| Because of the huge economic losses and disasters caused by the pitting problem ofstainless steel,the detection of pitting damage has attracted widespread attention.Previous studies have shown that the nonlinear surface wave can better characterize the pitting damage evolution of stainless steel.Further studies have found that pit morphology has a complex effect on the propagation of nonlinear surface waves,but the specimens of pitting corrosion is difficult to prepare during the experimental process.Based on this,this paper uses ABAQUS finite element software to simulate the propagation behavior of nonlinear surface waves in pitting damage models of different shapes and sizes,and compared with experiment of the nonlinearity of ultrasonic surface waves.The conclusions are as follows:1.Based on the finite element software ABAQUS,a nonlinear surface wave detection model was established and related parameter settings for the model were discussed,including meshing requirements,boundary conditions,analysis step definitions,and load.It was found that:(1)The wave field of surfaces contains surface wave R,shear wave S,longitudinal wave P and head wave,and the wave energy of the surface wave accounts for the total energy at most 91%;(2)The nonlinear effect of surface wave is closely related to the applied load,which makes the model show three stages: elasticity,plasticity and necking.The plasticity stage is sensitive to the nonlinearity of the material.Finally the load of the pitting damage model is determined.(3)Further comparing the spectrum of P-wave and R-wave,the response of the P-wave to the nonlinearity in the model is very small.2.Non-linear surface wave detection was performed on 304 stainless steel specimens soaked in Fe Cl3 solution for 2 h to 24 h.The size and morphology of the pits were measured by laser confocal scanning microscopy,and substituted into the model for simulation.The results show that the normalized nonlinear coefficients ?/?0 obtained by experiments and simulations are monotonous with the increase of corrosion time.Therefore,it is feasible to use analog method to detect pitting corrosion of stainless steel.3.Based on the nonlinear surface wave detection model,the effects of topographic parameters for the etch pit,such as depth H,width W and shape,and concentrated stress Rr on the propagation behavior of nonlinear surface waves are simulated.The results show that:(1)Compared with the results of elliptical,rectangular and trapezoidal pitting defects,it is found that the nonlinear coefficients of elliptical and rectangular pitting defects are similar,while the fundamental amplitude of trapezoidal flaws is much smaller than others the amplitudes of the fundamental waves of the first two;the amplitude and the nonlinear coefficient of harmonics increases significantly,which is related to the discontinuous interface area of the defect.(2)H< 0.2?,the nonlinear effect is not significant;H> 0.2?,the nonlinear coefficient increases with the depth of the defect.The analysis considers that it is related to the filtering effect of the defect;(3)The nonlinear coefficient ? decreases exponentially monotonously with the increase of the defect width W,which is related to the scattering and filtering;(4)Nonlinear coefficient ? increases first and then decreases with the increase of the trapezoidal pitted angle ?,which is related to the constraint effect of the left boundary of the defect on the surface wave at a specific frequency;(5)The second harmonic amplitude and the nonlinear coefficient monotonously increase with the increase of the concentrated stress on the surface.The rising trend is related to the strain stress-strain nonlinearity at the surface. |
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