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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The fracture surface of the PPS5 composite sample is shown in Fig. 6), the presence of PPS fibres in small amounts can be noted. The strengthening mechanism of the PPS veils is not entirely clear, though some markings from possible PPS fibre pull-out can be seen, along with the flax fibre pull-out. A much clearer impact of PPS veils pull-out is visible on the delaminated surface of the PPS10 sample as in Fig. 6(c). This is noted by the presence of smooth-sided cylindrical grooves in the matrix. This suggests poor adhesion between the matrix and the PPS fibres, as little damage has been done to the matrix by this pull-out. Quan et al. (2020d) noted similarly poor adhesion concluding the smoothness of the PPS fibre to be the leading cause. PPS pull-out is also notable by the directionally of some of the PPS fibres, with many directed upwards towards the opposite arm of the DCB sample suggesting extensive crack bridging. Some debris from the matrix can be seen suggesting a small amount of matrix cracking may have occurred. The fracture surface of the PPS20 sample is shown in Fig. 6(d) at 500x magnification, with the highlighted area being shown in greater detail at 1800x magnification in Fig. 6(e). As in the PPS10 sample, matrix cracking and debris can be noted in Fig. 6(d). This is again confined to areas with a high density of PPS fibres, perhaps suggesting that the entanglement of the fibres may have played a role. It is unlikely that such damage to the matrix would otherwise occur due to the poor adhesion between the matrix and PPS fibres (Quan et al., 2020a). PPS fibre pullout is again evidenced by grooves in the matrix material. Additionally, some fibre breakage appears to have occurred, this can be seen in Fig. 6(e) by the clear damage to the ends of the fibres. In Fig. 6(d), the difference in direction between the long pulled-out fibres and those that are broken should also be noted. 3.2. Mode II interlaminar fracture toughness The Mode II interlaminar fracture toughness of the flax fibre composites with and without the PPS veils are studied, under the shear mode condition using the three-point bending test. Fig. 7 shows the average Mode II interlaminar fracture toughness values for the composites with and without the PPS addition. Compared to the strengthening mechanisms provided by PPS veils in Mode I delamination, their contribution in Mode II is relatively minor, though still notable. The greatest average level of toughening was seen in PPS20, with a 13% increase over the reference from 1523.6 J/m 2 to 1724.8 J/m 2 as shown in Fig. 7. The addition of PPS veils at 10 g/m 2 resulted in an average increase of 9% over the reference, while PPS at 5 g/m 2 led to a 1% increase in interlaminar fracture toughness. The improvements seen are decidedly less than those achieved in Mode I testing, this is likely due to fibre bridging not being the main strengthening mechanism in Mode II (Kuwata and Hogg, 2011), and so the ability of the veils to improve fracture toughness is greatly reduced. From Fig. 7, PPS5 is a clear outlier resulting in minimal effect on the toughness of the material. Investigating further, the PPS5 samples held the best repeatability with the lowest standard deviation of the four composite variations. Looking at the literature, Quan et al. (2020b) investigated the effect of thermoplastic veil addition to non-crimp carbon fibre. The results showed a clear increase in fracture toughness from the introduction of PPS veils at any areal density, though the increase from PPS5 was less than that from higher densities. Assuming this trend would continue with flax fibre composites a larger increase may have been expected, perhaps suggesting some discrepancy in the manufacturing of the PPS5 composites. Factors such as fibre moisture content, fibre-matrix interaction, and fibre wetting differ between natural and synthetic fibres, which could influence the results. The small nature of the improvement seen across all the composites means the manufacturing process could have a large effect on test outcomes. Due to the large number of steps in the process combined with the inconsistency of the flax itself due to its natural origins, the variability between samples is incredibly difficult to route out. In the case of natural fibres, the delamination was not well controlled, with instances of rapid delamination, particularly in the Mode I samples. This could be attributed to poor fibre-matrix interaction between the hydrophilic flax fibres and the hydrophobic matrix. The rapid delamination made it difficult to monitor crack progression accurately, leading to deviations from the designated crack paths. As a result, large standard deviations were observed in the data. Based on this it could be recommended to use more samples than the 5 dictated by the standard when testing natural fibres.