Research On Ultrasonic Detection And Evaluation Technique For Porosity Of Carbon Fiber Composites

Carbon fiber composites are widely used. With the development of material structure and process, traditional ultrasonic testing techniques meet lots of problems in composite non-destructive testing (NDT) area. As great limitation has appeared in many places, new ultrasonic testing techniques and theories are needed. Based on foundation project relative to porosity ultrasonic detection, several techniques were studied in this article, such as model of porosity in composite with variable thickness, identification of layered concentrated voids and local defects, application of backscattered signals in porosity detection of thin layered composites, defects morphology and ultrasonic testing techniques of braided composites.In chapter1, the background of the research work and significance of the dissertation were discussed. Moreover, main defect types of composites, porosity testing technology and digital signal processing technology were analyzed. Finally, main research contents and chapters arrangement were introduced.In chapter2, ultrasonic testing methods in porosity of carbon fiber composites were overviewed. Evolutionary process of the model, existing research achievement, characteristic and limitation of this method were also analyzed.In chapter3, based on the principle of ultrasonic transmission method, a model between porosity and the number of the laminate, ultrasonic attenuation was established. Through fitting calculation with the test data, porosity calculation formula was given. This model could be used to test porosity of different laminate for composites. On the basis, a model of the relationship between porosity and the thickness, ultrasonic attenuation was established. Later, discussion of the model revealed that, the model was suited to composite with big radius of curvature. Finally, according to the advantage and limitation, workpiece types and test situations that were not suitable for ultrasonic transmission method were analyzed.In chapter4, based on identification of layered concentrated voids, porosity and defect detecting techniques for thick layered composites were studied. Firstly, total reflection coefficient and transmission coefficient of ultrasound propagating through layered medium were established, the influence of between medium thickness and acoustic resonance was discussed, and the transformation law was analyzed between reflection coefficient and thickness. Secondly, the characteristics of structure noises in thick layered composites were studied, which revealed that, using structure noises the defects under incident plane could be tested, and it could also be used to identify voids and rich resin close to the surface of the composite. Then, defects characteristics far from the surface, and the influence between voids and echo frequency component were studied. Finally, an improved correlation coefficient method was used to identify the position of layered concentrated voids, and differential evolution algorithm could be use to restore the echo waveform.In chapter5, according to the problem that back wall echo missed when testing composites using ultrasonic pulse echo method, the feasibility of using backscattered signals to extract porosity information was studied. Then, components of backscattered signal were introduced, and it could be found that energy and mean value had great correlation with the attenuation of back wall echo. Finally, ultrasonic C scanning using back wall echo attenuation method, energy method and mean value method was made. The result showed that, backscattered signal could reflect the area where the back wall echo was very weak, and could therefore be used to evaluate the porosity when there was no back wall echo.In chapter6, defects characteristics and ultrasonic testing techniques of braided composites were studied. Firstly, Analysis of main defects and morphology characteristics was made, and the distribution features of voids were statistically studied. Secondly, ultrasonic testing technique was preliminary discussed, and the influence of resin, fiber and voids on the ultrasound attenuation was analyzed. Finally, ultrasonic testing experiment techniques for braided composites were studied. Then, waveform and defects characteristics were given through the time-frequency analysis on two different braided composites.In chapter7, main results and conclusions of this dissertation were systematically summarized; the prospects and the future research work were discussed and forecast.