Unveiling Faults in Composite Materials by Suppressing Axial and Lateral Coherent Noise with Modulated and Time Coded Excitation

Layered, porous, and fiber reinforced materials, such as concrete and carbon-fiber-reinforced polymer (CFRP) contain a heterogeneous microstructure that produces coherent noise which mask faults in the material when excited acoustically. Therefore methods that can address the scattering of the incident excitation and the resulting coherent noise are undergoing much research. Frequency selective filtering has been partially successful in suppressing noise from some scatterers by attenuating frequencies that are introduced by off-axial reflections. Here frequency selective modulation was combined with time coding to generate an excitation signal that had properties for suppressing noise in both axial and lateral directions. The objective was to locate faults normally veiled by scattered excitation noise when using conventional ultrasonic equipment. Both a computer simulation model and a physical target were used to test the possibility of improving the signal-to-noise ratio (SNR) of faults with conventional transducers. Simulation and physical testing both revealed that the coded, modulated signal reflected by faults could be raised above the coherent noise floor and spatially located in situations where conventional excitation was insufficient.

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