Modelling the different mechanical response and increased stresses exhibited by structures made from natural fibre composites

Abstract

Natural fibres exhibit improved sustainability and similar mechanical properties to E-glass. However, for laminates there is a larger difference in properties and limited assessments of structural components. An analytical method for grillages is developed which is generally shown to predict the stress to within 5% of an FEA model. The simulations demonstrate a change in structural response between flax and carbon, with flax demonstrating higher stresses than expected for the lower Young's modulus for the same topology. Flax is shown to be more sensitive to transverse Young's modulus than standard composites and a better characterisation of this property is required.

Comparative design of E-glass and flax structures based on reliability

“…The flax turbine is 10% lighter than the Eglass structure and passes the structural requirements however the flax structure has a different failure mode and larger deflection than the E-glass ones and the effects of these deformations on the efficiency of the blade are not determined; the comparative safety of the blades is also not documented. Blanchard et al [14] also find a change in behaviour when flax is used in a structure with an increase in stress for flax grillage structures compared to E-glass and carbon equivalents. Bambach [15] experimentally investigates channel sections made of natural fibre composites where it is shown that natural fibre sections can be used in light structural applications because of their predictable compressive behaviour.…”

“…Whilst flax fibre and laminate scale properties are a popular topic, with 765 journal papers published on these topics in the top 4 composite journals in the last 5 years, studies at the structural scale are still limited, with only 4 papers. These studies have found that flax structures change their behaviour when compared to conventional composites, [13] [14] [15] [16]. This creates a requirement for further investigations as extrapolation of material properties from laminate scale does not account for these structural effects.…”

“…The influences of these material properties and manufacturing techniques on the structural properties are then investigated. To perform the analysis, Monte Carlo simulation is combined with an analytical grillage model successfully validated for flax reinforced epoxy composites by Blanchard et al [14].…”

“…The minimum value for the transverse strength is particularly low, 82% lower than the mean, even though these specimens were cured in an autoclave for which higher properties can be expected. The range for compressive data is smaller and can be attributed to the large influence of the resin on 9 Discarded by the box plot as a minimum outlier 10 Data from test 11 was discarded by the box plot as a maximum outlier 11 Discarded by the box plots as maximum outliers 12 Specimens made of FUD115 prepreg are discarded as the weft/wrap ratio is 1/8 13 Fabric described as UD but only 67% of the fibres in the 0° direction 14 Unknown resin 15 Fabric described as UD -ribs 4/4 16 Specimens fabricated by pultrusion 17 Fabric described as UD but weft/ wrap ratio is 84/16 wt% 18 Discarded by the box plot as a minimum outlier 19 Range of volume fraction 20 Specimens fabricated by filament winding are discarded compression properties. For many of the properties there is still not a large enough quantity of points to gain confidence in the values, despite coming from multiple references.…”

“…The structural response of the grillage structure, including the stresses, can then be calculated. This is performed according to a rapid analytical calculation used in Sobey et al [104] with an updated formula developed by Blanchard et al [14] which can account for the change in structural response exhibited by low stiffness materials such as flax; this approach has been shown to be within 5% accuracy of FEA based on the variable range used in the simulations. The third step it to compare the stresses to the failure criteria to determine if the grillage is failing based on first ply failure.…”