Peter J. Schubel

Modelling the effect of yarn twist on the tensile strength of unidirectional plant fibre yarn composites

The structural potential of plant fibres as reinforcing agents can only be realized when the highest reinforcement efficiency is employed. Hence, aligned plant fibre composites are of interest. However, due to the short length of technical plant fibres, the reinforcement needs to be in the form of staple fibre yarns, which have a twisted structure. Although twist facilitates yarn processability, it has several detrimental effects on the composites produced from such twisted yarn reinforcements; one of which is fibre obliquity and misalignment. This results in a drastic drop in composite mechanical properties. No analytical model currently exists to accurately predict the effect of yarn twist on aligned plant fibre composite tensile strength. In this paper, a novel mathematical model is developed. The model is based on (i) a modified rule of mixtures for plant fibre composites, (ii) well-defined structure-property relationships in an idealised twisted staple fibre yarn and (iii) the Krenchel orientation efficiency factor. The developed model includes a corrected orientation efficiency factor of cos2(2α), where α is the yarn surface twist angle. The model is validated with extensive experimental data from Goutianos and Peijs (Goutianos S, Peijs T (2003) Adv Compos Lett 12(6):237) and is found to be a near-perfect fit (R2 = 0.960). Experimental data from other studies are also used for further verification.

Hydroxyethylcellulose surface treatment of natural fibres: the new ‘twist’ in yarn preparation and optimization for composites applicability

The use of low-cost renewable natural fibres as reinforcements for structural composites is attractive but requires specific considerations over that of textile industry requirements. Textile yarns are twisted for processability and increased tensile strength. However, reinforcements employing twisted yarns produce poorer composites due to hindered yarn impregnation, inadequate wettability and compromised orientation efficiency. Whilst assessing the physical properties of select plant fibre yarns that determine reinforcement/composite properties, a strong correlation between yarn twist and compaction is observed. This manuscript also examines a novel plant fibre treatment method using hydroxyethylcellulose (HEC). HEC treatment not only enables intra- and inter-yarn binding thus allowing easy preparation of aligned fabrics, but also improves yarn mechanical properties whilst maintaining physical properties such as low twist. It is noticed that low twist yarns are more responsive to HEC treatment; the tenacity and stiffness of low twist flax is observed to increase by 230 and 75%, respectively.

Mechanical Property Characterization of Aligned Plant Yarn Reinforced Thermoset Matrix Composites Manufactured via Vacuum Infusion

This article evaluates the mechanical properties of vacuum-infused unidirectional plant fiber composites (PFRPs), composing of bast fiber yarns and thermoset matrices. PFRPs are found to have lower fiber volume fractions than E-glass composites (GFRPs). Apart from the expected (30–40%) lower density of PFRPs, they have 60–80% lower tensile strength, 30–60% lower tensile stiffness, 5–10 times lower impact strength, and 20–30% lower interlaminar shear strength than GFRPs. Importantly, critical fiber lengths are of the same order (0.2–0.5 mm). Composites reinforced with flax rovings exhibit exceptional fiber tensile modulus of 65–75 GPa and fiber tensile strength of about 800 MPa.

Furan matrix and flax fibre as a sustainable renewable composite: Mechanical and fire-resistant properties in comparison to phenol, epoxy and polyester

Due to environmental and fossil fuel supply concerns, polyfurfuryl alcohol or ‘furan’ resin has recently gained attention as a renewable alternative thermoset resin with reduced CO2 emissions in comparison to the existing petrochemical-based systems. When combined with natural flax fibres, it offers the potential to produce a fully bio-derived sustainable composite with structural mechanical capabilities and fire-resistant properties. In this study, the mechanical properties of furan flax and E-glass fibre laminates are characterised and directly compared to polyester, epoxy and phenolic laminates produced using identical reinforcing fibres and methods. In addition, the acidity and fire-resistant properties are also compared. Furan resin E-glass laminates were found to have mechanical properties comparable to existing resin systems with excellent fire-resistant properties, which are equal to that of phenolic and exceeding polyester and epoxy performance. However, both phenolic and furan flax laminates were found to give reduced mechanical performance in comparison to polyester and epoxy. This reduction in strength was attributed mostly to micro voids surrounding elementary flax fibres believed to be the result of post-cured fibre shrinkage due to moisture uptake.

Experimental determination and control of prepreg tack for automated manufacture

Abstract The automated tape laying (ATL) process has been examined and found to be sensitive to tack and stiffness properties of the prepreg material being laid. A comparison of existing aerospace and newly developed ATL prepreg tapes has revealed significant differences in tack response to temperature and feedrate. Examination of constituent resin rheology has found that tack, and the two observed failure modes, are somewhat dependent upon viscoelastic stiffness. Observation of temperature and feedrate response revealed a time–temperature superposition relationship. The Williams–Landel–Ferry equation was utilised to make predictions of the temperature response based on the feedrate response. Tack levels were stabilised over the feedrate range by making temperature adjustments. Results from the peel test, where mould conditions at lay-up were recreated, were found transferable to the ATL, where a suitable lay-up feedrate under ambient conditions was predicted.

