A Basic Study on the Damage Estimation of Composite Materials by Impact Sound.

点击翻译按钮获取中文摘要

A simple instrumentation system is developed for determining the elastic constants of materials from the sound produced by the impact of a steel ball on a small rectangular bar specimen (abt. 3*4*50mm) which is suspended by thin ceramic threads in a furnace. The natural frequencies of the flexural and torsional vibration modes are measured from the impact sound using a fast Fourier transform analyzer and elastic constants are computed from these frequencies using Timoshenko's beam theory and Saint-Venant's torsion theory. For isotropic materials, Young's modulus E, shear modulus G and Poisson's ratio v are determined from one specimen. It is demonstrated that the elastic constants obtained by the present method are in good agreement with those determined by the ASTM standard method.The elastic constants of SiC/SiC composite were measured at elevated temperatures up to 1,200ﾟC in air. When the specimen was kept at a constant temperature of 1,000 or 1,100ﾟC, the elastic constants decreas … More ed with time at the early holding stage, but they recovered at the later stage. A scanning Auger nanoprobe system was employed to examine the surfaces of the original and tested specimens. It was shown that the time variation of the elastic constants at high temperatures is attributable to the oxidation of the pyrocarbon coating of the SiC fiber and the formation of silica, and that the present method is conveniently used for evaluating the damage of materials from the reduction of the elastic constants.The present method is extended to include the measurement of elastic constants of a plate specimen. The natural frequencies of bending vibration of a rectangular plate (50.0*70.0*2.8mm) of SiC/SiC composite were measured from the impact sound and the damage of the material as the reduction of Young's moduli was determined by employing a finite element method. Finally, the natural frequencies of a jet engine nozzle flap made of SiC/SiC composite panel were measured from the impact sound before and after engine operation, and the damage of the flap was estimated as the reduction of the Young's modulus of the material. It may be noted that the present method is capable of measuring the shear modulus as well as Young's modulus at high temperatures by a simple and cost effective procedure using small specimens, and is thought to be useful for material evaluation at development and operation stages. Less