Investigation Of Crack-tip Opening Displacement Under Cyclic Loading

Crack tip opening displacement(CTOD),as one of the most viable parameters to characterize crack growth and fracture in ductile materials,has a significant role in integrity analysis of marine structures.Finite element method has been generally accepted by a large number of researchers to investigate CTOD.The main objective of this paper is a centercracked plate under cyclic loading.A relatively simple calculation model for CTOD is established with theoretical and finite element analyses,which provides a basis for fatigue and fracture evaluation.The major contents of this paper are summarized as follows.At first,the CTOD calculation model is established for constant amplitude cyclic loading.The maximum crack opening displacement(MCOD)is introduced in this paper.According to theoretical analysis,there are certain relationships between CTOD and MCOD,and the relationship is determined with finite element.And then effects of several factors,namely range of plate width,crack length,large-scale yielding,loading form(stress ratio and cyclic waveform)and material property(yield stress,elastic modulus and material hardening)are investigated.The calculation model is modified with the equivalent yield stress,which has a wide application.Subsequently,the CTOD calculation model is established for variable amplitude cyclic loading.Based on the results in constant amplitude cyclic loading scenario,the effect of loading sequence is explored with finite element analyses.According to the acceleration and retardation of fatigue crack growth,two loading scenario(high-low and low-high sequence loading)are applied and the calculation model of CTOD for variable amplitude loading is developed.At last,the CTOD calculation model is established for tensile-compressive loading.Based on the results in variable amplitude cyclic loading scenario,a quantitative study on the influence of compressive stress on crack-tip opening displacement is conducted,and a relationship between the variation of CTOD and compressive stress is obtained.And then series of simulation cases are performed to cover the effects of various parameters influencing the variation of CTOD,namely range of plate width,crack length,stress level(the maximum tensile and compressive stresses)and material property(yield stress,elastic modulus and material hardening).The established relationship is modified and the calculation model for CTOD is obtained.The results with the proposed method are also compared with those published.It is noticed that the calculation model is applicable to center-cracked finite plates of various materials,filling the research gap of CTOD estimation under tension-compression load at stress ratio R<0.