Wind Turbine Blade Design Review

A detailed review of the current state-of-art for wind turbine blade design is presented, including theoretical maximum efficiency, propulsion, practical efficiency, HAWT blade design, and blade loads. The review provides a complete picture of wind turbine blade design and shows the dominance of modern turbines almost exclusive use of horizontal axis rotors. The aerodynamic design principles for a modern wind turbine blade are detailed, including blade plan shape/quantity, aerofoil selection and optimal attack angles. A detailed review of design loads on wind turbine blades is offered, describing aerodynamic, gravitational, centrifugal, gyroscopic and operational conditions.

Surface quality prediction of thermoset composite structures using geometric simulation tools

Abstract The surface quality of polymer composite laminates was examined via geometric modelling techniques and compared to experimental data. TexGen software provided the platform for the development of a surface roughness simulation tool which accounted for textile architecture and specific cure kinetics of the matrix. The study focused on the influence of thermal and chemical shrinkage during cure and the change in localised volume fraction across the surface of a unit cell. A one-dimensional analysis was used to determine proportional dimensional changes to the matrix region, with the results stitched together to form a three-dimensional topological plot. Three demonstrator geometric models were developed to represent a carbon 2 × 2 twill weave fabric with 3000, 6000 or 12 000 tows. These models were analysed with low and high shrink resin properties. Optical microscopy was used to determine accurate tow forms for compacted tows which aided the development of the geometric model. Simulated profiles, topography and surface roughness measures were compared to experimental data which demonstrated the significance of matrix contraction and fabric architecture on the final surface quality. The simulations were shown to represent experimental data typically within 6%.

On recycled carbon fibre composites manufactured through a liquid composite moulding process

The recovery of carbon fibres from waste and end-of-life carbon fibre reinforced plastic materials is both economically lucrative and environmentally necessary. Here, we characterise the physical and mechanical properties of recycled carbon fibre reinforced plastics (rCFRPs) composed of random and oriented non-woven recycled carbon fibre mats that were impregnated with liquid epoxy matrices using a vacuum-infusion set-up. The low areal density and poor compactability of the non-woven mats implied that press-moulding upon impregnation was essential to control laminate thickness and improve fibre content; this may limit the applications of the resulting rCFRPs. Moreover, the press consolidation process is thought to degrade fibre length, and is a likely cause for the lower-than-expected tensile properties of the rCFRPs. Expectedly, the oriented rCFRPs exhibited better tensile and compressive properties than the random rCFRPs. Notably, while the tensile strength of the rCFRPs was only up to 2.5 times better than the matrix, the tensile modulus was 4–10 times enhanced. Through a comparative literature survey, we found that the liquid composite moulded rCFRPs were outperformed by rCFRPs fabricated through other manufacturing processes (e.g. prepregging), particularly those employing high compaction pressures, and utilising long fibres recovered through pyrolysis and chemical processes, rather than the fluidised-bed process.

A coupled structural and flow approach for numerical simulation of the light resin transfer moulding process. I: Model outline

The first part of this two-part study introduces an approach for a numerical model of the composite manufacturing method light resin transfer moulding. Discussion of process physics and relevant literature is used to develop a coupled finite element and infusion software simulation of both the structural and flow elements of the method, with the aim of producing an accurate and comprehensive model. This theoretical overview of the model acts as a precursor to the final part of the research where results are presented and verified and a wind energy case-study application will be demonstrated.

A coupled structural and flow approach for numerical simulation of the light resin transfer moulding process. II: Fabric permeability and compaction characterisation, model results and a 6-kW wind turbine blade case study

This is the second paper in a two-part study into the numerical simulation of the light resin transfer moulding (LRTM) process. It focuses on the development of empirically derived models for the permeability and compaction behaviour of Unifilo during light resin transfer moulding. A detailed review of relevant literature allowed identification of important material parameters, and appropriate testing procedures were devised accordingly. Previously published approaches were used to construct light resin transfer moulding specific permeability and compaction models. A material response for each stage of the process was quantified and then inputted into the light resin transfer moulding simulation developed in the Part I of this study. Results were verified using data from an empirical testbed, and the completed light resin transfer moulding model was then applied to a 6-kW wind turbine blade case study to evaluate different infusion strategies and minimise fill time. Finally, it was used to compare the light resin transfer moulding, resin transfer moulding and liquid resin infusion processes for the manufacture of this part.