Aluminium alloy sheet coupons with a central hole were tested in fatigue under specially designed programmed load sequences. Electron microscopy of the fatigue fracture surfaces showed striation mode fatigue crack extension up to more than a millimeter before onset of rapid failure. Crack opening stress was determined from striation patterns. Under variable amplitude loading and conditions of notch root cyclic inelasticity, crack opening stress exhibits noticeable hysteresis. The hysteresis is attributed to a difference between crack closure and opening stresses. A linear model of this phenomenon appears to describe observed behaviour.
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R. Sunder (1993) studied this question.
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