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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Flexural properties of the composites were studied using the Zwick-Roell Z005 UTM with a 10 kN load cell. The samples were of length 100 mm, width of 15 mm and thickness of 3 mm. The span length for the three-point test was 80 mm. The test was run at a crosshead speed of 2 mm/min until the sample failure was observed. The delaminated surface of the Mode I testing was observed using the Scanning electron microscopy (SEM) analysis. Emitech K575X was used for the gold sputtering and the SEM analysis was performed using Hitachi TM4000Plus operated at an accelerating voltage of 15 kV. 3. Results and discussions 3.1. Mode I Interlaminar fracture toughness The Mode I interlaminar fracture toughness of the flax fibre biobased composites with and without the addition of PPS veils are studied from the DCB test results. The introduction of the PPS veils significantly improved the Mode I interlaminar fracture toughness values. Fig. 4 presents the typical force-displacement curve from the DCB test and Fig. 5 shows the average interlaminar fracture toughness of the flax fibre composites with and without the PPS addition. PPS0 denotes the flax fibre biobased composites without any PPS veils and the PPS 5 denotes the flax fibre composites with PPS veils at 5 gsm, similarly for PPS 10 and PPS 20. The force-displacement curves of the composite sample with PPS veils interleaved show higher force values during the delamination signifying more resistance to the crack advancement. The addition of PPS veils at any density has a positive impact on the interlaminar fracture toughness values of the composite. A clear trend is visible whereby the interlaminar toughness improves as the areal density of the added veil increases. The addition of PPS veils at 5 gsm, 10 gsm and 20 gsm increased the fracture toughness values by 32.7%, 41.2% and 59.4% respectively. The fracture surface was observed using scanning electron microscopy to understand the mechanisms involved and the changes seen in the neat samples and PPS interleaved samples. Fig. 6 shows the delaminated surface of the composite samples at varying PPS densities. It can be noted that the PPS veils bridging is the mechanism that causes additional energy for delaminating the surfaces for higher PPS areal densities. The main strengthening mechanisms noted were both fibre pull-out and fibre breakage. From Fig. 6(a) flax fibre pullout can be noted, this is a result of the flax tow being fibrous, which results in the interface between the fibre and matrix not being as clean as traditionally manufactured fibres such as glass or carbon fibres. Additionally, worth mentioning is the poor adhesion of the matrix material to the flax fibres, this is noted by the small amounts of matrix material remaining attached to the fibres after breakage. Similar findings of the clean surfaces were noted in the literature, with poor fibre matrix adhesion seen as the prime reason for poor interlaminar fracture toughness for neat flax fibre composites (Prasad et al., 2019, 2020).

Fig. 4. Mode I interlaminar fracture toughness of flax fibre composites at different areal densities of PPS veils.