Investigating the Mechanism of Facet Formation and the Influence of Crack Initiation Size on a Cast Aluminum Alloy in Ultrasonic Fatigue Under Varied Humidity Environments

Extensive research was conducted to analyze the ultrasonic fatigue behavior of ASGU-T64 cast aluminum alloy under different humidity environments. The study placed particular emphasis on investigating the factors influencing crack initiation, as well as the propagation of both short and long cracks. By examining the alloy's performance in various moisture conditions, a comprehensive understanding of its fatigue behavior was achieved.

The primary objective is to elucidate the mechanism underlying crack initiation and accurately predict the lifespan of short and long cracks. The ultimate goal is to determine how crack initiation size affects the percentage of crack initiation life in relation to the overall fatigue life.

Scanning Electron Microscope (SEM) and Electron Back Scatter Diffraction (EBSD) were employed and provided valuable insights into the characteristics of the facets. Furthermore, computational methods were utilized, employing the Paris crack growth law, to accurately determine the growth lives of both short and long cracks. By combining experimental and computational approaches, a comprehensive understanding of the fracture behavior and crack growth mechanisms was achieved, contributing to the advancement of knowledge in this field.

Through this study, it was discovered that fatigue cracks in the AS7GU-T64 alloy consistently initiated on the surface of the sample, primarily due to the presence of persistent slip bands (PSBs). Each facet observed on the fracture surface corresponded to an entire grain within the short crack area. While the stress intensity factor fell within the range of 3.5 to 10 MPa·√m for all three environments, it was found that the stress intensity factor in dry air exceeded that of saturated air and distilled water conditions. Importantly, the percentage of fatigue life attributed to crack initiation was found to be heavily dependent on the humidity of the testing environment and the applied stress amplitude. These insights highlight the intricate relationship between environmental conditions, stress intensity factor, crack initiation, and the overall fatigue life of the AS7GU-T64 alloy.

Humidity negatively affects the ultrasonic fatigue life of the AS7GU-T64 alloy. Furthermore, the size of crack initiation was identified as a significant factor influencing the percentage of crack initiation life in relation to the overall fatigue life.