Mechanical properties of alkali treated plant fibres and their potential as reinforcement materials II. Sisal fibres

The tensile strength and Young’s modulus of sisal fibre bundles were determined following alkalisation. The results were then analysed with respect to the diameter and internal structure such as cellulose content, crystallinity index and micro-fibril angle. The tensile strength and stiffness were found to vary with varying concentration of caustic soda, which also had a varying effect on the cell wall morphological structure such as the primary wall and secondary wall. The optimum tensile strength and Young’s modulus were obtained at 0.16% NaOH by weight. The stiffness of the sisal fibre bundles obtained using the cellulose content also referred to as the micro-fibril content was compared with the stiffness determined using the crystallinity index. The stiffness obtained using the crystallinity index was found to be higher than that obtained using the cellulose content however, the difference was insignificant. Alkalisation was found to change the internal structure of sisal fibres that exhibited specific stiffness that was approximately the same as that of steel. These results indicates that the structure of sisal fibre can be chemically modified to attain properties that will make the fibre useful as a replacement for synthetic fibres where high stiffness requirement is not a pre-requisite and that it can be used as a reinforcement for the manufacture of composite materials.     

Mechanical properties of alkali treated plant fibres and their potential as reinforcement materials. I. hemp fibres

In this study a thorough analysis of physical and fine structure of hemp fibre bundles, namely surface topography, diameter, cellulose content and crystallinity index, have been presented. The fibre bundles have been alkalised and physical and mechanical properties analysed. Alkalisation was found to change the surface topography of fibre bundles and the diameter decreased with increased concentration of caustic soda. Cellulose content increase slightly at lower NaOH concentrations and decrease at higher NaOH concentrations. The crystallinity index decrease with increase in caustic soda concentration up to 0.24% NaOH beyond which, it decreases with increase in NaOH concentration. It was also found that the tensile strength and stiffness increases with increase in the concentration of NaOH up to a limit. Tensile strength and Young’s modulus increase with decrease in cellulose content, while crystalline cellulose decreases slightly but with improved crystalline packing order resulting in increased mechanical properties. Similar observations are elucidated by the crystallinity index. Alkalised hemp fibre bundles were found to exhibit a similar specific stiffness to steel, E-glass and Kevlar 29 fibres. The results also show that crystallinity index obtained following alkalisation has a reverse correlation to the mechanical properties. Stiffer alkalised hemp fibre bundles are suitable candidates as reinforcements to replace synthetic fibres. The improvement in mechanical properties of alkali treated hemp fibre bundles confirms their use as reinforcement materials.     

Tensile strength of pine needles and their feasibility as reinforcement in composite materials

A feasibility study concerning the use of pine needles from Maritime Pine (Pinus pinaster) trees as reinforcement in composite materials has been presented in this paper with the tensile strength being investigated for a total of 150 specimens at three gauge lengths, namely 50, 75 and 100 mm. In order to calculate the tensile strength for each specimen, a correlation was obtained between the cross-sectional area and external dimensions of the individual pine needles. The mean value of the tensile strength was noted to vary only slightly between 33.4 MPa for the 50 mm gauge length and 31.4 MPa for the 100 mm case with a minimum and maximum of 15 and 65 MPa, respectively. Analysis of the data using the standard Weibull model indicated the Weibull strength to vary between 33.5 and 36.0 MPa whereas the Weibull modulus varied between approximately 3.5 and 4.5. Further analysis using the Weibull model indicated the presence of a bimodal strength distribution at each gauge length that was consistent with the presence of two distinct flaw populations operating within the pine needles. Overall, it was concluded that the strength of the pine needles was sufficient for inclusion in polymer matrix composites subject to low stress or non-load bearing applications such as fibreboard and thermal or acoustic insulation.     

