Noncontact determination of elastic moduli by two-dimensional Fourier transformation and laser ultrasonic technique

A laboratory instrument that utilizes broadband laser ultrasonics and two-dimensional Fourier transformation for signal processing has been developed to characterize the properties of various foils and plates. Laser ultrasonics generation is achieved by using a pulsed laser which deposits pulsed laser energy on the surface of the specimen. The displacement of the resulting broadband ultrasonic modes is monitored using a two-wave mixing photorefractive interferometer. By means of the two-dimensional Fourier transformation of the detected spatial and temporal displacement wave forms, the image of density of state (DOS) for the excited ultrasound is obtained, and from it the materials properties are extracted. Results are presented for a 150 μ m thick paper sample, a 50 μ m stainless steel foil, and a 1.27 mm thick aluminum plate. The DOS image demonstrates the ability to measure the properties of each generated ultrasonic modes and provides a direct, nondestructive, measure of elastic moduli of the tested specimens.