Crack closure and effect of load variation on small fatigue crack growth behaviour

Constant amplitude, plus high and low and repeated two-step loading fatigue 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 was continuously monitored without interruption of the test. It was found that the growth process of a small crack is a transition process from the incipient, ideal crack to the conventional long fatigue crack. Fatigue cracks longer than 0.2 mm were shown to have a residual tensile deformed region in the wake of the crack tip and exhibited retardation after load reductions due to differences in closure levels. However, for cracks smaller than 0.1 mm, closure is insufficient to control the crack growth behaviour, and the high level loading may give rise to an acceleration of crack growth.