CRACK CLOSURE AND EFFECT OF LOAD VARIATION ON SMALL FATIGUE-CRACK GROWTH-BEHAVIOR

Constant amplitude, plus high and low and repeated two-step loading fa tigue tests were conducted on a structural steel and a titanium alloy. Detection and measurement of very small crack sizes were possible by using a specially designed small-sized test specimen. By improving the unloading elastic compliance technique, the crack closure behaviour w as continuously monitored without interruption of the test. It was fou nd that the growth process of a small crack is a transition process fr om the incipient, ideal crack to the conventional long fatigue crack. Fatigue cracks longer than 0.2 mm were shown to have a residual tensil e deformed region in the wake of the crack tip and exhibited retardati on after load reductions due to differences in closure levels. However , for cracks smaller than 0.1 mm, closure is insufficient to control t he crack growth behaviour, and the high level loading may give rise to an acceleration of crack growth.