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Crystallographic texture of wrought aluminum alloys has significant influence on the fatigue damage incubation and small-crack growth in the high cycle fatigue (HCF) regime. HCF lives have demonstrated fairly large scatters due to the random textural feature and its stochastic interaction with fatigue damage evolution. The fatigue life of an individual specimen is deterministic, which manifests the specific microstructure in the specimens and its interference with fatigue. In this paper, micromechanical simulations were conducted to quantify the grain orientation and grain boundary mismatch effects to fatigue damage incubation in a wrought 7075-T651 Al alloy. Fatigue damage incubated at fractured intermetallic particles near or at the surface [Xue et al. 2007]. Many intermetallic particles fractured after a few initial loading cycles; however, only one or a few fractured particles induced a crack into the matrix. One of these microcracks eventually grew and became a dominate crack and caused the final failure. The size of the particles, the orientation of the grain in which the particle resides, the particle spatial location to the grain boundary, the mismatch of the grain boundary, and the interaction of all these factors comprise the primary reason for the fatigue damage incubation. But no systematic investigation exists to provide general guideline for fatigue design and structural prognosis applications. In this research, a crystal plasticity constitutive model is implemented to simulate the microplasticity at the fractured intermetallic particles in single crystal and bicrystals, in which the crystallographic orientation mimics the typical rolling texture. The spatial location of the particle with respect to the grain and grain boundary is designed to represent those observed on the fractographs fatigued in the high cycle fatigue regime. The non-uniform loading distribution due to the heterogeneity of textured alloy is investigated by simulating on a representative unit cell with grain size and orientation taken from the realistic stereomicrograph of the sample. This approach is an initial high-order mechanistic multistage fatigue modeling approach, which can be implemented to estimate the uncertainty of the fatigue life prediction and fatigue life distribution using Monte Carlo simulation. Eventually, the reliability of fatigue life predictions will be accessed and the model evaluated as a mathematically rigorous tool for the structural integrity prognosis of aerospace structures that use wrought aluminum alloys.

Micromechanical simulations are conducted to quantify the influence of microstructure attributes to the formation of small fatigue cracks. Three wrought aluminum alloys (7075-T651, 2024-T3, virtual material) with fractured particle are studied to quantify the influence of material���s yield strength and ultimate strength to material���s fatigue resistance. Laser Engineered Net Shaping (LENS) material with pores of various spatial distribution and particles are simulated for the microplasticity and its effects on fatigue incubation. A cohesive zone model is used to study the interface cohesive behavior���s influence to the cyclic driving mechanisms. Different simulations based on different interfacial crack geometries and particle shapes are studied. A cohesive law with unloading-reloading cyclic behavior is introduced. A damage factor D is proposed to study the possibility of interfacial crack propagation. With this factor, plastic wake zone behind the debonding is studied.

A Laser Engineered Net Shaping (LENSTM) processed AISI 316L-grade stainless steel possesses unique microstructural features that affect its fatigue damage incubation mechanisms and fatigue life. As observed experimentally, fatigue damage was incubated almost exclusively at a relatively large pore located at or near the specimen surface, or at an incompletely melted powder particle on the surface in some rare cases. Micromechanical simulations were conducted on a series of representative volume elements to probe the micromechanism of fatigue damage incubation. A non-local plastic shear strain range-based fatigue manage parameter was introduced to represent the micronotch plasticity accumulation state under a large deformation gradient region. This microplasticity parameter was found to correlate to the fatigue endurance limit and transition between high cycle and low cycle fatigue regimes quite well. In the high strain amplitude regime (>0.3%), the spatial distribution extends no or little effect on fatigue damage incubation. The scatter in fatigue life is primarily dominated by small crack growth. In the low strain amplitude regime, the void on the edge of the specimen induced two times higher microplasticity compared to void inside the specimen. Due to the high population of the voids inside the alloy, the fatigue strength is limited by the voids on the surface. The microplasticity threshold is approximately 0.15% which corresponds to the fatigue endurance limit of 30.8 MPa.Micromechanical simulation demonstrated that the microplasticity induced by the unmelted powder particle is significantly lower than that generated by the void in the microplasticity threshold regime. Therefore, there must be an interactive effect between unmelted powder particles and the surround porosity in the alloy that incubated the fatigue damage.