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Description
In this work, a nanosecond pulsed laser was used to photoacoustically generate ultrasonic waves in an unsaturated polyester resin plate. The generated waves propagated through periodic arrays of stainless-steel cylinders immersed in water. The objective was to investigate how the center-to-center spacing between cylinders modifies the propagation of ultrasonic waves.
Three periodic configurations with spacings of 1.4, 1.5, and 1.6 mm were analyzed, while keeping the cylinder diameter, the number of rows, and all other experimental conditions constant. The transmitted ultrasonic signals were recorded using a 0.5 mm PVDF needle hydrophone. During the measurements, the periodic array was translated over 50 positions with a spatial step of 0.1 mm, while both the photoacoustic excitation point and the detector remained fixed. From these measurements, two-dimensional B-scan maps and their corresponding frequency-domain representations were obtained.
The results show that ultrasonic wave propagation is modified by the center-to-center spacing between cylinders. The arrival time of the photoacoustic signal depends on its propagation path through the periodic array. Owing to the periodicity of the array, these temporal variations repeat spatially, giving rise to well-defined curved patterns. For the arrays with 1.5 and 1.6 mm spacing, these curved patterns are clearly distinguished, whereas for the 1.4 mm configuration they become less distinguishable. Likewise, the spectral distribution exhibits differences among the three periodic configurations studied.
The analysis includes two-dimensional frequency-wavenumber (FFT2D) maps, which allow for the investigation of the spatial redistribution of acoustic energy and the identification of possible preferential propagation directions associated with each periodic configuration.