An ultrasonic method, which allows the determination of the in-plane elastic stiffnesses 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. With this technique, the shear coupling coefficients can be independently calculated, and the validity of the orthotropic assumption can be tested. 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.

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