In-Plane Bending and Failure Mechanism of Sandwich Beams with GFRP Skins and Soft Polyurethane Foam Core

The out-of-plane bending of sandwich panels has been widely investigated, but studies on their behavior under in-plane loading are quite scarce. In this study, 18 sandwich beam specimens were tested under three- and four-point in-plane-bending. The specimens are composed of low-density polyurethane core and glass fiber-reinforced polymer (GFRP) skins on both sides. The shear span-to-depth ($a/h$) ratio was varied from 1.33 to 4.67. Two different skin thicknesses were also used, providing beam depth-to-skin thickness ($h/t$) ratios of 47 and 94. It was found that the ultimate moment reduced by approximately one third as the $a/h$ ratio increased from 1.33 to 4.67 in sandwich beams with an $h/t$ ratio of 94 and decreased by about one quarter in beams with an $h/t$ ratio of 47. Doubling the skin thickness [i.e., from $h/t$ of 94–47] increased the moment capacity significantly. The percentage increase ranged from 55 to 95% as $a/h$ increased from 1.33 to 4.67. Failure of sandwich beams occurred by skin wrinkling in compression, mostly symmetrically, where skins move in opposite directions. A simplified analytical model accounting for shear and flexural rigidities of the sandwich beam and a skin wrinkling failure criterion in compression was developed and validated against experimental results.