Ultrasonic pulse-echo subwavelength defect detection mechanism: Experiment and simulation

The ultrasonic pulse-echo backscattered amplitude integral (BAI)-mode imaging technique [IEEE Trans. UFFC, 45:30 (1998)] has demonstrated sensitive detection of subwavelength channel defects (38-μm diameter reliably and 6-μm diameter occasionally) in flexible 220-μm-thick food package seals (17.3 MHz, λ ≈ 86 μm). However, the underlying subwavelength defect detection mechanism is poorly understood. In this contribution, a theoretical modeling study was undertaken to elucidate the mechanism. The subwavelength diameter channel was fused in-between two plastic package films by applying heat from one side of the films. The sample cross-section microstructure was characterized from both optical and acoustic images. The cross-section impedance profiles along sample thickness dimension were determined. Although identical in nominal impedance properties before sealing, the two binding films showed an asymmetric impedance profile after sealing. Transient finite-element heat conduction analysis and impedance profiles of multiple-sealed package samples showed that the single-sided heating process caused an asymmetric impedance profile. A generalized impedance model was proposed based on these observations. An efficient two-dimensional simulation tool using a finite-difference time-domain method and the perfectly matched layer numerically evaluated the defect detection behavior of the radio-frequency (rf) echo waveforms. The normalized correlation coefficients between the simulated and the measured rf echo waveforms were greater than 95% for this generalized model, which suggested the validity of the proposed impedance model.