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Instrumented impact testing and its application to the study of acicular ferrite steels McConnell, Paul

Abstract

An instrumented impact test (IIT) machine was constructed and calibrated using static and dynamic loading. The theory and fundamentals of IIT have been reviewed. Tests were performed to assess the proposed ASTM IIT validity criteria. The requirements that the fracture time be greater than 3 times the period of specimen oscillations and 1.1 times the electronic response time appear to be conservative. The data confirm that adoption of the criterion, B > 2.5 (K[sub Id]/σ[sub yd])², ensures plane strain fracture, whereas the acceptance of a linear load-to-failure condition (i.e., P[sub MAX] < P[sub GY]) may not be conservative enough. For general yield failures, crack initiation was shown to occur prior to the attainment of maximum load. Thus, initiation energies calculated by assuming that crack initiation occurs at the maximum load are nonconservative. The dynamic properties of two acicular ferrite pipeline steels were characterized by IIT. The Information obtained, particularly the fracture toughness parameters and the initiation energies, revealed significant inadequacies in the toughness specifications and test methods presently used by the pipeline industry. Tests performed to assess the significance of testing, standard Charpy V-notch specimens versus full pipe wall thickness Charpys showed that lower upper shelf energies were obtained for the full wall specimens. However, the magnitude of the transition and lower shelf energies and the transition temperatures were similar. Fatigue precracked standard Charpys specimens absorbed much lower energies and had higher transition temperatures than did the standard specimens. Tests were also performed to assess the strain aging behaviour of the two acicular ferrite steels. Strain aging the semi-killed steel resulted in a decrease in the propagation energy, with no change in the magnitude of the initiation energy. For this steel, strain-aging does not increase the potential for crack initiation. Tests also revealed that sites near the seam weld of the pipe made with that semi-killed steel had experienced sufficient pipe-forming strain and thermal energy from the welding process to exhibit strain age effects. The fully killed acicular ferrite steel did not strain age; its strength and toughness were increased upon aging. The instrumented impact test provided fracture toughness data that correlated very well with that obtained by more conventional fracture toughness testing techniques. The total fracture energy from a standard Charpy test was shown to often mask the fracture toughness value of a material. The initiation energy obtained from testing a precracked Charpy specimen accurately indicated the relative magnitude of the fracture toughness, however.

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