Full exploitation of Cannabis sativa as reinforcement/filler of thermoplastic composite materials

Hemp strands and cane straw of hemp have been used as reinforcement and filler of polypropylene composites obtained by injection moulding. The aim of the work was to improve the tensile properties of hemp composites and make them more similar to those obtained with glass fibre composites. For this reason, maleated polypropylene (MAPP) was used as compatibility agent in hemp strand and hemp straw composites. MAPP decreases the hydrophobic nature of PP matrix and enhances the dispersion and the adhesion at interface between both constituents. The hydrophilic property of material’s surface was determined by colloidal titration. The length and diameter of the fibre reinforcement during processing were analysed and their aspect ratio calculated. The tensile results obtained showed that the mechanical properties of hemp strand composites can amount to 80% of the mechanical properties of glass fibre composites.     

Artichoke (Cynaracardunculus L.) fibres as potential reinforcement of composite structures

The aim of this paper is to examine the use of artichoke fibres as potential reinforcement in polymer composites. The fibres are extracted from the stem of artichoke plant, which grows in Southern Sicily. In order to use these lignocellulosic fibres as potential reinforcement in polymer composites, it is fundamental to investigate their microstructure, chemical composition and mechanical properties.Therefore, the morphology of artichoke fibres was investigated through electron microscopy, the thermal behaviour through thermogravimetric analysis and the real density through a helium pycnometer. The chemical composition of the natural fibres in terms of cellulose, lignin, and ash contents was determinated by using standard test methods.Finally, the mechanical characterization was carried out through single fibre tensile tests, analysing the results through statistical analysis.     

Lateral stiffness and vibration characteristics of composite plated RC shear walls with variable fibres spacing

In this paper, a finite element model for static and free vibration analysis of reinforced concrete (RC) shear walls structures strengthened with thin composite plates having variable fibres spacing is presented. An efficient analysis method that can be used regardless to the sizes and location of the bonded plates is proposed in this study. In the numerical formulation, the adherents and the adhesives are all modelled as shear wall elements, using the mixed finite element method. Several test problems are examined to demonstrate the accuracy and effectiveness of the proposed method. Numerical results are obtained for six nonuniform distributions of E-glass, graphite and boron fibres in epoxy matrices. The fibre redistributions of the bonded plates are seen to increase the frequencies modes and reduce substantially the lateral displacements.     

On the reinforcement of concrete sleepers by composite materials

The most important role of sleepers in a railroad system is conveyance of the train load from rail to the ballast. Since the sleepers are under complicated loading in real conditions, the first stage is static analysis of them. Future railway traffic will certainly be even faster than that of today, and at the same time the demanded load capacity of the trains will probably increase. This implies that the demands for concrete sleepers will increase and the need for reliable analytical tools will be extensive in the near future.

The aim of this work is to increase the load capacity of the sleepers by means of composite materials and to make the railroad system ready for heavier and faster trains. The first step is applying a closed form solution. Then a finite element model of a concrete sleeper is established, in the nest step. Later a full-scale test of reinforced and non-reinforced sleepers is done, and at last, a validation of the finite element model through comparison with full-scale test results as well as closed form solution is conducted. The percentage of increased load capacity of sleepers and good agreements between the results make the work beneficial and reliable.     

Bending reinforcement of timber beams with composite carbon fiber and basalt fiber materials

This work shows a study based on data obtained experimentally using bending tests of pine timber beams reinforced with composite materials. Fibers used for the execution of the reinforcement are basalt and carbon. Basalt fiber composites are applied in different grammages, whereas with carbon composites, unidirectional and bidirectional fabrics are used. The behavior of the beams was analyzed regarding the reinforcement variables applied, and the results are compared with those of the tested beams without reinforcement. This work proves the good behavior of fiber reinforce plastic (FRP) with basalt fiber when applied to timber beams, and that of bidirectional carbon fabrics as opposed to the unidirectional ones.     

Estimation of composite strength by a modified rule of mixtures incorporating defects

Inadequacy of the rule of mixtures to estimate the true values of composite properties having a large concentration of defects has led to numerous tests being performed costing time and money. Statistical average of the properties from the Weibulls distribution law has thus been relied upon so far for the design of composite structures. Yet, to estimate the properties of a fibre reinforced composite material having sufficient flaw densities due to the methodologies adopted for its processing, more so for fibres, has long been appreciated. To avoid this inadequacy a modified rule of mixtures is developed incorporating defect concentration in the fibre and matrix enabling to arrive at a more realistic value of the composite.     

