Axial Loading Tests and Simplified Modeling of Sandwich Panels with GFRP Skins and Soft Core at Various Slenderness Ratios

Experimental and analytical studies were conducted to investigate the axial strength and failure modes of sandwich panels with glass fiber reinforced polymer (GFRP) skins and soft polyurethane core. A total of 45 specimens of $78\times 150\mathrm{mm}$ cross-section and of lengths $L_e$ varying from 500 to 2,400 mm [i.e., slenderness ratio ($K{L}_e:r$) of 15–70, where $r$ is the radius of gyration] were tested concentrically using pin-ends ($K=1$). The effects of skin thickness and an internal GFRP rib on axial behavior were studied. The study first assessed the level of out-of-straightness and concluded that all specimens generally fall within an acceptable limit of $\mathrm{span}/\mathrm{1,000}$. A model based on sandwich panel theory, accounting for excessive shear deformations of the soft core, was used to predict axial strength at a wide range of $K{L}_e:r$ and then used in a parametric study. It was shown that short panels with $K{L}_e:r$ of 15–17 experienced local failure, outwards skin wrinkling in non-ribbed panels, or skin crushing in ribbed panels. Slender panels with $K{L}_e:r$ of 41–70 experienced global buckling followed by secondary local failure. Panels with intermediate slenderness (17–41) showed mixed failure modes. The load at which local failure governs at low $K{L}_e:r$ appears to be a constant, independent of $K{L}_{e}r$, whereas the global buckling load decreases significantly with increasing $K{L}_e:r$. Increasing skin thickness appears to be more effective at high $K{L}_e:r$ whereas increasing core shear modulus appears to be more effective at low $K{L}_e:r$.