In-Situ Optical Microscopy Study on Plastic Zone size estimation of aluminum alloy 7075-T6 under cyclic loading

An in-situ optical microscopy fatigue testing is proposed in this paper to investigate the forward and reversed plastic zone under cyclic tensile loading. During the testing, every loading cycle is divided into a certain number of steps. Images of the crack tip are taken at each step. The full strain field around the crack tip is then determined using the digital image correlation (DIC) technique. The plastic zone size is obtained combining the DIC results and the material constitutive relationship. Experimental measurements are compared with theoretical predictions. It is observed that the crack closure effect has a large effect on the reversed plastic zone size. The plastic zone size remains almost constant when the unloading path is beyond a certain stress level. A modified model is proposed to estimate the crack tip plastic zone variation based on the experimental observations.