Abstract
As a retardation of fatigue crack propagation occurs under two step loading such as high to low loading, it is difficult to estimate the fatigue life in variable loading conditions. Therefore, the mechanism of retardation was investigated by changing the ratio of two-step loading in aluminum alloys in this paper.
It is found in this study that acceleration of crack growth following low-to-high load change is not apparent in this investigation, there exists a uniquely-shaped fracture surface with some width (about 50-200 um) right angle to the crack growth direction at the load-change area under high-to-low load change. This area shows some swelling, with no striation but minute and distinct unevenness.
When R=0 and -1, if maximum load decreases to 80%, crack retardation does not occur even though the crack growth rate falls slightly. When R=0.1, by varying low load in small increments (50%, 45%, 43%, and 40%) the number of cycles of crack retardation were investigated. The varying rate is in proportion to the number of cycles of crack retardation.
This study proposed a crack growth model wherein the plastic zone formed by tensile stress was compressed by the surrounding elastic zone, and the crack does not open immediately under tensile stress load.
