Structural Behavior of Sandwich Beams with Flax Fiber–Reinforced Polymer Faces and Cardboard Cores under Monotonic and Impact Loads

To meet the ever-increasing demand for more environmentally conscious building designs, it is important that sustainable building material options be available. Natural and recycled materials can be used in sandwich panels to reduce their environmental footprint. This study featured experiments with 12 sandwich beams constructed with flax fiber–reinforced polymer (FFRP) faces and recycled corrugated cardboard cores under monotonic and impact loading. Each sandwich beam was 1,200 mm long and 150 mm wide, and was constructed of two-layer FFRP faces and a 75-mm-thick corrugated cardboard core. Six specimens were prepared using a plain cardboard core and six with a waxed cardboard core. Two separate test methods were employed in this study: a three-point bending test and a drop-weight impact test. Three specimens of each type with a span length of 1,120 mm of each type were tested under monotonic load. The load was applied through a 150-mm-wide steel hollow structural section (HSS) and was measured with a 250 kN load cell. The midspan deflection was measured with a string potentiometer, and the strains in the top and bottom faces at midspan were measured using strain gauges. The monotonic test data were recorded at a rate of 10 Hz. Three specimens of each type were tested under a drop-weight impact load. The drop weight was applied to the midspan. To match the monotonic tests, the drop weight was affixed with a 150 mm HSS loading surface. The midspan displacement was measured with a fast-action string potentiometer, and the midspan face strains were measured using strain gauges. The impact data were recorded at a rate of 25 kHz. In addition, each impact test was filmed with high-speed video (500 frames per second). The residual monotonic flexural behavior after impact was also investigated for specimens that survived the impact testing (that is, they were additionally tested under monotonic three-point bending). The results of the tests were compared with the results of similar tests on sandwich beams with conventional petroleum-based foam cores and showed that the cardboard core beams behaved similarly to the foam core beams. It was determined that core manufacturing and specimen preparation had a significant effect on the overall specimen behavior and potentially caused premature failure in some of the tests. The residual monotonic tests of the specimens after impact showed that there was no significant reduction in specimen strength or stiffness after an impact event. Existing models used for predicting the behavior of foam-core FFRP sandwich beams were used to predict the behavior of the cardboard specimens tested in this study.