Fatigue analysis of brittle materials using indentation flaws

Abstract

The results of an experimental dynamic fatigue study on glass-ceramic specimens containing indentation flaws are analysed in terms of the theory developed in Part 1. A Vickers indenter is used to introduce the flaws, and a conventional four-point bend apparatus to break the specimens. Base-line data for testing the essential theoretical predictions and for evaluating key material/environment parameters are obtained from “polished” surfaces, i.e. surfaces prepared to a sufficient finish to ensure removal of any pre-existing spurious stresses. The fatigue tests are carried out in water. Inert strength tests in dry nitrogen are used to “calibrate” appropriate equilibrium fracture parameters, with “dummy” indentations on selected control specimens providing a convenient measure of the critical crack dimensions at failure. Regression analysis of the dynamic fatigue data yields values for “apparent” kinetic parameters, which are converted to “true” kinetic parameters via the transformation equations of Part I. Regeneration of the fatigue function from the theory using the parameters thus determined gives a curve which passes closely through the experimental data points, thereby providing a self-consistent check of the formalism. The implications of the results in relation to the use of macroscopic fracture parameters in the prediction of strength properties for materials with small-scale flaws is an important adjunct to this work. Finally, a recommended procedure for the general testing of dynamic fatigue properties of ceramics using indentation flaws is described.