On-line measurement of texture, thickness and plastic strain ratio on steel sheets using laser-ultrasound resonance spectroscopy

Laser-ultrasonics is used as a non-contact technique to determine reliably and rapidly the crystallographic texture, the average plastic strain ratio, and the thickness of sheet metal on the production line. With laser-ultrasonics, a short laser pulse is used to generate a wideband pulse of ultrasound and a laser interferometer is used for detection. In this paper, a large number of echoes are collected and analyzed together to measure the natural resonance frequencies in the thickness of the sheet. From these frequencies, two texture coefficients, W400 and W420 are calculated, as well as a highly accurate measurement of the sheet thickness. Using these texture coefficients and well-known relationships, the average and planar variation of the plastic strain ratio, respectively 'r' and Δr, can be evaluated. Both parameters are indications of the formability of metals sheets. Measurements on 1 mm thick, ultra-low-carbon steel sheets at LTV Steel Company have shown the following measurement accuracies: 'r' to within ±0.04, Δr to within ±0.1, thickness to better than ± 1 μm. (author)