FATIGUE CRACK-PROPAGATION IN THE THRESHOLD REGIME AFTER RAPID LOAD REDUCTION

In this work, the influence of rapid load reduction on fatigue crack growth in the threshold regime of the aluminium alloy 2024-T3 has been studied. It can be shown that fatigue crack growth may be severely influenced by crack closure due to oxide formation and fracture surface roughness. After rapid load reduction, crack arrest could be observed at K(max) values 10-100% above the constant amplitude threshold, depending on the environment. With measurements in different environments (humid air and vacuum), the oxide-induced crack closure effect could be recognized as being mainly responsible for an increase of the stress intensity threshold. Using high-frequency fatigue testing equipment, it was possible to show that after rapid load reduction in a vacuum, cracks may begin to grow again after crack arrest of more than 5 x 10(7) cycles.