Research Of Small Crack Growth Behavior And Fatigue Life Prediction In Al-Li Alloy

As the rapid development of Aeronautics and Astronautics,the demand for aircraft’s weight,performance and energy consumption is higher and higher and the demand for material is more rigorous.Developing high strength and light alloy is one of the most effective ways to low down aircraft’s weight and improve its performance.Al-Li alloy is a kind of Al alloy with low density,high strength to weight ratio and high stiffness to weight ratio,also it has good damage tolerance property,so it is one of the most important candidate material for Aeronautics and Astronautics structure.Small crack effect exists in many kinds of Al alloys,so if we do the fatigue life prediction using the long crack growth rule,the result can be dangerous.Therefore,studying Al-Li alloy’s small crack effect and fatigue life prediction will be of meaningful.This article studies Al-Li alloy’s small crack effect and its fatigue life prediction,the contents include three parts:(1)Small crack test.Study Al-Li alloy’s small crack effect and its growth rule through standard small crack test.The specimen used in this test is single edge notch specimen,this specimen can well obtain small surface crack and corner crack initiated from material’s flaws and can simulate the stress field near bolt hole which extensively exists in aircraft structures.The scanning method for small crack is replication,which is a standard method widely used in the world.The replication is simple to use,low cost and can accurately detect several microns long crack and the locations of several cracks,but it is troublesome and time consuming.Test results show that Al-Li alloy in this paper exists small crack effect under negative stress ratio.(2)Theoretical analysis.Small crack tends to initiate from inclusion particles,cavities,strong slip zone and weak interface and grains.The reasons for its growth rate higher than long crack includes crack closure,microstructure and plastic zone of crack tip.Using the modified method of fracture mechanics based on(35)K to correlate small crack and long crack is very attractive.According to research results of scholars like Newman and Wu,as long as we reasonably introduce the crack closure,the method of fracture mechanics used in the long crack growth analysis can extend to the small crack initiated naturally.Adopting this method can carry out the overall life prediction based on the small crack theory.This article adopts the method of Newman-Wu to predict Al-Li alloy’s fatigue life,the prediction results and test results are in good agreement,so the prediction method is feasible.(3)Simulation of cold expansion.Cold expansion is widely used to improve the fatigue life of parts with holes,so this paper studies the strengthening effect of cold expansion on Al-Li alloy.Using a mandril with certain magnitude of interference to expand the hole is called cold expansion process,which can induce an elastic-plastic deformation layer,also called residual compressive stress layer.The simulation of cold expansion is difficult to convergence,time consuming and needs much computational resources,because it involves nonlinearity of material and geometry and contact analysis.This article simulates the whole cold expansion process with the FEM software ABAQUS,including split-sleeve expansion and reaming.The results agree well with the test ones.Moreover,the simulation has highlighted the effect of expansion value and cuttings weight on the residual stress field.