۱٫ Introduction

۲٫ Experimental procedure

۳٫ Fatigue tests results

۴٫ Discussion

۵٫ FEA considering the effect of defects

۶٫ Conclusion

References

Since a decrease of the fatigue strength may result from punching operations, this study proposes a methodology for designing punched parts against high cycle fatigue crack initiation. To reach this goal, high cycle fatigue tests are performed on different specimens configurations with either punched or polished edges. Due to punching effects, the fatigue strength of punched specimens is significantly decreased. Fracture surfaces observations reveal that crack initiation occurs always on a punch defect. Additional investigations are combined to characterize how the edges are altered by the punching operations. High tensile residual stress levels along the loading direction are quantified using X-Ray diffraction techniques. Furthermore, micro-hardness measurements and X-Ray diffraction results reveals a strong hardness gradient due to punching operation. For a better understanding of crack initiation mechanisms, the edge geometries have been scanned with 3D optical microscopy, allowing us to identify the most critical defect (and its real geometry) by comparing the edges before and after fatigue failure. Finally, FEA are performed on identified defects. A non-local high cycle multiaxial fatigue strength criterion has been used as post-processing of FEA to take into account the effect on the HCF strength of defects and the strong stress-strain gradients around them.

Introduction

New electric motors generations require high-performance electrical steel sheets. These materials, which present better magnetic properties, are suitable for several applications especially high speed electric motors. The improved magnetic properties of Fe-Si alloys are provided by reducing thickness, adjusting silicon content and increasing the grain size. Electric motors are made from stacks of thin sheets. Punching process is usually used to produce these components because it offer a good production rate and low cost. However, this process generates different effects influencing the durability of components. Several studies have highlighted the influence of these effects which are hardening, tensile residual stresses and defects [1‒۴]. In order to study the effect of punching process on the high cycle fatigue strength of electric motor components and to take into account these effects when designing pieces, an experimental and numerical procedure are curried out. Fatigue tests have been performed on different specimen’s configurations. Punched, polished and punched-annealed specimens are used to quantify the contribution of each effect induced by the process on the high cycle fatigue strength of the studied alloy. SEM observations reveal that fatigue crack initiation occurs on the specimen’s edge. In order to study the fatigue crack initiation mechanisms, SEM observations and 3D surface topography of the specimens have been performed. Results show damaged edges with many types of defects. A specific procedure is used to identify the critical defects [5-6].