From the stems of agricultural crops to the structural trunks of trees, studying the mechanical b... more From the stems of agricultural crops to the structural trunks of trees, studying the mechanical behaviour of plant stems is critical for both commerce and science. Plant scientists are also increasingly relying on mechanical test data for plant phenotyping. Yet there are neither standardised methods nor systematic reviews of current methods for the testing of herbaceous stems. We discuss the architecture of plant stems and highlight important micro- and macro-structural parameters that need to be controlled and accounted for when designing test methodologies, or that need to be understood to explain observed mechanical behaviour. Then, we critically evaluate various methods to test structural properties of stems, including flexural-bending (two-, three-and four-point bending) and axial-loading (tensile, compressive and buckling) tests. Recommendations are made on best practices. This review is relevant to fundamental studies exploring plant biomechanics, mechanical phenotyping of plants, and the determinants of mechanical properties in cell walls, as well as to application-focussed studies, such as in agro-breeding and forest-management projects, aiming to understand deformation processes of stem structures. The methods explored here can also be extended to other elongated, rod-shaped organs (e.g. petioles, midribs and even roots).
Proceedings of the National Academy of Sciences of the USA, 2017
Inspired by biological systems, we report a supramolecular polymer-colloidal hydrogel (SPCH) comp... more Inspired by biological systems, we report a supramolecular polymer-colloidal hydrogel (SPCH) comprising 98 wt% water that can be readily drawn into uniform (ca. 6 µm thick) 'supramolecular fibers' at room temperature. Functionalized polymer-grafted silica nanoparti-cles, a semi-crystalline hydroxyethyl cellulose derivative and cucur-bit[8]uril undergo aqueous self-assembly at multiple length scales to form the SPCH, facilitated by host-guest interactions at the molecular level and nano-fibril formation at colloidal length scale. The fibers exhibit a unique combination of stiffness and high damping capacity (60-70%), the latter exceeding that of even biological silks and cellulose-based viscose rayon. The remarkable damping performance of the hierarchically-structured fibers is proposed to arise from the complex combination and interactions of 'hard' and 'soft' phases within the SPCH and its constituents. SPCH represent a new class of hybrid supramolecular composites, opening a window into fiber technology through low-energy manufacturing.
Laminated bamboo is emerging as a novel material in design and construction. As a natural fibre c... more Laminated bamboo is emerging as a novel material in design and construction. As a natural fibre composite, it has unique mechanical properties that allow for innovations that are not possible in other materials. Here, we discuss one new application of those properties: the development of a novel bending technique using high temperature, and we explore its implications for design. We have explored the fundamental properties of laminated bamboo and its thermal relaxation as it passes the glass transition temperatures of its constituent polymers. By mechanically thinning engineered bamboo material, score lines allow precise, controlled and localised heating that promotes limited but essential elasto‐plastic behaviour. Concentrated heating above the glass transition temperature induces property evolution and structural morphology changes, which results in thermal relaxation with minimal recovery and full set upon cooling. This original technology is then deployed in the design and construction of a folded plate helical shell composed of thin laminated bamboo sheets.
Lumen impregnation, unlike most other wood modification methods, is typically assessed by the por... more Lumen impregnation, unlike most other wood modification methods, is typically assessed by the pore-filling ratio (PFR) (i.e. the fraction of luminal porosity filled) rather than by weight percentage gain (WPG). During lumen impregnation, the impregnants act on the voids in the wood rather than on the solid mass (e.g. cell walls), but the PFR cannot be measured as conveniently as the WPG during processing. Here, it is demonstrated how the PFR can be calculated directly from the WPG if the bulk density of the untreated wood is known. The relationship between the WPG and bulk density was examined experimentally by applying a pressured impregnation on knot-free specimens from Sitka spruce with a liquid mixture of methacrylate monomers. Based on the validated model, it was possible to further study the effect of different process-related parameters, such as hydraulic pressure, on lumen impregnation. Skeletal density is another key parameter in this model, which directly reflects the amount of inaccessible pores and closed lumens, and can be independently determined by helium pycnometry. The permeability can be qualitatively evaluated by PFR as well as skeletal density. For instance, poor permeability of knotty wood, due to the large extractives content around knots, was reflected by a lower skeletal density and inefficient lumen impregnation (low PFR). Although this model was examined on a laboratory scale, it provides guidance on the precise effect of different parameters on lumen impregnation, thereby improving the fundamental understanding of and enabling better control over the modification of wood by impregnation.
Polymer composites are found throughout the world both natural and artificial in origin. In the v... more Polymer composites are found throughout the world both natural and artificial in origin. In the vast majority of applications, composites serve as structural support or reinforcement roles. Demand for lightweight tough composites is growing in multiple application spaces such as areospace, biomaterials, and infrastructure with physical properties as diverse as the applications. The unifying component in all composites is the presence of an interphase. Many measurement techniques and measurement tools have been developed for the study of this crucial region in composite materials. Many of these methods are great for the measurment and study of bulk properties or model systems. However, development of methods that permit the direct observation of interactions at the interphase during applied stress are needed. Here we employ fluorescence lifetime imaging and hyperspectral imaging to observe activation of a fluorogenic dye at the composite interface as a result of applied stress. The advantages of this sytem include commercial availability of the dye precursor, and simple one-pot functionalization. The attachment of the dye at the interface is easily monitored through emission wavelength shifts and fluorescence lifetime variations. Interfacial mechano-responsive dyes have potential for both fundamental studies as well as industrial use as a structural health monitoring tool.
Trees, and their derivative products, have been used by societies around the world for thousands ... more Trees, and their derivative products, have been used by societies around the world for thousands of years. Contemporary construction of tall buildings from timber, in whole or in part, suggests a growing interest in the potential for building with wood at a scale not previously attainable. As wood is the only significant building material that is grown, we have a natural inclination that building in wood is good for the environment. But under what conditions is this really the case? The environmental benefits of using timber are not straightforward ; although it is a natural product, a large amount of energy is used to dry and process it. Much of this can come from the biomass of the tree itself, but that requires investment in plant, which is not always possible in an industry that is widely distributed among many small producers. And what should we build with wood? Are skyscrapers in timber a good use of this natural resource, or are there other aspects of civil and structural engineering, or large-scale infrastructure, that would be a better use of wood? Here, we consider a holistic picture ranging in scale from the science of the cell wall to the engineering and global policies that could maximise forestry and timber construction as a boon to both people and the planet.
The increasing use of high-value carbon fibre in composites is linked with increasing waste gener... more The increasing use of high-value carbon fibre in composites is linked with increasing waste generation: from dry fibre and prepreg offcuts during manufacturing to end-of-life parts. In this work, a novel thermoplastic tape was produced from 60 wt.% manufacturing waste carbon fibres (60 mm long) and 40 wt.% polyester (PET) fibres using a thermal consolidation technique. The thin (0.2 mm) and narrow (20mm wide) tapes were then used to fabricate laminated composite panels in two 0/90 tape architectures: cross-ply and woven ply. Various mechanical properties, including tensile, flexural, compression, and impact, were evaluated. It was found that cross-ply performed better than woven ply laminates, with failure in the latter materials typically initiating at the tape interlacement points.
