Nonlinear Ultrasonic Evaluation Of Fatigue Damage Of Aluminum Alloy

6082 aluminum alloy has medium strength,good corrosion resistance and weldability which is widely used in structural engineering and rail transportation industry.In practical applications,engineering components are often subjected to cyclic stress or strain loading,resulting in the accumulation of microplastic deformation and the initiation of micro cracks,and eventually leading to fracture failure,which seriously threatens the safe operation of engineering components.Therefore,it is of theoretical significance and engineering application value to realize the detection and characterization of aluminum alloy fatigue damage.In this research,the fatigue damaged samples were acquired by fatigue tests based on6082 aluminum alloy for different cyclic loading,which can represent the simulation of various fatigue damage levels.The ultrasonic testing platform was set up using RAM-5000 high energy ultrasonic measurement system.The experimental parameters of ultrasonic nonlinear measurement were adjusted with the original sample.The ultrasonic velocity,attenuation coefficient and second-order ultrasonic nonlinearity parameters of the specimens with different fatigue damage degrees were measured,and the susceptibility to fatigue damage was analyzed.The results show that the change of ultrasonic velocity and attenuation coefficient is not obvious with the increase of fatigue life of 6082 aluminum alloy,the relative change of sound velocity is 1.36 %,the relative change of the attenuation coefficient of fatigue damaged specimen is 5.01 %,and the ultrasonic velocity and attenuation coefficient can not effectively characterize the fatigue damage degree of the 6082 aluminum alloy.The normalized nonlinear parameters showed a trend of increasing first and then decreasing with the increase of the fatigue life of 6082 aluminum alloy.Before the fatigue life of 68.7 %,the normalized ultrasonic nonlinear parameters increased significantly,and the increased variation was 43 %.The parameters of ultrasonic nonlinearity can be used to characterize the early fatigue damage of 6082 aluminum alloy.And the maximum value can be used as a reference value for the material fatigue damage to some extent.At the same time,XRD and TEM were used to calculate the dislocation density and observe and analyze the dislocation microstructures of 6082 aluminum alloy specimens with different degrees of fatigue damage.The dependence of the dislocation density and thechange of material damage and the nonlinear parameters was established.Before the fatigue life of 68.7 %,the change of the second order ultrasonic nonlinear parameters is positively correlated with the change of the dislocation density in the material,which indicates that the change of the dislocation density in the material is the main reason for the increase of the ultrasonic nonlinear response in the early fatigue damage of the 6082 aluminum alloy.In the analysis of TEM photos,the original sample is plane dislocation structure with few dislocation lines.With the increase of fatigue damage,dislocation density gradually increases,dislocation tangles begin to appear,and dislocation wall structures gradually form.When the nonlinear parameter reaches its maximum value,it becomes mature dislocation cell structure.The fatigue damage specimen was observed by SEM.It was found that the fatigue crack appeared in the later period of the damage.The attenuation of the finite amplitude acoustic wave in its propagation process increases,and the attenuation of the second harmonic is greater than that of the fundamental wave,resulting in the decrease of the nonlinear response of the ultrasonic.