High-Cycle Fatigue Properties of Automobile Cold-Rolled Steel Sheet with Stress Variation

Abstract

Since many components in the automotive body and chassis are produced by various manufacturing processes, have different chemical compositions, and are subjected to complex loading cycles, it is important to understand their loading mechanisms and susceptibility to damage. This research examined the mechanical properties of cold-rolled steel sheets and evaluated the effects of stress variations on fatigue behavior. Specifically, a series of load-controlled high-cycle fatigue tests were conducted by varying the stress levels of SPCC and SPRC340 sheet materials. The results showed that fatigue life and the fatigue limit increased with higher tensile and yield strengths. In addition, testing results indicated that the fatigue limit was higher than the monotonic yield strength due to cyclic hardening with plastic deformation during fatigue cycling. Regarding tensile properties upon pre-deformation, the yield strength increased with a higher amount of pre-deformation and was greater than the fatigue limit after deformation. Based on these experimental results, two types of fracture modes were observed under the applied stress range. General fatigue fracture mode, which denotes failure by crack initiation, propagation and final rupture at low stress amplitude, was observed with fatigue lives larger than 4 × 10⁵ cycles. On the other hand, constrained fracture mode occurred at stress levels higher than 0.89 times the tensile strength and exhibited a fracture surface without fatigue crack initiation or propagation.