The reinforcement of natural rubber (NR) with particles and fibres enables their use in even high... more The reinforcement of natural rubber (NR) with particles and fibres enables their use in even high performance applications, such as in road-racing bicycle tire casings. Here, for the first time, we examine the potential of silk textiles as reinforcements in NR to produce a fully-green, flexible yet strengthened elastomeric composite material. Various material properties were evaluated and compared with similar nylon textile reinforced NR composites. Two types of NR were used: whole and purified natural rubbers. The composite samples were prepared by sandwiching a single layer of textile between layers of NR. NR/silk composites exhibited higher static and dynamic mechanical properties than NR/nylon composites. In addition, silk textiles in whole NR composites performed significantly better than purified NR composites, due to stronger fibre/matrix adhesion and better wettability in the former, as indicated by surface energy measurements and scanning electron microscopy micrographs. Such bio-based natural rubber/silk composites might find interesting applications in soft robotics and as flexible, inflatable tubes.
The recovery of carbon fibres from waste and end-of-life carbon fibre reinforced plastic material... more 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.
The drive towards sustainability, even in materials technologies, has fuelled an increasing inter... more The drive towards sustainability, even in materials technologies, has fuelled an increasing interest in bio-based composites. Cellulosic fibres, such as flax and jute, are being considered as alternatives to technical synthetic fibres, such as glass, as reinforcements in fibre reinforced polymer composites for a wide range of applications. A critical bottleneck in the advancement of plant fibre composites (PFRPs) is our current inability to predict PFRP properties from data on fibre properties. This is highly desirable in the cost- and time-effective development and design of optimised PFRP materials with reliable behaviour. This study, alongside limited other studies in literature, have found that the experimentally determined (through single fibre tests) fibre properties are significantly different from the predicted (‘back-calculated’ using the popular rule-of-mixtures) fibre properties for plant fibres. In this note, we explore potential sources of the observed discrepancy and identify the more likely origins relating to both measurement and errors in predictions based on the rule-of-mixtures. The explored content in this discussion facilitates the design of a future investigation to (1) identify the sensitivity of the discrepancy between measured and predicted fibre properties to the various potential origins, (2) form a unified hypothesis on the observed phenomenon, and (3) determine whether the rule-of-mixtures model (in specific cases) can be improved and may be able to predict properties precisely.
The non-linear stress-strain behaviour of plant fibre composites is well-known in the scientific ... more The non-linear stress-strain behaviour of plant fibre composites is well-known in the scientific community. Yet, the important consequences of this, in terms of the evolution of stiffness as a function of applied strain and cycles to failure, are not well-studied in literature. This is despite the fact that stiffness degradation is a well-accepted indicator of damage in a composite material, and is regularly used as a component failure criterion. This article systematically explores the evolution of stiffness of various aligned plant fibre composites, subjected to i) monotonic loading, ii) low-cycle, repeated progressive loading, and iii) fatigue loading. The evolution in stiffness in plant fibre composites is found to be complex: structural changes in the elementary fibre cell wall and damage development in the composite have often competing effects on stress-strain behaviour. Indeed, the evolution in stiffness of plant fibre composites is found to be unlike that typically observed in traditional composites, and therefore needs to be taken into account in the design of structural components.
Here we characterise the thermal properties of engineered bamboo panels produced in Canada, China... more Here we characterise the thermal properties of engineered bamboo panels produced in Canada, China, and Colombia. Specimens are processed from either Moso or Guadua bamboo into multi-layered panels for use as cladding, flooring or walling. We utilise the transient plane source method to measure their thermal properties and confirm a linear relationship between density and thermal conductivity. Furthermore, we predict the thermal conductivity of a three-phase composite material, as these engineered bamboo products can be described, using micromechanical analysis. This provides important insights on density-thermal conductivity relations in bamboo, and for the first time, enables us to determine the fundamental thermal properties of the bamboo cell wall. Moreover, the density-conductivity relations in bamboo and engineered bamboo products are compared to wood and other engineered wood products. We find that bamboo composites present specific characteristics, for example lower conductivities—particularly at high density—than equivalent timber products. These characteristics are potentially of great interest for low-energy building design. This manuscript fills a gap in existing knowledge on the thermal transport properties of engineered bamboo products, which is critical for both material development and building design.
With the growing interest in material technologies incorporating bio-based constituents, substitu... more With the growing interest in material technologies incorporating bio-based constituents, substitutes to both conventional polymer foams and their synthetic reinforcements are being sought. In this article, we characterise the physical, mechanical, thermal and environmental resistance properties of novel syntactic foams that are based on silkworm cocoons as natural, hollow particulate fillers for a rigid, bio-based polyurethane matrix. Syntactic foams were cast with the natural cocoons replacing 60–90% by weight (40–70% by volume) of the polymer matrix. Although reinforcing the foam with domesticated and wild cocoons increased its density from 45 kg/m3 to ~60 kg/m3 and ~120 kg/m3 respectively, a marked increase in both absolute and specific compressive properties was also observed. Comparable thermal and dimensional stability of the proteinaceous cocoon reinforced polymer foams with the unreinforced foam attest to comparable environmental resistance.
The ultrastructure of the self-constructed tube housing of the bioluminescent marine worm, Chaeto... more The ultrastructure of the self-constructed tube housing of the bioluminescent marine worm, Chaetopterus sp. reveals that the bio-nanocomposite tube comprises of multiple non-woven plies of multi-axially oriented organic nanofilaments (ø 50-1,100 nm) cemented together by an unstructured organic matrix binder. The thin-walled, impermeable tubes are bio-inspirational for conventional pipe technology. Orientation distribution analyses revealed that the dominant orientation angles of nanofilaments in the tube were 0°, ±45° and ±65°, which correlate well with optimal winding angles for ‘man-made’ fibre reinforced composite pipes subjected to specific loading conditions. Such a use of high aspect ratio nanofilaments in multi-axial laminates would impart toughness and flexibility to the tube structure, and facilitate rapid tube growth. While the tube production mechanism is not entirely known at this stage, our time-lapse studies show that, contrary to generic assumptions in literature, the worm actively, rapidly and sporadically produces and expands the tube.
