Effects of fluctuation of fibre orientation on tensile properties of flax sliver-reinforced green composites

Plant-based natural fibres are often used as a reinforcing material for environmentally friendly green composites. Especially, the form of slivers of natural fibres is anticipated for increasing their stiffness and strength. However, the sliver structure has fluctuations in fibre orientation, which decreases their mechanical properties. This paper describes the effects of such fibre orientation fluctuation on tensile properties of fibre-reinforced fully green composites. The composites were reinforced with slivers of high-strength flax fibres, for which a fabrication method called ‘direct method’ was applied. To quantify the morphology of the fibre orientation, fibre orientation angles were measured on fine segments, which were divided into 1 mm × 1 mm squares on a photograph of the whole composite surface. Although it is well-known that tensile strength of unidirectional composites decreases with increasing fibre orientation angle, the tensile strength obtained here did not show any appreciable relation to the statistical properties of measured fibre orientation angles such as average and standard deviation. The concept of two-dimensional (2D) autocorrelation was used in the present study to express the degree of similarity between fibre orientation angles in two different local areas. Results show that, if high 2D autocorrelation coefficients occupy more area on a composite surface, then this composite possesses more regular fibre orientation and tends to exhibit higher tensile strength. This tendency is stronger in the composites close to on-axis alignment, whereas it became weak in the off-axis composites angled more than 15° because of shear fracture.     

High mechanical performance of fibre reinforced cementitious composites: the role of “casting-flow induced” fibre orientation

Governing the dispersion and the orientation of fibres in concrete through a suitably balanced set of fresh state properties and a carefully designed casting procedure, is a feasible and cost-effective way to achieve a superior mechanical performance of fibre reinforced cementitious composites, which may be required by the intended application, even keeping the fibre content at relatively low values (e.g. around 1% by volume). In this paper the possibility of pursuing the above said “integrated” approach has been addressed in the framework of larger project focused on developing a deflection-hardening FRCC (DHFRCC), reinforced with 100 kg/m³ (1.27% by volume) of short steel fibres (13 mm long and 0.16 mm in diameter). The material has to be employed to manufacture thin (30 mm) roof elements, without any kind of conventional reinforcement, which have been anticipated to work, as simply supported beams, over a 2.5 m span. The study hence paves the way to the possibility of exploiting at an industrial level the correlation among fresh state performance, fibre dispersion and hardened state properties of self consolidating steel fibre reinforced concrete to achieve enhanced structural performance tailored to the specific application.     

Influence of formwork surface on the orientation of steel fibres within self-compacting concrete and on the mechanical properties of cast structural elements

The influences of formwork surface on the final orientation of steel fibres immersed in self-compacting concrete and on the resulting mechanical response of the cast structural elements are investigated. Experimental observations of fibre orientation within cast slabs, obtained via computed tomography, indicate that fibres tend to orient according to the flow patterns during casting, but such tendencies are suppressed near rough formwork surfaces. Fibre orientation, in turn, affects the mechanical properties of the concrete as demonstrated by the load testing of beams extracted from the cast slabs. These processes and results are simulated using a computational fluid dynamics model of the casting process, in tandem with a lattice model of the fracture of the beam specimens. The computational fluid dynamics model determines the coordinates of each fibre within the concrete, which serve as input to the lattice model. Through comparisons with the experimental data, it is shown that these simulations correctly predict the phenomena of interest. We conclude the paper by highlighting a relationship between the number and orientation of the immersed steel fibres crossing the fracture plane and the mechanical response of the structural elements.     

