Method of ultrasonic inspection of materials through opaque barriers

Abstract

An improved apparatus and method for ultrasonic inspection of materials through barriers such as gaps in manufactured parts is disclosed. The improvement herein is directed to enabling such ultrasonic testing to bridge ambient gaps such as intentionally formed gaps in composite structures having a first structure for originally receiving and transmitting sound separated by the gap from another structure to be inspected. Preferably, the gap is flooded with a gas having a predictable and optimum speed of sound relative to the material of the first and second structures. Sound is propagated to the first structure in a wave packet that is transmitted through the couplant fluid. The sound is generated in a wave packet having a spatial width at least twice the dimension of the gap to be bridged. The wave packet has a contained frequency having a wavelength (relative to the speed of sound of the gas flooding the gap) to create a constructively interfering standing wave node within the gap. The sound propagated to the gas-filled gap has a wavelength which is a half-integer with respect to the gap dimension. Sound passes through the first structure, creates a standing wave node in the gas-filled gap, passes into and acoustically interrogates the second structure for flaws and reflects. Reflected ultrasoound from the interrogated second structure again bridges the gap as a constructively interfering standing wave, passing through the primary structure and then through the couplant fluid to a transducer for receipt and analysis of the received ultrasound.