On Crack Closure and Crack Tip Shielding during Fatigue Crack Growth

Fatigue crack growth can be strongly influenced by crack closure, i. e., the contacting of the opposing faces of a fatigue crack above the minimum load in a loading cycle. This crack closure shields the crack tip from the full stress field, but different views are held regarding the extent of this shielding. The type of closure, whether it be plasticity-induced, or roughness-induced may also playa role. According to Elber, who considered plasticity-induced closure, the crack is effectively closed as soon as the first detectable closure is developed in the wake of the crack tip. However, Otto Buck and his colleagues, who used ultrasonic waves to investigate crack closure, have been in the forefront of those who claim that the crack tip continues to close even after the first closure contacts have been made, thereby leading to a lesser degree of shielding than in Elber's view. The purpose of the present paper is to review the extensive work in the area highlighting the ultrasonic approaches as well as the current state of the art concerning crack closure, and to discuss the various views concerning the extent of shielding provided by crack closure. In the 1970s when Elber [1,2] first conceived of the concept of crack closure Otto Buck and his co-workers [3-11] were using ultrasonic approaches to follow fatigue crack growth in structural alloys. The ultrasonic signatures clearly showed direct evidence for crack closure and the continuing closing of the crack as the load was reduced after closure was first observed. Fatigue crack growth ahead of the propagating crack is dominated by the cyclic plastic deformation that occurs ahead of the propagating crack. The mechanics of the crack opening is such that a crack is not completely open under a tensile load unless a critical crack closure stress is exceeded. The crack closure phenomenon has the effect of reducing crack propagation rate by reduction of the range in stress intensity,