Ultrasonic guided wave inspection of a titanium repair patch bonded to an aluminum aircraft skin

Adhesively bonded aircraft repair patches provide stiffness improvement and life extension to aging aircraft. Repair patches are mainly metal or composite plate like structures bonded onto the aircraft skin after adequate surface preparations. Any damage to the bonding in an adhesive repair patch, like adhesive and cohesive weaknesses similar to that found in adhesively bonded joints, can result in less than expected life extension of the aircraft. A conventional inspection approach using normal or oblique incidence of ultrasonic waves has only limited capability in detecting adhesive and cohesive weaknesses. An ultrasonic guided wave based inspection method is developed and presented. Theoretical work using wave structures is done to select guided wave modes with large in-plane displacement at the aluminum skin and adhesive epoxy interface. Aluminum plate with epoxy bonded titanium repair patches were fabricated in the laboratory with simulated interfacial and bulk weaknesses representing a small representative class of the defects possible in a bonded joint. Defect sensitive guided wave modes were selected and implemented successfully in an angle beam wedge based inspection set-up. This was further successfully extended to a linear scan, termed as a G (*) scan, which has defect sizing ability also.