Physical properties of interlocking core structures of flax fibre-reinforced polylactic acid composite

In the composite industry, natural fibre flax has great potential to replace synthetic fibres like carbon and glass due to their versatile characteristics like biodegradability, lightweightness, low cost, and environmentally friendly materials. Therefore, this thesis presents the findings of investigating the compression and fire properties of a range of sandwich structures made from flax fibre reinforced polylactic acid (flax/PLA) composite. Initial testing was focused on the effect of variation in manufacturing processes on the tensile properties of flax/PLA composites. Then, a novel double cell wall interlocking square core with three different cell sizes and honeycomb cores with three different lengths were fabricated and tested under quasi-static compression loading to determine their strength and specific energy absorption (SEA) capabilities. Following this, the interlocking square and honeycomb cores were filled with two different high-density foams and their strength and SEA values were determined. Furthermore, the fire properties of empty interlocking square and honeycomb core sandwich structure composites were investigated by cone calorimeter measurements at three different heat fluxes. The findings on tensile testing revealed that flax/PLA composites manufactured at a temperature of 180 ℃ and pressed for 10 mins gave the highest value of tensile strength, whereas a temperature of 190 ℃, pressed for 15 mins, was the lowest. Furthermore, testing on empty and foam-filled double cell wall interlocking square core sandwich structures showed that empty double cell wall significantly increased the compressive strength and SEA values for foam-filled interlocking square core increased up to 126 % and 64 %, respectively, as compared to the empty core structure. It was also demonstrated that the higher the foam density filled in a smaller cell size core structure, the greater the effect on its strength and SEA. In addition, the variation of the cell core sizes was more pronounced in the case of the empty core structure compared to their foam-filled core counterparts. It was also observed that the average SEAs of foam-filled and empty honeycomb cores showed no significant difference, lying around 8.7 kJ/kg and 8.2 kJ/kg, respectively. In contrast, its compressive strength was clearly pronounced, in which the foam-filled core outperformed the empty counterparts by around 33 %. Clearly, the addition of foam inside both core structures improved the literal support to the core cell wall, resulting in a considerable delay in cell wall buckling and a significant improvement in strength and SEA values relative to the empty core structures. The mechanical strength and SEA values of these structures compare very favourably with data from tests on a wide range of cores with and without foam reported in the literature. Finally, the cone colorimeter and the results showed that, in terms of time-to-ignition and heat release rate (HRR), the honeycomb core structure performed better than the square core structure in almost all three heat fluxes. However, this advantage was only really noticeable in the case of 35kW/m2, where the time–to–ignite was higher by 17 seconds. It is critical to note that the skin of sandwich structures played a significant role in increasing HRR; as a result, improving the skin of the sandwich structures by applying fire retardant is necessary for these structures to be used in engineering applications.

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