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.     

Liquid sensing properties of fibres prepared by melt spinning from poly(lactic acid) containing multi-walled carbon nanotubes

Poly(lactic acid) (PLA)/multi-walled carbon nanotube (MWNT) composites were melt spun with different take-up velocities (max. 100 m/min) to obtain electrically conductive fibres. The incorporation of MWNT contents between 0.5 and 5.0 wt.% was realised in a previous melt mixing process using twin-screw extrusion. The relative resistance change of the fibres caused by contact with different solvents (water, n-hexane, ethanol, methanol) and solvent concentrations was used as liquid sensing response, whereas the time dependent resistance was recorded during immersion and drying cycles. Transmission electron microscopy and Raman spectroscopy indicated enhanced orientation of MWNT along the fibre axis with take-up velocity, resulting in decreased sensitivity during solvent contact. Additionally, sensitivity decreased as the weight content of MWNT increased and was furthermore dependent on the characteristics of used solvents. In context with the targeted application of leakage detection, fibres with low MWNT amount and low draw down ratio (as extruded fibres with 2 wt.% MWNT) are suitable, as they showed relative resistance changes of up to 87% after 10 min immersion in methanol even if the recovery upon drying was suppressed significantly.     

Enhancement of carbon nanotube fibres using different solvents and polymers

Liquid infiltration is an efficient way to densify carbon nanotubes (CNTs) and was used to strengthen CNT fibres in the method of array spinning. Rather than the volatility, the dipole moment of solvent plays a more important role in determining the densification level. The fibres densified by highly polar but non-volatile solvents such as N,N-dimethylformamide, dimethyl sulphoxide, and N-methyl-2-pyrrolidone were 100–200 MPa stronger than those by ethanol and acetone. Ethylene glycol is the most efficient solvent due to its two polar OH groups and improved the fibre strength to 1.45 GPa. Long chain or cross-linked polymers like polyvinyl alcohol, polyimide, and bismaleimide (BMI) were introduced into CNT fibres by infiltration with aid of polar solvents. These polymers reinforced the fibres significantly, as they can connect non-neighboring CNTs and benefit the load transfer. The strongest CNT/BMI fibre was 2.38 GPa in strength and 110 GPa in modulus.     

On the orientation of fibres in structural members fabricated with self compacting fibre reinforced concrete

The incorporation of fibres into concrete produces important benefits, mainly on the residual load-bearing capacity. These improvements depend on the type, content and orientation of the fibres, being a strong relationship between the number of fibres in the fracture surfaces and the post peak parameters. Although the fibres could be homogeneously distributed after mixing, the casting and compaction processes can significantly affect the fibre distribution and orientation, and consequently the mechanical performance of the material. In the case of Fibre Reinforced Self Compacting Concrete (FR-SCC) the existence of significant flow and wall effects may influence fibre orientation. This paper analyzes the fibre orientation in thin structural elements cast with FR-SCC and its effects on the residual mechanical properties. A slab of 0.90 × 1.80 × 0.09 m, a wall of 0.50 × 2.00 × 0.08 m, and a beam of 0.15 × 0.15 × 2.50 m were selected as representative elements where different concrete flow conditions take place. A strong heterogeneity in the orientation of the fibres was found. The fibre orientation varied with the flow rate and with the wall effect; the thickness of the elements or the proximity to the bottom of the moulds appeared as important variables. It was demonstrated that in thin elements the residual mechanical properties can be quite different when diverse zones and/or directions of the structural elements are considered.     

Fracture characterisation of short bamboo fibre reinforced polyester composites

In the study, fracture behaviour of short bamboo fibre reinforced polyester composites is investigated. The matrix is reinforced with fibres ranging from 10 to 50, 30 to 50 and 30 to 60 vol.% at increments of 10 vol.% for bamboo fibres at 4, 7 and 10 mm lengths respectively. The results reveal that at 4 mm of fibre length, the increment in fibre content deteriorates the fracture toughness. As for 7 and 10 mm fibre lengths, positive effect of fibre reinforcement is observed. The optimum fibre content is found to be at 40 vol.% for 7 mm fibre and 50 vol.% for 10 mm fibre. The highest fracture toughness is achieved at 10 mm/50 vol.% fibre reinforced composite, with 340% of improvement compared to neat polyester. Fractured surfaces investigated through the Scanning Electron Microscopy (SEM) describing different failure mechanisms are also reported.     

