TWI Industrial Member Report Summary 54/1978
By M G Dawes
The paper reviews the definition, fracture characterising 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 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
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 over-estimates of KIc, in materials having yield strength less than approximately 700/mm2.