Inspection Method of Metal-to-Metal Adhesive Bond Strength During Depot Level Maintenance

The elimination of fasteners to join two composite components together and reliance on adhesive bonds in a bonded structure is an important factor in the design and development of adhesively bonded composite structures for the Air Forces’ next generation of fighter and bomber.  Minimizing or eliminating fasteners is also important in the of repair of adhesively bonded metal-to-metal aircraft structures at the depots as part of a sustainment operation to extend the operational life of aircraft.  Present process control to repair these structures emphasizes surface preparation as the primary means of controlling quality and bond strength. To date, ultrasonic and thermographic inspection is the only means, aside from visual inspection, that is used to judge the quality of adhesively bonded structures and bond repairs.  The current nondestructive inspection techniques involving ultrasonic inspection are not capable of determining bond strength for certification of bonded structures, limiting bonded joint acceptance for critical structures. Laser Bond Inspection (LBI) offers a method to characterize adhesive bond strength that are absent from traditional NDT techniques (Ultrasonic Inspection).  The LBI method has been matured for evaluating the composite-to-composite adhesive bonds.  LBI can be also used to validate the integrity of adhesive bonds during the repair process by identifying weak bonded repairs and to detect the presence of “kissing bonds.”  The implementation of the LBI method is to verify the adhesive bonding process is under control and the bond meets the intended design requirements.  Laser Bond Inspection (LBI) will benefit the repair process by verifying the repair strength, ensuring aircraft safety and minimize rework of metal-to-metal bonds. This SBIR Phase I proposal is focused on demonstrating LBI can be used to characterize metal-to-metal bonds by detecting the characteristics of the returning stress wave as is currently done with composite-to-composite bonds. The investments made by Industry and Government to generate data and build the various LBI system components to conduct this inspection process can be leveraged to support this proposed Phase I for maturation and implementation for the inspection of bonded repairs.  LBI can be used to validate the integrity of repairs that are made to either a metallic or composite aircraft by identifying weak bonded repairs and to detect the presence of “kissing bonds.”