Experimental Evaluation of Elastic and Shear Properties of Expanded Polystyrene Foam

Rapid prototyping using foam could be a very useful tool for the engineering experimental research community, owing to its cost-effectiveness and the present-day availability of precise laser cutting technologies and advances in computer numerical control (CNC). Thus, Expanded Polystyrene (EPS) foam was chosen to model scaled-down wing geometries for the flutter test in a 0.7m X 0.7m open-jet wind-tunnel at Virginia Tech. The experiences from this test would later be leveraged in studying full-scale, composite, inflatable wing at a 1.85m X 1.85m Stability Wind Tunnel facility at Virginia Tech. Since the structural dynamic behavior of the model is highly dependent on its material properties, ASTM-based material tests were carried out to determine material properties of the EPS foam of density 1 pcf, used in the model. For the elastic modulus (E), a three-point bending test was carried out as per ASTM C203, using a standard universal testing machine. The determination of the shear modulus (G) proved to be more challenging, and, after some preliminary studies, ASTM C393/C393-M-16 was used, together with digital image correlation (DIC) for non-contact mid-point deflection measurement. These tests yielded the results of E≈967 psi and G≈447 psi, compared to the respective values of 895 psi and 305 psi, estimated by correlating the experimental modal data and numerical vibration modal results from tuning the material properties of its finite element model. It was noted that, compared to the manufacturer-provided material data (E≈200 psi and G≈300 psi for foam with density 1pcf), the foam elastic modulus from the above two approaches are in better agreement. However, the material shear modulus appears to be more sensitive to experimental uncertainties, and hence, leads to more disparity. These observations demonstrate the importance of carrying out in-house material tests when the system dynamics are highly dependent on the material properties involved.