Air-coupled ultrasonic vertical reflection method using pulse compression and various window functions: feasibility study

Abstract

Nondestructive inspection using the air-coupled ultrasonic vertical reflection method has not thus far been realized because of the large mismatching of acoustic impedances between the test objects and air; this study aims to realize the air-coupled ultrasonic vertical reflection method. In order to separate and detect the faint defect-reflected wave from the surface-reflected wave, we examine the use of ultrasounds using linear frequency modulation (chirp signal) with window functions and the pulse compression technique. As a first step, we investigate the required conditions of the excited chirp signal to detect the signal received after transmittance into the test objects. We find that chirp signals with larger bandwidths and appropriate window functions are required for successful defect inspections. In addition, we investigate the effectiveness of using multiple probes and merging chirp signals with different frequency ranges in order to enhance the bandwidth of the excited chirp.