Effect of loading sequences on fatigue crack growth and crack closure in API X65 steel

Abstract

Abstract This paper presents a predictive model for fatigue crack growth under loading sequences. Several parameters had been quantified mainly under variable amplitude loading due to its peculiarities. The applicability of the crack closure model was studied to predict the fatigue life span resulting from the continuous effect of loading blocks. The API grade X65 steel, which is widely used in the crude oil and gas industries, was used in this study. Spectrum loading with a two-level block sequences, designated as high-to-low and low-to-high, were designed to correspond to variable amplitude loading. A stress ratio of 0.1 and 0.7 was retained at each block sequences. The effect of block transition from high-to-low caused a delay in crack growth due to the residual plastic wake left behind the crack tip. The crack growth under low-to-high loading blocks were faster than that under high-to-low loading blocks. A modified Walker was established and demonstrated to be capable of describing the features of crack arrest and crack growth acceleration under continuous loading sequences. The normalised root-mean-square error of the predictive model ranged from 0.05 to 0.22\u202fat both load ratios. The integrated crack closure into Walker can reasonably predict the influence of loading sequences on the fatigue crack growth in the API X65 steel.