Research On Acoustic Method Of The Fatigue Damage Of T700 Carbon Fiber Composite Materials

Carbon fiber composite materials with high specific strength,large specific stiffness,corrosion resistance,high temperature resistance,light weight and other excellent material properties are widely used in high-precision scientific research and other industries.However,in the service process,due to long-term operation in harsh environments,such as load,fatigue,creep,mechanical damage and other factors,carbon fiber composite materials may produce undetectable internal damage.The mechanical properties and service lifes of CFRP sharply declined,which seriously affected the usability and reliability.Therefore,it is very important to develop methods of effective detection and evaluation for the damage mechanism and performance prediction of carbon fiber composites.In this paper,two kinds of experiments are carried out on experimental specimens,loading and unloading experiments and fatigue tests.Acoustic emission detection technology and nonlinear ultrasonic detection technology are used for detection.The trend between the Felicity ratio and the relative nonlinear coefficient during the loading and unloading experiment was analyzed,and the corresponding relationship between the curve of the Felicity ratio and the nonlinear coefficient and different loading stages was mutually verified.In the fatigue experiment process,nonlinear ultrasonic signals were collected to analyze the relationship between the relative nonlinear coefficient and the loading cycle,providing new ideas for the non-destructive evaluation and detection technology of the fatigue damage process of composite materials.Based on the macroscopic damage mechanics,the single-layer laminated plate is taken as the research object to analyze the mechanical properties of the composite material;the fatigue damage mechanism of the composites with different layers was understood,and the basic principles of the nonlinear ultrasonic testing technology were combined to analyze the mechanism of the nonlinear effect of composite damage process.In this paper,during the loading and unloading experiments of composite plies with different layers,acoustic emission testing technology was used to monitor to obtain the variation curve of Felicity ratio with the load.The acoustic emission parameters analysis and time domain analysis are used to reflect the dynamic changes of the damage,and the acoustic emission characteristics of the specimens at different stages of tensile damage are analyzed.It can be seen that as the load increased,Felicity ratio presented a significant downward trend.In this paper,in the process of loading and unloading of composite materials with different layers,the nonlinear ultrasonic technology was used for on-line detection to calculate the ? values of the specimens under different loads.According to the Felicity ratio variation trend of acoustic emission,it can be seen that the relative nonlinear coefficients of the two lay-up specimens shows a rising trend with the increase of load,and the trend of Felicity ratio decreases with the increase of load.The variation of the two curves shows consistency.The results show that the relative nonlinear coefficient can characterize the severity of the damage or structural defects of the material,which can be used as a new important assessment criterion for the severity of the damage during the static tensile test of the specimen.In this paper,the fatigue strength test of carbon fiber composites under different stress ratios is used to analyze the relationship between the relative nonlinear coefficient ? and the loading cycle at different measurement positions with different stress ratios.Gaussian function fitting and quadratic polynomial fitting were used to fit the curve with the relative nonlinear coefficient ? as a parameter,and the obtained fitting result was compared and analyzed.The experimental results show that the correlation coefficient of the Gaussian fitting function is larger than the correlation coefficient of the quadratic polynomial fitting function for each stress ratio,which shows that using Gaussian function fitting can well fit the relationship between relative nonlinear coefficients and loading cycles.The damage factor D is defined by The ratio of ? value to fatigue failure at different loading cycles,which is used to characterize the severity of fatigue damage of materials.It is proposed to establish a nonlinear ultrasonic quantitative evaluation method for T700 carbon fiber composites.