An Heuristic Approach to a Defect-Based Fatigue Life Estimation Via Optical Microscopy and Fatigue Crack Growth Rate Data

Abstract Despite the progress additive manufacturing has made into delivering near-net shape components with minimal defects, the presence of gaseous pores, lack-of-fusion and keyhole pores, and material inclusions present a pressing challenge to the prediction of a component’s fatigue life. Depending on the defect location, its influence in fatigue life ranges from negligible to overwhelming, increasing the scatter associated with fatigue even further, posing difficulties during the design phase of components. Interesting work has been performed to improve fatigue life estimation through the use of computerised tomography (CT) to map out existing defects, detecting the killing defect based on the defect volume and surrounding stress field, and applying a fatigue crack growth law. While this approach is valuable, its application is dependent on the size of the sample; furthermore, use of CT scanning equipment is expensive. Hence, the chosen approach in this work used optical microscopy imaging along different cross-sections of the same component to parametrise statistical distributions for both the defect size and location. This information is then coupled with the stress field the part will be subjected to, and by applying a fatigue crack growth rate model of the same material, an estimate service life is obtained. Assessing the feasibility of this approach is the objective of this work, by applying the methodology in optical microscopy images fully defining one deposition from which specimens were obtained, identifying the killing defect and computing the expected service life.