C/SiC复合材料空气耦合超声检测数值模拟

针对高孔隙率C/SiC复合材料空气耦合超声检测，引入考虑孔隙形貌的随机孔隙模型开展数值模拟研究。结合力学和声学性能测试计算材料弹性刚度矩阵，借助组织分析建立考虑孔隙微观形貌、孔隙率分别为5%、10%、15%的随机孔隙有限元模型，研究了空气耦合超声透射法检测过程中超声波传播特征及典型缺陷的响应规律。结果表明:材料纵波声速约2830 m/s，横观各向同性五个独立弹性常数分别为158.149、88.589、34.141、15.288和13.793 GPa。孔隙呈长条状，随孔隙率增加，超声衰减逐渐增大；孔隙尺寸与波长的比值约在0.05~0.22范围，主要为瑞利散射机制。高孔隙率、复杂孔隙形貌显著影响超声波的传播过程，导致个别条件下声场指向性发生偏转，影响缺陷检测。当分层缺陷长度由0增加到25 mm时，接收信号幅值衰减增大，与无分层模型相比最大衰减增加33.9 dB。随着复合材料层板厚度的增加，超声衰减进一步增强，声场也将产生一定偏转，主要体现孔隙和分层的共同作用。计算结果与实验吻合较好，为高孔隙率C/SiC复合材料的高质量无损检测提供支撑。 

Simulation of air-coupled ultrasonic testing on C/SiC composites

Numerical simulation based on finite element method was carried out for the air-coupled ultrasonic testing of C/SiC composites with high porosity. The elastic stiffness matrix was calculated based on mechanical and acoustical analysis. Finite element models with random void morphology were established, of which the porosity was 5%, 10%, and 15%, respectively. The characteristics of ultrasonic propagation and the response to typical defects were studied in the transmission testing profile. Results indicate that the longitudinal wave velocity in the direction vertical to laminate is about 2830 m/s, and five independent elastic constants of transverse isotropy are 158.149, 88.589, 34.141, 15.288 and 13.793 GPa. The voids are strip-like, and the ratio of width to wavelength is between 0.05~0.22. It is found that the ultrasonic attenuation gradually increases with the increasing porosity, which is mainly in Rayleigh scattering regime. Meanwhile, the directivity pattern of ultrasonic field under some conditions is also changed due to the high porosity and complex morphology. As the length of delamination increases from 0 to 25 mm, the attenuation of received signal increases, and the maximum is about 33.9 dB compared with that without delamination. It is of similarity for the evolution of the ultrasonic field with the increasing thickness of laminate, mainly owing to the combined effects of the delamination and voids. The simulation results show a good consistency with experiments and provide support for high-quality non-destructive testing on C/SiC composites.