Damage in flax/epoxy quasi-unidirectional woven laminates under quasi-static tension

Abstract

The damage initiation and development in flax/epoxy laminates under quasi-static tension is studied. The laminates are made of quasi-unidirectional woven prepregs in different configurations [0°]8, [0°, 90°]2S, [−45°, 45°]2S and [0°, 90°, +45°, −45°]S, and processed using an autoclave. The damage was monitored during the tensile test using acoustic emission and observed by post-mortem microscopy of the samples. The stress–strain curves illustrate the ductile behaviour of the [+45°, −45°]2S composite, whereas in the other composites a more brittle behaviour was observed. Non-linearity of the stress–strain curves is explained by the intrinsic non-linearity of flax fibres in tension. The combination of the stress–strain data and the registered acoustic emission data is used to identify the damage initiation and propagation thresholds. The damage thresholds are the lowest in the [0°]8 laminate and the highest in the [+45°, −45°]2S laminate. The observed fracture zones and damage mode are cracks inside and on the boundary of technical fibres, cracks on the boundary of tows, matrix cracking, fibre pull-out and fibre breakage. A notable feature of the damage behaviour is almost full absence of transverse matrix cracks inside tows in 90° plies, which are the major damage modes in glass- and carbon-reinforced plastics. This is attributed to the low stress concentrations in transverse direction due to the low transverse modulus of flax fibres.

Damage in biocomposites: Stiffness evolution of aligned plant fibre composites during monotonic and cyclic fatigue loading

“…4a). This difference in stressstrain behaviour of plant fibre composites and glass fibre composites has been reported earlier, both in coupon tests [23,26,27] as well as in large-scale components (such as wind turbine blade case studies) [4]. A difference in damage accumulation mechanisms has been suggested for natural fibre and synthetic fibre reinforced composites.…”

“…Thereafter, in general, the stiffness remained fairly constant up to their failure strain (ranging from 1.2% to 1.7%), if not increasing or decreasing marginally. This has also been observed in few other studies on plant fibre composites [23,25,26,30], including testing the effect of off-axis loading of unidirectional laminates [23], and the effect of twisted yarns and untwisted rovings as reinforcements [25].…”

“…1), the initial elastic modulus E 0 was determined from the initial tangent modulus in the strain range of 0.025-0.100%, as prescribed in literature given the low elastic limit (of around 0.10-0.15%) of plant fibre composites [23,[25][26][27]. The residual or apparent stiffness E at e% strain was thereafter calculated using the tangent modulus at e% strain ( Fig.…”

“…A difference in damage accumulation mechanisms has been suggested for natural fibre and synthetic fibre reinforced composites. Plant fibre composites, due to their weak(er) fibrematrix interface, are suggested to demonstrate fibre/matrix debonding and interface failure, while the prevailing damage mechanism in glass fibre composites is suggested to be matrix transverse cracking [26]. In addition, unlike glass fibres which are isotropic, homogenous materials and display linear-elastic behaviour, plant fibres are natural materials which have inhomogeneous properties, natural defects (such as kink bands), and are intrinsically non-linear materials [23,26,27].…”

Quasi-unidirectional flax composite reinforcement: Deformability and complex shape forming

“…Focussing the attention on the first cycle, as for a possible infusion process, the fibre volume fraction ranges from 23.5% for a pressure of 1 kPa to 39.4% applying 250 kPa. In [49], fibre volume fraction of 47% was reached for laminates made of the same LINEO prepreg at a pressure of 400 kPa. These results are comparable to the data presented in [50] for a flax plain weave fabric and a flax non-crimped stitched unidirectional fabric which feature a fibre volume fraction in the range 40-50% after compression up to 250 kPa.…”