PSFBG-ultrasonic Damage Detection Of Matrix Cracks In Carbon Fiber Reinforced Composites

At present,Carbon fiber-reinforced plastics(CFRPs)are playing an increasingly important role in aerospace,automotive and marine applications due to their high strength-to-weight ratio.However,commonly used CFRP laminates are prone to have complex damages during long-term use.Of these,matrix cracks are always the earliest appearing damages.The occurrence and subsequent expansion of matrix cracks will reduce the strength of the structure and increase the risk of potential damage.Therefore,a non-destructive testing(NDT)technique is important to accurately and sensitively detect and locate cracks in the structure and evaluate the degree of structural damage.In this paper,the high-sensitivity phase-shift fiber Bragg grating(PSFBG)is applied to the ultrasonic damage detection and localization of CFRP.The experimental and finite element simulation are combined to study the linear characteristics of Lamb waves propagating in CFRP,and based on these characters the damage degree and the locating of cracks were studied.The main research work is as follows:Firstly,this paper introduced the significance and background of this research.Studied the research status of damage detection technology based on wave theory and the ultrasonic detection technology of FBG sensor were investigated.The research progress of PSFBG sensors in ultrasonic testing was analyzed.It was found that the application of PSFBG in the ultrasonic testing of composite matrix cracks remained to be studied.Secondly,this paper analyzed the theory of fiber grating and grating sensing.The structure of the fiber grating,the calculation of the resonant wavelength and its strain theory were introduced.The spectral characteristics and theoretical analysis methods of PSFBG were described.Finally,the response characteristics of fiber grating to ultrasonic waves were analyzed.Then,the CFRP laminate was modeled by LS-DYNA finite element software,and the micro crack was simulated by the node splitting method.The influence of the number of cracks on the propagation characteristics of Lamb wave was analyzed,including the attenuation of the amplitude of Lamb wave and the variation of the damage index with the changing of crack numbers and propagating distance.The crack was located by the linear fitting of damage index.Wavelet basis decomposition was applied to further demonstrate the declining of Lamb wave.Finally,the reflection signal of the crack was extracted,and the crack is accurately positioned by the Time of Flying method.Finally,a CFRP laminate with particular carbon fiber layers was designed and fabricated,and the law of matrix cracks in the three-point bending experiment was summarized.Cracks were observed and measured under a microscope to demonstrate the failure mode of the composite sheet.A PSFBG ultrasonic sensing system with high sensitivity was built,and based on this system,the ultrasonic damage detection experiment of the CFRP laminate was conducted.The experimental results were consistent with the simulation results,which proved the effectiveness of the experimental method.At the same time,the detection signals of PSFBG sensors and traditional piezoelectric ceramic sensors were compared,and the effectiveness and superiority of PSFBG sensors were proved.