Evidence of fracture surface interference for cracks loaded in shear detected by phase-shifted speckle interferometry

Abstract

Interference of the asperities on a crack loaded in pure, remote shear wedges the crack faces open, thereby inducing a mode I stress intensity factor (SIF). The interference also shields the crack tip from the applied mode II SIF. Three-illumination beam, phase-shifted speckle interferometry was used to measure the three-dimensional incremental displacement fields in a 7 x 11 mm area around a mode I fatigue precrack in a Al 7075 specimen loaded in 94 increments of increasing shear. The displacement fields were accumulated relative to the unloaded state by sampling at appropriate locations in the incremental fields to optimize spatial resolution and compensate for large rigid body motions. The induced mode I SIF and the effective mode II SIF were estimated from the crack tip shear displacement (CTSD) and crack tip opening displacement (CTOD). A digitized fracture surface profile was numerically shifted according to the experimentally measured crack face displacements to determine the locations of fracture surface interference as a function of applied load.