Non-contact and nondestructive evaluation of grain-size in thin metal sheets

It is desirable to measure the grain-size in a production-line for thin metal-sheets such as cold-rolled sheets in a non-contact way. This enables one to control the production parameters by means of computer-aided signal processing. Of course such a system should be user-friendly and should work reliably in particular under heavy duty conditions. At present we develop such a system. A short laser pulse impinging on the surface of a thin metal-sheet launches an ultrasonic pulse of large bandwidth. The higher frequency components are more strongly scattered by the ensemble of the polycrystals according to Rayleigh's law. The ultrasonic attenuation is evaluated by measuring the change of the Fourier-spectrum of two subsequent back-wall-echoes whose displacement fields at the surface are picked up by an interferometer. The measured strain amplitudes as a function of frequency are compared to the theoretically exspected one by a least square fit with the grain-size as a parameter. In order to be able to evaluate grain-sizes from 5 - 50 mym in metal-sheets thinner than 1 mm, the bandwidth of the ultrasonic pulses and of the detection system should extend up to 80 MHz. Ultrasonic pulses generated by short laser pulses via the thermoelastic effect and ablation of a thin surface layer exhibit a complicated directivity pattern, and not only P-waves (signals which arrive with the velocity for longitudinal waves) are produced but also signals arriving with S-velocity. These signals make a clear separation of the P-echoes impossible. Therefore, we have studied experimentally the various parts of the ultrasonic modes at a given absorbed intesity and energy of the laser pulses. As it turned out it is possible to suppress the generation of the S-modes almost completely. Furthermore, we discuss the types of interferometers suitable to detect ultrasonic pulses at optically rough surfaces, a prerequisite for industrial inspection system. We shall present a critical review of our wor