Crack Nucleation, Growth, and Arrest Under Subcritical Crack Growth

Abstract

Component failures occur by crack initiation, growth and fast or overload fracture. The energetics demands that the crack tip driving forces must exceed the material resistance to crack growth. Crack being a high energy defect cannot nucleate spontaneously. However, it can nucleate in the presence of applied and localized internal stresses. Internal stresses can be of mechanical, chemical, electrochemical in origin. We show that crack nucleation and its continuous growth can occur only if the applied and internal stresses, and its gradients exceed some minimum values. Otherwise, cracks do not nucleate or nucleated cracks get arrested resulting in non-propagating cracks. Crack nucleated at sharp notches can get arrested if the notch-internal stresses decrease sharply with distance from the notch tip. Failure diagrams have been developed defining the mechanical equivalent of chemical driving forces by extending Kitagawa–Takahashi diagram developed for simple fatigue. We show the applicability of these concepts to various cases of subcritical crack growth. The available experimental data on the growth kinetics of incipient cracks nucleated at stress concentrations are limited in the literature for validation. Nevertheless, these concepts are based on physical principles and should be valid for all subcritical crack nucleation and growth, and should provide guidelines for design.