PSI - Issue 82

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

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J. Best et al. / Structural Integrity Procedia 00 (2026) 000–000

Nomenclature CFRP Carbon fibre reinforced polymer PES Polyethersulfone (PES) ILFT Interlaminar fracture toughness UD-CF Unidirectional carbon fibre EC IN2 Easy Composites IN2 epoxy resin FVF Fibre volume fraction VaRTm Vacuum-assisted resin transfusion method SEM Scanning electron microscopy DMA Dynamic mechanical analysis ILSS Interlaminar shear strength G IC Mode I fracture critical energy release rate

1. Introduction Fibre Reinforced Polymer (FRP) composite materials see extensive use in aviation, high-performance automotive and marine applications, as well as in sporting goods, wind-turbines and more. FRPs are renowned for their high strength-to-weight ratio, excellent tensile strength, modulus, and resistance to corrosion and fatigue. FRP composites fall short however when it comes to the handling of out-of-plane loads and impacts, leading to premature and unexpected failures. These weaknesses have limited the potential applications of FRPs. Significant research has been conducted in attempts to understand and mitigate factors leading to FRP failure. The inhomogeneous, anisotropic nature of composite structures creates a various and complex array of failure modes. Common failure modes in FRP composites consist of interlaminar fibre/matrix delamination, matrix fracture, fibre breakage, and fibre pull out (Naebe et al., 2016). Researchers have utilised a wide range of different techniques to improve composite fracture properties, including the stitching together of laminate layers (Tan et al., 2012), modification of fibre surfaces (Downey and Drzal, 2016), the introduction of additional interlaminar layers (Xue et al., 2024) and the addition of strengthening particles into the composite matrix (Wang et al., 2024). The method of including interlaminar or “interleaving” polymer layers to improve composite fracture resistance is of particular interest to this research. This method has seen increasing attention in recent years, owing to the significant benefits it can bring to composite fracture properties, its ease-of-implementation and its general lack of detriment to composite mechanical performance (Palazzetti and Zucchelli, 2017). Interleaving polymer layers can be broken down into two sub-categories: nanofibre mats and polymer films. For a nanofibre mat it is often desirable to retain the original structure of the nanofibrous interleave, allowing the matrix to infuse and benefit from the mechanical properties of the nanofibre mat, and promoting other fracture resisting effects such as nanofibre-bridging and nanofibre-pullout (Beckermann and Pickering, 2015). For an interleaving polymer film, it is more desirable that the polymer homogenise with the epoxy matrix during composite curing (Cheng et al., 2020). This can create a range of new structures within the matrix, from isolated microspheres of strengthening polymer to fully phase-inverted structures depending on the concentration of additional polymer (Kinloch et al., 1994). These new structures can offer elevated matrix mechanical properties and promote fracture resisting mechanisms such as the inhibition of crack-tip propagation and crack diversion (Xue et al., 2024). In this work, the effect of interleaving polymer films of polyethersulfone on the mechanical and fracture properties of a CFRP composite manufactured using a standard room-temperature curing resin will be examined.