With the growing interest in bio-based composites as alternatives to traditional glass fibre rein... more With the growing interest in bio-based composites as alternatives to traditional glass fibre reinforced composites (GFRPs), there has been a persistent rise in the commercial use of plant fibre composites (PFRPs). In contrast, nature’s ‘wonder-fibre’ silk has had no commercial applications, and only limited scientific investigations, as a composite reinforcement. To produce silk fibre composites (SFRPs) with useful properties, three key recommendations from our critical literature review were followed: i) a high-failure strain, low-processing temperature thermoset matrix was used to a) maximise the reinforcing effect of low-stiffness, ductile silk, and b) facilitate impregnation and avoid fibre degradation, ii) high fibre volume fractions were employed to ensure that fibres carried a larger fraction of the load, and iii) given the lack of studies investigating fracture energy dissipation mechanisms in SFRPs, interface modification was avoided due to its complex, sometimes detrimental, effects on toughness. In directly addressing the question, ‘is there a case for silks as polymer reinforcements?’, we evaluated various mechanical properties of nonwoven and plain woven SFRPs against similar flax and glass composites. In all cases, woven composites performed better than nonwoven composites. While SFRPs were weak in terms of stiffness, their flexural and tensile strength was comparable to PFRPs, but much below that of GFRPs. Notably, the low density of SFRPs, like PFRPs, made them comparable to GFRPs in terms of specific flexural properties. Woven SFRPs exhibited much higher fracture strain capacities than both flax and glass composites, making SFRPs suitable for applications where high compliance is required. The Achilles’ heels of PFRPs have been their reportedly i) inadequate interfacial properties, ii) inferior impact properties, iii) poor strength performance, and iv) high moisture sensitivity. We found that SFRPs outperformed their flax counterparts in areas i)-iii), and were more comparable to, but not better than, GFRPs. While concerns such as cost and ‘sustainability’ of silk are acknowledged, potential applications for SFRPs are discussed.
The housing tube material of the marine worm Chaetopterus sp. exhibits thermal stability up to 25... more The housing tube material of the marine worm Chaetopterus sp. exhibits thermal stability up to 250 °C, similar to other biological materials like mulberry silkworm cocoons. Interestingly, however, dynamic mechanical thermal analysis conducted in both air and water elucidated the lack of a glass transition in the organic tube wall material. In fact, the visco-elastic properties of the anhydrous and undried tube were remarkably stable (i.e. constant and reversible) between –75 and 200 °C in air and 5 and 75 °C in water, respectively. Moreover, it was found that hydration and associated-water plasticization were key to the rubber-like flexible properties of the tube; dehydration transformed the material behaviour to glass-like. The tube is made of bio-nanocomposite fibrils in highly-oriented arrangement, which we argue favours the biomaterial to be highly crystalline or cross-linked, with extensive hydrogen- and/or covalent-bonds. Mechanical property characterisation in the longitudinal and transverse directions ascertained that the tubes were not quasi-isotropic structures. In general, the higher stiffness and strength in the transverse direction implied that there were more nanofibrils oriented at ±45° and ±65° than at 0° to the tube axis. The order of the mechanical properties of the soft-tough tubes was similar to synthetic rubber-like elastomers and even some viscid silks. The complex structure-property relations observed indicated that the worm has evolved to produce a tubular housing structure which can i) function stably over a broad range of temperatures, ii) endure mechanical stresses from specific planes/axes, and iii) facilitate rapid growth or repair.
To aid design engineers in closing the existing gap between current scientific knowledge and actu... more To aid design engineers in closing the existing gap between current scientific knowledge and actual market applications of plant fibre reinforced plastics (PFRPs), this article provides comprehensive Ashby-type materials selection charts for PFRPs to facilitate product design and development. General tensile mechanical property profiles are presented for a variety of PFRPs to enable the design for i) optimal stiffness and strength, ii) minimal weight (i.e. optimal specific properties), iii) minimal cost, and iv) minimal eco-impact. A large database is used to capture the range in properties of different i) reinforcements forms (short fibres: pellets and nonwovens; long fibres: multiaxials and unidirectionals), ii) polymer matrices (thermoplastic and thermosetting), and iii) manufacturing techniques (injection moulding, compression moulding, hand lay-up, vacuum infusion, resin transfer moulding and prepregging). As PFRPs are often viewed as alternatives to glass fibre composites (GFRPs), for demonstrative purposes the tensile properties of the various PFRPs are compared with similar GFRPs. Moreover, highlighting that other mechanical parameters may be critical performance indices for specific products and applications, a materials property chart for a fatigue-limited design is also produced.
Composites Part A: Applied Science and Manufacturing, 2014
While it is common knowledge in natural fibre composites manufacture that plant fibre reinforceme... more While it is common knowledge in natural fibre composites manufacture that plant fibre reinforcements are considerably less compactable than synthetic fibre reinforcements, the through-thickness compaction behaviour of animal-fibre silk reinforcements has not been characterised thus far. We find that not only are silk reinforcements significantly more compressible than plant fibre reinforcements, but their compactibility exceeds that of even glass fibre textiles. For instance, the fibre volume fraction (at a compaction pressure of 2.0 bar) of woven biaxial fabrics of silk, plant fibres and E-glass are 54-57%, 30-40% and 49-54%, respectively. Therefore, silks provide an opportunity to manufacture high fibre content natural fibre composites; this is a bottleneck of plant fibre textiles. Analysing the structure of silk textiles through scanning electron microscopy, we show that favourable fibre/yarn/fabric geometry, high degree of fibre alignment and dispersion, and suitable technical fibre properties enable optimal packing and arrangement of silk textiles.
This article evaluates the mechanical properties of vacuum-infused unidirectional plant fibre com... more This article evaluates the mechanical properties of vacuum-infused unidirectional plant fibre composites (PFRPs), composing of bast fibre yarns and thermoset matrices. PFRPs are found to have lower fibre 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 fibre lengths are of the same order (0.2-0.5 mm). Composites reinforced with flax rovings exhibit exceptional fibre tensile modulus of 65-75 GPa and fibre tensile strength of about 800 MPa.
Plant fibres, perceived as environmentally sustainable substitutes to E-glass, are increasingly b... more Plant fibres, perceived as environmentally sustainable substitutes to E-glass, are increasingly being employed as reinforcements in polymer matrix composites. However, despite the promising technical properties of cellulose-based fibres and the historic use of plant fibre reinforced plastics (PFRPs) in load-bearing components, the industrial uptake of PFRPs in structural applications has been limited. Through an up-to-date critical review of literature, this manuscript presents an overview on key aspects that need consideration when developing PFRPs for structural applications, including the selection of I) the fibre type, fibre extraction process, and fibre surface modification technique, II) fibre volume fraction, III) reinforcement geometry and interfacial properties, IV) reinforcement packing arrangement and orientation, and V) matrix type and composite manufacturing technique. A comprehensive materials selection chart (Ashby plot) is also produced to facilitate the design of a PFRP component, based on (absolute and specific) tensile properties.
