| It is well known that fatigue crack growth rate is controlled by the crack tip plasticity under cyclic loading. Direct measurements of plastic zone variation (reversed plastic zone and forward plastic zone) and cracking behavior are of great importance for the development of physics-based fatigue crack growth model. The objective of this study is to design and perform new fatigue crack growth testing to investigate the crack growth mechanisms and behaviors, and then develop a novel time-based physical model based on the testing observation for the fatigue crack growth and fatigue life prediction. In this research, two types of the in-situ testing are proposed. One is under the optical microscopy and the other is under the scanning electron microscopy (SEM). The in-situ optical microscopy fatigue testing is developed to investigate the strain field variation ahead of the crack tip in the continuous time domain. The sizes of forward and reversed plastic zones are estimated using digital image correlation and the material constitutive relation. The in-situ SEM fatigue testing is conducted to investigate crack growth kinetics and the corresponding crack tip opening displacement (CTOD) at any small time instant during the constant and variable cyclic loadings. The experimental measurements (such as the plastic zone size, crack growth, CTOD, and so on) are used to establish a methodology for the fatigue crack growth prediction. The proposed methodology is fundamentally different from most existing cycle-based fatigue crack growth models due to its small time scale concept and direct experimental observations at the very small temporal and spatial scales. The methodology consists of three major components: the CTOD estimation framework, the crack closure model (virtual crack annealing model), and the geometrical relationship between the CTOD and the crack increment (da). All the components are demonstrated and validated under uniaxial constant and/or variable amplitude loadings for aluminum alloys. Finally, extensive experimental data are used to validate the developed methodology. The proposed study provides novel and promising experimental techniques and modeling for the fatigue crack growth analysis, which greatly enhances the understanding of fatigue damage mechanisms of materials. |
