The sensitivity of ultrasonic features to the condition of the adherend-adhesive interface in composite joints was studied. The inspected specimens represent different bonding characteristics due to the variety of surface pretreatments to the composite adherends. Each signal obtained from ultrasonic inspection of a specimen was characterized by a feature vector, consisting of features derived from both the time and frequency domain representations of the signal. Some of the features were found to be significantly sensitive to the interfacial characteristics of the joints. The sensitivity was verified by means of conventional statistical tests. The sensitivity of the features enabled the application of basic pattern recognition techniques for the classification of the joints according to the surface preparation of the adherends.
The influence of impact damage on the structural performance of composite laminates was investigated by a joint effort and included nondestructive evaluation (NDE), mechanical testing and analysis. The impacted NASA Industry Standard Compression After Impact test specimen was chosen. Damage characterization incorporated ultrasonic and radiographic methods. Computerized and manual ultrasonic testing from both sides of the test panels provided accurate information on the damaged area. The findings were confirmed by a destructive X-ray microfocus technique combined with an opaque penetrant for damage enhancement. From the NDE data, a simplified damage model was constructed and analyzed for strength and stability. Failure loads and modes were predicted and compared to compression after impact test results. Preliminary tests showed very good agreement.
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