High-cycle fatigue characteristics of squeezed cast aluminum alloy smooth specimens cut from car wheels

摘要

High-cycle fatigue tests of smooth specimens cut from squeeze cast Ai alloy car wheels were carried out to clarify the fatigue behaviour under constant stress amplitudes. In addition to this, fatigue tests under two-step stressing were performed to study the effect of stress change on fatigue damage. Fatigue cracks were initiated from eutectic Si particles and sometimes from a slip band in the matrix. But, no cracks were initiated from microscopic defects like pin-holes and shrinkage porosities. The In/ vs. N/Nf relation (l: crack length, N_f: number of cycles to failure) under constant stress amplitudes exhibited stress dependency, namely the crack length initiated at a given value of N/N_f was larger in higher stresses. With regard to the fatigue damage under two-step stressing, the cumulative cycle ratio, ?(N/N_f), for low-to-high block stressing was smaller than 1, however it was larger than 1 for high-to-low block stressing. The value of ?(N/N_f)) was closely related to the growth characteristics of a small crack.