From the stems of agricultural crops to the structural trunks of trees, studying the mechanical b... more From the stems of agricultural crops to the structural trunks of trees, studying the mechanical behaviour of plant stems is critical for both commerce and science. Plant scientists are also increasingly relying on mechanical test data for plant phenotyping. Yet there are neither standardised methods nor systematic reviews of current methods for the testing of herbaceous stems. We discuss the architecture of plant stems and highlight important micro- and macro-structural parameters that need to be controlled and accounted for when designing test methodologies, or that need to be understood to explain observed mechanical behaviour. Then, we critically evaluate various methods to test structural properties of stems, including flexural-bending (two-, three-and four-point bending) and axial-loading (tensile, compressive and buckling) tests. Recommendations are made on best practices. This review is relevant to fundamental studies exploring plant biomechanics, mechanical phenotyping of plants, and the determinants of mechanical properties in cell walls, as well as to application-focussed studies, such as in agro-breeding and forest-management projects, aiming to understand deformation processes of stem structures. The methods explored here can also be extended to other elongated, rod-shaped organs (e.g. petioles, midribs and even roots).
Proceedings of the National Academy of Sciences of the USA, 2017
Inspired by biological systems, we report a supramolecular polymer-colloidal hydrogel (SPCH) comp... more Inspired by biological systems, we report a supramolecular polymer-colloidal hydrogel (SPCH) comprising 98 wt% water that can be readily drawn into uniform (ca. 6 µm thick) 'supramolecular fibers' at room temperature. Functionalized polymer-grafted silica nanoparti-cles, a semi-crystalline hydroxyethyl cellulose derivative and cucur-bit[8]uril undergo aqueous self-assembly at multiple length scales to form the SPCH, facilitated by host-guest interactions at the molecular level and nano-fibril formation at colloidal length scale. The fibers exhibit a unique combination of stiffness and high damping capacity (60-70%), the latter exceeding that of even biological silks and cellulose-based viscose rayon. The remarkable damping performance of the hierarchically-structured fibers is proposed to arise from the complex combination and interactions of 'hard' and 'soft' phases within the SPCH and its constituents. SPCH represent a new class of hybrid supramolecular composites, opening a window into fiber technology through low-energy manufacturing.
Laminated bamboo is emerging as a novel material in design and construction. As a natural fibre c... more Laminated bamboo is emerging as a novel material in design and construction. As a natural fibre composite, it has unique mechanical properties that allow for innovations that are not possible in other materials. Here, we discuss one new application of those properties: the development of a novel bending technique using high temperature, and we explore its implications for design. We have explored the fundamental properties of laminated bamboo and its thermal relaxation as it passes the glass transition temperatures of its constituent polymers. By mechanically thinning engineered bamboo material, score lines allow precise, controlled and localised heating that promotes limited but essential elasto‐plastic behaviour. Concentrated heating above the glass transition temperature induces property evolution and structural morphology changes, which results in thermal relaxation with minimal recovery and full set upon cooling. This original technology is then deployed in the design and construction of a folded plate helical shell composed of thin laminated bamboo sheets.
Lumen impregnation, unlike most other wood modification methods, is typically assessed by the por... more Lumen impregnation, unlike most other wood modification methods, is typically assessed by the pore-filling ratio (PFR) (i.e. the fraction of luminal porosity filled) rather than by weight percentage gain (WPG). During lumen impregnation, the impregnants act on the voids in the wood rather than on the solid mass (e.g. cell walls), but the PFR cannot be measured as conveniently as the WPG during processing. Here, it is demonstrated how the PFR can be calculated directly from the WPG if the bulk density of the untreated wood is known. The relationship between the WPG and bulk density was examined experimentally by applying a pressured impregnation on knot-free specimens from Sitka spruce with a liquid mixture of methacrylate monomers. Based on the validated model, it was possible to further study the effect of different process-related parameters, such as hydraulic pressure, on lumen impregnation. Skeletal density is another key parameter in this model, which directly reflects the amount of inaccessible pores and closed lumens, and can be independently determined by helium pycnometry. The permeability can be qualitatively evaluated by PFR as well as skeletal density. For instance, poor permeability of knotty wood, due to the large extractives content around knots, was reflected by a lower skeletal density and inefficient lumen impregnation (low PFR). Although this model was examined on a laboratory scale, it provides guidance on the precise effect of different parameters on lumen impregnation, thereby improving the fundamental understanding of and enabling better control over the modification of wood by impregnation.
Polymer composites are found throughout the world both natural and artificial in origin. In the v... more Polymer composites are found throughout the world both natural and artificial in origin. In the vast majority of applications, composites serve as structural support or reinforcement roles. Demand for lightweight tough composites is growing in multiple application spaces such as areospace, biomaterials, and infrastructure with physical properties as diverse as the applications. The unifying component in all composites is the presence of an interphase. Many measurement techniques and measurement tools have been developed for the study of this crucial region in composite materials. Many of these methods are great for the measurment and study of bulk properties or model systems. However, development of methods that permit the direct observation of interactions at the interphase during applied stress are needed. Here we employ fluorescence lifetime imaging and hyperspectral imaging to observe activation of a fluorogenic dye at the composite interface as a result of applied stress. The advantages of this sytem include commercial availability of the dye precursor, and simple one-pot functionalization. The attachment of the dye at the interface is easily monitored through emission wavelength shifts and fluorescence lifetime variations. Interfacial mechano-responsive dyes have potential for both fundamental studies as well as industrial use as a structural health monitoring tool.
Trees, and their derivative products, have been used by societies around the world for thousands ... more Trees, and their derivative products, have been used by societies around the world for thousands of years. Contemporary construction of tall buildings from timber, in whole or in part, suggests a growing interest in the potential for building with wood at a scale not previously attainable. As wood is the only significant building material that is grown, we have a natural inclination that building in wood is good for the environment. But under what conditions is this really the case? The environmental benefits of using timber are not straightforward ; although it is a natural product, a large amount of energy is used to dry and process it. Much of this can come from the biomass of the tree itself, but that requires investment in plant, which is not always possible in an industry that is widely distributed among many small producers. And what should we build with wood? Are skyscrapers in timber a good use of this natural resource, or are there other aspects of civil and structural engineering, or large-scale infrastructure, that would be a better use of wood? Here, we consider a holistic picture ranging in scale from the science of the cell wall to the engineering and global policies that could maximise forestry and timber construction as a boon to both people and the planet.
The increasing use of high-value carbon fibre in composites is linked with increasing waste gener... more The increasing use of high-value carbon fibre in composites is linked with increasing waste generation: from dry fibre and prepreg offcuts during manufacturing to end-of-life parts. In this work, a novel thermoplastic tape was produced from 60 wt.% manufacturing waste carbon fibres (60 mm long) and 40 wt.% polyester (PET) fibres using a thermal consolidation technique. The thin (0.2 mm) and narrow (20mm wide) tapes were then used to fabricate laminated composite panels in two 0/90 tape architectures: cross-ply and woven ply. Various mechanical properties, including tensile, flexural, compression, and impact, were evaluated. It was found that cross-ply performed better than woven ply laminates, with failure in the latter materials typically initiating at the tape interlacement points.
