Application of acoustic wavefield imaging to non-contact ultrasonic inspection of bonded components

Through transmission ultrasonic (TTU) methods are widely used for inspection of critical aerospace structural components because of their simplicity and the high sensitivity of TTU methods to bonding and delamination type defects. TTU inspection requires access to both sides of a component, which is generally not a problem before components are installed in final assemblies. However, limited access to the back side of a component on final assemblies usually precludes using TTU methods. Pulse echo (PE) methods are often used when only single side access is available, but PE ultrasonic methods have a limited penetration range from the outer surface, and do not have the sensitivity of TTU, particularly when surfaces are non-parallel. Thus, structural assemblies are often disassembled when a thorough TTU inspection is required. The work presented here addresses the need for a field deployable ultrasonic inspection method which has the sensitivity of TTU methods, is non-contact, i.e., couplant is not required, and does not require access to the back side of the part. These goals are accomplished by attaching a sparse array of ultrasonic transducers to the back side of a component or embedding them within the component. These transducers are excited to generate ultrasonic waves which propagate through the structure, and the resultant acoustic wavefields are imaged using a non-contact, air-coupled transducer. This ultrasonic wavefield imaging method is referred to as Acoustic Wavefield Imaging (AWI). Results are presented for a bonded aluminum plate specimen demonstrating that recorded wavefield images clearly show bonding flaws at internal interfaces.