Study On The Damage Tolerance And Multiple Cracks Propagation Behavior Of Al-Li Alloy

Al-Li alloys are increasingly used in aerospace structures.At present,2060 and 2195 Al-Li alloys are emerging aerospace materials,but the understanding of their damage tolerance performance is not sufficient,and the interaction between cracks and its influence mechanism on crack propagation process and propagation life are not clear.For the above questions,this thesis firstly made a research on the damage tolerance properties of 2060 and 2195 Al-Li alloys,then analyzed the interactions between multiple cracks and their influence on the crack propagation process by the methods of experimental research and numerical analysis.On this basis,a method for predicting the life of multiple cracks propagation was presented.The main research contents are as follows:(1)A study on the damage tolerance of 2060 and 2195 Al-Li alloys.Through tensile tests,fracture toughness tests,fatigue crack growth rate tests and fracture analysis of 2060 and 2195 Al-Li alloys,it was found that their tensile fracture modes are plastic fracture and plastic and quasi-cleavage mixed fracture respectively,so the tensile property of 2060 Al-Li alloy is better than that of 2195 Al-Li alloy.To characterize the fracture toughness of 2060 and 2195 Al-Li alloys,the crack tip opening displacement(CTOD)was used.It turned out that the CTOD value of the L-T oriented sample of 2060 Al-Li alloy is greater than that of the TL oriented sample,and the CTOD value of 2060 Al-Li alloy is greater than that of 2195 Al-Li alloy.The fracture modes of the fracture toughness tests of these two alloys are plastic fracture and plastic fracture plus mixed fracture along the grain respectively.The fatigue crack growth rate of 2060 Al-Li alloy is less than that of 2195 Al-Li alloy.Based on the above comparative analysis,it was found that the the damage tolerance of 2060 Al-Li alloy is better than that of 2195 Al-Li alloy.Therefore,focusing on 2060 Al-Li alloy,this paper studied the interaction between cracks and fatigue crack propagation behavior.(2)Interaction analysis and mechanism research on multiple cracks of 2060 Al-Li alloy.Based on the extended finite element method,the accuracy of the single crack stress intensity factor calculated by the displacement extrapolation method,the equivalent integral region method and the interaction integral method were compared,and it was found that the interaction integral method has the highest accuracy.The accuracy of M-integral for calculating the mode I and mode II stress intensity factors at multi-crack tips was verified,and the interactions between collinear cracks,non-collinear parallel cracks and one oblique and one horizontal cracks were analyzed by this method.The mechanism of crack interaction was revealed from the perspective of stress field.At last,the interaction expression between collinear double cracks and non-collinear equal-length double cracks was fitted by the least square method.(3)Fatigue crack growth process and fatigue life prediction of 2060 Al-Li alloy.The interactions of single crack propagation process,collinear cracks and non-collinear parallel cracks propagation process were analyzed by fatigue crack propagation test and numerical method.It was found that the growth rate of collinear cracks is greater than that of single crack,and the growth rate of short crack of collinear unequal length cracks is less than that of long crack.The growth direction of non-collinear parallel cracks will deflect after the crack tips overlaps and the deflection directions of the two crack tips are opposite.On the basis of the Paris formula,the fatigue single crack propagation life was predicted by combining the C and m parameters of single crack and the interaction integral method.Compared with the experimental results,the error is 3.8%.By employing the C and m parameters of multiple cracks,the fatigue multiple cracks propagation life was predicted and reliable results were obtained.This thesis found the damage tolerance of 2060 Al-Li alloy is better than that of 2195 Al-Li alloy,so 2060 Al-Li alloy is more suitable for use in the key parts of structure.At the same time,this paper also made a research on the fatigue multicracks propagation behavior of 2060 Al-Li alloy,and a method for predicting multi-cracks propagation life was proposed,which has important scientific significance and application value for establishing and improving the current damage tolerance analysis method and evaluation system.