Determination of local stress intensity factor at crack tip using image correlation techniques

The extraordinary mechanical properties of high strength aluminum alloys such as AA7075-T6 are caused by coherent nanoprecipitations. These nanoprecipitations generate local stress fields and interact with moving dislocations and propagating microcracks. In this paper, image correlation techniques are used to determine the local strain and stress field in the vicinity of fatigue crack tips during the loading of compact tension (CT) specimen. The fatigue crack tip was sharpened with decreasing fatigue loading after fatigue cracks initial appearance. Images of the crack tip were taken using atomic force microscopy/ultrasonic force microscopy (AFM/UFM) and white light interference microscopy (WLIM) before and after mechanical loading of the specimen. Both techniques are applicable for measuring the out-of plane displacement during the loading process. In addition, image correlation techniques can be used to determine the in-plane displacement resulting from mechanical loading. This information is used to calculate the local stress intensity factor in the vicinity of the crack tips.