An Investigation On Crack Propagation Based On The Effect Of Crack Plastic Zone

One of the main failure modes of the marine structures is fatigue and fracture.Due to the effect of the crack tip plastic zone,crack closure occurs during crack propagation and the crack opening stress is closely correlated with fatigue crack propagation rate.Therefore,the crack opening stress assessment is an important issue in security assessment involved in crack components.Up to now,most numerical analysis of crack opening stress is associated with the calculations of crack tip field,but relevant fracture parameters in crack tip field could not be conveniently obtained in practice.Analytical models generally adopted in theoretical study are usually based on relatively simplified assumptions.Besides,most empirical equations above are limited to the experimental range of material types and stress ratios.The crack opening stress assessment is applied relatively less in engineering practice.In this work,through theoretical considerations,elastic-plastic finite element analysis and experimental verifications,the maximum crack opening displacement is introduced to investigate the crack propagation behavior considering the effect of crack tip plastic zone.Theoretical considerations are given to the relationship between the crack opening stress and the variable quantity of maximum crack opening displacement.Based on elastic-plastic finite element analysis of center cracked finite plate,and accounting for the effects of crack geometry size,stress ratio,Young's modulus,yield stress and strain hardening,the explicit expression of relationship between crack opening stress and the variable quantity of maximum crack opening displacement is presented.Further studies show this relationship could be applied to predict crack growth rates,which correlate well with experimental results.Afterwards,accounting for the effects of stress ratio,overload ratio and material properties,the relationship between crack opening stress and the variable quantity of maximum crack opening displacement of a center-cracked specimen is verified under tensile overloading.In addition,the effect of distance between overloads on crack propagation is investigated.The results show that the maximum retardation occurs when the distance between overloads is 0.233 of first overload plastic zone size,which can provide reference for getting the maximum retardation in practice.Finally,the effect of compressive loads on crack opening stress is investigated.Based on elastic-plastic finite element analysis of center cracked finite plate,and accounting for the effects of crack geometry size,Young's modulus,yield stress and strain hardening,the explicit expression of crack opening stress versus maximum crack opening displacement is presented.And the experimental results show that crack propagation rates(da/dN–?K)curves at different stress ratio are condensed to a single curve.This work presents a crack propagation model which describes crack closure based on the variable quantity of maximum crack opening displacement.And this work contributes to the assessment of fatigue life of crack components in engineering practice.