ABSTRACT
Since for deep notch three-point-bend and CKS pieces 1'/ is only a weak function of a/W (1'/ ~ 2 for the standard bend piece and approximately 2·2 for CKS; see Chapter 3) early derivations of a J-R-curve used
dJR/da = (I'//Bb)(dU/da) (2.100) where the whole term ( U + .1 U) was the total work done, U up to initiation of cracking plus .1U afterwards (see Fig. 2.72). Multiple test piece techniques were at first used as a necessary means of ascertaining the amount of crack growth by cutting open the test piece after the required degree of deformation. The precise value of ligament used to evaluate JR varied from initial (i.e. bo) to current (i.e. bo - .1a) or some average but was not important whilst .1a ~ b.
