An Ultrasonic Technique for Testing the Orthotropic Symmetry of Polymeric Sheets by Measuring Their Elastic Shear Coupling Coefficients

An ultrasonic method which'allows one to determine the in-plane elastic stiff- nesses of thin polymeric sheets is described. The determination is complete, as it includes the shear coupling coefficients. Sheets are often assumed to display orthotropic symmetry, which means that the shear coupling coefficients are zero along the principal axes. The shear coupling coefficients can now be independently calculated, and there is a test of 'the validity of theorthotropic assumption. A quantity called the nonorthotropic angle is presented as a coordinate-independent measure of the lack of orthotropic response. Results from a polyester sheet demonstrate that significant nonorthotropic behavior is encountered in commercial plastic sheets. Stiffnesses calculated from tests conducted on laminated sheets are shown to be in good agreement with stiffnesses predicted from measurements on individual plies. The technique is argued to be self-consistent and an attempt is made to convince the reader that this is a legitimate way to characterize all the in-plane elastic stiffnesses of thin sheets.