Characterization of composite materials made from discontinuous carbon fibres within the framework of composite recycling

Recycling carbon fibres from waste composite materials would only be efficient if it were possible to separate the fibres and the matrix and to re-use the recycled fibres as new reinforcements. The challenge is to use non-continuous fibres to produce high-strength materials. The formation of defects in “semi-long” fibre composites has not yet been taken into account. In this paper the influence of fibre length and fibre alignment on the strength and the modulus of composite materials is illustrated. It is shown that the presence of defects may be modelled in order to understand what the quality of a second generation composite material would be.     

Carbon nanotubes as reinforcement elements of composite nanotools

Nanotechnology is stimulating the development of nanomanipulators, including tips to interact with individual nanosystems. Fabricating nanotips fulfilling the requirements of shape (size, aspect ratio), mechanical, magnetic, and electrical properties is a material science challenge. Here, we report the generation of reinforced carbon-carbon composite nanotools using a nanotube (CNTs) covered by an amorphous carbon matrix (shell); the CNT tip protruded and remained uncoated to preserve apex size. Unsuitable properties such as flexibility and vibration could be controlled without deteriorating the CNT size, strength, and resilience. Nanomanipulation experiments and molecular dynamics simulations have been used to study the mechanical response of these composite beams under bending efforts. AFM probes based on these C-C composite high aspect ratio tips generated excellent image resolution and showed no degradation after acquiring several hundred (400) images.     

Characterization of a novel natural cellulose fabric from Manicaria saccifera palm as possible reinforcement of composite materials

Identifying novel natural fibers/fabrics with proper properties as reinforcement material is a new challenge in the field of bio-composites. Hence, the aim of this paper is to study the possibility of using a natural fabric extracted from Manicaria saccifera palm as a novel reinforcement in composites. This fabric was extensively characterized by chemical composition analysis, infrared spectroscopy (FTIR) analysis, morphological studies (SEM), thermo-gravimetric analysis (TGA) and physical /mechanical properties studies. From SEM analysis it was identified globular protrusions spread uniformly over the fiber which could help the mechanical interlock with the resin. As well, Manicaria fabric showed good thermal stability, low density, low moisture content and good tensile properties. Further, their properties are comparable to most natural cellulose fabrics and some synthetic fabrics, such as fiber glass fabrics. Manciaria saccifera fabric showed to be a suitable candidate as natural reinforcement material for the development of bio- composite.     

Tensile properties of chemically treated hemp fibres as reinforcement for composites

Natural fibres, unlike synthetic fibres fabricated in-house, grow naturally. Their geometrical and physical properties are highly affected by environmental issues such as climate change. For instance, inconsistent cross-sectional areas and shapes along the length of a natural fibre can result from environmental changes. These irregularities in natural fibres affect the ultimate load that can be carried by these fibres in structural engineering applications. In this study, the tensile properties of single hemp fibres were measured by taking into account, the variations in fibre diameters. Alkali, acetyl and silane treatments of fibres were carried out to obtain a better surface finish. The treatment effects on the fibres with respect to tensile properties were discussed. The relationship between tensile properties of treated fibres and the variation of their diameters was also studied. It was found that the tensile strength of chemically-treated fibres was lower than that of untreated fibres.     

Effect of reinforcement and solvent content on moisture absorption in epoxy composite materials

The effect of fibrous reinforcement and solvent content on moisture uptake in composite laminate was investigated. Two materials using identical epoxy resin systems but different reinforcements—glass vs. carbon fibers—and of different solvent content—low vs. normal—were examined. Samples were characterized in terms of water absorption and desorption. Mechanical and thermal properties including flexural modulus, flexural strength, and glass transition temperature were measured. Results clearly show the contribution of the fiber reinforcement as well as solvent content on the water absorption rate and mechanical property changes.     

