Staging of fatigue crack kinetics: patterns and features

Abstract

The concept of staging of fatigue crack kinetics (FCK) is presented. According to the developed concept the staging of FCK is attributed to changes in critical (preceding the fracture) deformation structures localized at the crack front and in micromechanisms of fracture operating in these structures. At the first stage of the FCK, these critical structures are clusters of dislocation braked near various barriers. In this case, the crack propagates through the low-energy-type mechanisms of brittle fracture with the formation of cleavages of various scale. At the second and third stages of the FCK, a critical two-level fragmented structure (two-level nanostructured state) is formed near the crack front. Brittle microlaminations along the boundaries of large-scale fragments of this structure oriented in the direction of maximum principal deformation play a key role in the high-energy-type mechanism of stable crack growth at the second stage of the FCK and form fatigue striations on the fracture surface. At the third stage of the FCK the mechanism of stable crack growth is added with the mechanism of origination and development of fracture at the boundaries of small-scale fragments of the critical fragmented structure with the formation of micropores which results in the formation microdimples in the corresponding areas of the fracture surface. The universal nature of the FCK staging and features of manifestation are illustrated by fractographic study of the fatigue crack propagation in highly stressed parts of aero-engines under various cyclic loading conditions, i.e., under self-induced and random vibration of compressor blades, repeated-static loading of compressor disks in the course of a cyclic test under vacuum conditions and during operation in the presence of crystallographic texture.