In situ SEM testing for crack closure investigation and virtual crack annealing model development

In this paper, an in situ scanning electron microscope (SEM) fatigue testing approach is proposed to investigate the crack closure phenomenon within one cyclic loading under plane stress conditions. One objective of this experimental study is to verify the existence and significance of crack closure by directly measuring the variation of crack tip opening displacement (CTOD) and evaluating the crack opening stress levels. During the testing, the loading cycle is divided into a certain number of levels. At each level, high resolution images are taken around the crack tip region by SEM. Following this, imaging analysis is used to process these images in order to quantify the crack tip behavior at any time instant. Four constant amplitude loading cases with different stress ratios are investigated. Crack closure phenomenon is directly observed and measured. A simple mechanical model (named virtual crack annealing model) is developed based on the in situ SEM testing observations. The calculated crack opening stress level is compared with experimental observations. A detailed discussion is given based on the current investigation to explain some well-known issues in the classical fatigue theory.