The study of ultrasonic pulse-echo subwavelength defect detection mechanism

Popularly used flexible food packages are produced by hermetically heat-fusing the opposing seal surfaces to avoid postprocess contamination of the product. Seal integrity issues such as channel leaks and imperfect bonds must be inspected reliably and efficiently for food safety and quality assurance purpose.; The ultrasonic pulse-echo backscattered amplitude integral (BAI)-mode imaging technique has been developed to detect microchannels in flexible food package seals. Requiring no a priori sample information, this technique detects 38-μm-diameter channels reliably and 6-μm-diameter channels occasionally using a 17.3-MHz focused transducer in degassed water (20°C, λ ≈ 86 μm, total sample thickness 220 μm), which are in subwavelength scale and much smaller than human observer inspection limit (∼50 μm). The technique can perform blind inspection and has the potential to become a real-time inspection methodology.; To bring the BAI-mode image closer to real world application, a zigzag raster scanning protocol has been proposed to simulate the real-time package product line inspection scenario. An experimental procedure was established to investigate the engineering trade-off between the BAI-mode image quality and the zigzag spatial sampling pattern. Quantitative image quality assessment showed that the zigzag pattern prompted scanning speed and the optimal spatial sampling step size was between one and two times the −6-dB pulse-echo transducer focal beam lateral diameter.; A theoretical modeling study has been undertaken to understand the subwave-length channel defect detection mechanism associated with the BAI-mode image. A generalized impedance model was proposed through both microscopic optical imaging and acoustic impedance measurement of package sample cross-section. The validity of the generalized model was evaluated numerically by simulating the echo signal behaviors using finite-difference time-domain simulation. The normalized correlation coefficients between the simulated and the measured RF echo waveforms were greater than 95% for this generalized model.; In summary, the dissertation identified and solved two research challenges in ultrasonic pulse-echo nondestructive evaluation through both experimental and numerical approaches. The spatial sampling study will provide quantitative engineering guideline for real-time on-line package inspection implementation. The subwavelength defect detection mechanism study prompts our understanding about nondestructive characterization and evaluation of materials.