The reinforcement of natural rubber (NR) with particles and fibres enables their use in even high... more The reinforcement of natural rubber (NR) with particles and fibres enables their use in even high performance applications, such as in road-racing bicycle tire casings. Here, for the first time, we examine the potential of silk textiles as reinforcements in NR to produce a fully-green, flexible yet strengthened elastomeric composite material. Various material properties were evaluated and compared with similar nylon textile reinforced NR composites. Two types of NR were used: whole and purified natural rubbers. The composite samples were prepared by sandwiching a single layer of textile between layers of NR. NR/silk composites exhibited higher static and dynamic mechanical properties than NR/nylon composites. In addition, silk textiles in whole NR composites performed significantly better than purified NR composites, due to stronger fibre/matrix adhesion and better wettability in the former, as indicated by surface energy measurements and scanning electron microscopy micrographs. Such bio-based natural rubber/silk composites might find interesting applications in soft robotics and as flexible, inflatable tubes.
The recovery of carbon fibres from waste and end-of-life carbon fibre reinforced plastic material... more 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.
The drive towards sustainability, even in materials technologies, has fuelled an increasing inter... more The drive towards sustainability, even in materials technologies, has fuelled an increasing interest in bio-based composites. Cellulosic fibres, such as flax and jute, are being considered as alternatives to technical synthetic fibres, such as glass, as reinforcements in fibre reinforced polymer composites for a wide range of applications. A critical bottleneck in the advancement of plant fibre composites (PFRPs) is our current inability to predict PFRP properties from data on fibre properties. This is highly desirable in the cost- and time-effective development and design of optimised PFRP materials with reliable behaviour. This study, alongside limited other studies in literature, have found that the experimentally determined (through single fibre tests) fibre properties are significantly different from the predicted (‘back-calculated’ using the popular rule-of-mixtures) fibre properties for plant fibres. In this note, we explore potential sources of the observed discrepancy and identify the more likely origins relating to both measurement and errors in predictions based on the rule-of-mixtures. The explored content in this discussion facilitates the design of a future investigation to (1) identify the sensitivity of the discrepancy between measured and predicted fibre properties to the various potential origins, (2) form a unified hypothesis on the observed phenomenon, and (3) determine whether the rule-of-mixtures model (in specific cases) can be improved and may be able to predict properties precisely.
The non-linear stress-strain behaviour of plant fibre composites is well-known in the scientific ... more The non-linear stress-strain behaviour of plant fibre composites is well-known in the scientific community. Yet, the important consequences of this, in terms of the evolution of stiffness as a function of applied strain and cycles to failure, are not well-studied in literature. This is despite the fact that stiffness degradation is a well-accepted indicator of damage in a composite material, and is regularly used as a component failure criterion. This article systematically explores the evolution of stiffness of various aligned plant fibre composites, subjected to i) monotonic loading, ii) low-cycle, repeated progressive loading, and iii) fatigue loading. The evolution in stiffness in plant fibre composites is found to be complex: structural changes in the elementary fibre cell wall and damage development in the composite have often competing effects on stress-strain behaviour. Indeed, the evolution in stiffness of plant fibre composites is found to be unlike that typically observed in traditional composites, and therefore needs to be taken into account in the design of structural components.
Here we characterise the thermal properties of engineered bamboo panels produced in Canada, China... more Here we characterise the thermal properties of engineered bamboo panels produced in Canada, China, and Colombia. Specimens are processed from either Moso or Guadua bamboo into multi-layered panels for use as cladding, flooring or walling. We utilise the transient plane source method to measure their thermal properties and confirm a linear relationship between density and thermal conductivity. Furthermore, we predict the thermal conductivity of a three-phase composite material, as these engineered bamboo products can be described, using micromechanical analysis. This provides important insights on density-thermal conductivity relations in bamboo, and for the first time, enables us to determine the fundamental thermal properties of the bamboo cell wall. Moreover, the density-conductivity relations in bamboo and engineered bamboo products are compared to wood and other engineered wood products. We find that bamboo composites present specific characteristics, for example lower conductivities—particularly at high density—than equivalent timber products. These characteristics are potentially of great interest for low-energy building design. This manuscript fills a gap in existing knowledge on the thermal transport properties of engineered bamboo products, which is critical for both material development and building design.
With the growing interest in material technologies incorporating bio-based constituents, substitu... more With the growing interest in material technologies incorporating bio-based constituents, substitutes to both conventional polymer foams and their synthetic reinforcements are being sought. In this article, we characterise the physical, mechanical, thermal and environmental resistance properties of novel syntactic foams that are based on silkworm cocoons as natural, hollow particulate fillers for a rigid, bio-based polyurethane matrix. Syntactic foams were cast with the natural cocoons replacing 60–90% by weight (40–70% by volume) of the polymer matrix. Although reinforcing the foam with domesticated and wild cocoons increased its density from 45 kg/m3 to ~60 kg/m3 and ~120 kg/m3 respectively, a marked increase in both absolute and specific compressive properties was also observed. Comparable thermal and dimensional stability of the proteinaceous cocoon reinforced polymer foams with the unreinforced foam attest to comparable environmental resistance.
The ultrastructure of the self-constructed tube housing of the bioluminescent marine worm, Chaeto... more The ultrastructure of the self-constructed tube housing of the bioluminescent marine worm, Chaetopterus sp. reveals that the bio-nanocomposite tube comprises of multiple non-woven plies of multi-axially oriented organic nanofilaments (ø 50-1,100 nm) cemented together by an unstructured organic matrix binder. The thin-walled, impermeable tubes are bio-inspirational for conventional pipe technology. Orientation distribution analyses revealed that the dominant orientation angles of nanofilaments in the tube were 0°, ±45° and ±65°, which correlate well with optimal winding angles for ‘man-made’ fibre reinforced composite pipes subjected to specific loading conditions. Such a use of high aspect ratio nanofilaments in multi-axial laminates would impart toughness and flexibility to the tube structure, and facilitate rapid tube growth. While the tube production mechanism is not entirely known at this stage, our time-lapse studies show that, contrary to generic assumptions in literature, the worm actively, rapidly and sporadically produces and expands the tube.
