PSI - Issue 82

Jet Best et al. / Procedia Structural Integrity 82 (2026) 98–106 J. Best et al. / Structural Integrity Procedia 00 (2026) 000–000

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2. Experimental 2.1. Materials

The non-crimp unidirectional carbon fibres, U-C-PB-317g/m²-1270 mm of areal weight 317 g/m 2 and carbon fibre bundle type 24 K were sourced from Saertex GmbH, Germany. The carbon fibre had a ply thickness of 0.17 mm and density of 1.78 g/cm 3 . IN2 Epoxy Infusion Resin was sourced from Easy Composites UK. The density of the resin is 1.12 g/m 3 at 20 ℃. Polyethersulfone (PES, Ultrason® E2020P SR) granules were supplied by BASF Germany, supporting this research. N,N- dimethylformamide (DMF, 99.5 wt%) was sourced from Sigma Aldrich. 2.2. Epoxy characterization, PES film production & phase-separation Easy Composites recommended a curing scheme of 25℃ for 24 hours and post-curing of 100℃ for 3 hours for IN2 resin. Research by (Cheng et al., 2019) demonstrated that PES phase-separation would occur in E51 resin once a temperature of 120℃ was reached. Thus, it was decided to modify the curing scheme of the IN2 resin. DMA testing was conducted on IN2 resin samples cured under the manufacturer's recommended scheme; 120℃ for 3h; and 120℃ for 3h followed by a 180℃ post-cure, to determine the resin’s response to the elevated curing temperatures necessary for PES phase separation. Production of PES films involved drying the PES granules at 140℃ for 4h. PES of 10 wt% is dissolved into a 90 wt% DMF solvent by mechanical stirring for 1 hour. The solution is poured onto a custom-made polycarbonate tray, where it is spread using a glass rod. The thickness of the film is controlled by placing metal strips of specific thickness at the edges of the tray over which the glass rod is rolled. Once the solution is spread out, the tray containing the solution is then immediately immersed in a de-ionised water bath. A film of PES immediately forms as the solution contacts the water. After 24 hours, the film is removed from the water bath and dried at 60℃ for 4 hours. The phase separation behaviour of PES films of varying thicknesses was tested under different curing regimes. Ultimately, a curing scheme of 140℃ for 3h was used for composite sample production. 2.3. Composite Fabrication The cold vacuum-assisted resin transfer moulding method was used to fabricate the carbon fibre epoxy composites. The epoxy and amine hardener were mixed in the ratio of 100:30 and mechanically stirred for 15 minutes. Composites were cured at 140 ℃ for 3 hours. For the relevant samples, interleaving PES films were included between carbon fibre fabric layers during layup. For both interlaminar shear strength (ILSS) and 3-point bending tests, the stacking sequence consisted of 8 layers of carbon fibres for reference samples. For PES interleaved samples, PES films were included after every second layer of carbon fibre (a total of three films). For Mode I fracture toughness tests, samples consisted of 16 layers of carbon fibre. PES interleaved specimens contained a single layer of PES film at the mid-layer. Test specimens were cut from the composite lamina according to the recommended dimensions of the respective ISO standards. FVF of Mode I control and PES interleaved samples were 68% and 75% respectively. FVF of ILSS and flexural control and PES interleaved samples were 75% and 65% respectively (Gill et al., 2009). 2.4. Characterisation The flexural strength and modulus of the composite samples were evaluated by conducting the ISO 14125 three point bending test. Tests were performed on five samples with a thickness of 2.5 mm, width 15 mm, and length 100 mm. Support span was set as 80 mm, and the rate of crosshead displacement as 1 mm/min. The interlaminar shear strength (ILSS) properties of the composite samples were studied following ISO 14130. Sample thickness was 2.5 mm, with sample width five times the thickness, and overall length ten times the thickness. Support span was set as five times the thickness, and the rate of crosshead displacement as 1 mm/min. The Mode I fracture toughness of the composite samples was determined according to ISO 15024. A crack was simulated by applying Kapton (Teflon) tape along the edge of the middle ply stack sequence. Samples were cut to dimensions of length 150 mm and width 25 mm. Load blocks were affixed using adhesive. Correction fluid was applied