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

Abstract

Laser-ultrasound resonance spectroscopy, a non-contact ultrasonic technique, was used to determine reliably and rapidly the crystallographic texture, the average plastic strain ratio, and the thickness of sheet metal on the production line. As with laser-ultrasonics, a short laser pulse is used to generate a wide-band pulse of ultrasound and a laser interferometer is used for its detection. In this paper, a large number of echoes are collected and analyzed together using Fourier techniques to measure the natural resonance frequencies in the thickness of the sheet. One longitudinal and two shear resonance frequencies were measured together with their harmonics. From these frequencies, two crystallographic orientation distribution coefficients, W 400 and W 420, are obtained, as well as a highly accurate measurement of the sheet thickness that is corrected for changes in ultrasonic velocity caused by texture variations. Using these coefficients, the average and in-plane twofold and fourfold variations of the plastic strain ratio, respectively r̄, Δ 2 r, and Δ 4 r, can be evaluated. These parameters are indications of the formability of metals sheets, which is of industrial interest. Measurements on 1 mm thick, low carbon steel sheets have shown the following measurement accuracies: r̄ to within ±0.08, Δ 2 r, and Δ 4 r to within ±0.1, and thickness to better than ±1 μm. On-line tests at LTV Steel Company showed that the sensitivity of the apparatus is sufficient to detect systematic variations in texture along the length of similar production coils and that the on-line repeatability for r̄ was of order ±0.02.