Investigative Method for Fatigue Crack Propagation Based on a Small Time Scale

In-situ scanning electron microscopy (SEM) testing based on a small time scale is proposed to integrally investigate the fatigue crack growth behavior and mechanisms, which is different from the widely-used, cycle-based approach due to its small time scale and comprehensive analysis of the effects of microstructure, crack closure and applied loading on crack growth. In the proposed methodology, the behavior of fatigue crack growth at any time within a loading cycle is observed by SEM to investigate the influence of microstructure on crack growth. Images with high resolution are taken to measure the crack tip opening displacement (CTOD), and the correlation between CTOD and the stress intensity factor (SIF) K is studied. A model based on experimental data is used to predict the CTOD variation. The unstable crack growth of aluminum alloy 7050-T7451 is investigated using the proposed method. Results show that this method has great potential in fatigue crack growth mechanism research compared with the traditional cycle-based approach.