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%.     

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.     

Lead magnesium niobate-lead titanate fibres by a modified sol-gel method

Lead magnesium niobate-lead titanate (PMN-PT) ceramic fibres with the nominal composition of 0.65Pb(Mg_1/3Nb_2/3)O₃-0.35PbTiO₃ have been fabricated by a modified sol-gel method. Due to the difficulty of dissolving the magnesium component, the mixed oxide method was used together with the traditional sol-gel method. To obtain crack-free fibres, pyrolysis was carried out at a very slow heating rate under specific atmosphere to control the organic burnout. The thermal and microstructural properties were investigated using thermogravimetric analysis, scanning electron microscopy and X-ray diffraction. The optimum sintering temperature is 1200 °C and yields a fibre with a final diameter of around 100 μm. A single PMN-PT fibre has been poled and its electrical properties were measured. The properties of the fibre are found to be better than that of a ceramic disc.     

Impression creep behaviour of magnesium alloy-based hybrid composites in the transverse direction

The creep behaviour of a creep-resistant AE42 magnesium alloy reinforced with Saffil short fibres and SiC particulates in various combinations has been investigated in the transverse direction, i.e., the plane containing random fibre orientation was perpendicular to the loading direction, in the temperature range of 175–300 °C at the stress levels ranging from 60 to 140 MPa using impression creep test technique. Normal creep behaviour, i.e., strain rate decreasing with strain and then reaching a steady state, is observed at 175 °C at all the stresses employed, and up to 80 MPa stress at 240 °C. A reverse creep behaviour, i.e., strain rate increasing with strain, then reaching a steady state and then decreasing, is observed above 80 MPa stress at 240 °C and at all the stress levels at 300 °C. This pattern remains the same for all the composites employed. The reverse creep behaviour is found to be associated with fibre breakage. The apparent stress exponent is found to be very high for all the composites. However, after taking the threshold stress into account, the true stress exponent is found to range between 4 and 7, which suggests viscous glide and dislocation climb being the dominant creep mechanisms. The apparent activation energy Q_c was not calculated due to insufficient data at any stress level either for normal or reverse creep behaviour. The creep resistance of the hybrid composites is found to be comparable to that of the composite reinforced with 20% Saffil short fibres alone at all the temperatures and stress levels investigated. The creep rate of the composites in the transverse direction is found to be higher than the creep rate in the longitudinal direction reported in a previous paper.     

Yb-doped NaBi(WO4)2 fibre single crystals grown by the micro-pulling down technique and emission spectroscopic characterization

NaBi_1−xYb_x(WO₄)₂ fibres single crystals were successfully grown by micro-pulling down technology (MPD). The Yb³⁺-doped NaBi(WO₄)₂ fibres single crystals have been pulled using MPD technique with controlled diameter and stationary stable growth conditions corresponding to flat crystallization interface with meniscus length equal to the fibre radii and pulling rate range [6-48 mm h⁻¹]. We have determined the monophased field of NaBi_1−xYb_x(WO₄)₂ for x ≤ 0.3. The lattices parameters decrease as a function of Yb³⁺ substitution in Bi³⁺ sites. The melt behaviour has been study by DTA/TG analysis. We have found that the stoichiometric compounds NaBi(WO₄)₂ melt congruently at 935 °C. The fibre diameters varied from 0.5 to 1 mm depending on the capillary die diameter, pulling rate and the molten zone temperature. Complementary Yb³⁺ spectroscopic characterization in the NaBi(WO₄)₂ lattice has been done by IR emission measurements under laser pumping at room temperature.     

Microwave heating as a means for carbon fibre recovery from polymer composites: a technical feasibility study

Carbon fibre composites with an epoxy resin matrix were subjected to microwave-heating experiments in order to volatilise the polymer content and to produce clean fibres for potential reuse in high-grade applications. The composites were processed at 3 kW for 8 s in a multimode microwave applicator. The recovered fibres were characterised by tensile tests and electron microscopy. The results compare favourably with virgin fibre properties.     

