Discerning the Fatigue Crack Growth Behavior of API X65 Steels Under Sequence Loading

Abstract

This paper presents the effect of sequence loading on fatigue crack growth of API X65 steel tested in room temperature. Most of pipeline steels are subjected to wide spectrum of loading during the length of service. Influence of load history effect can cause a trickier assessment to the fatigue behaviour. The objective of this study is to determine the fatigue crack growth behaviour under sequence loading. The material used in this study is X65 steel grade, broadly used in transporting the oil and gas. The constant amplitude loading under mode-I loading and stress ratio of 0.1 and 0.7 are investigate. The effects of sequence load are presents by the two-level block loading. The retardation effect is more intense for block sequence loading. At least about 9.7x10 4 delayed numbers of cycles was observed which is two times longer compared to constant amplitude loading. The load sequence was suggested affected by retardation consequently delayed the crack growth. It was further observed an increase of overload load ratio consequently resulted a crack grows faster about 18%. It was concluded a lengthy in crack growth was associated with the delay cycles suggesting that the fatigue life depends mainly on size of delay zone and compressive residual stress near the crack tip.

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