Study On Nonlinear Ultrasonic Testing Method For Metal Early Plastic Damage

With the rapid development of industry,metal materials are more and more widely used in various industries,so the detection of early plastic damage of metal materials has become a hot issue.Many researches indicate that the ultrasonic nonlinear characteristic parameters are sensitive to the early degradation of mechanical properties and early damage of the material.In this paper,the finite element analysis model of electromagnetic ultrasonic nonlinear detection was established based on the Murnaghan model for the plastic damage aluminum alloy specimens with different thicknesses.The simulation and experimental research were carried out.The main research work is as follows:(1)Based on the strain energy function of the Murnaghan model,the relationship between the Murnaghan parameters and the degree of plastic damage was derived.By changing the Murnaghan parameters to study the nonlinear characteristics of ultrasonic waves,the simulation results show that with the increase of the Murnaghan parameter,the plastic damage degree of the test piece increases,and the amplitude of the second harmonic also increases.(2)Based on the Murnaghan model,an ultrasonic nonlinear finite element analysis model was established.The electromagnetic wave nonlinearity detection was carried out on the aluminum alloy specimens with a thickness of 10 mm using longitudinal waves.By analyzing the nonlinear effects of different constitutive relations,ultrasonic propagation distance and different excitation frequencies,the second harmonic amplitude exhibits a cumulative effect with the increase of ultrasonic propagation distance,and the nonlinear effect has nothing to do with the excitation frequency.(3)The nonlinear response of Lamb waves in aluminum alloy specimens with a thickness of 1 mm was simulated.The results show that as the degree of plastic damage increases,the amplitude of the second harmonic also increases.With the increase of the ultrasonic propagation distance,the second harmonic amplitude and the relative nonlinear coefficient show a cumulative effect;when the ultrasonic wave is propagated from the intact region to the plastic damage region,the second harmonic amplitude and the relative nonlinear coefficient will have a large jump,and this characteristic can be used to quantitatively detect the length of the plastic damage region.(4)An experimental platform of electromagnetic ultrasonic non-linear detection was built.The ultrasonic wave was used to detect the specimen in the thickness direction.The experimental results show that as the degree of plastic damage increases,the amplitude of the second harmonic increases.This characteristic can be used to qualitatively measure the degree of plastic damage of thick plates;Using Lamb wave to do the ultrasonic nonlinearity detect of the specimens with thickness of 1mm,the amplitude of second harmonic and relative nonlinear coefficient exhibits a cumulative effect with the ultrasonic propagation distance,which enables quantitative measurement of the length of the plastic damage zone.