In-situ microscopy study of strain field variation ahead of fatigue crack tip

In this paper, an experiment based on the optical microscopy is performed to investigate the continuous variation of strain field near the fatigue crack tip within the load cycle. The specimen is a small thin Al2024-T3 plate with an edge-crack. Its surface is speckled with tiny and dense spots. The applied load cycle is divided into a certain number of steps, at which high resolution images around the crack tip are recorded by a measuring microscope system. And then, the strain distribution at each load step is evaluated through the digital image correlation (DIC) technique. Thus, the continuous variation of the strain field in the vicinity of the crack tip within the applied load cycle can be achieved. Combining the experimental measurements with the material constitutive relationship, the plastic zone size can be estimated. Furthermore, it is observed that the crack closure has an obvious influence on the plastic zone size curve for different stress ratios. Besides, the present investigation is also extended to the plastic-induced loading interaction under the single overload.