UC San Diego

Blunt impact damage (BID) is known to potentially cause significant strength reduction on composite structures even when the damage is barely noticeable. Therefore, non-destructive evaluation (NDE) to assess blunt-impacted composite aircraft structures is a crucial procedure to determine existence and severity of the damage to examine airworthiness. Ultrasonic guided waves (UGWs) test method has been studied herein as an NDE technique to evaluate large composite structures, because the typically thin-walled aircraft structures effectively act as has guided waves propagation paths.

UGW scanning methods were performed only from the external skin-side using pitch-catch method: an actuator and a receiver located at a distance apart to scan the structure within. The actuator and the receiver transducers were placed on the adjacent skin-to-shear tie bolted joint regions for scanning of internal shear tie and C-frame components, and placed at opposite sides of the hat stringer on the skin bay for investigation of co-cured stringer components. UGWs transmission through bolted joint structure was examined to determine the test frequency that allowed wave transmission through bolted joints at low attenuation. And UGW tests from the skin-to-shear tie joint regions identified significant wave energy drop at the test frequency from the comparison of the pristine and the damaged (HEWABI) shear ties. Furthermore, damage characteristics were analyzed from the UGW measurements of the stringer scan followed by successful blunt impacted stringer delamination damage detection. Features extracted from the UGW test results showed exceptional match to the damage location, severity, and modes determined from the conventional NDE methods. Finally, UGW measurements-based residual strength estimation was studied from simple notched coupon tests and is an on-going study for more complex structures with realistic damage modes.