NONDESTRUCTIVE EVALUATION OF FATIGUE-INDUCED CHANGES IN THE STRUCTUREOF COMPOSITES BY AN ULTRASONIC METHOD USING A LASER

Studies were conducted to nondestructively evaluate the damage to a gl ass-fiber-reinforced plastic during cyclic and static loading. The eva luation was made by an ultrasonic method employing a laser. In both th e unloaded and loaded specimens, the ultrasound attenuation spectrum h as a resonance peak attributable to the periodic nature of the structu re of the composite. This peak is shifted to the low-frequency region during static loading, due to a decrease in the elastic modulus. The s pectra obtained after cyclic loading have no resonance peaks, due to a ttenuation of the ultrasound over a broad range of frequencies by a la rge number of fatigue cracks. Additional static loading results in con centration of the cracks near the boundary between the glass fibers an d the polymer matrix, which leads to the formation of a resonance peak in the high-frequency region of the spectrum.