Effect Of Residual Stress On Fatigue Crack Propagation Of 7N01 Aluminum Alloy Joints

Aluminum alloys with low density and high specific strength are widely used in industries such as ships,automobiles and trains.Fatigue failure is a major problem in the structure of aluminum alloys.The existence of residual stress affects the fatigue life of the aluminum alloy structure,so it is particularly important to study the effect of residual stress on the fatigue crack propagation behavior of aluminum alloy.As the sample is extracted from the structure,the residual stress will redistribute,and the residual stress also redistribute with the crack propagation.Therefore,how to accurately determine the re-distribution law of residual stress and the effect of redistribution on fatigue crack propagation behavior is very important.In this thesis,the residual stress redistribution patterns in the samples are discussed before and after the compact tensile specimen(CT specimen)are extracted from the welded 7N01 aluminum alloy sheet by finite element simulation and experiment.With the aid of the extended finite element method(XFEM),a crack propagation model for a threedimensional CT specimen with static cracks of various lengths is established to facilitate the equivalent replacement of a dynamic crack growth process,so that the residual stresses redistributed along with crack propagation can be analysised.The stress intensity factors generated at the crack tip are also calculated.The Walker formula is used to fit the experimental fatigue crack propagation data,and the variation law of fatigue crack growth rate with crack tip average stress intensity factor 8)and stress intensity factor amplitude ? is obtained.The fatigue expansion behavior of specimens with and without residual stress under external load is analyzed with Walker's surface.The results showed that when the specimens contained residual stress,the crack growth rate may increase or decrease with the increase of cracks.Before the stress intensity factor generated at the crack tip gets to zero,the residual stress significantly affects the rate of fatigue crack propagation.When the crack length is large,the factors affecting the fatigue expansion rate are mainly external loads.