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

Abstract

The development of composites based on plant fibres has considerably grown in last decades; however, their high sensitivity to moisture limits their use in structural applications. The objective of this study is to determine and understand the impact of moisture variations during one-year cyclic hygrothermal ageing i.e. 52 cycles on the transverse properties of a unidirectional epoxy flax composite. A cycle consists of 3.5 days at 90% RH followed by 3.5 days at 40% RH. Compared to the studies usually carried out to determine the impact of ageing on the longitudinal properties of unidirectional composites, the determination of the transverse mechanical properties during this type of ageing allowed to focus on the contribution of the matrix and the interface on the evolution of the composite properties. The multi-scale analyzes provided identification of the phases responsible for the evolution of transverse mechanical properties induced by ageing. The transverse ultimate tensile strength drops by about 20% after the first week of ageing and slightly decreases up to 2 months. This evolution seems to be induced by the creation of flaws, in particular the phenomena of fibre-fibre and matrix-fibre debonding and the creation of cracks in the fibres initiated at the lumen level, revealed by SEM. The transverse modulus also strongly drops after

A review of the effect of hydrothermal aging on the mechanical properties of 2D fiber‐reinforced resin matrix composites

“…The unaged FFRP composite appeared to show "cracks" along the flax fibers due to the formation process of the composite, where the cavities and lumens of the flax fibers are flattened to look like embedded cracks. 111 Consequently, stress concentration occurs at the cracks, and the prolonged action of moisture and degradation of the binder material in the fibers further promotes the rupture of the fiber portion along the lumen, resulting in a loss of tensile strength. After aging at 60 C for 2 weeks, the sample F I G U R E 1 3 SEM images of bending fractures of T700/TR1219B composites in different states.…”

On the response of flax fiber reinforced composites under salt-fog/dry conditions: Reversible and irreversible performances degradation

“…This material showed moderate decreases in the mechanical performance (i.e., about -10% of the moduli and about -14% of the ultimate tensile stress) after 1 year of exposure and 52 cycles of aging. In another paper of the authors [24], they evaluated the effect of the same aging conditions on the composite transverse mechanical properties, showing noticeable decrements in both transverse tensile strength (i.e., about -20% after the first week of ageing) and modulus (i.e., about -18% already after the first week to reach about -45% after 1 year). These results were mainly ascribed to the plasticization of the epoxy matrix.…”

Performances Recovery of Flax Fiber Reinforced Composites after Salt-Fog Aging Test

“…Furthermore, some research activities specifically evaluated the effects of a wet/dry cyclic aging on the mechanical performances of natural fiber composites [15,16], evidencing noticeable decrements in both strength and stiffness.…”