Abstract
An interferometric strain/displacement gauge (ISDG) system with a laser diode was developed to dynamically measure the crack closure behavior of small surface cracks. The timing of the laser radiation was controlled by a computer and was synchronized with the fatigue load. Smooth low-carbon steel specimens were fatigued. The crack mouth opening displacement of small fatigue cracks was measured dynamically as well as statically with the ISDG system. The crack opening stress was determined using the unloading elastic compliance method. The crack opening stress obtained at dynamic loading was smaller than that obtained at stepwise loading. The relationship between the crack propagation rate and the effective stress intensity range obtained at dynamic loading agreed very well with that for long cracks.
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