Date of Award
Doctor of Philosophy
Mechanical and Aeronautical Engineering (to 2013)
Load excursions, in the form of tensile and compressive overloads are known to produce interaction effects during subsequent constant amplitude loading. In this work, the fatigue crack growth and crack closure levels, under both constant amplitude and variable amplitude loading, are investigated in the aluminium-lithium alloy 2090-T84, and the aluminium alloys 2024-T351 and 7150-T651. The crack closure measurements were obtained using both a crack tip opening displacement (CTOD) gauge and a crack mouth opening displacement (CMOD) gauge.
Fatigue crack growth tests were performed on specimens of thickness 1.6mm, 3mm, 6mm and 14mm. Tests were performed at load ratios of 0.1 and 0.7, and a multitude of maximum stress levels ranging from 8% to 33% of the material yield stress. Load excursions were applied at 1.75 times the maximum load. The amount of post-overload retardation cycles obtained after the application of a tensile overload, were found to be largest in the thinner material. Limited load interaction effects were found after the application of a compressive underload, however, the effect of a compressive underload applied directly after a tensile overload was to reduce the beneficial retardation effects produced by the single overload cycle. The measured crack opening stresses were in good agreement with the trends obtained for the fatigue crack growth results. Greater fatigue lives were obtained in the overload tests, with subsequent higher crack opening stresses measured. Following additional post-overload cycles, constant amplitude crack growth rates were restored. However, the crack opening stresses continued to "ratchet-up" after each subsequent tensile overload application.
Stoychev, Stoyan Ivanov, "Load Interaction Effects on Fatigue Crack Growth" (2005). Dissertations. 1063.