Numerical Fatigue Analysis for Crack Propagation Using XFEM Method

Additive manufacturing (AM) enables highly complex metallic components, but process-induced defects such as porosity, lack-of-fusion discontinuities, and residual stresses can markedly reduce fatigue resistance and accelerate crack initiation and growth. Because experimental fatigue crack propagation tests are costly, time-consuming, and not always transferable from standard specimens to real geometries, this dissertation develops a predictive numerical framework for fatigue crack growth in AM metals based on the eXtended Finite Element Method (XFEM). The work first establishes the fracture-mechanics background required to model fatigue crack growth, with emphasis on stress intensity factors and Paris-law behaviour. Building on this foundation, a mesh-independent XFEM formulation is implemented in Abaqus by combining enrichment functions, a partition-of-unity approach, and a level-set representation of the crack surface, enabling crack propagation without remeshing. Crack growth under cyclic loading is introduced through an energy-based fracture criterion, implemented via the *FRACTURE CRITERION keyword and calibrated using Paris-law constants, coupled with “Direct Cyclic” analyses to efficiently simulate large numbers of cycles. Validation is carried out in three stages. First, XFEM simulations of Compact Tension (CT) specimens are compared against experimental fatigue crack growth data, reproducing the crack-length evolution (a–N), fatigue crack growth rate trends, and Paris-law parameters with strong agreement. Second, the approach is tested against literature-based configurations including multiple-crack scenarios, demonstrating robustness in tracking complex crack paths. Finally, the framework is applied to a realistic AM component—a hip prosthesis stem—under service-like cyclic loading, comparing different material states (as-built versus post-processed conditions such as heat treatment and HIP) and showing the model’s capability to distinguish the influence of post-processing and load orientation on crack trajectory and predicted fatigue life.