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Nondestructive characterization of layered composite materials with a laser optoacoustic sensor

A laser optoacoustic sensor (LOAS) for the nondestructive characterization of layered structures of composite materials is presented. The method employs laser pulses for the generation of wide-band acoustic transients in the ultrasonic frequency range from 100 kHz to 10 MHz. The acoustic transient signal is generated at the very surface of materials under testing. The laser wavelength is chosen to achieve the desirable optical attenuation in the material and, thus, to induce acoustic transients within desirable frequency range. The detection of laser-induced ultrasonic signal, its temporal profile and the ultrasonic spectrum, may be performed either in transmission mode or in reflection mode. The laser optoacoustic sensing in transmission mode (LOAS-T) is performed at the surface opposite to the irradiated surface. LOAS-T utilizes measurements of acoustic attenuation coefficient as a function of ultrasonic frequency within the range of laser-induced frequencies. The laser optoacoustic sensing in reflection mode (LOAS-R) permits measurements of ultrasonic transients scattered from the volume of the material under testing. The ultrasonic signal is detected at the site of laser irradiation. The measurements of the reflected ultrasonic signal is frequently the only mode available for the nondestructive evaluation, because usually only one side of the evaluated object is accessible. Samples of layered graphite-epoxy composite materials with volume percentage of micropores from 0 to 1.2% were studied with LOAS. In the ultrasonic frequency range from 1 to 5 MHz, the acoustic attenuation coefficient was found to increase with an increase of relative volume of micropores. The 'noise' component of the scattered acoustic pulse sharply increases with the increase of relative volume of micropores. A 1% increase in the relative volume of micro-pores results in a 3 times greater 'noise' component in the laser optoacoustic signal. Results indicate possibility for the LOAS to be utilized as a diagnostic system for the quantitative characterization of composite materials.