Research On Laser Ultrasonic Detection Technology Of CFRP Sheet Defects Based On Optical Microphone

Carbon Fiber Reinforced Polymer(CFRP)is one of the most rapidly developed and widely used composite materials in recent years.It has the merits of high-specific strength,high-specific modulus,anti-fatigue performances and other excellent properties,which lead to the wide application of CFRP in the fields of aerospace,shipbuilding,automotive,medical and energy industries.Many defects are usually exhibited during the application and fabrication of CFRP sheets.In order to ensure the high quality and reliability of CFRP structural parts,it is necessary to detect the defect during the application and fabrication.The limitations of traditional testing methods can not meet the requirements for testing accuracy in engineering.Therefore,we need a reliable and effective non-destructive testing and evaluation technique to detect the defects in CFRP sheets.As an emerging non-destructive testing technology,laser ultrasonic testing technology can meet the requirements for CFRP sheet defect detection.The laser ultrasonic testing technology with optical microphone as the signal receiving device has non-contact,high sensitivity to ultrasonic signals,high detection accuracy,and other advantages.It is suitable for dangerous environment and has a broad range of applications.Therefore,the laser ultrasonic testing technology for CFRP sheet defects based on the optical microphone was deeply investigated.Firstly,the process of pulse laser acting on CFRP sheet to excite ultrasonic waves was theoretically analyzed.A thermo-mechanical coupling numerical simulation model of pulsed laser excitation of CFRP monolayer sheet was developed.The computer simulations of laser ultrasonic were carried out to analyze the propagation law of ultrasonic waves in CFRP monolayer sheet excited by pulsed laser and the influence of parameters and the thickness of CFRP internal defects on the propagation law and amplitude of ultrasonic waves.Secondly,a non-contact CFRP sheet defect laser ultrasonic testing system based on the optical microphone was developed.The hardware platform was constructed and the data acquisition and processing program based on LabVIEW software were developed.The hardware part and software part can coordinately work.The wavelet threshold method of the ultrasonic signal,noise reduction algorithm and defect recognition and imaging methods were studied.The stability of the system was verified by testing ultrasonic signals under different laser energies.Finally,the developed laser ultrasonic testing system was used to conduct laser ultrasonic A,B,and C scanning experiments on the simulated defects of different depths and sizes in the prepared CFRP samples.The test images which based on the minimum value,the sound path,and the time domain and the frequency domain of internal defects with different diameter-to-depth ratios were obtained.The C-scan images of upper and lower shallow surface defects and carbon fiber laying direction were obtained.Compared with infrared thermal imaging detection,it was verified that the laser ultrasonic testing technology was more suitable for detecting the deeper defects.The calculation method of internal defect thickness was established to realize the threedimensional imaging of CFRP flat-bottom hole defects.