Research On Voids Testing For Carbon Fiber Reinforced Plastics Based On Ultrasonic Backscattered Signal Processing

Carbon fiber reinforced plastics(CFRP)have been widely used in many fields such as aerospace,automobile,large engineering construction and so on.In the process of manufacture and use,defects will inevitably appeared in CFRP.The ultrasonic testing technology is one of the main nondestructive testing means for CFRP defects.With the progress of CFRP manufacturing technology,CFRP components containing thick section parts and variable-thickness curved parts have been used gradually,which make the traditional composite ultrasonic testing methods and signal processing technologies hard to meet the testing precision requirements.Aimed at the difficulties of ultrasonic testing for these materials,the ultrasonic testing technologies for void defects in thick section CFRP and variable-thickness curved CFRP were studied based on fund projects relative to ultrasonic testing for CFRP,and the the research work and achievements of full article are as follows.(1)All kinds of common ultrasonic testing methods for voids in CFRP were analyzed,the features of every ultrasonic testing method were obtained.Then the voids of CFRP studied in this paper were metallographically observed and analyzed,and the features of void shape and distribution were obtained.Based on the above analysis results,the ultrasonic backscattered signal processing method was put forward for void testing of thick section CFRP and variable-thickness curved CFRP.(2)The sound wave reflection and transmission in laminar viscous medium was deduced and the frequency model of sound wave reflection coefficient distribution function was established.The sound wave reflection coefficient of multilayered CFRP was calculated based on this model,and the ultrasonic resonance condition in layered CFRP as well as the influence of CFRP layer number on resonance was obtained.The sound wave reflection coefficient of layered CFRP within voids was calculated further,and the influence of void content and distribution on ultrasonic resonance was obtained.Meanwhile,the features of each component of ultrasonic pulse echo signal for layered CFRP testing were analyzed in detail,and the time domain model as well as the frequency domain model of ultrasonic testing signal was established based on this analysis.(3)The identification method for local concentrated void defects in thick section CFRP based on ultrasonic backscattered signal processing was put forward.The ultrasonic backscattered signal can be divided into near surface signal and far surface signal according to its features.For the processing of near surface signal,two methods were proposed.One was based on resonance structure noise features and the other was based on resonance structure noise removing.For the processing of far surface signal,the wavelet transform modulus maxima denoising method based on signal correlation analysis was proposed.The local concentrated voids in thick section CFRP were experimentally identified by the above methods,then the destructive metallographic experiment was implemented,which proved the feasibility of the above signal processing methods.(4)The identification method for the void defects in variable-thickness curved CFRP based on lifting wavelet decomposition of ultrasonic backscattered signal was put forward.The porosities of CFRP specimens which had been tested by ultrasound were measured by metallographic experiment,and the features of ultrasonic pulse echo signals were analyzed.Then the ultrasonic backscattered signals were decomposed by lifting wavelet transform,and the features of both original signals and decomposed signals were analyzed.The relationships between the porosity and the features of both original signals and elect decomposition signals were analyzed respectively further.The results showed that the material porosity representational capacity of optimal decomposition signal features was better than that of the original signal features.(5)The ultrasonic C-scan imaging method for thick section CFRP based on the features of backscattered signal energy as well as the ultrasonic C-scan imaging method for variable-thickness curved CFRP based on the features of backscattered decomposition signal energy was put forward.Meanwhile,the three-dimensional imaging technology for ultrasonic backscattered signal features was put forward based on the research results of No.3,and the generated three-dimensional images can intuitionally show the features of the local concentrated voids in thick section CFRP.