Impact Fatigue Damage Detection In Carbon Fiber Reinforced Polymer Composites Based On Nonlinear Vibro-Acoustic Modulation

In recent years,carbon fiber reinforced polymer(CFRP)has been widely used in various important fields of engineering structure due to their advantages of superior comprehensive performance,significant designability and lower production cost.However,accidental impact may cause impact damage,and evolve into fatigue source,which seriously affects the overall performance and service life of the material structure,and finally brings the potential safety risks.Therefore,how to quickly and effectively carry out nondestructive testing of CFRP composite structures and provide early warning of micro damage of structures has always been a hot research topic.This study mainly carries out the research on the impact fatigue damage detection in CFRP composites based on nonlinear vibro-acoustic modulation(VAM),and explores how to effectively identify and evaluate impact fatigue damage in composite materials as well as predict their residual fatigue life.In this paper,the related principles of nonlinear VAM system are described,and the sources of different nonlinear effects in materials and structures are analyzed,the nonlinear ultrasonic wave equation and its approximate solution are derived from the one-dimensional nonlinear elastic wave equation in free plate structure,based on the derived formula,the corresponding damage eigenvalue is selected and the theoretical model of VAM are introduced.Finally,the fast Fourier transform is selected as the signal processing method,and the corresponding fatigue damage theoretical model is cited,which lays a theoretical foundation for the research of nonlinear VAM of CFRP composites.Through the frequency response test of intact composite specimen and modal analysis of finite element simulation model,the corresponding low-frequency(LF)and highfrequency(HF)modal frequency are obtained.Three composite plates with different damage degrees were selected as the comparative specimens to study the effects of the excitation frequency and voltage amplitude of LF vibration signal and HF ultrasonic signal on the results of VAM detection,and the experimental parameters were optimized.The nonlinear VAM test was carried out on intact specimen and impact fatigue damage specimens,the relationship between the amplitude of HF ultrasonic main peak,amplitude of modulated side-bands,modulation parameter and impact fatigue life was analyzed.On this basis,the acoustic resonance method detection was also used to detect the impact fatigue damage of composites.Based on the theory of material stiffness degradation,the corresponding fatigue damage assessment models of the two methods were established to evaluate the impact fatigue damage and predicted the residual fatigue life of composites.The two methods were compared and analyzed in detail from the aspects of detection mechanism,excitation mode and nonlinear parameters.Finally,the micro morphology characteristics of the composite specimens were observed by optical microscope,and the evolution process of the damage was discussed.The experimental results showed that in order to obtain strong nonlinear modulation effect,the excitation frequency of LF vibration signal should be selected as the natural frequency of material structure with larger frequency response,and the amplitude of excitation voltage should be appropriately increased.According to the modulated side-bands nonlinear signal in the frequency spectrum of the received signal,the nonlinear VAM can distinguish the intact specimen from the damaged specimens successfully.By comparing the nonlinear VAM with the nonlinear acoustic resonance method,it can be seen that the two methods can effectively detect the impact fatigue damage in CFRP composites based on the modulation of the sidebands and the resonance frequency shift in the frequency spectrum of the received signal,and the corresponding fatigue damage model showed good sensitivity to the evaluation of impact fatigue damage and prediction of residual life of composite materials,in the initial stage of impact fatigue life,the fatigue damage in composite materials was usually dominated by matrix micro-cracks,at this time,the material strength decreased gradually,and the corresponding nonlinear parameters increased slowly.At the final stage of the fatigue life,the carbon fiber broke,resulting in a significant decrease in the strength of the material,and the nonlinear parameters increased rapidly till the failure of the composites.