Advanced non-linear ultrasonic sideband peak count-index technique for efficient detection and monitoring of defects in composite plates

Increasing the service life and therefore the efficiency of a structure can be achieved by implementing appropriate Non-Destructive Testing and Evaluation techniques. Detecting and monitoring of defects is not always feasible when using Linear Ultrasonic techniques especially at the initiation stage of damage. In this investigation, a new Non-Linear Ultrasonic method is developed by properly tuning the standard Sideband Peak Count-Index (SPC-I) technique and will be called tuned SPC-I technique. It is shown that the tuned SPC-I technique is more efficient for detecting defect initiation and defect progression at both micro- and macro-scales. The efficiency of the tuned SPC-I technique is demonstrated by monitoring impact induced damage progression in glass fiber reinforced composite plates. This tuned SPC-I technique is shown to be very sensitive to defects at the micro-scale level and it remains sensitive to larger defects as the micro-cracks coalesce to form macro-cracks whereas other Non-Linear Ultrasonic techniques start to lose their sensitivity for larger cracks. By properly tuning the SPC-I technique it can be used for various materials/geometries for monitoring non-linearity generated by micro and macro scale damages. This is achieved by experimentally tuning the most sensitive frequency for the SPC-I analysis using a pristine specimen. This sensitive frequency is then used to detect and monitor defects. Once defects progress to form macro-cracks, the specimen properties are altered causing a shift of the sensitive frequency and allowing continued monitoring of the defects. Glass fiber composite plate specimens that are impacted with increasing impact energies (0J, 5J, 10J, 20J, 30J, 40J, and 50J) are investigated. It is determined that by using the approach discussed above, it is possible to robustly detect impact damage, monitor the progression of impact damage at both micro- and macro-scales by shifting the tuned frequency.