In this work, a high-resolution imaging method for the inspection of isotropic plate-like structures using linear arrays and Lamb waves is proposed. The evaluation of these components is limited by the low dynamic range resulting from main lobe and side lobe field patterns, and from the narrowband nature of the Lamb waves. Based on a full matrix array, synthetic aperture technique using all emitter-receiver combinations, different images from the same object are obtained by using different apodization coefficients, which are related to a trade-off between main lobe width and relative side lobe level. Several image compounding strategies have been tested and a new algorithm, based on apodization and polarity diversities between signals, is proposed. However, some effects, such as the dead zone close to the array and reverberations caused by interactions of the wavefront and defects, still limit the quality of the images. The use of spatial diversity, obtained by an additional array, introduces complementary information about the defects and improves the results of the proposed algorithm, producing high-resolution, high-contrast images. Experimental results are shown for a 1-mm-thick isotropic aluminum plate with artificial defects using linear arrays formed by 30 piezoelectric elements, with the low dispersion symmetric mode S0 at the frequency of 330 kHz.
SAFT techniques are based on the sequential activation, in emission and reception, of the array elements and the post-processing of all the received signals to compose the image. Thus, the image generation can be divided into two stages:(1) the excitation and acquisition stage, where the signals received by each element or group of elements are stored; and (2) the beamforming stage, where the signals are combined together to obtain the image pixels. The use of Graphics Processing Units (GPUs), which are programmable devices with a high level of parallelism, can accelerate the computations of the beamforming process, that usually includes different functions such as dynamic focusing, band-pass filtering, spatial filtering or envelope detection. This work shows that using GPU technology can accelerate, in more than one order of magnitude with respect to CPU implementations, the beamforming and post-processing algorithms in SAFT imaging.
An assessment of the standard fabrication Micro-Electro-Mechanical Systems (MEMS) process Multi-User MEMS Processes (MUMPs) for complex air-coupled capacitive Micromachined Ultrasonic Transducer array aperture manufacture is reported. A 1-D linear array and a 2-D sparse symmetric binned-array have been designed and manufactured, and then characterised experimentally using electrical impedance measurements, laser vibrometry and air-coupled field measurement; the experimental data are supported by simulated data using Finite Element technique and field simulation based on Huygens’ principle. A methodology for the manufacture of the array structures using the MUMPs process is described. Electrical characterisation shows the devices operation at 770 kHz and the existence of large parasitic capacitances and electrical losses. Mechanical crosstalk of array substrate has been measured at -40 dB using laser vibrometry. Moreover, the laser vibrometry measurement and the field characteristics of one element reveal that each element operates as a piston radiator
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