Laser Ultrasonic Characterisation of Rolled Steel Strip: Wave Propagation in Inhomogeneous Thin Sheets

For the production of hot rolled steel it is of special interest to assess the homogeneity of the material along the length of the strip. Being couplant free, laser ultrasound as a method is especially suited for the task because it opens up the opportunity of measuring material parameters early in the production process. Detailed models of the ultrasonic signal are needed though in order to extract robust figures from the experimental data, which give information about the elastic constants and damping rates resulting from the underlying microstructure. Micro-alloyed and dual-phase steel sheets in the thickness range between 3.5 mm and 4 mm were used in a test case for the propagation of Lamb waves in a weakly inhomogeneous and damping medium. Using a scanning setup we are able to determine the full dispersion relation of the in-plane sound field and thereby determine the contribution of the varied vibrational modes to the sound field. In particular we are able to determine the frequencies of the non-dispersing surface modes. These are closely related to the Poisson’s ratio of the steel, which is a reliable figure for the homogeneity of the strip and independent of the thickness of the sheet. Likewise we determined damping rates for different surfaces modes taking into account the geometric spread of the sound waves. Inhomogenities of the material, e.g., texture from rolling or scattering from grains, break the symmetry of the sheet geometry and necessitate numerical treatment of the wave propagation problem. Numerical simulations of ultrasonic wave propagation are used to investigate the influence of grain size and texture.