Numerical evaluation of ultrasonic pulse-echo subwavelength defect detection

The ultrasonic pulse-echo Backscattered Amplitude Integral (BAI)-mode imaging technique [UFTC Trans, 45:30, 1998] has been developed to detect small channel defects in flexible food package seals. This technique detects 38-mum-diameter channels reliably and 6-mum-diameter channels occasionally using a 17.3-MHz; focused transducer in water (20degreesC, lambda approximate to 86 mum, total sample thickness 220 mum). However, interaction between ultrasound and sample microstructure - the underlying detection mechanism is poorly understood. Experimental evidence showed that the subwavelength channel was fused inside the two binding trilaminate plastic package films. Each trilaminate film had three sublayers. Package sample impedance profiles along the ultrasound beam axis were examined. Although identical in nominal impedance properties before sealing, the two binding films showed an asymmetric impedance profile after sealing. A generalized impedance profile model was proposed. The defect detection behavior of the echo signal was investigated by solving the 2D linear acoustic wave equations in fluid with finite-difference time-domain method and the perfectly matched layer absorbing boundary. The normalized correlation coefficients between the simulated and the measured RF echo waveforms were greater than 95% for this generalized model.