Cyclic hygrothermal ageing of flax fibers’ bundles and unidirectional flax/epoxy composite. Are bio-based reinforced composites so sensitive?

Abstract

The use of vegetal fibers to reinforce polymeric matrix composites is challenging and goes with questions of durability issues, especially when exposed to damp conditions. The aim of this study is to quantify, up to 1 year, the impact of humidification/drying cycles, i.e. 3.5 days at 90% HR and 3.5 days at 40% HR, both at 55°C, on the longitudinal mechanical properties of a unidirectional flax/epoxy composite. Then, by a multi-scale analysis, the objective is to identify the causes of mechanical properties evolutions. According to the results of this study, a cyclic hygrothermal ageing induces irreversible degradations of fibers (matrix-embedded or not) and interfaces within the composite. Indeed fiber/fiber and fiber/matrix debondings have been observed by morphological analyses and a loss of fibers hydrophilic components has been highlighted by chemical and microstructural characterizations. A plasticization of flax fibers has also been shown during this kind of ageing. All these modifications induce a drop of composite's moduli after the first cycles of exposure and a decrease of ultimate tensile stress and strain after long time exposure. But these mechanical properties evolutions are much lower than the values found in the

Inverse approach for flax yarns mechanical properties identification from statistical mechanical characterization of the fabric

“…temperature, relative humidity) (Abida et al, 2020). Cadu et al (2019) showed that the morphology, the microstructure and the chemical composition of flax fiber bundles change with cyclic hygrothermal ageing (between relative humidity (RH) of 90% and RH of 40%). These modifications could be responsible for the mechanical behavior change according to water content (Abida et al, 2019;Célino et al, 2014;Perrier et al, 2017).…”

Why cyclic hygrothermal ageing modifies the transverse mechanical properties of a unidirectional epoxy-flax fibres composite?

“…On the unaged material (Figure 14-a), an imperfect fibre/matrix cohesion was observed in some very localized areas that exhibited residues of the cortex of the flax stem. This phenomenon was also highlighted for the 0° test samples (Cadu et al, 2019). After 1 week of ageing (Figure 14-b), fibre/fibre and fibre /matrix debondings appear larger and more numerous than for the unaged material.…”

“…As the shear strengths of the interface between two elementary fibres and of the interface between epoxy resin and flax fibres are respectively about 2.9 MPa (Beakou and Charlet, 2013) and 22.5 MPa (Antoine Le , these debondings may explain the drop in the ultimate tensile strength in the composite, observed after the first week of ageing thanks to the mechanical tests (Figure 2). Debonding can be due to the differential swelling between the fibres and the matrix during hygrothermal ageing cycles (Cadu et al, 2019;Li and Xue, 2016) and the degradation of the covalent bonds created between the epoxy and amine functions of the resin and the hydroxyl functions of the fibres (Antoine Le . The decrease in the ultimate tensile strength of the composite observed between the first and the ninth week indicates that the debonding phenomenon must be accentuated during these ageing cycles.…”

“…More recently, Cadu et al (Cadu et al, 2019) and Koolen et al (Koolen et al, 2020) have carried out hygrothermal ageing cycles in order to stimulate the differential swelling inside the epoxy/flax composite. These ageing cycles seem to lead to less dramatic damage evolution.…”

“…After 52 cycles (one cycle is composed of a 3.5 days step at 90% RH followed by a 3.5 days step at 50% RH) of ageing at 55°C, i.e. one year, Cadu et al (Cadu et al, 2019) have determined a slight decrease in longitudinal mechanical performance of a unidirectional composite. The modulus and ultimate tensile strength respectively drop by 12% and about 13%.…”