ULTRASONIC MEASUREMENTS FOR THE EVALUATION OF THERMAL FATIGUE DAMAGE
Forsyth, David Sean
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An ultrasonic test system was assembled to evaluate the ability of ultrasonic measurements to measure thermal fatigue damage in composite material systems. The ultrasonic test system used a modification of the sing-around technique. The sample to be studied was placed between a transmitting and a receiving transducer, and the time between the first peak on the transmitted waveform and the first peak of the received waveform was measured as the transit time through the sample. The peak to peak voltage at both transducers was also measured, and used to define an attenuation. Cyclic thermal fatigue loading was carried out on three fibre-reinforced plastics, two uniaxial and one randomly oriented. Before any loading, and at intervals during loading, measurements of ultrasonic velocities and attenuations at frequencies of 3.00, 4.00, and 5.00MHz were performed. These measurements were made for a number of samples, normalized, and averaged. Generally it was found that ultrasonic attenuations increased and ultrasonic velocities decreased with increasing number of thermal fatigue cycles. This depended on the material tested and the propagation direction of the ultrasound. Scanning electron microscopy (SEM) was used as a qualitative tool to examine specimens after fatigue to provide information on the modes of stress relief in the specimens. It was found that ultrasonic attenuation is sensitive to thermal fatigue damage in these materials. The main mode of damage is fibre-matrix debonding at the fibre ends, occasionally accompanied by matrix cracking. This is demonstrated by the SEM examinations of the fatigued specimens. A damaged surface layer in the uniaxial composites did not have a great effect on the bulk properties of the specimens, and ultrasonic velocities were not always sensitive to damage, due to the nature of the stress relief.