Microstructural,physico-chemical and mechanical characterisation of Sansevieria cylindrica fibres – An exploratory investigation

The microstructural, physical, chemical and mechanical properties of Sansevieria cylindrica fibres are described for the first time in this work. A microstructural analysis of S. cylindrica leaves showed the presence of structural fibres and arch fibres. Polarised light microscopy and scanning electron microscopy of these fibres revealed a hierarchical cell structure that consisted of a primary wall, a secondary wall, a fibre lumen and middle lamellae. The cross-sectional area and porosity fraction of the fibre were estimated to be approximately 0.0245 mm² and 37%, respectively. The fibre density and fineness were approximately 0.915 ± 0.005 g/cm³ and 9 Tex, respectively. An X-ray diffraction and Fourier transform infrared analysis of the fibres showed the presence of cellulose I_β with a crystallinity index of 60%. Tensile tests showed that the corrected Young’s modulus was approximately 7 GPa, the tensile strength was 658 MPa, and the total elongation was between 10% and 12%.     

On flow properties,fibre distribution,fibre orientation and flexural behaviour of FRC

For improving the mechanical properties of fibre reinforced concrete one can either increase the fibre content, use hybrid fibre systems, or one can attempt to align fibres in the direction of stress. In this paper, it is attempted to use the flow-properties of the fresh (self-compacting) concrete to change the fibre distribution and orientation. Using a single mixture of fibre reinforced concrete, containing 3% of 30 mm long straight steel fibres, the fibre distribution and orientation was determined in three different parts of a ‘U-shaped specimen’ where the concrete could flow in three different directions. The fibre distribution and orientation was determined from a CT-scan. Flexural tests show that the mechanical behaviour depends on the fibre distribution and orientation, which can be affected by changing the viscosity of the fresh mixture.     

Influence of the ion-atom flux ratio on the mechanical properties of chromium nitride thin films

In this paper we report the mechanical properties of chromium nitride (CrN) thin films deposited at different levels of ion bombardment and their relationship with the microstructural parameters, such as grain size, preferred orientation and residual stress. The samples were deposited by unbalanced magnetron sputtering changing the substrate-target distance and the substrate bias, keeping other deposition condition fixed. The mechanical properties were obtained by nanoindentation performed on 1.8 μm thick samples. Under the different deposition conditions all of the CrN films were approximately stoichiometric, but clear variations in the microstructure were seen. The hardness was nearly constant at 24-27 GPa even when the grain size, residual stress and crystalline orientation changed. However, the elastic modulus showed a steady increase from 300 to 350 GPa, proportional to the variations in grain size and the residual stress level.     

Terahertz spectroscopy to study the orientation of glass fibres in reinforced plastics

We employ terahertz time-domain spectroscopy (THz TDS), a novel, non-destructive testing method, to study the fibre orientation and fibre content in reinforced plastics. The birefringent properties of plastics filled with differing amounts of short glass fibres are measured at frequencies from 100 GHz up to 1 THz. To predict the permittivity of the experimentally examined composite materials, we use an effective medium theory first introduced by Polder and van Santen. On the basis of the measured data and this model, we deduce the additive content ξ, the preferential orientation of the fibres φ and the fraction of orientated fibres a. Our findings agree well with corresponding mold flow simulations performed with commercially available software.     

Mechanical properties of 3D KD-I SiCf/SiC composites with engineered fibre-matrix interfaces

Three-dimensional (3D) silicon carbide (SiC) matrix composites reinforced with KD-I SiC fibres were fabricated by precursor impregnation and pyrolysis (PIP) process. The fibre-matrix interfaces were tailored by pre-coating the as-received KD-I SiC fibres with PyC layers of different thicknesses or a layer of SiC. Interfacial characteristics and their effects on the composite mechanical properties were evaluated. The results indicate that the composite reinforced with as-received fibre possessed an interfacial shear strength of 72.1 MPa while the composite reinforced with SiC layer coated fibres had a much higher interfacial shear strength of 135.2 MPa. However, both composites showed inferior flexural strength and fracture toughness. With optimised PyC coating thickness, the interface coating led to much improved mechanical properties, i.e. a flexural strength of 420.6 MPa was achieved when the interlayer thickness is 0.1 μm, and a fracture toughness of 23.1 MPa m^1/2 was obtained for the interlayer thickness of 0.53 μm. In addition, the composites prepared by the PIP process exhibited superior mechanical properties over the composites prepared by the chemical vapour infiltration and vapour silicon infiltration (CVI-VSI) process.     

