PSI - Issue 68

Robert Lowe et al. / Procedia Structural Integrity 68 (2025) 173–183 R. Lowe et al. / Structural Integrity Procedia 00 (2025) 000–000

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While many differing methods of improving interlaminar fracture resistance have been discussed, each comes with drawbacks. An additional method that has been shown to greatly improve interlaminar fracture resistance is the interleaving of veils (Quan et al., 2020d). Interleaving involves the addition of an interlayer between the layers of plies within the composite before infusion (Quan et al., 2020a). Thermoplastic veils are groups of extremely fine fibres which can be random or orientated (Kuwata and Hogg, 2011) with low areal densities ranging from 5 to 20 g/m 2 . The addition of thermoplastic veils to the composite does not require substantial change to the manufacturing procedure and so is a cost-effective and accessible method (Quan et al., 2020d). The toughening effect of the veil material is reliant on the adhesion between the thermoplastic material and the matrix. In NFRCs the fibre matrix interaction is poor, leading to low adhesion between the two, as a result, NFRCs are prone to delamination which severely hampers further application (Prasad et al., 2019). The interleaving of veils between fibre layers improves interlaminar fracture toughness while minimizing the impact on the in-plane properties of the composite (Quan et al., 2020a). This research aims to investigate the effect of the addition of PPS veils at different areal densities on the mechanical properties of the final composite. The project's broad goal is to characterize the mechanical properties of the composite and ascertain how these are impacted by the addition of PPS veils of varying areal densities. To the best of the author's knowledge, there weren’t many studies that highlight the use of veils that evaluate their effects on natural fibres and recyclable epoxy resin. The properties of the material will be characterized through interlaminar fracture toughness in both Modes I and II, in addition to flexural testing. The strengthening mechanisms will be discussed and characterized based on the fracture surfaces of the test samples using scanning electron microscopy. Additionally, the interesting fact is that the PPS is a thermoplastic material that can be easily recycled at the end of use. The chemically cleavable thermoset epoxy also allows the recycling of the matrix at the end of life (Ferrari et al., 2021; Saitta et al., 2022). The composite is therefore fully recyclable and environmentally friendly. Due to the potential of the material, the goal of the project is to mechanically characterize the impact of PPS veil addition at varying densities through empirical methods. Nomenclature NFRC Natural Fibre Reinforced Composites TiO 2 titanium dioxide PPS Polyphenylene Sulphide UTM Universal Testing Machine SEM Scanning Electron Microscopy Plain weave flax fibre mat with an average thickness of 0.75 mm and an areal density of 400 g/m 2 is used as reinforcement material. The density of the flax fibre is 1.45 g/cm 3 . Matrix materials included Polar Bear biobased epoxy, recyclable hardener Recyclamine TM R*101 and a reactive diluent, Polar Bear RD, purchased from R*Concept, Spain. These materials have a bio-content of over 28%, with the hardener allowing the matrix to be cleavable by a mild acid, allowing for recycling of the matrix without damage to the fibres. PPS veils were received from Technical Fibre Products, UK, for use as an interleaving material with areal densities of 5 g/m 2 , 10 g/m 2 and 20 g/m 2 . 2.2. Composite fabrication The flax fibres were cut into respective sample dimensions and were dried in a hot air oven before the composite fabrication at 110 degrees for 3 hours to remove any moisture content. For the Mode I and Mode II test samples, additional layers of unidirectional carbon fibres were stacked on both sides so that the samples restrict the bending failure and completely see the delamination failure during the interlaminar fracture tests. The fibre-reinforced composites are manufactured by cold vacuum-assisted resin infusion technique. The fibres are laid out in an 2. Experimental 2.1. Materials