Study On Damage Detection Of Materials Based On Nonlinear Ultrasonic

Composites have been widely used in engineering fields such as aerospace and automotive because of their high specific strength,high specific modulus and light weight.The nonlinear ultrasonic testing technology has a wide range of detection,and is sensitive to microcracks and micro-deformation generated in the early stage of materials service.In recent years,it has been the focus of research by scholars at home and abroad.Based on the nonlinear effect of ultrasound,the fatigue damage of aluminium plate and the tensile damage of composite laminates are studied by nonlinear ultrasonic non-destructive testing.Firstly,based on the theoretical knowledge of non-linear ultrasound,the nonlinear ultrasonic dynamic equation in solid medium is derived.The expression of the relative second-order ultrasonic nonlinear coefficient which can evaluate the damage of the material is obtained,and the propagation characteristics of the Lamb wave are explored.Secondly,the fatigue testing and nonlinear ultrasonic testing are carried out on the complete aluminum plates,the aluminum plates containing the hole and the aluminum plates repaired with the composite patches.The amplitudes of fundamental ultrasonic and the second harmonic of the three sets of specimens under different fatigue cycles are extracted.The relative second-order ultrasonic nonlinear coefficients are obtained,and the graph of the relative second-order ultrasonic nonlinear coefficient with the fatigue cycles is fitted to study the corresponding relationship between the relative second-order ultrasonic nonlinear coefficients and the fatigue damage process.Then,the principle and process of FFT and HHT transform are introduced.The time-frequency analysis of the same group of Lamb wave signals is performed,and the results of FFT and HHT transform are compared.Finally,the numerical simulation and experimental validation of tensile damage detection for intact composite laminates and composite laminates with holes are carried out respectively.It includes the construction of damage detection simulation system in ABAQUS software,such as the setting of unit type and unit size,the selection of time increment step and the application of load and boundary,etc.The numerical simulation and experiment of tensile damage detection for composite laminates have been completed.The amplitudes of fundamental wave and second harmonic wave are obtained by HHT transformation of the Lamb wave signals obtained from simulations and experiments.The relative second-order ultrasonic nonlinear coefficients under different tensile loads are calculated and fitted into curves to study the relationship between the relative second-order ultrasonic nonlinearity coefficients and tensile damage of composite laminates.