Research On Nonlinear Ultrasonic Testing Methods Of Porosity Of Composite Materials

Composite materials have been widely used in construction,automotive,aerospace and other industries due to their advantages such as good anti-friction and shock absorption properties,oxidation resistance,high temperature resistance,and low thermal expansion coefficient.Pore is one of the most common defects in composite materials and porosity has a significant impact on the properties of the material.Therefore,non-destructive testing methods related to porosity are receiving more and more attention.The ultrasonic detection technology has the advantages of high efficiency,strong penetrability,and harmless to the human body.Therefore,it is used for quantitative evaluation of porosity of composite materials.In order to solve the problem of low detection accuracy of current porosity ultrasonic detection,this thesis conducted research on the extraction of ultrasonic signal characteristic parameters and porosity characterization based on linear ultrasonic detection method and nonlinear ultrasonic detection method.Firstly,in order to explain the mechanical behavior of the pores in the material under the external force,based on the spring model,the response models of the pores under the action of linear ultrasonic and nonlinear ultrasonic were established.At the same time,the length of the pore is proportional to the amplitude of the second harmonic,and the width is inversely proportional to the amplitude of the second harmonic,which indicates the direction for the selection of the nonlinear ultrasonic detection method of porosity.Based on the theory of linear ultrasound and nonlinear ultrasound,it is pointed out that the ultrasonic attenuation coefficient and ultrasonic nonlinear coefficient can be used to characterize the porosity of composites.Secondly,a random pore model was constructed using random medium theory,and COMSOL software was used to perform linear ultrasound simulations on random pore models with different porosities,and the ultrasonic attenuation coefficient was calculated.At the same time,based on the linear ultrasound theory,the sample was tested by the pulse transmission method,and the ultrasonic attenuation coefficient was extracted by collecting and processing the received signals.Simulation and experimental results show that the attenuation coefficient can reflect the difference in the porosity of the material.Then,COMSOL software was used to perform nonlinear ultrasonic simulation on random pore models with different porosities,and nonlinear characteristic parameter was extracted.The variation trend between pore length,width,porosity and ultrasonic nonlinear coefficient were studied.The results show that the pore length and porosity are respectively proportional to the nonlinear coefficient,and the pore width is inversely proportional to the nonlinear coefficient.It is compared with the linear ultrasonic simulation results,and it can be seen that the accuracy of nonlinear ultrasonic detection of porosity is higher.Finally,nonlinear ultrasonic testing experiments were performed on samples with different porosities,and the received signals were processed using dynamic wavelet fingerprint technology.It is found that suitable wavelet basis functions,scale ranges and proportion of white slices are favorable for extracting nonlinear effects.The number of white pixels was extracted from the dynamic wavelet fingerprint image as a nonlinear feature parameter,and the porosity detection was successfully implemented.By analyzing the scanning electron microscope image of the sample,the porosity of the sample was measured and the parameters such as pore diameter and standard deviation were calculated.The analysis results show that the pore size and size distribution have important effects on the porosity detection error.