Thermal deformation of carbon-carbon composite materials with different schemes of reinforcement under thermal cycling conditions

We study the results of experimental investigation of the thermal deformation of carbonized carbon fiber-reinforced plastics used as a heat-shielding material for aircrafts and shuttle spacecrafts. It is shown that, in the process of thermal cycling of laminated carbon-carbon composite materials with chaotic or three-dimensional reinforcement, the degree of thermal expansion increases independently of the gaseous environment of testing up to a certain number (4–5) of heating cycles. As the number of thermal cycles increases, the thermal strains induced in the materials gradually decrease due to the structural changes in the matrix, fiber, and at the interfaces of materials in the composite. __________ Translated from Problemy Prochnosti, No. 3, pp. 118–133, May–June, 2007.     

A new principle in design of composite materials: reinforcement by interlocked elements

We propose a new materials design concept based on the use of regular assemblies of topologically interlocked elements. A particular implementation of this concept, viz. a layer of tetrahedron-shaped elements, was studied in some detail. The packing arrangement in the layer is such that each individual element is held in place by its immediate neighbours. This structure can provide a load-bearing skeleton of a composite material. A second phase, serving as a matrix or binder, can be selected to provide special structural or functional properties such as thermal or sound insulation, fluid transport, controlled electrical conductivity, etc. It is envisaged that strong and flexible composite materials with high impact resistance can be created on this basis. A model specimen assembled according to this topological principle was tested with respect to its stiffness and load bearing capacity. First experimental and theoretical results show that a layer consisting of many interlocked elements has a much larger mechanical compliance than its monolithic counterpart, and can withstand considerable deformations. Other possible shapes of three-dimensional elements interlocked into a monolayer and the principles of their generation are discussed. The design principle proposed opens up new avenues for creating multifunctional composite materials.     

The potential of using date palm fibres as reinforcement for polymeric composites

Interfacial adhesion of natural fibres as reinforcement for fibre polymeric composites is the key parameter in designing composites. In the current study, interfacial adhesion of date palm fibre with epoxy matrix is experimentally investigated using single fibre pull out technique. The influence of NaOH treatment concentrations (0–9%), fibre embedded length and fibre diameter on the interfacial adhesion property was considered in this study. Scanning Electron Microscopy (SEM) was used to observe the surface morphology and damage feature on the fibre and bonding area before and after conducting the experiments. The results revealed that 6% concentration of NaOH is the optimum solution for treating the date palm fibre to maintain high interfacial adhesion and strength with epoxy matrix. The embedded length of the fibre controlled the interfacial adhesion property, where 10 mm embedded length was the optimum fibre length.     

Axial compressive behaviour of reinforcing fibres and interphase in glass/epoxy composite materials

Axial compressive behaviour of reinforcing fibres and interphase in glass fibre/epoxy resin composites were examined. Axial compressive strengths of glass fibres were evaluated by the tensile recoil method. The effects of silane-based coupling surface treatment agent on the fibre compressive strengths were investigated. The glass fibres showed higher compressive strengths when coated by the surface treatment. Interphase behaviour was also investigated by means of the single-fibre embedded compressive test. The particular stress and strain distributions inside the specimen were examined by a three-dimensional finite element analysis. The parameter interfacial transmissibility instead of the conventional critical fibre length theory was introduced as an index of interfacial properties. This parameter was useful to estimate the interfacial properties at the elastic state apart from the complicated critical state. It was confirmed that the surface treatment improved the glass/epoxy interphase under axial compressive load.     

Bending stiffness of composite plates with delamination

Cross-ply GFRP circular plates have been impacted repeatedly at increasing input energies. The global bending stiffness of each plate was measured before and after each impact through quasi-static bending tests. The effects of local thickening as well as matrix cracking and delamination on global bending stiffness have been discussed. Approximate analytical solutions for bending of damaged and undamaged plates under uniform ring load have been obtained. Two types of models have been used to separate the effects of changes in material and geometrical properties on global bending stiffness. The trends of bending stiffness changes have been successfully simulated by analytical calculations.