Closure and growth behavior of short fatigue cracks at notches in 3% silicon iron.

Abstract

Sharply notched specimens of 3 % silicon iron were fatigued under several stress ranges, either above or below the notched fatigue limit for complete fracture. The growth behavior of short fatigue cracks near the notch root was analyzed based on the crack closure. A computer-aided high-resolution unloading elastic compliance method was employed to measure the closure of short fatigue cracks. The etching susceptibility of 3 % silicon iron was used to observe the plastic zones around notches and cracks. The growth rate of short fatigue cracks first decelerated with increasing crack length, and then accelerated or became non-propagating, depending on the applied stress range. Similar deceleration was observed when the growth rate was correlated with the stress intensity range. The effective stress intensity range was a unique parameter in correlating the growth rate of short fatigue cracks, and the relation was identical to that obtained for long cracks. Notch plasticity was not a direct cause for the deceleration. The highly strained zone size, measured by the etch pit technique, was useful for estimating the effective stress intensity range for short cracks.