EFFECTS OF CHANGING STRESS AMPLITUDE ON THE RATE OF FATIGUE-CRACK PROPAGATION IN TWO ALUMINUM ALLOYS

Abstract

A series of fatigue tests with specimens subjected to constant-amplitude and two-step axial loads were conducted on 12-in. wide sheet specimens of 2024-T3 and 7075-T6 aluminum alloy to study the effects of a change in stress level on fatigue-crack propagation. Comparison of the results of the tests in which the specimens were tested at first a high and then a low stress level with those of the constant-stress-amplitude tests indicated that crack propagation was generally delayed after the transition to the lower stress level. In the tests in which the specimens were tested at first a low and then a high stress level, crack propagation continued at the expected rate after the change in stress levels. (Author)