Reducing weight of chassis components is an important task that does not need any further motivation or background. It can be read in a large part of the technical papers in the field. A tempting approach to achieving lighter designs is to increase the material stress bearing capacity, allowing for higher in-service stresses and thus enabling material thickness reduction and shape modifications. However, when cut edges from manufacturing processes are present, or when formed radii are found in critical locations, this design approach could be associated with high risks. The main aim of this thesis is to provide a framework for quantitative and qualitative estimation of the effect of sheet metal forming on high cycle fatigue strength on specimen level. 

Increased steel grades often mean increased sensitivity for notches and surface properties, having implications both on formability and fatigue strength. Hence, the product developer might design for higher nominal in-service stresses, while selecting an alloy that is less suited to handle this increase. One solution is to increase the safety factors, decreasing the weight saving potential. Another alternative is to account for forming and post processing effect on fatigue life to find the most efficient solution. If sufficient and understandable estimation methods are lacking, the engineer has to rely on fatigue testing which is expensive.

In the synopsis an overview of metal fatigue in the context of sheet metal formed components is presented. Important aspects regarding fatigue modeling, common forming processes and post-forming operations are outlined along with a description of relevant numerical and experimental methods and considerations. Details of the conducted research are then presented in the appended papers, where the first introduces a simplified approach for numerical simulation of shear cutting to obtain residual stresses. The simplification mainly lies in the failure model calibration. The second paper studies the possibility of using the obtained residual stresses together with measured values of surface roughness to quantify fatigue life reduction of uniaxially loaded shear cut specimens. In the third paper the approach is extended to handle bending load cases and compressive residual stresses, while in the fourth paper the applicability to other forming processes and post-processes is studied. 

For shear cutting it is shown that a simplified failure model calibration is sufficient for estimating the cut edge characteristics to be used in fatigue life estimations. The life estimations can, under certain circumstances, be done for various load cases and load ratios using only residual stress results from finite element simulations, surface roughness measurements, uniaxial tensile test results and a base material S-N curve. The method could also be used to estimate the effect from stamping and shot peening on fatigue. It is suggested that engineers can use the proposed framework as a complementary tool to testing for assessment of the fatigue life implications of different alloys, grades, design choices and manufacturing processes. This could reduce the cost and time of product development and ultimately contribute to lighter and safer chassis designs.