Study On Local Residual Stress At Impact Pit Defect On Surface Of 2A12 Aluminum Alloy

2A12 aluminum alloy is widely used in aviation field,Many kinds of defects may appear in its components during service life of aircraft,and the impact pit is the typical one of them.The defect will cause local stress concentration,thus is one of the main fatigue crack sources.In fatigue analysis,the stress concentration at defect,e.g.impact pit,is concerned.However,high level local residual stress and plastic strain usually exist there and their influences on fatigue life still have not well understood.Therefore,deep studies on the local residual stress and its relaxation under cycle load at impact pit defect on 2A12 aluminum alloy have important theoretical and engineering application values for the fatigue life assessment of aircraft.The main works are as follows:(1)In order to study the local residual stress distribution at impact pit defect on 2A12 aluminum alloy,the impact process is simulated through FEM.Meanwhile impact pits are made by dropweight test device with various impact energy.The finite element model is verified by comparing the numerical and experimental impact pit depths under the same impact energy.(2)The applicability of the un-equiaxial residual stress estimation by some common used theoretic models which based on indentation method is studied,and a modified model is proposed.Micro indentation tests of equiaxial and un-equiaxial residual stress of 2A12 aluminum alloy are simulated using FEM,then the errors of three models,i.e.Suresh model,Yun-Hee Lee model and energy model,are calculated.A correction factor is introduced in Yun-Hee Lee model to improve the accuracy of un-equiaxial residual stress estimation.Residual stresses at bottom and edge of impact pits of 2A12 aluminum alloy are measured by indentation method,and the results are used to validate FEM simulations.(3)The relaxation of residual stress at impact pit of aluminum alloy 2A12 under cyclic load is studied numerically.The influences of the amplitude,the mean and the ratio of the cyclic load are considered.The simulation reveals that residual stress relaxation mainly occurs during the first period of cyclic load.Residual stress decreases exponentially with the increase of amplitude of applied cycle load.The relaxations of residual stresses at bottom and edge of pit increase with the mean and absolute values of cycle load,respectively.(4)Low cycle fatigue load is applied to specimen of 2A12 aluminum alloy containing impact pit according to LCFTS(Low Cycle Fatigue Test Standard).The dependences of residual stress reductions at bottom and edge at impact pit on cycle load are measured by micro indentation method.It also derives the relationship between residual stress relaxation and cyclic stress amplitude based on Morrow Model.The experimental residual stress relaxation well agrees with FEM prediction.