Identifying fatigue crack initiation through analytical calculation of temporal compliance calibrated with Computed Tomography

Fatigue failure is ubiquitous in engineering applications. While the total fatigue life is critical to understanding a component's operational life, for safety, regulatory compliance, and predictive maintenance, the characterization of initiation life is important. Traditionally, initiation life is characterized by potential drop method, acoustic emission technique, and strain-based measurements. However, the primary challenge with these methods lies in the necessity of calibration for each new material system. The difficulties become even more aggravated for additively manufactured components, where fatigue properties are reported to vary widely in the open literature. In this work, an analytical methodology is utilized to evaluate the initiation life of two different materials such as AlSi10Mg and SS316L, fabricated via laser-powder bed fusion (L-PBF) technique. The processing parameters are selected such that AlSi10Mg behaves like a brittle material while SS316L shows ductile behavior. A custom fatigue testing apparatus is used inside Computed Tomography (CT) for evaluating fatigue initiation. The apparatus reports load-displacement data, which is post-processed using an analytical approach to calculate the evolution of material compliance. The results indicate that crack initiation during fatigue loading is marked by a noticeable change in compliance. The analytical technique shows a maximum difference of 4.8% in predicting initiation life compared to CT imaging. These findings suggest that compliance monitoring can effectively identify fatigue initiation in various materials.