Exploring effects of In-situ Hygrothermal conditions on Fracture Toughness of Flax Fibre Epoxy Composite
G. Elangovan (TU Delft - Mechanical Engineering)
V. Popovich – Mentor (TU Delft - Mechanical Engineering)
J.A. Pascoe – Mentor (TU Delft - Aerospace Engineering)
Yasmine Mosleh – Mentor (TU Delft - Civil Engineering & Geosciences)
Lotfollah Pahlavan – Graduation committee member (TU Delft - Mechanical Engineering)
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Abstract
Composites are widely used in structural applications due to their high strength-to-weight ratio, stiffness, and design flexibility, but their sustainability remains a limitation. To address this, flax fibre-reinforced polymer (FFRP) composites offer competitive mechanical performance while being biodegradable and less energy-intensive to produce. However, the use of FFRPs in load-bearing structural applications is constrained, in particular, by the susceptibility of flax fibres to environmental conditions such as temperature and humidity, and by a limited understanding of their delamination behaviour in the primary loading modes. Therefore, this study investigates the interlaminar fracture toughness of FFRP composites under various hygrothermal conditions in Mode I loading.
The experimental analysis was conducted under environmental conditions representative of natural weathering, including hot-wet, hot-dry, room, and cold environments. The samples were initially conditioned at the respective hygrothermal conditions and subjected to quasi-static and fatigue loading in an environmental chamber. The results demonstrate a strong dependence of fracture toughness on the applied hygrothermal conditions, indicating that FFRP composites are highly sensitive to both temperature and relative humidity. Under quasi-static loading, the fracture toughness increased with higher humidity and lower temperature, indicating enhanced crack growth resistance due to moisture and improved
fibre bridging, while a reduction in fracture toughness was observed under low-humidity conditions. Fatigue results showed distinct Paris curves, with a rightward shift observed under high-humidity and low-temperature conditions, indicating improved resistance to fatigue crack propagation, whereas Paris curves corresponding to low-humidity environments shifted leftward, reflecting decreased resistance to fatigue crack growth.
Fractographic analysis using optical microscopy and scanning electron microscopy (SEM) revealed common microstructural features such as technical fibre bridging, fibre pull-out, yarn loosening, fibre patches, scarps, and matrix cracking. The nature of fracture transitioned from ductile under high humidity and elevated temperature to brittle at low temperature, highlighting a shift in the dominant failure mechanism from interfacial debonding to matrix-dominated cracking. Surface roughness measurements, however, exhibited considerable statistical scatter across all environmental conditions, likely due to the strong influence of technical fibre bridging on the measured roughness. Consequently, the observed changes in Mode I interlaminar fracture toughness with humidity and temperature were not clearly reflected in the roughness parameters.
Overall, the findings emphasise the strong dependence of the fracture behaviour of FFRP composites on environmental exposure. Understanding these effects is critical for the reliable design and durability prediction of FFRP composites in structural applications. The results contribute to establishing a foundational understanding of the fracture mechanics of FFRPs.