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Analysis: tensile behaviour of unidirectional flax/epoxy plies

News International-French

16 Aug 2011

Flax fibres exhibit many advantages as a reinforcement for polymers, including good mechanical properties, low density and biodegradability. Producing them requires little energy, and they come from a renewable source. This work describes the development of a composite material reinforced by flax fibres.

(Published on May-June 2006 – JEC Magazine #25)





General information on flax fibre and project objectives



Flax fibre has always been considered as a noble and delicate material for textiles; it requires careful attention. It is well suited for the fabric industry, and used to make haute couture clothing (long fibres).


The flax plant is a veritable “blue” gold, giving rise to many byproducts. Flax tow is used in the fabrication of different types of paper, from bank notes to cigarette paper, and it even enters into the composition of some brake pads. Flax shives serve to make fibreboard. The seeds are either sown, made into oil and paint, or used in the food industry, as they are the richest in omega-3 fatty acids. The dust is used as organic soil improvers (fertilisers) for crops.



Where does the blue flax flower grow?


France is the world’s leading producer of long flax fibres, accounting for 50% of the quantities grown (see graph below). In France, about 6,000 farmers work for the textile flax industry and roughly 80,000 hectares are devoted to flax cultivation – accounting for 80% of the total surface area for flax in Europe.


Economic activity in Normandy has been linked for many years to its crops of textile flax. The region is recognised the world over for its position as the leading flax producer.


To give you a better idea, here are some significant figures: the surface area in Normandy sown in textile flax represents more than 60% of the national total for flax, or about 50,000 ha in 2004; this represents 50% of the total surface area devoted to textile flax in Europe (source: Agreste, 2004). Scutched flax from Normandy – or more than 30,000 metric tons of long fibres – accounts for 30% of the world’s flax production.



Growing and processing traditional flax


The process of growing and processing flax is highly specific and requires the appropriate technology. Today, the rolled-up flax straw is brought from the fields to be scutched, an operation that separates out five different co-products.


These are the shives (50%), the long fibres (20%), the dust and other separated waste material (15%), the short fibres (10%) and the flaxseed (5%).


It is important to keep in mind that developing new outlets for flax will require innovating in fibre selection, in the optimisation of crops and harvesting equipment, and in processing technologies.


From agricultural applications to the composite industry


The plant fibres used most in technical applications are wood, flax and hemp. So far, these materials have been integrated in the form of short synthetic or natural fibres (mats or compounds) combined with thermoset or thermoplastic matrices. The fibres are used mainly in paper mills, in the building industry for heat and sound insulation, and in the automotive industry for overmoulded parts, interior door trim, bumper beams, and more.


But in structural applications where short fibres cannot provide a solution for the loads involved, or for replacing high-modulus or high-strength reinforcement fibres such as glass, carbon, etc., we have to use long flax fibres. When they are processed into continuous tapes, they can be applied in weaving, filamentwinding, spray-up, LFT, fibre-placement and extrusion processes.


Our project involved starting with a choice of base materials to finally arrive at a high-performance product for use in the composite industry (by Acome).


The intermediary stages required expertise in biochemistry, physical chemistry and mechanics. Thanks to complementary partnerships with universities, we were able to see these stages through successfully.


Mechanical properties of flax fibres and unidirectional composites


The flax fibres selected (“Hermes” variety, grown in Normandy, France) were laid out on the ground for retting and then subjected to mechanical scutching and combing processes.


The research objectives were to:
- examine the mechanical properties of the fibre as a function of sampling area (where on the plant the fibres are taken); - compare the mechanical properties of flax fibre of the “Hermes” variety with those of glass fibre;
- study the behaviour of plies reinforced with unidirectional flax fibres in connection with the fibre properties.


Tensile characterisation of flax fibres as a function of the sampling area


Many factors influence the mechanical properties of a flax fibre, including:
- variety;
- specific growth conditions (cultivation techniques, soil, climate);
- plant maturity at harvest;
- extraction techniques used (retting, scutching, combing);
- storage conditions;
- strain rate;
- the length of fibre under load (L0 = 10mm);
- the amount of water absorbed (about 8%).


This entails considering different types of flax fibre. Analysing the ply behaviour requires knowing the properties of the flax fibres used, so we worked on fibres that were fully identified as to the variety, geographical origin and growing conditions, the degree of retting, and the mechanical processes used to extract the fibres.


