Fatigue Behavior of Smooth Specimen Cut from Squeeze Cast Al Alloy Car wheel and Effect of Stress Change

摘要

Fatigue tests of smooth specimens cut from squeeze cast Al alloy car wheels were carried out to clarify the fatigue behavior under the constant stress amplitude. In addition to this, fatigue tests under the two-step loading were performed to study the effect of stress change on fatigue damage. To minimize the scatter in fatigue tests due to the fluctuation of laboratory atmosphere, all the tests were made in controlled air (t = 30 deg C, h = 60 percent). Fatigue cracks were initiated principally from the eutectic Si particles and sometimes from a slip band in the matrix. But, no initiation from microscopic defects such as pin-holes and shrinkage porosities was observed. The lnl vs. N/N_f relation under constant stress amplitude exhibited stress dependency, namely the crack length initiated at same N/N_f was larger in higher stress. With regard to the fatigue damage under the two-step loading, the cumulative cycle ratio sum (N/N_f) for the low to high block loading was smaller than unity, however it was larger than unity for the high to low block loading. The value of sum (N/N_f) was closely related to the growth characteristics of a small crack.