Microstructure,Properties Characterization And Ultrasonic Evaluation Of CFRP Laminate With Thermal Oxidative Degradation

Carbon Fiber Reinforced Plastic(CFRP)is prone to thermal oxygen degradation resulting in performance degradation and a threat to component service safety.The existing evaluation methods of thermal oxygen degradation are mostly carried out by weighting,and the degree of degradation is judged according to the mass change of the samples.This method is not realistic for large components.How to achieve in-situ and non-destructive field detection for the degree of degradation is an urgent need for long-term service evaluation of large composite components.Ultrasonic non-destructive evaluation technology can effectively characterize changes of microstructure.It is an important method to evaluate the performance of components.However,there is little research on ultrasonic evaluation of CFRP degradation performance.Therefore,ultrasonic non-destructive evaluation technology is carried out.The research on microstructure and performance evaluation of CFRP after thermal oxygen degradation has important significance for the development of CFRP in practical engineering applications.In this paper,T300/AG-80 composites were studied.The microstructure and mechanical properties of CFRP during thermal oxygen degradation were studied.The thermal oxygen degradation mechanism of CFRP was discussed.The CFRP delamination damage was characterized based on ultrasonic non-destructive testing and evaluation techniques.The feasibility of evaluating the degradation properties of CFRP using acoustic properties is discussed.The main research contents and results are as follows.1.The mechanism of thermal oxygen degradation of CFRP at 150°C was studied by means of weight loss test,microstructure observation and Fourier infrared technique.In the early stage of degradation(72h),the weight loss rate of CFRP increased rapidly and reached20% of the total weight loss.The microscopic morphology observation showed that the interface between carbon fiber and resin was seriously damaged,and the interface bonding ability was decreased.Fourier infrared spectroscopy analysis of CFRP structure revealed that after 72 h,the characteristic peak of ester group C=O bond was formed.After 360 h,the characteristic peak of epoxy group disappeared.The thermal stability of CFRP was studied by thermogravimetric analysis.It can be seen from the change of activation energy that thedegree of CFRP deterioration is intensified after thermal degradation.Combined with the above analysis of the structural changes of CFRP during thermal degradation,the mechanism of thermal degradation of CFRP was discussed.It was mainly divided into the following aspects: at first,low molecular weight such as water molecules volatilized,and then the bonding ability of carbon fiber and resin matrix decreased,and at last the resin matrix further cures the cross-linking and the molecular chain is broken.2.Three-point bending test was carried out on CFRP short beam specimens with different degradation time.The variation of bending strength decreased with degradation time and then increased and finally decreased.Ultrasonic C-scan technique was used to detect delamination damage of curved specimens.The metallographic photograph of the crack and the ultrasonic C-scan can be used to characterize the delamination damage.The dynamic mechanical parameters of CFRP during the degradation process of CFRP were studied by dynamic mechanical analysis.At room temperature,the storage modulus and loss modulus increase first and then decrease with the degradation time,which is attributed to the post-cure effect,damping and the fiber/resin interface state.From the dynamic mechanical parameter temperature spectrum,it is known that with the degradation time increasing,both the CFRP loss factor peak and the loss modulus peak move towards the high temperature direction,and the glass transition temperature increase,which indicates that there is always post-cure effect during the degradation process,and the heat resistance of CFRP is improved.3.During the thermal degradation process,the longitudinal sound velocity of CFRP decreases first,then rises and then decreases.This is the change of elastic modulus caused by the volatilization of low molecular weight substances such as water molecules and the post-cure effect.The result of a combination of fiber/resin interface states.The attenuation coefficient increase first and then decreases at 10 MHz,which is related to the absorption attenuation of the resin material and the scattering attenuation at the fiber resin interface.Therefore,changes in acoustic characteristics can be used for the evaluation of CFRP thermos-oxidative degradation.