Influence of shear cut holes on the fatigue performance of hot-rolled 800 MPa automotive steel grades

Experimental fatigue tests were carried out for two t = 3 mm 800 MPa hot-rolled (HR) high-strength steel grades. These steels have good formability, and it is possible to use shear cutting to enable high productivity. The test campaign included unnotched base metal specimens to evaluate the fatigue performance of HR surface and notched specimens with shear-punched and electrical discharge-machined circular holes to evaluate the effect of cutting process and geometrical shape on the fatigue strength capacity. In this context, the properties of the shear-cut affected zone (SAZ) and resulting high tensile residual stresses, and their effects on the fatigue performance, were evaluated. The fatigue test results were first evaluated based on the nominal stress approach, followed by the applications of local approaches. The mean fatigue strength of the specimens with shear cut holes in the nominal stress system was 125 MPa (with a slope parameter of m = 3, stress range at the net cross section), which is at the comparable level to laser cut holes. Fractography analysis, conducted with scanning electron microscopy (SEM), showed local crack-like defects at the SAZ, and the characterization of local defect feature was utilized in the fatigue strength assessment using a multiparametric notch stress method applying effective stresses obtained by the critical distance approach.