Effects of dispersion on the inference of metal texture from S0 plate mode measurements. Part I. Evaluation of dispersion correction methods.

Ultrasonic S0 waves (fundamental symmetric Lamb modes) are being considered in several laboratories for the nondestructive characterization of the texture (preferred grain orientation) and formability of metal sheets and plates. In a typical experimental setup, the velocities of the S0 waves are measured as a function of wave propagation angle with respect to the rolling direction of the plate. However, the S0 waves are known to be dispersive, and that dispersion must be considered in order to isolate the small, texture-induced shifts in the S0 wave velocity. Currently, there are two approximate dispersion correction methods, one proposed by Thompson et al. [Met. Trans. A 20, 2431 (1989)] and the other introduced by Hirao and Fukuoka [J. Acoust. Soc. Am. 85, 2311 (1989)]. In this paper, these two methods will be evaluated using an exact theory for wave propagation in orthotropic plates. Through the evaluation, the limits of the current texture measurement techniques are established. It is found that when plate thickness to wavelength ratio is less than 0.15, both Thompson's and Hirao's methods work satisfactorily. When the thickness to wavelength ratio exceeds 0.3, neither Thompson's nor Hirao's dispersion correction method provides adequate corrections for the current texture measurement techniques. Within the range of 0.15-0.3, Thompson's method is recommended for weakly anisotropic sheets and plates and Hirao's method may be more appropriate for some strongly anisotropic cases.