Analysis And Prediction Of Fatigue Life Of Fiber Reinforced Al-Li Alloy Laminate Under Variable Amplitude Loading

Fiber-reinforced metal laminates(FRMLs),also known as fiber metal laminates(FMLs),are sandwich-type composites in which metal layers and fiber-reinforced resin composite layers are alternately laminated,and have excellent fatigue and damage tolerance performance.So far,for FMLs,a great deal of research has been done on the material properties of ARALL and GLARE laminates,while the research on the material properties of other new FMLs is relatively less.Meanwhile,these research mainly focused on the tensile behavior and the fatigue crack propagation behavior under constant amplitude(CA)and variable amplitude(VA)loading;the research on fatigue crack initiation life is less,and the research on the total fatigue life of laminates is even less.Through the systematic research on the fatigue performance and life prediction method under typical VA loading for a new FMLs named glass-reinforced 2060 aluminum-lithium alloy laminate,the present thesis aims at providing theoretical basis and analytical method for the structure life design and life prediction of FMLs.In this thesis,through fatigue S-N curve test of the new type of FMLs and its component metal materials under CA and typical overload(periodic single overload,periodic single underload and periodic high and low overload)loading,the relationship of fatigue S-N curves between the laminates and its component metal materials is systemically studied under the same overload condition,and the effect of overload form on the fatigue performance of different materials under CA loading is analyzed.According to the characteristics of the S-N curves,under each typical overload loading,the two fatigue life prediction models of the new FMLs are proposed based on different theories.Through fatigue S-N curve test of the new type of FMLs under Mini-Twist spectrum loading,the life prediction method for the laminate materials is studied under complicated spectrum loading,and the effects of mean stress modification method and damage failure criterion on the fatigue life prediction are discussed.(1)In order to study the fatigue characteristics of the new FMLs under different loading conditions and predict the fatigue life of laminates under VA loading,a reasonable fatigue test program is designed.By carrying out the fatigue S-N curve test of the new FMLs and its component metal materials under CA,periodic single overload,periodic single underload,periodic high and low overload and spectrum loading,a large amount of fatigue life test data was obtained.(2)In order to determine the metal layer stress of FMLs,an on-line measurement method of the metal layer strain for FMLs,the digital image correlation(DIC)technology,is investigated.In order to verify the effectiveness of DIC method,taking the new type of FMLs studied in this paper as an example,DIC method and finite element method were respectively used to solve the metal layer stress of laminates.The result shows that the metal layer stress calculated by DIC method agrees well with the finite element solutions.Therefore,this paper separates oneself from the traditional strain measurement method and uses DIC method to successfully achieve online measurement of metal layer strain.(3)The analytical method for stress prediction of metal layers in fiber metal laminates is studied.The insufficiency of the traditional classical theory to solve the metal layer stress of laminates is analyzed.To solve this problem,the concept of equivalent performance for laminates is introduced to modify the laminate theory in this paper.That is,the sub-layer stiffness method and the energy method are used to modify the laminate theory.By comparing with the results of classical laminates theory and DIC,the progressiveness of modified method is verified.It provides preconditions for the performance analysis and accurate prediction of FMLs.(4)Through the analysis of fatigue life and S-N curves for new FMLs and its component metal materials under CA R=0.06 and typical overload loading,the study found that the S-N curve characteristics of laminates are affected by bridging effects and metal layer stress under typical overload loading,and the more the number of layers or the lower the applied stress,the more obvious the bridging effect;2/1 and 3/2 laminates occur the overload delay effect under single overload and high-low overload loading,and 2/1 laminates occur the damage effect of high-low loading under high and low overload loading,while the loading effect of 2/1 and 3/2 laminates is not obvious under single underload loading.Based on the characteristics of the S-N curves,the two fatigue life prediction models of new FMLs are proposed under each typical overload loading.(5)The effects of mean stress modification method and damage failure criterion on the fatigue life prediction of new FMLs are studied under spectrum loadings.In addition,taking into account the characteristics of irregular load spectrum,based on the Hashin hypothesis,Yao and Himmel model is modified by combining with Miner damage theory.The results show that for mean stress modification model,the prediction accuracy of prediction method based on piecewise linear model is best;for damage failure criterion,the prediction accuracy of prediction method based on modified model is best.