Using Excitation Modulation and Coding to Achieve Submillimeter Resolution In Materials Previously Impenetrable

The probability of detecting a discontinuity in a material is dependent upon the wavelength of the excitation used. The choice of wavelength becomes less straightforward when grains, pores, and fibers are similar in size to the discontinuity. Reducing the wavelength to less than one-half the size of the discontinuity guarantees that detection resolution will be sufficient, however, it also reduces sample penetration in heterogeneous materials. Furthermore, combining materials with unique favorable wavelengths complicates the choice for excitation wavelength. Measuring the thickness of a material behind a coating suffers when one wavelength is suitable for the coating and a different wavelength is suitable for the coated material. Here a solution is verified on multiple samples by relating detection resolution to excitation bandwidth. By maximizing the excitation’s bandwidth, and lowering the excitation’s center frequency, previously impenetrable heterogeneous materials were permeated while maintaining detection resolution. Excitation modulation was used to control the center frequency while excitation coding was used to overcome immense losses imposed by the material and transducer. For verification, the implementation was modeled both analytically and through computer simulation, and validated by physical lab testing with conventional transducers. Lab samples included, ½ inch thick carbon fiber, 1-inch thick fiberglass, 1-inch thick Acetal, 1¼ inch layered epoxy, 1¾ inch subsea coating on steel, and 3-inch thick high-density polyethylene (HDPE). Submillimeter resolution measurements were possible using conventional ultrasonic probes with 2.25 MHz and 500 kHz resonant frequencies while the center frequency delivered to the transducer was as low as 10 kHz.

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