Fatigue crack growth of alumiunium 7075 alloy under multiple periodic overloads

Abstract

Most structural components are subjected to variable amplitude loading rather than constant amplitude loading during their service life. Variable amplitude loading can be simply described as constant amplitude loading with occasional high peak loads in between. Some examples of this type of loading acting on structural components include an aircraft flying in turbulent conditions or ships sailing through storms.

In the current experiment, ASTM standard middle tension (MT) specimens were used to experimentally study the effects of overload ratio in periodic multiple overloads on fatigue crack growth behaviour of 7075-T651 aluminium alloy in ambient air. Tensile tests were first conducted to determine the yield stress of the material and a certain percentage of the yield stress were used for the baseline and overload stress. A number of specimens were used to ensure the accuracy of the results.

Significant overload ratio effect was identified. An overload ratio of 1.25 and 1.5 yielded crack growth acceleration. Increasing overload ratio shows an increasing crack growth rate. In addition, the surface of the specimens was viewed under the scanning electron microscope (SEM) to observe the fracture surfaces of the specimens. Marker bands may not be visible under the SEM at overload ratio below 1.25.