The Research On Fatigue Crack Growth Process Of Aluminum Alloy Material Based On Elastic-Plastic Mechanics

Fatigue crack growth rate,as one of the fracture mechanics properties,is of great significance to accurately evaluate the remaining life of the component.At present,the Paris formula is the most widely used fatigue crack growth rate model in engineering practice,but it can not describe the stress ratio effect and overload retardation phenomenon.The essence is that the Paris formula is based on the assumption that the material is linearly elastic.In fact,the local plastic zone caused by the stress concentration at the crack tip has a more complicated influence on the crack propagation.In order to explore the mechanism of material crack propagation more deeply and to evaluate the damage tolerance of structures more accurately,the study of fatigue crack growth process based on elastic-plastic mechanics is an inevitable trend of development.In this paper the experimental study theoretical analysis and computer simulation techniques are used to study the fatigue crack growth process of aluminum alloy based on elastic-plastic mechanics.The main contents include:(1)The study on simulation model of aluminum alloy elastic-plastic mechanics.The DIC static tensile test was designed to obtain the strain distribution data of the crack tip of the aluminum alloy specimen under different loads.By comparing with the finite element simulation results,the reasonable elastic-plastic mechanical parameters of the aluminum alloy material were determined.It is more reasonable for 7N01 aluminum to use 280Mpa as a plastic yield point than that of 309Mpa,and the correctness of the elastoplastic finite element model is verified.(2)DIC constant amplitude crack growth test and fatigue crack growth test under different overload were designed.The elastic-plastic finite element model under different crack length was established,and the simulation of crack growth process under constant amplitude loading was realized.The method of inserting cohesive units and binding nodes between cracks is proposed,and the simulation of crack propagation process before and after overload is realized.(3)Based on the elastic-plastic incremental plasticity theory,combined with experimental and computer simulation methods,a model of crack propagation rate based on elasto-plastic mechanics is established.Compared with the crack growth rate curve based on linear elasticity,the crack growth established by incremental plastic Jep integral The rate curve well characterizes the fracture mechanics properties of the material,verifying the rationality of the incremental ?Jep integral as a driving force parameter for crack propagation under constant amplitude.(4)Comparing with the conventional simulation method and the crack growth rate curve obtained by the simulation method of the crack propagation simulation before and after the overload presented in this paper.It is verified that ?Jact is reasonable as the driving force of the crack propagation after the overload,a theoretical model based on ?Jact is given.Based on the experimental data of 7N01 aluminum alloy a simple normalization model and overload delay model are proposed.The influence of stress ratio and overload ratio on crack propagation rate is considered.It is suitable for engineering fatigue crack propagation life estimation.