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

182 10

modulus follows closely the trends of the flexural strength of the composite. Combining the results of both flexural strength and flexural modulus the addition of PPS has a clear impact. From the literature, the flexural properties of a composite are largely reliant on the transfer of stress from the matrix to the fibres (Quan et al., 2020b), it is unclear in what way the addition of PPS veils is influencing this and as such gives rise to potential future work on the subject. 4. Conclusions Natural fibre-based composites have a large role to play in the future of lightweight materials, due to their inherent sustainability. Recycling at the end of life was examined from our previous works (Saitta et al., 2022) and it was found that most composites were being sent to landfills or incinerated. Flax fibres in a biobased resin matrix were therefore identified as an eco-friendly and recyclable solution, due to acid cleavable nature of the crosslinks in the matrix. Material properties were also investigated with interlaminar fracture toughness identified as a key drawback of the material. Three mechanical tests, Mode I, Mode II interlaminar fracture toughness and flexural properties, were investigated as key to characterizing the mechanical performance of composite materials. The addition of non-woven veils was selected due to the method having the greatest potential as the materials were accessible and required minimal changes to standard manufacturing processes. The Mode I fracture toughness of the composite saw large improvements from the addition of PPS veils at all areal densities. PPS20 resulted in the greatest increase, with a 59% increase in fracture toughness over the reference sample. PPS5 and PPS10 saw similar improvements equating to 33% and 41% respectively. Scanning electron microscopy was carried out on the fracture surface of the highest-performing sample of each areal density. Flax fibre pull-out was noted in all samples, this was due to the fibrous nature of the flax fibre tow. The low density of PPS fibres in the PPS5 sample made it difficult to confidently identify strengthening mechanisms though some grooves suggesting PPS fibre pull-out were noted. PPS pull-out was noted in both the PPS10 and PPS20 samples, the clean nature of the pull-out grooves suggested poor PPS fibre matrix adhesion. Damage to the matrix material due to PPS fibre entanglement was seen in both samples, fibre breakage was also noted in PPS fibres whose directionally was perpendicular to the crack growth direction. Testing the effect of different areal densities of PPS veils on the Mode II fracture toughness of the composite, it is clear that the addition of PPS veils of any density will result in an increase in fracture toughness over that of the reference. Additional improvements in fracture toughness were achieved by the addition of higher areal density veils with 20 g/m 2 resulting in the greatest increase in fracture toughness over the reference values. The reference samples without any PPS veil addition resulted in an average fracture toughness of 1523.6 J/m 2 . The addition of PPS veils at areal densities of 5 g/m 2 , 10 g/m 2 , and 20 g/m 2 , resulted in fracture toughness values of 1532.3 J/m 2 , 1668.3 J/m 2 , and 1724.8 J/m 2 respectively, equivalent to increases of 1, 9 and 13% over the reference. From the results, the addition of PPS veils leads to a small but positive influence in both the flexural strength and flexural modulus of the composite over the reference. A general trend of increased flexural strength and flexural modulus with an increase in PPS density was noted. The inclusion of PPS veils with an areal density of 20 g/m 2 led to the greatest improvement in both properties, with the flexural strength of the composite increasing by 15.5% while the flexural modulus increased by 21.8%. These increases equated to values of flexural strength and flexural modulus of 114.5 MPa and 6.48 GPa respectively. Acknowledgements Thanks to Technical Fibre Products, UK (Advanced Materials, UK), for providing the PPS veils used in this research. References AL-Oqla, F.M., Sapuan, S.M., 2014. Natural fiber reinforced polymer composites in industrial applications: feasibility of date palm fibers for sustainable automotive industry. Journal of Cleaner Production 66, 347–354. https://doi.org/10.1016/j.jclepro.2013.10.050 ASTM D2344, n.d. ASTM D2344/D2344M-22 Standard Test Method for Short-Beam Strength of Polymer Matrix Composite Materials and Their Laminates. ASTM Int. Beylergil, B., Tanoğlu, M., Aktaş, E., 2017. Enhancement of interlaminar fracture toughness of carbon fiber–epoxy composites using polyamide 6,6 electrospun nanofibers. Journal of Applied Polymer Science 134, 45244. https://doi.org/10.1002/app.45244 Clyne, T.W., Hull, D., 2019. An Introduction to Composite Materials [WWW Document]. Higher Education from Cambridge University Press.