Fatigue Reliability Analysis Of Cracked Structures Under Random Loadings

With the increasingly complex and harsh working environment of mechanical structures and the increasing influence of various random factors,fatigue reliability of cracked structure becomes more and more important in structural integrity analysis.To analyze the fatigue life and reliability,it is necessary to have a full understanding of the crack location and length in the components.Traditional fatigue crack analysis is mostly based on Paris' law or modified Paris' law,but this fatigue crack analysis method ignored the effect of multiple cracks location and boundary relationship in crack propagation.For components under complex working conditions,load variations also have shown vital influence on fatigue life.Therefore,according to the structural reliability theory,it is necessary to analyze the fatigue reliability of cracked structures from the perspective of probabilistic physics of failure.In order to solve this problem,this dissertation conducted the following research:(1)Analysis of crack location and boundary conditions.The main failure type of cracked structure is the surface crack failure for 30NiCrMoV12 steel.By analyzing different probabilistic models of surface crack and combining the randomness of initial position and length of crack,a new probabilistic model for judging surface crack is proposed,which provides a basis for judging the failure type of cracked structure.(2)Analysis of stress intensity factor and interaction of multiple cracks.For the flat plate model with two parallel cracks,the stress intensity factors at the crack tip are calculated by ANSYS simulation,and the interaction factors between neighboring cracks are fitted.The interaction factors are used to predict the stress intensity factors,which are verified by comparison with simulations.The accuracy and feasibility of the proposed interaction factors are presented.(3)Residual life prediction and reliability analysis of cracked structures.In order to consider the influence of random loads and the uncertainty of crack length on the residual life of cracked structures,the residual life and its distribution are calculated by different distributions of loads and crack length,and the fatigue reliability analysis is carried out.(4)Simulation and probability modeling of surface multi-crack propagation.By combining the tested surface cracks location and length of random cracks and using them as input conditions,the simulation of crack growth is carried out.Due to the interaction between cracks,the coalescence phenomenon of neighboring cracks may occur in the process of crack propagation,and considering the coalescence effect in the process of crack growth is more accurate with practical engineering.According to the theoretical basis of crack coalescence and the simulation results,combined with various uncertain factors in the crack coalescence phenomenon of cracked structures,the probability model of multiple surface cracks coalescence is established.