In this work, fatigue crack behavior of additively manufactured (AM) metallic alloy is simulated using phase field method. AM metallic alloys have found use in many space and
automotive applications including engine tubes, brackets, beams and other support structures. The fatigue response of AM materials, from a micromechanical perspective, depends on surface roughness, porosity,pore morphology, and crystalline structure (grain distribution, orientation and grain boundary structure). Empirical laws have been created, such as Murakami law and Multistage fatigue models, to predict fatigue limit and fatigue life of such parts. However, these approaches are very limited in capturing microstructural effects such as pore shape and orientation on macroscopic response.
This work develops a phase field crack growth model to study the effect of pore morphology on crack propagation under fatigue loading. The cost functional includes a spatial fracture toughness ratio that depends on plastic strain at the crack trip. As the material is loaded and unloaded, the plastic strain degrades the fracture toughness ratio, and enables crack growth. This is implemented in a scalable computational framework (Alamo) that solves conservation and evolution laws on an adaptively refined block structured grid.
A classic problem of a single pore within a grain is studies, where pores of ellipsoidal, cuboidal, hyposellipsoid shapes are considered. The grain structure is varied to account for plasticity along different directions, enabling anisotropy within fracture toughness ratio. Mode-I type cyclic loading is enforced for pores of different shapes and orientation and crack growth is studied. Results are compared against predictions from Murakami law to understand contributions of pore shape and orientation towards macroscopic fatigue laws.