Localization And Characterization Of Structural Damage Using The Non-Collinear Wave Mixing

In recent years,the development of the chemical,energy and aerospace industries has placed new requirements on service equipment.The equipment is in service under extreme conditions.Under the influence of high temperature,high pressure and heavy load,the material of the component gradually evolves to produce plastic zones and micro-cracks.In order to avoid sudden breaks of the structure caused by evolution of micro-crack and formation of macro-cracks,it is urgent to develop new non-destructive testing methods that can detect micro-cracks and the plasticity around the crack tips.In this thesis,plastic zone around the crack tips in damaged structural material was localized and characterized using non-collinear ultrasonic wave mixing,in which the method proposed could be used to track the crack propagation.The specific research contents are as follows:1.Using the finite element simulation method,the mode of the sum frequency and longitudinal wave produced by the two shear waves was selected to study the resonance conditions of nonlinear ultrasonic mixing based on the perturbation approach and vector analysis.By setting models with different parameters,the relationship of the size of the non-linear region on the specimens and the non-linear ultrasonic mixing was studied.The simulation results shown that the non-linear ultrasonic mixing effect has a positive correlation with the non-linear region size.2.Taking A17075-T6 material as the test object,it is experimentally proved that the longitudinal waves generated by the two shear waves could be used to locate the plastic zone around the crack tips.In the experiments,Two Ultrasonic shear waves were excited via plexiglass wedge with a 20-cycles Hanning-windowed sinusoidal tone burst at frequency of 5MHz-5MHz and 4.75MHz-5.25MHz,respectively.The nonlinear ultrasonic mixing parameters were used to characterize the plastic zone around the crack tips.The experimental results shown that the peak area of the nonlinear ultrasonic parameters generated by the two frequency pairs fits well with the actual location of plastic zone in the material.It can be seen that the nonlinear ultrasonic mixing method could locate the plastic zone around the crack tips more effectively.And the frequency of 5MHz-5MHz was better than 4.75MHz-5.25MHz.3.Taking A17075-T6 material as the test object,it has been experimentally proved that the longitudinal waves generated by the two shear waves could be used to track the evolution of the plastic zone around the crack tips.Specimens with plastic zone of different sizes were made.And two shear waves with a center frequency of 5MHz-5MHz were excited to locate and characterize the location and size of the plastic zone around the crack tips.The experimental results shown that the nonlinear ultrasonic mixing method could track the size and location of the plastic zone around the crack tips effectively.