The tensile mechanical properties of single flax fibres were determined in compliance with the NFT 25-704 and ASTM D 3379-75 standards. Given the fibre length and geometry (spindle shaped, average 33-mm length), the free length under load is 10mm. The tests served to determine the variation in Young’s modulus and in tensile strength with the diameter. Table 1 shows the properties of fibres taken from different parts of the plant, e.g. the top, middle, or base.


A look at table 1 shows that:
- the average diameter of the fibres changes according to the sampling area;
- the mechanical properties of fibres are influenced by the sampling area. This result could be explained by the different climate conditions throughout the 100 days of the plant’s growth. The fibres at the base, and closer to the root, are older and started to develop during the cold rainy season (April). The fibres in the middle received the benefit of the sun and rising temperatures during May and June, and the fibres at the top were sometimes subjected to drought and – especially – experienced slower growth as the seeds were developing. The biochemical analyses indicate differences in wall composition in terms of quantity, for example of cellulose, and also of the quality of the pectic and hemicellulosic polysaccharides encrusted on the cellulose microfibrils.


Table 1: properties of flax fibres as a function of sampling area. Df: fibre diameter, E: longitudinal Young’s modulus, UE: ultimate elongation, and Σ: tensile strength.

  Df (mm) E (MPa) UE % fu sfl (MPa)
Top 14.82 (±1.197) 60,463 (±11,456) 2.73 (±0,35) 1,644 (±328)
Middle 15.47 (±1.25) 69,252 (±10,764) 3.18 (±1,03) 1,874 (±663)
Base 18.76 (±0.73) 45,179 (±8,308) 1.73 (±0,66) 774 (±281)


Table 2: tensile mechanical properties of flax and glass fibres.

Fibres Density Efl (MPa) EfL/d sfl (MPa) sfl/d
Flax (middle) 1.53 69,252 45,262 1,874 1,225
E-glass 2.54 72,500 28,543 2,200 866


Table 2 shows a comparison between the specific tensile properties of flax fibre taken from the central part of the plant and those of glass fibre. It can be seen that the flax fibre used has good specific mechanical properties, so choosing this type of reinforcement fibre is justified when the objective is to optimise the weight of a part made of composite materials.


Characterisation of plies reinforced with unidirectional flax fibres


The unidirectional-fibre-reinforced specimens were compression- moulded following wet impregnation of the fibres. Axson’s 2015 epoxy resin was used for the laminate matrix. The tensile behaviour analysis on the unidirectional plies (epoxy resin reinforced with flax fibres from plant centre) shows, as a function of fibre volume percentage: 1) a quasi-linear increase in stiffness (fig.1) and strength, and 2) a decrease in ultimate elongation.




The average apparent Young’s modulus for the flax fibres is 60,603 MPa, which is close to the value found in tensile tests on fibres, or 69,252 MPa (±20,764).


A comparison of the tensile mechanical properties of plies reinforced with 30% by volume of unidirectional fibres shows the effect of the chosen fibre-sampling area in the plant (fig.2).


More information..........
The activities focus on high performance bio-composites made while integrating vegetable fibres associated with other polymers of natural or unnatural origins. The research concentrated on: the performance and composition of vegetable fibres, the dampening of fibres by a polymer and the adhesion between fibre/matrix.




Bearing in mind factors such as the fibre structure, the fact that the fibres are natural (dispersion of properties) and the effect of absorbed water, among other things, the performance of a flaxreinforced ply is quite complex.


There are differences between different types of flax, as well, so it is essential to test the performance of the fibres being used.


The chemical and mechanical treatments required to separate the fibres, clean their surface, and improve the fibre/matrix compatibility also tend to alter fibre performance and wettability mechanisms. Further study of the forming processes used for this type of fibre is needed.


Analysis of the tensile strength of a unidirectional-fibre-reinforced ply must take the specific features of flax and the impact of a large number of parameters into account. Such analysis should make it possible in future to create a system of predictive tools adapted for this type of material.




The authors would like to thank the French energy conservation agency, ADEME, and its scientific interest group AGRICE (Agriculture for Chemicals and Industry) for their financial support in this project, referenced Programme No. 03.01C.0084 - 2003-2005 and titled “De la filière de production au procédé de transformation : élaboration d'un matériau composite unidirectionnel hautes performances renforcé par des fibres de lin” (From the production chain to the converting process: developing a high-performance unidirectional flaxfibre- reinforced composite material”).