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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aluminium mold, typically inside a vacuum bag. Four layers of carbon fibre plies were placed on both the top and bottom to form the outer layers of the composite, while four flax fibre plies were positioned in the middle. For both Mode I and Mode II testing, a layer of 3M™ Polyimide Film Tape 5413 Teflon tape was added to the centre plane of the composite fibre layup to act as a crack insert for the composite. The presence of the tape would prevent the bonding between the layers to a specified length (80 mm here) creating a crack between the central plies. Mode II samples were manufactured using eight UD carbon fibre plies and four flax fibre plies giving an average thickness of 5.35 mm. Flexural samples were made using four plies of flax fibres without any further reinforcement or Teflon Polyimide tape, resulting in an average lamina thickness of 3.1 mm. Due to the high viscosity of the epoxy resin, it was placed inside a pre-heated oven at 40°C for 60 minutes before mixing. The preheating adjusts the viscosity making the infusion process easier. The epoxy is then mixed with the reactive diluent and recyclable hardener. The recyclable hardener was added with a mass ratio of 100:22 (22 parts hardener for each 100 parts epoxy), as recommended by the manufacturer, similarly the reactive diluent was added at 10 wt.% of the epoxy (Saitta et al., 2022). A high-speed shear mixer (Silverson L4R mixer) was used to completely mix the three constituent elements. Mixing was first carried out at 800 rpm for 5 minutes and then at 1500 rpm for the next 10 minutes. Uniform mixing was ensured by adjusting the height of the mixer at regular intervals. The mixed blend was then degassed under vacuum for 5 mins, this process removed bubbles, aiding in ensuring the integrity of the finished composite. The resin is then infused into stacked fibre inside the vacuum bag. The composite lamina was cured at room temperature for 24 hours and then post-cured in a preheated oven at 100°C for 3 hours as recommended by the resin manufacturer. Post curried sample was then left to cool down to room temperature. The fully cured composite plate panels were then removed from the vacuum bag. The test samples were trimmed from the panel based on the respective test conditions following the standards. The average fibre volume fraction was obtained as 34.6%. The value was very much in keeping with those reported in the literature which generally ranges from 28% to 40% (Prasad et al., 2019; Ravandi et al., 2016; Saitta et al., 2022), though significantly lower than those reported for carbon fibre composites (Quan et al., 2020a). The likely cause of this difference is the hydrophilic nature of the flax fibres, resulting in a relatively more amount of resin being absorbed by the fibres. The weight of resin in the composite is, therefore, greater than it would be with a hydrophobic fibre, resulting in a decreased reduction in the calculated fibre volume fraction.

Fig. 1. Mode I interlaminar fracture test.

2.3. Characterization Mode I interlaminar fracture toughness test was performed on a Hounsfield universal testing machine (UTM) with a 10 kN load cell. The test samples were of length 150 mm, width 25 mm and thickness 7 mm. Aluminium blocks were glued to both ends and the crack insert region is properly marked on the sample. Measurements from the crack insert tip were then marked on the sample edge. The most crucial were the initial 15 mm and final 5 mm crack lengths.