Effect of steel fibres on mechanical properties of high-strength concrete

Steel fibre reinforced concrete (SFRC) became in the recent decades a very popular and attractive material in structural engineering because of its good mechanical performance. The most important advantages are hindrance of macrocracks’ development, delay in microcracks’ propagation to macroscopic level and the improved ductility after microcracks’ formation. SFRC is also tough and demonstrates high residual strengths after appearing of the first crack. This paper deals with a role of steel fibres having different configuration in combination with steel bar reinforcement. It reports on results of an experimental research program that was focused on the influence of steel fibre types and amounts on flexural tensile strength, fracture behaviour and workability of steel bar reinforced high-strength concrete beams. In the frame of the research different bar reinforcements (2∅6 mm and 2∅12 mm) and three types of fibres’ configurations (two straight with end hooks with different ultimate tensile strength and one corrugated) were used. Three different fibre contents were applied. Experiments show that for all selected fibre contents a more ductile behaviour and higher load levels in the post-cracking range were obtained. The study forms a basis for selection of suitable fibre types and contents for their most efficient combination with regular steel bar reinforcement.     

Impact of fibre orientation on tensile,bending and shear behaviors of a steel fibre reinforced concrete

Fibre orientation and density are known to have a significant influence on steel fibre reinforced concretes (SFRC) mechanical properties. In practice, parameters such as fresh state properties, restriction to concrete flowability and placing methods are likely to induce different fibre orientations in characterisation specimens and structural components. This difference in fibre orientation can impact the mechanical behavior of the structural component and therefore provide an unsafe design if not considered. This project consisted to produce a large SFRC slab, extract specimens with different fibre orientations, and submit specimens to tensile, bending and shear tests to evaluate the impact of fibre orientation and density on mechanical and post-peak strengths. Test results have shown that tensile and bending behaviors are mainly influenced by the fibre orientation, while the shear behavior is mainly impacted by fibre density. Test results were processed to allow comparison between tensile and bending tests. Linear correlations between tensile residual stresses and fibre orientation where found, linear or power type correlations according to bending residual stresses, as well as linear correlations between shear stresses and fibre density.     

Preparation and properties of unidirectional boron nitride fibre reinforced boron nitride matrix composites via precursor infiltration and pyrolysis route

The unidirectional boron nitride fibre reinforced boron nitride matrix (BN_f/BN) composites were prepared via the precursor infiltration and pyrolysis (PIP) route, and the structure, composition, mechanical and dielectric properties were studied. The composites have a high content and fine crystallinity of BN. The density is 1.60 g cm⁻³ with a low open porosity of 4.66%. The composites display good mechanical properties with the average flexural strength, elastic modulus and fracture toughness being 53.8 MPa, 20.8 GPa and 6.88 MPa m^1/2, respectively. Lots of long fibres pull-out from the fracture surface, suggesting a good fibre/matrix interface. As temperature increases, both of the flexural strength and elastic modulus exhibit a decreasing trend, with the lowest values being 36.2 MPa and 8.6 GPa at 1000 °C, respectively. The desirable residual ratios of the flexural strength and elastic modulus at 1000 °C are 67.3% and 41.3%, respectively. The composites have excellent dielectric properties, with the average dielectric constant and loss tangent being 3.07 and 0.0044 at 2-18 GHz, respectively.     

Analysis of short fibres orientation in steel fibre-reinforced concrete (SFRC) by X-ray tomography

The mechanical properties of fibre composite materials are largely determined by the orientation of fibres within the matrix. Which orientation distribution short fibres follow in different parts of a structural element is still a subject for research and discussions in the scientific community. In this article, we present a modern and advanced method for measuring the orientation of short fibres in steel fibre-reinforced concrete (SFRC) by X-ray microtomography. With this method, a voxel image of the fibres is obtained directly in 3D, and the orientation of each individual fibre is calculated based on a skeletonized representation of this image. Scans of 12 SFRC samples, taken from the central height region of real-size floor slabs, reveal the fibres to be mostly horizontally oriented near the centre of a floor slab and more vertically oriented near the edge; here the alignment with the formwork dominates. The fibre orientation distributions are characterized by several orientation parameters as quantitative measures for the alignment. On the practical side, this method has the potential to be incorporated into the development and production process of SFRC structures to verify how the fibres contribute to capacity.     

