Strains and deflections of GFRP sandwich panels due to uniform out-of-plane pressure

Rectangular orthotropic glass fiber reinforced plastic sandwich panels were tested under uniform out-of-plane pressure and the strains and deflections were compared with those from finite-element models of the panels. The panels, with 0.32 cm (0.125 in.) face sheets and a 1.27 cm (0.5 in.) core of either balsa or linear polyvinylchloride foam, were tested in two sizes: 183 x 92 cm (72 x 36 in.) and 121 x 92 cm (48 x 36 in.). The sandwich panels were fabricated using the vacuum-assisted resin transfer molding technique. The two short edges of the sandwich panels were clamped, while the two long edges were simply supported. Uniform external pressure was applied using two large water inflatable bladders in series. The deflection and strains were measured using dial gages and strain gages placed at quarter and half points on the surface of the panels. Measurements were made up to a maximum out-of-plane pressure of 0.1 MPa (15 psi). A total of six balsa core and six foam core panels were tested. Finite-element models were constructed for the 183-cm-long panel and the 121-cm-long panel. Correlation between numerical and experimental strains to deflect the sandwich panel was much better on the top (tensile) side of the panels than on the bottom (compressive) side of the panels, regardless of panel aspect ratio or core material. All sandwich panels exhibited the same compressive strain reversal behavior on the compressive side of the panel. This phenomenon was thought to be due to nonlinearly induced micro-buckling under the strain gages, buckling of the woven fabric, or micro-cracking within the resin.