Quantitative Evaluation Study Of Porosity Of Carbon Fiber Composites By Ultrasonic

The advantages and characteristics of carbon fiber composite materials make them widely used in many fields such as aerospace,automobiles,energy,marine vessels,etc.The two most commonly used manufacturing processes are autoclave process and RTM process.A common defect in carbon fiber composites is voids.The porosity of voids affects the properties of the material.Therefore,the porosity of voids must be detected.The material with porosity higher than the permissible porosity limit should be discarded and no longer used so as not to affect the overall performance of the product.There are two main methods for the detection of porosity,non-destructive testing and destructive testing.In general,the porosity is evaluated in a combination of non-destructive and destructive methods,the porosity is non-destructively evaluated,and the destructive testing finally determines the porosity.The commonly used non-destructive testing methods for porosity are ultrasonic methods such as attenuation test method and nonlinear test method.At present,there is a detection model in the attenuation test method,but there is no model for the relationship between the nonlinear coefficient and the porosity.There is a certain degree of difficulty in the quantitative detection of porosity.Aiming at the problem of quantification of porosity of carbon fiber composites,a comparative study of ultrasonic testing was conducted in this paper.Firstly,by setting different fabrication parameters,different porosity carbon fiber laminates and RTM carbon fiber composites samples were made.Secondly,attenuation test systems and nonlinear test systems were built.For different characteristics of the material,different detection parameters were set,and the attenuation coefficient and nonlinear coefficient were tested.Metallographic analysis and acid etching were then performed to measure the porosity.Finally,the attenuation coefficient and porosity were fitted according to the existing model,and the attenuation coefficient and porosity of the same batch of products were tested to verify the accuracy of the model.The empirical formula was obtained by fitting the second-order and third-order nonlinear coefficients to the porosity respectively The accuracy of empirical formulas was verified on the same batch of products.At the same time,to determine the smallest and most sensitive ultrasonic parameters and its empirical formula for the porosity quantitative,the error of porosity quantification by attenuation coefficient and nonlinear coefficient were compared,and the attenuation coefficient,second-order and third-order nonlinear coefficients were normalized and compared respectively.The contrast between ultrasound bottom wave imaging and nonlinear imaging results was analyzed,and the applicable range of the two imaging methods was analyzed.The results show that the second-order and third-order nonlinear coefficients can be used to evaluate the porosity of carbon fiber composites.The second-order and thirdorder nonlinear coefficients have higher sensitivity and precision than the attenuation coefficients for quantitative quantification of porosity.The nonlinear coefficient is more suitable for the quantitative evaluation of the porosity of carbon fiber composites.For carbon fiber composite porosity imaging,when the porosity is less than 2%,non-linear imaging is required,when the porosity is higher than 2%,ultrasonic bottom wave imaging can reflect the difference in porosity.