Influence of the rheological properties on the steel fibre distribution and orientation in self-compacting concrete

The interest in potential applications produced with self-compacting fibre reinforced concrete continues to grow, but in practice, problems associated with an uneven distribution and orientation of fibres in the concrete structure occur. It is not clear what exactly influences uneven distribution of fibres in self-compacting concrete (SCC) mixtures, especially during the casting and how different factors influence fibre orientation. The objective of this work was to investigate how rheological properties influence the steel fibre distribution in self-compacting concrete. This work also focuses on the investigation of steel fibre spatial orientation dependence on rheological properties of SCC, while keeping other casting parameters and the proportions of mixture components constant. Mixtures with three different rheological properties were chosen based on slump flow, slump flow time t₅₀₀ and static segregation values. The steel fibre orientation, volumetric concentration and spatial distribution values were determined in separate beam sections using three different non-destructive testing methods: electromagnetic induction, image analysis and computed tomography (CT scan). The comparison of the results is presented. The results show how different rheological properties of SCC affect the steel fibre orientation and distribution for the case of beams produced with the flow-induced casting method.     

Coating of bioactive glass 13-93 fibres with biomedical polymers

The aim of this study was to coat bioactive glass 13-93 fibres with biomedical polymers. Two methods were used to coat the fibres, namely, dipping and pulling through a viscous solution. With both methods the fibres were successfully coated. Dipping was preferred for thin fibres (20–50 μm) and with this method approximately 2–5 μm thin polymer coat was obtained on the fibre surface. Pulling through viscous solution was preferred for thicker fibres (150–250 μm) and with this method approximately 10–30 μm polymeric coat was obtained. Coating the fibres enables further processing of the bioactive glass fibres and improves the mechanical properties and processibility of fibres.     

Dielectric and ferroelectric properties of Ba(Sn0.15Ti0.85)O3 thin films grown by a sol-gel process

Ferroelectric Ba(Sn_0.15Ti_0.85)O₃ (BTS) thin films were deposited on LaNiO₃-coated silicon substrates via a sol-gel process. Films showed a strong (1 0 0) preferred orientation depending upon annealing temperature and concentration of the precursor solution. The dependence of dielectric and ferroelectric properties on film orientation has been studied. The leakage current density of thin films at 100 kV/cm was 7 × 10⁻⁷ A/cm² and 5 × 10⁻⁵ A/cm² and their capacitor tunability was 54 and 25% at an applied field of 200 kV/cm (measurement frequency of 1 MHz) for the thin films deposited with 0.1 and 0.4 M spin-on solution, respectively. This work clearly reveals the highly promising potential of BTS compared with BST films for application in tunable microwave devices.     

Structural behaviour of thin planar and curved microconcrete plates reinforced conventionally and with steel fibres

This paper examines the load-deformation response of thin planar and doubly curved microconcrete plates and highlights the benefits they obtain from membrane action. The behaviour of elements containing stainless steel melt-extract fibre reinforcement, conventional reinforcement and fibres combined with conventional reinforcement was established by load-testing to failure many small-scale specimens. The results show the benefits and limitations of melt-extract fibre reinforcement for thin concrete structural elements.     

