Abstract
Laser Shock Peening (LSP) offers the retardation the Fatigue Crack Propagation (FCP) in metallic components by introducing high compressive residual stress fields. Besides the high value of compressive residual stresses, which can be close to the yield stress, these residual stress fields are characterized by a relatively high penetration depth compared to other methods such as shot peening. LSP uses short high-energy laser pulses to vaporize the surface material. The vaporized material is turned into plasma, whereby the plasma pressure causes a mechanical shock wave travelling through the material. A sacrificial layer, also referred to as ablative layer, can be used to produce high surface quality. After relaxation of the system, local plastic deformations resulting from the mechanical shock wave lead to the residual stress field. As the residual stresses need to satisfy the equilibrium, tensile residual stresses are always generated as well. Tensile residual stresses are able to accelerate the FCP, so the introduced residual stress field must be known in detail to guarantee efficient application of LSP.
In this work, a multi-step simulation strategy is applied and validated to predict the FCP in LSP-induced residual stress fields.The multi-step simulation consists of (i) an LSP process simulation (ii) a transfer method of plastic strains from the LSP process model to an FCP model based on the eigenstrain method, (iii) the calculation of the residual stresses in the FCP model and the FCP simulation to predict the crack-driving stress intensity factors and the calculation of the FCP rate using FCP equations, particularly the NASGRO equation, Walker's equation and Paris' law. The multi-step simulation is validated using an `experimental simulation', where predicted crack-driving stress intensities are applied to an unpeened specimen. The `experimental simulation' agrees well with experiments using peened specimens and thus validates the predicted stress intensity factors. In addition to the multi-step simulation and validation, the fatigue crack retarding and accelerating mechanisms are shown.
