Structural Behaviour of Sandwich Structures Constructed of Bio-Based Materials Under Monotonic and Impact Loads
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As a part of this thesis project, 69 sandwich specimens with flax fibre-reinforced polymer (FFRP) faces were manufactured, tested and analysed, specifically 57 one-way sandwich beams (1200 mm long x 150 mm wide x 80 mm thick) and 12 two-way sandwich panels (1200 mm x 1200 mm x 80 mm thick). The cores were made of either polyisocyanurate (PIR) foam or corrugated cardboard. A total of 1192 tests were performed as a part of this thesis research, including quasi-static three-point bending tests of one-way beams, quasi-static concentrated loading of two-way panels, impact loading of one-way beams, impact loading of two-way panels, and post-impact flexural testing. The testing program showed that sandwich structures with FFRP faces are viable alternatives to sandwich structures constructed with synthetic FRP faces. They exhibit high relative strength and resiliency. The structures were modelled analytically and numerically. For the one-way behaviour of the sandwich structures, a design-oriented analysis procedure was developed which can be feasibly used by practicing engineers. Additionally, a nonlinear energy balance model to predict the deflection of FFRP-foam sandwich beams under impact was developed. For the two-way behaviour of the FFRP-foam sandwich structures, the Mindlin Plate Theory was used to create a nonlinear model to predict the flexural load-deflection and load-strain responses. However, the model was not able to predict the localized deformation and failure present in thick-faced sandwich panels. Therefore, a finite element (FE) model was created to predict the quasi-static and impact behaviour of the panels and was verified using the test data. Based on the FE model, a parametric study was performed to observe the effect of core density, core thickness, face thickness and loading size. Panels with low-density cores were more susceptible to face wrinkling failure and panels with high-density cores were susceptible to both tensile rupture and core shear failure. It was also shown that the impulse duration and maximum displacement experienced under low energy impacts increased with a decrease in core thickness, face thickness and core density.