Determination of the elastic properties of thin layers and graded materials using generalized Love waves

摘要

The elastic properties of coatings and graded materials are very important in the design and evaluation for engineering purposes. It is well known that the velocity of the ultrasonic surface waves propagating in the layered structures and graded materials is strongly dependent on the elastic properties of the medium. Thus, by using an appropriate inverse algorithm, the elastic properties can be deduced from the measured phase velocity dispersion curves (dependence of velocity on frequency) of the surface wave. In this study we applied generalized shear surface waves (i.e., generalized Love waves) to investigate the elastic parameters of the layered media and graded materials. Generalized Love waves posses only one component of the mechanical displacement what is an advantage. Due to this reason, the mathematical description of the propagation of generalized Love waves is simpler than that using Rayleigh waves. In this article an inversion procedure for determining the elastic and geometrical parameters of thin coating layers from the measured dispersion curves of ultrasonic shear surface waves (i.e., Love waves) is presented. The inverse problem is formulated as an optimization problem with appropriately developed objective function. The objective function depends on the material parameters of the coating layer, frequency, and experimental data (phase velocity of the surface Love wave). The minimization of the objective function leads to a set of the optimum mechanical parameters of the thin layers (e.g., thickness, shear elastic constants). Good conformity between the experimental dispersion curves and those resulting from the inverse method can prove the correctness of the proposed inverse procedure.