Investigation On Key Problems Of Ultrasonic Detection For Fiber-reinforced Ceramic Matrix Composites

Continous Fibre-reinforced Ceramic Matrix Composites are favored by aerospaceand other fields of domestic and overseas for their light weight and high temperatureresistance. At present, part of these materials have been applied to the national defenseproducts in domestic. However, the study of ultrasonic detection for these kind of newmaterials is still rarely, and the study of acoustic characteristic is hard to seebreakthrough for their complexity. The acoustic characteristic of continousfibre-reinforced silicon carbide ceramic matrix composites is taken as the researchobject. The transverse isotropic model is applied to the material analysis on the basis ofhaving mastered the acoustic characteristic of silicon carbide matrix. The slownessgraph is obtained by the utilization of acoustic wave propagation theory, and thetationality of this model is verified by experimental method. Three acoustic parameters(velocity, attenuation, nonlinear coefficient) are measured, on the acoustic parametertesting platform which is setted up with pulse generator and oscilloscope, or throughthe nonlinear acoustic parameter detecting systerm which consists function generator,power amplifier and oscilloscope. Ultrasonic coupling effect of waterproofingmembrane, which is wrapped around the CFCC-SiC to solve the problem that thesematerials can’t be immersed during practical detecting process, is analyzed in terms ofwaveform, amplitude and frequency spectrum.The results show that the longitudinal and shear wave velocities are11500m/s and7000m/s respectively, and the attenuation is larger than the one of high temperaturealloy in general. The velocity of every direction in the CFCC-SiC preform plane isaround7800m/s, and the one of the direction perpendicular to the CFCC-SiC preformplane is2460m/s which is smaller than a half of the previous one. The measuringresults variation of acoustic parameters which are attained by point-to-point testing onthe materials is obvious, which shows that their distribution has regularity. Meanwhile,the attenuation coefficient of CFCC-SiC is varying from1dB/mm to3dB/mm, which isthe highest one among the materials having measured. The waveform, amplitude andfrequency spectrum of the water immersion ultrasonic detecting signal, which isattained from the sample having wrapped by waterproofing membrane, is inaccordance with the ones, which is attained from the sample not having wrapped by waterproofing membrane.It can be seen that the CFCC-SiC matrix has the characteristics of high velocity,high attenuation, and the directional of the preform lied up, the complexity ofcomposite structure. These cause a higher attenuation, a larger velocity in a certaindirection and a strong anisotropy. The large discreteness of acoustic parameters makesit possible to assess material features by them. It can achieve the desired detectioneffect by wrapping the materials with waterproofing membrane to protect them fromwater. This method applies to the immersion ultrasonic detecting of CFCC-SiC underthe condition of waterproofing.