Ultrasonic Evaluation For Shear Damage Of Carbon/Carbon Composites

Composite material is a good new material, which is widely used in aviation and space industries due to their outstanding physical and high-temperature properties, including high specific strength and modulus, excellent ablation and friction resistance, good heat-shock tolerance, low creep rate and density. All kinds of damage will seriously affect the performance of materials and components in the process of using, and the shear damage is one of the damages. In this paper, to take aerospace carbon/carbon composite materials as the object for study, using finite element method with three-point bending test to simulate the different shear damage degrees of the materials under different loads, to get the changes of ultrasonic attenuation> acoustic emission and resistance signals, explore composites, and to explore the non-destructive evaluation methods of composite materials.ALGOR finite element analysis software has been used in this study, established the shear test model of three-point bending to analysis the stress of C/C composites which are the Non-homogeneous anisotropic materials, the author designed a test fixture first, and then produced standard samples in accordance with the "Three-fiber reinforced composite test methods for flexural properties", finally got the load-distort curves and peak shear strength data., The results showed that:the fracture of C/C composite specimens in three-point bending test is divided into three stages:matrix cracking, force distribution of fiber, fiber damage caused fracture. The relationship between the degree of shear damage and ultrasonic attenuation is: After analysis of attenuation, found that the ultrasonic attenuation of C/C composite materials are increasing as the load increases. In the matrix cracking stage, AE signals collected (including the amplitude of the signal, counting and energy signals) are less, the sudden increase of resistance is caused by matrix cracking; as the load increases, fibers gradually uniform force distribution, AE signals and the resistance changes are relatively smooth at this stage; when the load is about to reach the sample’s critical point, it is into the fiber failure stage, the number of AE signals are significantly increasing, from the resistance curve can also find that the resistance changes suddenly. Throughout the process, the better the bearing capacity of the sample, the less the AE signals.In this paper, the established finite element model, the actual test results and the establishment of ultrasonic attenuation model can be used to evaluate the C/C composite damages. Changes in AE and resistance signals can be evaluated the C/C composite damages too. Ultrasonic attenuation coefficient is suitable for C/C composite material production process quality evaluation, and acoustic emission and resistance method are good ways to monitor the qualities of materials in using. This results in ensuring the safe use of composite components is important.