Fatigue crack growth and crack closure of structural materials under random loadings

ABSTRACT Fatigue crack growth tests were carried out under various programmed and pseudo-random loadings on a carbon steel, a high tensile strength steel and an alminum alloy, over a wide range of growth rate regime and crack closure behavior was investigated. Crack length and closure were measured by a minicomputer aided unloading elastic compliance method. The crack opening point was unchanged during one block of programmed load sequence and also during a period of pseudo-random loading. The crack opening stress intensity Kop was found to be controlled by the maximum stress intensity range-pair and its stress ratio, and furthermore, could be predicted from constant amplitude crack opening data. Cyclic counting methods were compared and the effective stress intensity range-pair counting was found pertinent to assessment of crack growth under irregular load histories. A prediction procedure was proposed for fatigue crack growth rates under stationary variable loadings.