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The paper reviews the definition, fracture characterizing roles, and measurement of critical COD and J-values. It is proposed that COD should be defined as the opening displacement at the original crack tip position. This definition avoids much of the ambiguity of previous definitions based on the crack tip profile and the elastic-plastic interface. Attention is drawn to a fundamental problem which limits the general application of the J-contour integral concept to elastic-plastic descriptions of the crack tip environment when cracks occur in overmatching yield strength weld regions. A comparison of recent three-point single-edge notch bend (SENB) testing techniques, based on the standard instrumentation used in KIc tests, shows there is a close mathematical link between the estimated values of COD and J.
Experimental data, obtained over a wide range of temperatures, are used to demonstrate how the critical values of COD and J for unstable fracture are affected by variations in specimen geometry. Also, it is shown that measurements of JIc may lead to overestimates of KIc in materials having yield strengths less than approximately 700 N/mm2.
mechanical properties, fracture tests, crack initiation, toughness, crack opening displacement, J, -integral, elastic-plastic cracking (fracturing), fracture properties, structural steels, crack propagation
Principal research engineer, The Welding Institute, Cambridge, England