Process-Dependent Porosity and Mechanical Properties of Flax and Carbon-Flax Epoxy Laminates

Process-Dependent Porosity and Mechanical Properties of Flax and Carbon-Flax Epoxy Laminates

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Flax fibre–reinforced polymers (FFRPs) are attractive for lightweight structures due to their low density, favourable specific stiffness and partially bio-based origin. However, their performance is strongly affected by manufacturing route, porosity and moisture uptake, which are typically more critical than for conventional glass or carbon fibre composites. This study compares flax, glass, carbon and carbon–flax hybrid epoxy laminates produced by vacuum infusion, hand lay-up and autoclave prepreg processing. Fibre volume fraction and void content are determined from density measurements, optical microscopy and X-ray computed tomography. Tensile and flexural properties are measured according to EN ISO 527-4 and EN ISO 14125 in the dry state and after accelerated conditioning at 35 ± 2 °C and 100 % relative humidity. Infused unidirectional flax laminates reach a tensile strength of about 259 MPa and a specific tensile strength of approximately 0.21 MPa·m³·kg⁻¹, comparable to glass laminates. Moisture exposure increases thickness by 11.8– 13.9 % for infused flax laminates and about 2.3 % for an infused carbon–flax hybrid laminate and leads to strength reductions up to roughly 30 % in flax-rich laminates, while autoclave-processed laminates show only minor losses. The results provide process-dependent design guidelines for FFRPs in moisture-exposed, weight-sensitive components.

Flax fibre–reinforced polymers (FFRPs) are attractive for lightweight structures due to their low density, favourable specific stiffness and partially bio-based origin. However, their performance is strongly affected by manufacturing route, porosity and moisture uptake, which are typically more critical than for conventional glass or carbon fibre composites. This study compares flax, glass, carbon and carbon–flax hybrid epoxy laminates produced by vacuum infusion, hand lay-up and autoclave prepreg processing. Fibre volume fraction and void content are determined from density measurements, optical microscopy and X-ray computed tomography. Tensile and flexural properties are measured according to EN ISO 527-4 and EN ISO 14125 in the dry state and after accelerated conditioning at 35 ± 2 °C and 100 % relative humidity. Infused unidirectional flax laminates reach a tensile strength of about 259 MPa and a specific tensile strength of approximately 0.21 MPa·m³·kg⁻¹, comparable to glass laminates. Moisture exposure increases thickness by 11.8– 13.9 % for infused flax laminates and about 2.3 % for an infused carbon–flax hybrid laminate and leads to strength reductions up to roughly 30 % in flax-rich laminates, while autoclave-processed laminates show only minor losses. The results provide process-dependent design guidelines for FFRPs in moisture-exposed, weight-sensitive components.

Composites | Flax | Manufacturing | Mechanical properties | Porosity

Composites | Flax | Manufacturing | Mechanical properties | Porosity

ZOUHAR, J.; KOLOMÝ, Š.; SLANÝ, M.; PETŘÍK, M.

01.12.2025

Trans Tech Publications Ltd

Solid State Phenomena

Solid State Phenomena

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