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Investigation of Plastic Zone Development in Dynamic Tear Test Specimens

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Technical Report

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Defence Research Establishment Atlantic Dartmouth, Nova Scotia

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The J-integral is an elastic-plastic fracture criterion, which permits measurement of the fracture toughness of a specimen that has been fractured after general yielding. An understanding of the ratio of plastic zone size radius to the crack tip blunting stretch zone is required to determine the upper limit of temperature relative to full size transition curves where elastic plastic fracture becomes invalid. This study endeavours to acquire this ratio using finite element techniques. The development of the plastic zone in dynamic tear DT specimens and a non-standard three point bending fracture test specimen used to measure fracture properties was the main focus of the study. The ABAQUS finite element software was used to model the elastic-plastic behaviour of the specimens. For the DT specimen, a crack was induced by pressing the notch, followed by fatigue cracking at a limit load level of 40 of the specimen limit load, whereas, the crack shape for the non-standard specimen was a fatigue crack defined at approximately 30 of the limit load. The shapes of these cracks were adequately modelled in the finite element analysis. The specimens were made of 350WT steel and 304 stainless steel materials and were loaded until fixed amounts of permanent deformation were recorded. Results were obtained in the form of plots, showing the progression of the plastic zone around the crack tip. For each case, mid point plastic deflection, stretch zone width and plastic zone radius were computed. The finite element results obtained were compared to experimental elastic plastic testing where available, and reasonably accurate agreement was achieved.

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