Preparation, structure and mechanical properties of all-hemp cellulose biocomposites

All-hemp (Cannabis Sativa L.) cellulose composites were prepared by a mechanical blending technique followed by hot pressing and water–ethanol regeneration. The alkali treated fibres were ground and sieved to a size ranging from 45 μm to 500 μm. Introduction of fibres into 12% w/v cellulose N-methyl-morpholine-N-oxide (NMMO) solution was performed with low solution viscosity at 100 °C. The solid mixtures were cut and heat pressed between heated glass and PTFE plates at 85 °C to obtain a flat smooth-surfaced composite sheet of approximately 0.2 mm thickness. The cellulose was regenerated in a 50:50 water–ethanol mixture that subsequently removed NMMO and stabilizer (Irganox 1010, Ciba) from the composite. FTIR and X-ray diffraction measurements were performed to investigate the structural change of cellulose from fibre into partially regenerated composite. Composition and thermal stability of composites were investigated using thermogravimetry. A broadening of the scattering of the main crystalline plane (0 0 2) and a depression of the maximum degradation temperature of fibre were observed. The observations revealed a structural change in the fibres. The mechanical properties of composites depended on size, surface area, crystallinity and the structural swelling of fibres.     

Dynamic properties of pultruded natural fibre reinforced composites using Split Hopkinson Pressure Bar technique

The paper presents results on dynamic mechanical properties of jute, and kenaf fibre reinforced composites at various strain rates using compression Split Hopkinson Pressure Bar technique. The stress–strain curves for both pultruded natural fibre reinforced composites at strain rates of nearly 1400 s⁻¹ are illustrated and then compared with statically determines stress–strain curve (1.0 × 10⁻³ s⁻¹). Results show that the strain rate does affect the value of dynamic compressive properties of both pultruded natural fibre composites. Higher dynamic compression modulus and 2.5% flow stress were recorded for higher strain rates as compared to lower strain rate over the range of strain rates investigated. Under dynamic loading, jute fibre reinforced composites recorded the highest value of dynamic response in terms of compression modulus, 2.5% flow stress and compressive strength than that of kenaf fibre reinforced composites. In addition, kenaf fibre reinforced composites is more severely damaged as compared to jute fibre reinforced composites for all tested strain rate.     

Measurement of fibre waviness in industrial composite components

Fibre orientation is measured from polished sections of the unidirectional plies of two industrial CFRP components made by resin transfer moulding (RTM) or prepreg/vacuum consolidation. The image analysis technique described by Creighton et al. [Composites: Part A 2001; 32: 221–229] is used to determine the fibre orientation over typically 5 × 5 mm sections. The standard deviation in fibre orientation angle is in the range 0.6–1.8°, being smallest for in-plane waviness of the prepreg component. The length and width of the waviness region along and transverse to the fibres is characterised using autocorrelation. The length is in the range 1.1–4.4 mm, being significantly greater in the prepreg than in the RTM component. The width is in the range 0.37–1.30 mm and is broadly similar across the sample types. It is demonstrated that the image analysis method can also be applied to X-ray images, giving good agreement with results from the polished prepreg samples.     

Structural and thermoelectric properties of HfNiSn half-Heusler thin films

The bulk thermoelectric properties of half-Heusler alloys have recently been extensively studied due to their potential as thermoelectric materials. However, only a few publications have been addressed on thin film systems. The present study investigated the structural and thermoelectric properties of HfNiSn half-Heusler alloy thin films grown at different substrate temperatures: 25 °C, 200 °C, and 400 °C. The crystalline phase and structural variation of the films were determined by X-ray diffraction and scanning electron microscopy. Polycrystalline thin films were obtained for utilizing lower substrate temperatures. The HfNiSn thin films exhibited preferred (111) orientation when substrate temperature was higher than 400 °C. The in-plane Seebeck coefficient and resistivity of HfNiSn thin films with preferred orientation were much lower than those of films without orientation. This implies the thermoelectric properties of HfNiSn alloy may exhibit anisotropic characteristics. The best Seebeck coefficient and power factor of HfNiSn thin films obtained in this work are −68 μV/K and 1.3 μW/K²cm, respectively, measured at room temperature. The effects of partial substitution of Sn by Sb on thermoelectric properties of HfNiSn thin films were also studied with a “pseudo-combinatorial” approach.