FRACTURE SURFACE INTERFERENCE IN SHEAR .2. EXPERIMENTAL MEASUREMENTS OF CRACK-TIP DISPLACEMENT FIELD UNDER MODE-II LOADING IN 7075-T6 A1

The in-plane and out-of-plane displacement field around the tip of a c rack (produced by Mode I fatigue precracking) loaded in pure, remote M ode II was measured as a function of K-IIapp by electro-optic holograp hic interferometry. The effective Mode II stress intensity factor and the induced Mode I stress intensity factor were estimated from a fit t o the near-tip expressions for the crack face displacement fields. The effective Mode II stress intensity factor was always less than the ap plied stress intensity factor demonstrating that the fracture surface asperities shield the crack tip. The induced Mode I stress intensity f actor was roughly half that of the applied Mode II stress intensity fa ctor, demonstrating that pure Mode II cracks cannot be obtained for cr acks with microscopically rough surfaces. Plateaus and jumps in the de pendence of the effective Mode II and induced Mode I stress intensity factor on the applied Mode II were correlated further supporting the n otion that the fracture surface interference is based on asperity inte ractions. Analysis of the data shows that force transfer across the cr ack faces cannot be simply described by a frictional interaction acros s the asperities. It is suggested that deformation of the asperities m ust be considered to accurately model the shielding and induced crack face opening under remote Mode II loading.