Inversion of ultrasonic, plane-wave transmission data in composite plates to infer viscoelastic material properties

Stiffness and damping properties of viscoelastic materials are given by the real and imaginary components, respectively, of the material constants. A new technique is proposed to experimentally measure the real and imaginary components of anisotropic (and isotropic) viscoelastic plates. Main advanta ge of this technique is that material properties of thin plates can be meas ured where many other techniques fail. Material properties are obtained by numerically inverting the transmitted ultrasonic fields, obtained for diffe rent incident angles. Simplex inversion algorithm is applied to initial est imates of plate thickness and plate properties. By this iterative technique the values of the unknown parameters (material properties and plate thickn ess) are continuously modified to give better agreement between the experim ental and theoretical transmitted fields. After a certain number of iterati ons the speed of convergence of the Simplex scheme is significantly reduced . To improve the accuracy of convergence the Newton-Raphson inversion techn ique is adopted at that point. By this technique material properties of dif ferent types of plates are measured. These is a glass plate (isotropic plat e with no damping), a polymer plate (isotropic plate with damping), and gla ss fiber reinforced epoxy plates with different fiber orientations (anisotr opic plates with damping). Both real and imaginary components are successfu lly measured for all these plates. In a relative scale the measurement erro r for the imaginary components is higher. Reliability of the measured mater ial constants of fiber reinforced epoxy plates is verified by the method of invariance. All experiments are carried out in the frequency range that is appropriate for satisfying two conditions-the specimen homogeneity and the plane wave conditions. (C) 2000 Elsevier Science Ltd. All rights reserved.