Grain-size measurements in thin metal sheets by laser-generated ultrasound

We use short laser pulses to generate broadband ultrasonic pulses in polycrystalline materials and interferiometric techniques to detect the ultrasonic signals. The damping of the spectral components of the ultrasonic pulses can be exploited to measure the total attenuation alpha(f) as a function of frequency f. Knowing alpha (f), it is then straightforward to deduce the grain size by using the Rayleigh-approximation describing the scattering contribution to alpha(f). This technique has been applied to metal plates with a thickness from 1.5 to 20 mm and grain size between 20 mym and 80 mym.