Currently, a number of non-destructive testing techniques are available to evaluate microstructural damage and integrated effect of defects in composites. However, it is still a long way before quantitative and standard methods are established. Our present attempt is to develop methods for quantitative characterization of different types of flaws in the advanced composite honeycomb structure. Three particular techniques, viz. Acoustic Impact Technique (AIT), Ultrasonic C-scan and Fokkerbond testing were chosen for detailed study and analysis. In this, ultrasonic C-scan was chosen as the reference technique to ascertain the validity of the other two techniques. Test coupons fabricated with/without defects using the autoclave facility were subjected to examination. These coupons were subjected to Ultrasonic C scan (system developed by ULTRAN, USA). After different trials, 5 MHz focussed lambda probe was chosen for scanning purposes. Pulse echo method in both interface and emission modes were tried out. With suitable gate setting, defects could clearly be identified using C-scan technique.

As part of the evaluation program, we tried to understand Acoustic Impact Technique (AIT) in detail. AIT is based on sensing and analyzing localized vibrations in the materials which are sensed by transducers. A defect or flaw changes the local stiffness of a structure or part and influences the vibration response. In the non-destructive evaluation program, test coupons with/without defects were subjected to AIT. AIT has shown to be consistent in its ability to detect delaminations introduced both by teflon tapes and various organic chemicals and presence of impurities. We could clearly establish the defective/nondefective areas in the coupons using this technique. This 'advanced coin tapper' could take away the subjectivity of the tapping technique and replace it with thorough engineering judgement and we could establish decisive results. Further, the coupons were subjected to Fokkerbond testing to identify the defects. Results obtained in Acoustic Impact Technique, ultrasonic C-scan and Fokkerbond testing could corroborate with each other.

To summarize, we could clearly establish the utility of the three different NDE techniques viz., Acoustic Impact Technique (AIT), Ultrasonic C-scan and Fokkerbond testing, for the Non-destructive Evaluation of the advanced composite honeycomb structure, with certain limitations on each technique. Hence, they could be used as complimentary techniques for the evaluation.