With the growing interest in bio-based composites as alternatives to traditional glass fibre rein... more With the growing interest in bio-based composites as alternatives to traditional glass fibre reinforced composites (GFRPs), there has been a persistent rise in the commercial use of plant fibre composites (PFRPs). In contrast, nature’s ‘wonder-fibre’ silk has had no commercial applications, and only limited scientific investigations, as a composite reinforcement. To produce silk fibre composites (SFRPs) with useful properties, three key recommendations from our critical literature review were followed: i) a high-failure strain, low-processing temperature thermoset matrix was used to a) maximise the reinforcing effect of low-stiffness, ductile silk, and b) facilitate impregnation and avoid fibre degradation, ii) high fibre volume fractions were employed to ensure that fibres carried a larger fraction of the load, and iii) given the lack of studies investigating fracture energy dissipation mechanisms in SFRPs, interface modification was avoided due to its complex, sometimes detrimental, effects on toughness. In directly addressing the question, ‘is there a case for silks as polymer reinforcements?’, we evaluated various mechanical properties of nonwoven and plain woven SFRPs against similar flax and glass composites. In all cases, woven composites performed better than nonwoven composites. While SFRPs were weak in terms of stiffness, their flexural and tensile strength was comparable to PFRPs, but much below that of GFRPs. Notably, the low density of SFRPs, like PFRPs, made them comparable to GFRPs in terms of specific flexural properties. Woven SFRPs exhibited much higher fracture strain capacities than both flax and glass composites, making SFRPs suitable for applications where high compliance is required. The Achilles’ heels of PFRPs have been their reportedly i) inadequate interfacial properties, ii) inferior impact properties, iii) poor strength performance, and iv) high moisture sensitivity. We found that SFRPs outperformed their flax counterparts in areas i)-iii), and were more comparable to, but not better than, GFRPs. While concerns such as cost and ‘sustainability’ of silk are acknowledged, potential applications for SFRPs are discussed.
The housing tube material of the marine worm Chaetopterus sp. exhibits thermal stability up to 25... more The housing tube material of the marine worm Chaetopterus sp. exhibits thermal stability up to 250 °C, similar to other biological materials like mulberry silkworm cocoons. Interestingly, however, dynamic mechanical thermal analysis conducted in both air and water elucidated the lack of a glass transition in the organic tube wall material. In fact, the visco-elastic properties of the anhydrous and undried tube were remarkably stable (i.e. constant and reversible) between –75 and 200 °C in air and 5 and 75 °C in water, respectively. Moreover, it was found that hydration and associated-water plasticization were key to the rubber-like flexible properties of the tube; dehydration transformed the material behaviour to glass-like. The tube is made of bio-nanocomposite fibrils in highly-oriented arrangement, which we argue favours the biomaterial to be highly crystalline or cross-linked, with extensive hydrogen- and/or covalent-bonds. Mechanical property characterisation in the longitudinal and transverse directions ascertained that the tubes were not quasi-isotropic structures. In general, the higher stiffness and strength in the transverse direction implied that there were more nanofibrils oriented at ±45° and ±65° than at 0° to the tube axis. The order of the mechanical properties of the soft-tough tubes was similar to synthetic rubber-like elastomers and even some viscid silks. The complex structure-property relations observed indicated that the worm has evolved to produce a tubular housing structure which can i) function stably over a broad range of temperatures, ii) endure mechanical stresses from specific planes/axes, and iii) facilitate rapid growth or repair.
To aid design engineers in closing the existing gap between current scientific knowledge and actu... more To aid design engineers in closing the existing gap between current scientific knowledge and actual market applications of plant fibre reinforced plastics (PFRPs), this article provides comprehensive Ashby-type materials selection charts for PFRPs to facilitate product design and development. General tensile mechanical property profiles are presented for a variety of PFRPs to enable the design for i) optimal stiffness and strength, ii) minimal weight (i.e. optimal specific properties), iii) minimal cost, and iv) minimal eco-impact. A large database is used to capture the range in properties of different i) reinforcements forms (short fibres: pellets and nonwovens; long fibres: multiaxials and unidirectionals), ii) polymer matrices (thermoplastic and thermosetting), and iii) manufacturing techniques (injection moulding, compression moulding, hand lay-up, vacuum infusion, resin transfer moulding and prepregging). As PFRPs are often viewed as alternatives to glass fibre composites (GFRPs), for demonstrative purposes the tensile properties of the various PFRPs are compared with similar GFRPs. Moreover, highlighting that other mechanical parameters may be critical performance indices for specific products and applications, a materials property chart for a fatigue-limited design is also produced.
Composites Part A: Applied Science and Manufacturing, 2014
While it is common knowledge in natural fibre composites manufacture that plant fibre reinforceme... more While it is common knowledge in natural fibre composites manufacture that plant fibre reinforcements are considerably less compactable than synthetic fibre reinforcements, the through-thickness compaction behaviour of animal-fibre silk reinforcements has not been characterised thus far. We find that not only are silk reinforcements significantly more compressible than plant fibre reinforcements, but their compactibility exceeds that of even glass fibre textiles. For instance, the fibre volume fraction (at a compaction pressure of 2.0 bar) of woven biaxial fabrics of silk, plant fibres and E-glass are 54-57%, 30-40% and 49-54%, respectively. Therefore, silks provide an opportunity to manufacture high fibre content natural fibre composites; this is a bottleneck of plant fibre textiles. Analysing the structure of silk textiles through scanning electron microscopy, we show that favourable fibre/yarn/fabric geometry, high degree of fibre alignment and dispersion, and suitable technical fibre properties enable optimal packing and arrangement of silk textiles.
This article evaluates the mechanical properties of vacuum-infused unidirectional plant fibre com... more This article evaluates the mechanical properties of vacuum-infused unidirectional plant fibre composites (PFRPs), composing of bast fibre yarns and thermoset matrices. PFRPs are found to have lower fibre 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 fibre lengths are of the same order (0.2-0.5 mm). Composites reinforced with flax rovings exhibit exceptional fibre tensile modulus of 65-75 GPa and fibre tensile strength of about 800 MPa.
Plant fibres, perceived as environmentally sustainable substitutes to E-glass, are increasingly b... more Plant fibres, perceived as environmentally sustainable substitutes to E-glass, are increasingly being employed as reinforcements in polymer matrix composites. However, despite the promising technical properties of cellulose-based fibres and the historic use of plant fibre reinforced plastics (PFRPs) in load-bearing components, the industrial uptake of PFRPs in structural applications has been limited. Through an up-to-date critical review of literature, this manuscript presents an overview on key aspects that need consideration when developing PFRPs for structural applications, including the selection of I) the fibre type, fibre extraction process, and fibre surface modification technique, II) fibre volume fraction, III) reinforcement geometry and interfacial properties, IV) reinforcement packing arrangement and orientation, and V) matrix type and composite manufacturing technique. A comprehensive materials selection chart (Ashby plot) is also produced to facilitate the design of a PFRP component, based on (absolute and specific) tensile properties.
Plant fibres, perceived as environmentally sustainable substitutes to E-glass, are increasingly b... more Plant fibres, perceived as environmentally sustainable substitutes to E-glass, are increasingly being employed as reinforcements in polymer matrix composites. However, despite the promising technical properties of cellulose-based fibres and the historic use of plant fibre composites (PFRPs) in load-bearing components, the industrial uptake of PFRPs in structural applications has been limited. In developing PFRPs whose mechanical properties are well-characterised, optimised and well-predicted, this thesis addresses the question: Can PFRPs replace E-glass composites (GFRPs) in structural applications?
