Investigation of surface roughness, hardness, and stiffness degradation of aluminum sheets subjected to flexural fatigue

Abstract

In this study, the correlations between the bending fatigue lives and various surface properties of commercial pure aluminum sheets damaged under different stress levels and cycles were assessed. Particularly, surface roughness, surface Brinell hardness, and stiffness on several top surface regions of the fractured AA1100-H14 aluminum sheets, damaged at different fatigue stages, were examined with straightforward test methods. These tests also revealed contrasting surface properties for the as-such and fatigue-damaged specimens. Specifically, for the fatigued specimens, which were cut along the rolling (RD) and long-transverse (LT) directions, the measured surface roughnesses were 4.53 and 4.92 times higher, the Brinell hardnesses were 10.0 and 20.1% lower, and the stiffnesses were 38.8 and 40.8% lower than the corresponding values obtained for the as-such specimens. The minimum degradation corresponded to the Stage III (HCF) of the stress amplitude versus number of cycles curve (S-N). An increased degradation was observed in Stages I and II (LCF) under high stress amplitude and low cycle fatigue. The degradation rates of surface roughness, surface hardness, and stiffness (elasticity) can be considered as the fatigue damage parameters for aluminum operating under variable loads.