Crack tip shielding by asperity contact as determined by acoustic measurements

Effects of Imperfect Interfaces on Acoustic Transmission and Diffraction

“…For all three of the signals shown, the first feature evident, as the scan proceeds from position 0, is a minor diffraction peak due to the crack tip indicated by the arrow. Such a peak has also been observed in the diffracted signal from a crack grown under constant AK conditions [5]. For the results shown here, the amplitude of this peak is relatively constant between samples, suggesting that conditions at the crack tip are unaffected by the changes in closure which occur deeper into the crack.…”

“…3 for the crack grown with a decreasing 6K exhibits partial closure all along the crack as evidenced by the relatively high transmission coefficient values of approximately 0.25. This is in contrast to fatigue cracks grown under constant ~ conditions, where the transmission coefficient drops below 0.05 [5]. It is speculated that the present result is caused by the continually decreasing plastic zone size, affecting plasticity induced in a fashion similar to the overload induced closure.…”

“…Earlier work [5] had suggested that for a fatigue crack that had undergone tensile overload the degree of contact between the crack faces was indicative of the length of the retardation in the crack growth and thus of the size and amount of the overload applied. Three cracks were grown with the growth results shown in Fig.…”

“…This shielding occurs below a stress intensity factor Klclosure at which the first contact during unloading occurs. Thus, the appl~ed stress intensity range, AK = K 1max -KI~in will have to be modified to include the effects of crack t~p shield~ng [1,5], leading to an effective stress intensity range, AK eff • This paper deals with variations in the degree of contact between the crack faces caused by changes in AKeff (from a constant AK) used during growth of the crack and the resultant changes in the transmitted and diffracted waves. The changes used include an overload block, a single cycle overload and continually decreasing AK.…”

On the mechanics of sinusoidal interfaces between dissimilar elastic–plastic solids subject to dominant mode I

“…The resulting non-planar crack propagation process along patterned interfaces is complex, and several models have been proposed to describe it. For instance, toughening of patterned interfaces has been attributed to mechanical interlocking and plastic deformation around a crack tip using contact-friction models [18][19][20][21][22][23]. More recently, the increase in interfacial toughness in rough polymer-metal interfaces was related to the adhesive-cohesive transition and crack deflection [24].…”