Environmental and frequency effects on fatigue crack growth rate and paths in aluminium alloy

The environmental and frequency effects on fatigue crack growth in aluminium alloys are studied theoretically and experimentally. 2024-T351 and 7075-T651 tested in corrosive environments ( humid air or technically purified nitrogen) show a constant crack growth rate (da /dN) at low values of the effective stress intensity range (DeltaK(eff)). Typical well-known fits of this curve (da /dN vs DeltaK(eff)) do not reflect the plateau-like region. A new model of crack growth is presented, which physically attributes this region to the formation and subsequent fracture of a crack tip oxide layer. The thickness of this layer is measured with X-ray photon electron spectroscopy. At higher loads, other mechanisms are understood to be active. The model parameters are determined from constant amplitude tests, and are valid for a given material and environment. In 7075-T651 tested in nitrogen, with R = 0.1 and 83 Hz, unexpected macroscopical crack branching is observed when DeltaK(eff) reaches approximately 3.0 MPa rootm.