Propagation Threshold and Crack Closure of Small Fatigue Cracks

Abstract

The propagation behavior of fatigue crack from pre-cracks in smooth or notched specimens of a low-carbon steel was studied. The development of crack closure with crack growth was measured through the compliance method. An interferometric displacement gage was used to measure the opening displacement of surface cracks. All cracks started to grow from the pre-cracks at the maximum stress intonsity factor equal to the threshold value of the effective stress intensity range ΔK_effth∞ for long cracks. As a crack grew, the crack closure developed and the effective stress intensity range ΔK_eff decreased. The crack stopped growing when ΔK_eff dropped below ΔK_effth∞. The relation between the crack propagation rate and the effective stress intensity range was unique, irrespective of the crack length or the atress level, and agreed well with that for leng cracks. Non-propagating cracks made under the fatigue limit in smooth specimen were mostly Stage I cracks, and the fatigue limit of smooth specimen was determined by the propagation threshold of these Stage I cracks.