f o r 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 elevaii  ted  temperature reactions i n maraging s t e e l ,I.I.M. Symposium, Suratkal,  March,1976.  

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