Ensuring that the highest reinforcement potential is exploited, this research examines the mechanical properties of aligned PFRPs based on bast fibre yarns/rovings and thermoset matrices. Although aligned GFRPs are found to outperform aligned PFRPs in terms of absolute mechanical properties, PFRPs reinforced with flax rovings exhibit exceptional properties, with a back-calculated fibre tensile modulus of up to 75 GPa and fibre tensile strength of about 800 MPa.
To identify the processing window which produces composites with useful properties, the minimum, critical and maximum fibre volume fraction of PFRPs have been determined, and compared to that of synthetic fibre reinforced composites. The effect of fibre volume fraction on the physical and tensile properties of aligned PFRPs has also been investigated. Furthermore, micro-mechanical models have been developed and experimentally validated, to reliably predict the effect of (mis)orientation, in the forms of yarn twist/construction and off-axis loading, on the tensile properties of aligned PFRPs.
To provide a complete set of fatigue data on aligned PFRPs, the effect of various composite parameters on PFRP cyclic-loading behaviour has been illustrated through S-N lifetime diagrams. A constant-life diagram has also been generated to enable the fatigue design and life prediction of a PFRP component. At each stage, the fatigue performance of PFRPs has been compared to that of GFRPs.
Finally, in directly addressing the main theme, this thesis adopts a novel comparative case study approach to investigate the manufacture and mechanical testing of full-scale 3.5-meter composite rotor blades (suitable for 11 kW turbines) built from flax/polyester and E-glass/polyester. The study claims that under current market conditions, optimised plant fibre reinforcements are a structural, but not low-cost or sustainable, alternative to conventional E-glass reinforcements.
Distribution for FRPs in EU [tonnes] d Moderate (~60,000) Wide (600,000) Low (15,000) Cost of raw... more Distribution for FRPs in EU [tonnes] d Moderate (~60,000) Wide (600,000) Low (15,000) Cost of raw fibre [£/kg] Low (~0.5-1.5) Low (~1.3-20.0) High (>12.0) Technical Density [gcm -3 ] Low (~1.35-1.55) High (2.50-2.70) Low (1.70-2.20) Tensile stiffness [GPa] Moderate (~30-80) Moderate (70-85) High (150-500) Tensile strength [GPa] Low (~0.4-1.5) Moderate (2.0-3.7) High (1.3-6.3) Tensile failure strain [%] Low (~1.4-3.2) High (2.5-5.3) Low (0.3-2.2) Specific tensile stiffness [GPa/gcm -3 ] Moderate (~20-60) Low (27-34) High (68-290) Specific tensile strength [GPa/gcm -3 ] Moderate (~0.3-1.1) Moderate (0.7-1.5) High (0.6-3.7) Abrasive to machines No Yes Yes Ecological Energy demand of raw fibre [MJ/kg] Low (4-15) e Moderate (30-50) High (>130) Renewable source Yes No No f Recyclable Yes Partly Partly Biodegradable Yes No No Hazardous/toxic (upon inhalation) No Yes Yes a Includes bast, leaf and seed fibres, but does not include wood and grass/reed fibres. b Includes E-and S-glass fibres. c Includes PAN-and pitch-based carbon fibres. d Estimated values for the year 2010, from [32] for global fibre production values and from [24, 25, 27] for values on the distribution of fibres for FRPs in EU. e While the energy required in the cultivation of plant fibres is low (4-15 MJ/kg), further processing steps (e.g. retting and spinning) can significantly increase the cumulative energy demand, for instance, to up to 146 MJ/kg for flax yarn [10]. f Carbon fibres based on cellulosic precursors currently account for only 1-2% of the total carbon fibre market [33]. including the stress-strain behaviour of each phase (that is, the fibre and the matrix), the volumetric composition, the geometrical structure and arrangement of the phases, and the interface properties [51]. While the EU automotive industry has principally focussed on three bast fibres, namely flax, jute and hemp, for PFRP production [10, 11, 25, 52], noting their regional availability, other fibres like sisal [13], bamboo [53], cotton [54], coir [55]
This chapter aims to provide a broad understanding of plant fibres and their composites, specific... more This chapter aims to provide a broad understanding of plant fibres and their composites, specifically commenting on critical factors influencing the mechanical properties of plant fibre composites, thereby dictating their applicability in structural components. In essence, this chapter serves as a relevant background and literature review to the work described in this thesis.
This chapter is based on the peer-reviewed journal article: Shah DU. Developing plant fibre composites for structural applications by optimising composite parameters: a critical review. Journal of Materials Science, 2013, 48(18): p. 6083-6107.
Employing plant fibre yarns as continuous reinforcements for unidirectional composites, this chap... more Employing plant fibre yarns as continuous reinforcements for unidirectional composites, this chapter evaluates the mechanical properties of aligned plant fibre composites (PFRPs), against aligned E-glass composites (GFRPs), to appreciate the true potential of biofibres as stiffness-inducing reinforcements. As composite materials are heterogeneous, the reinforcement and matrix type will obviously affect composite properties. Noting the effectiveness of aligned bast fibre reinforcements (e.g. flax, hemp and jute) and thermoset matrices (e.g. unsaturated polyester and epoxy) for load-bearing composites (as highlighted in Chapter 2), this study examines the effect of plant yarn type/quality and thermoset matrix type on composite properties.
This chapter is based on the peer-reviewed journal article: Shah DU, Schubel PJ, Clifford MJ, Licence P. Mechanical property characterization of aligned plant yarn reinforced thermoset matrix composites manufactured via vacuum infusion. Polymer-Plastics Technology and Engineering, 2014, 53(3): p. 239-253.
The purpose of this chapter is to analyse the relationship between structure and properties of al... more The purpose of this chapter is to analyse the relationship between structure and properties of aligned PFRPs. Specifically, the effect of fibre volume fraction on PFRP physical properties (porosity and fibre packing arrangement) and tensile properties is discussed. Parameters such as minimum, critical and maximum obtainable fibre volume fraction are also determined to identify the range of fibre volume fractions that produce twisted yarn reinforced PFRPs with useful properties.
This chapter is based on the peer-reviewed journal article: Shah DU, Schubel PJ, Licence P, Clifford MJ. Determining the minimum, critical and maximum fibre content for twisted yarn reinforced plant fibre composites. Composites Science and Technology, 2012, 72(15): p. 1909-1917.
