Abstract
A criterion of the transition from stable fibrous crack growth to unstable fracture is investigated on the basis of J-integral R curve concept.
The slope of JR curve is compared with that of crack driving force curve under fixed grip condition. Instability condition is expressed by
dJR/da≤dJF.G./da
And the expression of above formula for bending specimen is obtained which is a general form of the expression after Paris et al.
Three-point bending tests with spring bars were carried out at temperatures above brittle-ductile transition. Instability occurred after stable fibrous crack growth at the lower level of J value than conventional bending tests.
The proposed instability criterion compares well with the test results.
