Fatigue Crack Closure and Residual Displacement Measurements on Aluminum Alloys

Abstract

Ultrasonic bulk measurement technique was used to measure crack closure and a clip-on gage surface measurement technique was used to measure residual displacement at the crack tip of a propagating fatigue crack in 2219 and 7075 aluminum alloys. Experiments were carried out in vacuum, humid air, dry O2 gas, N, gas, and deuteriated water vapor environments to evaluate effects of environments on residual strain and crack closure loads. It was observed that the closure load detected by ultrasonics decreased with introduction of humidity while the residual strain parameter d increased with the humidity. The results are modeled as a build-up of oxide layer in addition to strengthening of asperities on the fracture surface. In the light of these observations, the effectiveness of surface and bulk measurements is evaluated. The results demonstrate that surface measurements are not always representative of the bulk behavior and use of such measurements in predictive analysis for fatigue can be misleading. pc P, and P, P m a n Nomenclature Crack length Residual displacement at the crack tip Thickness of the compact tension specimen Half height of the compact tension specimen Width of the specimen Stress intensity factor Maximum stress intensity factor Minimum stress intensity factor Range in the stress intensity factors (Kmx-Kmin) Crack opening stress intensity factor Effective stress intensity factors range (Kmx-Kop)