Research On Characterization And Localization Of Structures With Crack Damage Based On Nonlinear Ultrasonic

Before the occurrence of fatigue fracture,metal components will experience the propagation of micro-cracks and evolve into macro cracks,so it is necessary to detect the cracks in time in order to avoid sudden failure and fracture.Traditional linear ultrasound is not sensitive to crack defects,but nonlinear ultrasound can effectively characterize the early crack damage of materials,so the interaction between ultrasound and crack damage in metal components is explored,thus,the nonlinear ultrasonic evaluation of crack damage degree is realized.This research work promotes the application of nonlinear ultrasonic technology in the damage detection of practical engineering structures.In this paper,the superiority of surface wave in the detection of crack structure was determined from the comparison of the three wave patterns firstly,and then optimized the excitation signal,signal calculation and processing and the selection of coupling agent of the nonlinear surface wave testing system.The 6005 A aluminum alloy commonly used in the field of rail transit was taken as the test object,6005 A specimens with different crack lengths were prefabricated by fatigue loading,and the relative nonlinear coefficients of each specimen were tested respectively.The results suggest that under the experimental conditions,the relative nonlinear coefficient increases at first and then decreases with the increase of crack length,which may be related to the closing and opening motion of the crack region,the starting point of the crack opening motion is about 26.4% of the limit length of the crack growth,and the starting point of the crack in the fully open state is about 58.9% of the limit length of the crack growth,the crack size corresponding to the relative nonlinear coefficient measured beyond this range is not unique.The cracks of different depths are prefabricated by mechanical wire cutting,and the test results indicate that the relative nonlinear coefficient increases with the increase of crack depth.The nonlinear ultrasonic testing process of crack damage defects of6005 A aluminum alloy was simulated by using ABAQUS.When the crack length is greater than 6mm,the simulation results of the relative nonlinear coefficient varying with the crack length are different from the experimental results,which may be due to the fact that the crack in the range of simulation length is in the crack closed state region or transition region,the crack area within this length range is not enough to weaken the propagation of the signal.The relationship between the relative nonlinear coefficient with the sensing path and the crack deflection angle were explored,the results suggest that the relative nonlinearity increases with the increase of the crack deflection angle,and the longer the crack is,the more sensitive the relative nonlinear coefficient is to the crack deflection angle,and increasing the offset distance will lead to the decrease of the relative nonlinear coefficient.Based on this law,the crack damage position in aluminum alloy plate was initially located by using relative nonlinear coefficient,and the crack length and deflection angle were predicted.The deviation between the predicted results and the actual results may be due to the difference in geometric structure between the central crack prefabricated by mechanical wire cutting and the actual crack,and the nonlinear effect caused by the crack is the contact nonlinear response,the difference in geometric structure may lead to the change of interface collision mode and lead to test deviation.