In this chapter, the effect of (mis)orientation on the mechanical behaviour of aligned PFRPs is i... more In this chapter, the effect of (mis)orientation on the mechanical behaviour of aligned PFRPs is investigated. In particular, this chapter aims to i) review the effect of the microfibril angle on the tensile properties of plant fibres, ii) model the effect of reinforcing yarn twist on PFRP tensile properties, and iii) evaluate the effect of off-axis loads on PFRP tensile properties. This will i) provide an improved understanding on the mechanical behaviour and response of PFRPs, ii) enable the design and optimisation of PFRPs, and iii) enable the development of models to predict the mechanical properties of PFRPs. All of these are key to the employment of PFRPs for load-bearing applications.
This chapter is based on the peer-reviewed journal articles: Shah DU, Schubel PJ, Clifford MJ. Modelling the effect of yarn twist on the tensile strength of unidirectional plant fibre yarn composites. Journal of Composite Materials, 2012, 47(4): p. 425-436. Shah DU, Schubel PJ, Clifford MJ, Licence P. The tensile behavior of off-axis loaded plant fiber composites: an insight on the non-linear stress-strain response. Polymer Composites, 2012, 33(9): p. 1494-1504.
At present, there are limited papers that enable the preliminary design of PFRPs against fatigue.... more At present, there are limited papers that enable the preliminary design of PFRPs against fatigue. The objective of this study is to provide a complete set of fatigue data on aligned PFRPs to enable the design of a PFRP component against fatigue. A primary aim of the study described in this chapter is to thoroughly characterise the fatigue performance of aligned PFRPs through S-N lifetime diagrams, and specifically investigate the effect of i) plant fibre type, ii) fibre volume fraction, iii) textile architecture, and iv) stress ratio, on PFRP cyclic loading behaviour. At each stage, the fatigue performance of PFRPs is compared to that of E-glass/polyester composites. In addition, to facilitate fatigue life prediction of a PFRP component, a comprehensive constant-life diagram is generated. Recently, the author of this thesis has applied the data for the fatigue design and life prediction of a 3.5-meter hemp/polyester rotor blade.
This chapter is based on the peer-reviewed journal article: Shah DU, Schubel PJ, Licence P, Clifford MJ, Fatigue life evaluation of aligned plant fibre composites through S-N curves and constant-life diagrams. Composites Science and Technology, 2013, 74: p. 139-149.
This chapter is based on the peer-reviewed journal article:
Shah DU, Schubel PJ, Clifford MJ. Can... more This chapter is based on the peer-reviewed journal article: Shah DU, Schubel PJ, Clifford MJ. Can flax replace E-glass in structural composites? A small wind turbine blade case study. Composites Part B: Engineering, 2013, 52: p. 172-181.
The aim of this chapter is to present the major conclusions established from the work described i... more The aim of this chapter is to present the major conclusions established from the work described in this thesis, with reference to the overall theme described in Chapter 1. In addition, several recommendations are made for future work.
With the growing interest in developing high-performance natural fibre composites (NFRPs) for (se... more With the growing interest in developing high-performance natural fibre composites (NFRPs) for (semi)-structural applications, researchers are increasingly considering liquid composite moulding (LCM) processes and investigating key manufacturing-related issues. Here, we critically review the literature on LCM of NFRPs. Consequently, we identify key findings concerning the reinforcement-related factors (namely, compaction and permeability) that influence, if not govern, the mould filling stage during LCM of NFRPs. In particular, the differences in structure (physical and chemical) of natural and synthetic fibres, their semi-products (i.e. yarns and rovings) and their textiles are shown to have a perceptible effect on their processing via LCM.
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Papers by Darshil Shah
Ensuring that the highest reinforcement potential is exploited, this research examines the mechanical properties of aligned PFRPs based on bast fibre yarns/rovings and thermoset matrices. Although aligned GFRPs are found to outperform aligned PFRPs in terms of absolute mechanical properties, PFRPs reinforced with flax rovings exhibit exceptional properties, with a back-calculated fibre tensile modulus of up to 75 GPa and fibre tensile strength of about 800 MPa.
To identify the processing window which produces composites with useful properties, the minimum, critical and maximum fibre volume fraction of PFRPs have been determined, and compared to that of synthetic fibre reinforced composites. The effect of fibre volume fraction on the physical and tensile properties of aligned PFRPs has also been investigated. Furthermore, micro-mechanical models have been developed and experimentally validated, to reliably predict the effect of (mis)orientation, in the forms of yarn twist/construction and off-axis loading, on the tensile properties of aligned PFRPs.
To provide a complete set of fatigue data on aligned PFRPs, the effect of various composite parameters on PFRP cyclic-loading behaviour has been illustrated through S-N lifetime diagrams. A constant-life diagram has also been generated to enable the fatigue design and life prediction of a PFRP component. At each stage, the fatigue performance of PFRPs has been compared to that of GFRPs.
Finally, in directly addressing the main theme, this thesis adopts a novel comparative case study approach to investigate the manufacture and mechanical testing of full-scale 3.5-meter composite rotor blades (suitable for 11 kW turbines) built from flax/polyester and E-glass/polyester. The study claims that under current market conditions, optimised plant fibre reinforcements are a structural, but not low-cost or sustainable, alternative to conventional E-glass reinforcements.
This chapter is based on the peer-reviewed journal article:
Shah DU. Developing plant fibre composites for structural applications by optimising composite parameters: a critical review. Journal of Materials Science, 2013, 48(18): p. 6083-6107.
This chapter is based on the peer-reviewed journal article:
Shah DU, Schubel PJ, Clifford MJ, Licence P. Mechanical property characterization of aligned plant yarn reinforced thermoset matrix composites manufactured via vacuum infusion. Polymer-Plastics Technology and Engineering, 2014, 53(3): p. 239-253.
This chapter is based on the peer-reviewed journal article:
Shah DU, Schubel PJ, Licence P, Clifford MJ. Determining the minimum, critical and maximum fibre content for twisted yarn reinforced plant fibre composites. Composites Science and Technology, 2012, 72(15): p. 1909-1917.
This chapter is based on the peer-reviewed journal articles:
Shah DU, Schubel PJ, Clifford MJ. Modelling the effect of yarn twist on the tensile strength of unidirectional plant fibre yarn composites. Journal of Composite Materials, 2012, 47(4): p. 425-436.
Shah DU, Schubel PJ, Clifford MJ, Licence P. The tensile behavior of off-axis loaded plant fiber composites: an insight on the non-linear stress-strain response. Polymer Composites, 2012, 33(9): p. 1494-1504.
This chapter is based on the peer-reviewed journal article:
Shah DU, Schubel PJ, Licence P, Clifford MJ, Fatigue life evaluation of aligned plant fibre composites through S-N curves and constant-life diagrams. Composites Science and Technology, 2013, 74: p. 139-149.
Shah DU, Schubel PJ, Clifford MJ. Can flax replace E-glass in structural composites? A small wind turbine blade case study. Composites Part B: Engineering, 2013, 52: p. 172-181.