Study of the contribution of different effects induced by the punching process on the high cycle fatigue strength of the M330-35A electrical steel

Abstract

Because of their improved magnetic properties, Fe-Si alloys are widely used for new electric motor generations. The use of punching process to obtain these components specially affects their mechanical behavior and fatigue strength. This work aims at studying the influence of punching operations on the fatigue behavior of a Fe-Si alloy. High cycle fatigue tests are performed on different smooth specimen configurations with either punched or polished edges. Results show a significant decrease of the fatigue strength for punched specimens compared to polished ones. To understand the origin of the fatigue failure on punched specimens, SEM observations of the fracture surfaces are carried out. They reveal that crack initiation always occurs on a punch defect. Additional experimental techniques are combined to characterize how the edges are altered by punching. The impact of punching operations on residual stresses and hardening is then investigated. Residual stresses are quantified on punched edges using X-ray diffraction techniques. Important tensile residual stresses exist in the loading direction as a result of punching operations. Also, according to XRD analyses and micro-hardness measurements, teh hardened zone depht is about 200µm. To dissociate teh respective influences of strain hardening, residual stresses and geometrical defects, a heat tratment is applied to both punched and polished specimens in order to quantify the contribution of each parameter to the high cycle fatigue resistance. Results show that the geometry of defects is one of teh most influent parameters. Consequantly, a finite element model is developed to simulate teh influence of edge defects on the fatigue strength of punched components. A non-local high cycle fatigue criterion is finally used as a post-processing of FEA to consider the effect of defets and teh associated stress-strain gradients in the HCF strength assessment.