Recycled carbon fibre for high performance energy absorption

This paper compares the mechanical properties of virgin and recycled woven carbon fibre prepreg and goes on to assess the potential for recycled carbon fibre reinforced plastic (rCFRP) to be used in high performance energy absorption structures. Three sets of material were examined: fresh containing virgin fibres and resin, aged which was an out of life but otherwise identical roll and recycled which contained recycled fibre and new resin. The compressive strength and modulus of rCFRP were approximately 94% of the values for fresh material. This correlated directly with the results from impact testing where rCFRP conical impact structures were found to have a specific energy absorption of 32.7 kJ/kg versus 34.8 kJ/kg for fresh material. The tensile and flexural strength of rCFRP were 65% of the value for fresh material. Tensile and flexural moduli of rCFRP were within 90% of fresh material and ILSS of rCFRP was 75% that of fresh.     

Fibre orientation effects on the fracture of short fibre polymer composites: on the existence of a critical fibre orientation on varying internal material variables

The dependence of fracture toughness on fibre orientation, in short fibre reinforced polymers, was investigated using materials with different polymer matrix (polyamide 6.6, polyarylamide and polyoxymethylene), fibre sizing, fibre content, mean fibre length and fibre length distribution.To assess the dependence on fibre orientation, plates with unidirectionally oriented fibres were prepared and cut at various angles with respect to the direction of the aligned fibres. The fracture behaviour was investigated by single-edge notch three-point bending tests. In addition the stress-strain behaviour was examined by performing uniaxial tension and compression tests.Both the critical stress intensity factor K _C and the fracture energy G _C measured at fracture initiation were found to present a bi-linear relationship to the factor characterizing fibre orientation, with different slopes over different ranges of the orientation factor. This suggested the occurrence of a transition between different failure mechanisms with varying fibre orientation, namely matrix fracture and fibre debonding at low values of the fibre orientation factor, fibre breakage and pull-out at high values of the fibre orientation factor. This interpretation is supported by the observation of the crack growth direction (which varies with varying fibre orientation) and the analysis of the fracture surfaces. The slopes of the two linear branches of the toughness vs. fibre-orientation-factor plot and the critical fibre orientation angle depend on all internal variables investigated: constituent polymer matrix, degree of fibre-matrix adhesion, fibre content, mean fibre length and fibre length distribution.     

On the effect of different polymer matrix and fibre treatment on single fibre pullout test using betelnut fibres

This study is aimed at exploring the possibility of improving the interfacial adhesion strength of betelnut fibres using different chemical treatments namely 4% and 6% of HCl and NaOH respectively. The fibre specimens were partially embedded into different thermosetting polymer matrix (polyester and epoxy) as reinforcement blocks. Single fibre pullout tests were carried out for both the untreated (Ut) and treated betelnut fibres with different resins and tested under dry conditions. Scanning electron microscopy was used to examine the material failure morphology. The studies revealed the differences of interfacial adhesion strengths for the various test specimens of betelnut fibres treated with the polyester and epoxy matrix which followed in the order of: N6 ? N4 > H4 > Ut > H6. It was proven that fibres treated with 6% of NaOH exhibits excellent interfacial adhesion properties. The interfacial adhesion shear strength of these fibres using polyester and epoxy has improved by 141% and 115% correspondingly compared to untreated fibre under the same treatment.     

Woven hybrid biocomposites: Dynamic mechanical and thermal properties

The dynamic mechanical and thermal analysis of oil palm empty fruit bunch (EFB)/woven jute fibre (J_w) reinforced epoxy hybrid composites were carried out. The storage modulus (E′) was found to decrease with temperature in all cases, and hybrid composites had showed better values of E′ at glass transition temperature (T_g) compared to EFB and epoxy. Loss modulus showed shifts in the T_g of the polymer matrix with the addition of fibre as reinforcing phase, which indicate that fibre plays an important role in case of T_g. The Tan δ peak height was minimum for jute composites and maximum for epoxy matrix. Complex modulus variations and phase behaviour of the hybrid composites was studied by Cole-Cole analysis. Thermal analysis result indicates an increase in thermal stability of EFB composite with the incorporation of woven jute fibres. Hybridization of EFB composite with J_w fibres enhanced the dynamic mechanical and thermal properties.     

Fixed-gain CMOS differential amplifiers with no external feedback for a wide temperature range

We present original CMOS amplifiers designed for the DC to 10 MHz frequency range and operating in the 70-380 K temperature range. Aimed applications concern readout circuitry to be associated with THz bolometric pixels (either high-T_c superconducting or uncooled semiconducting), which require accuracy, low noise and low power consumption. Two designs are described that both exhibit high fixed-gain (40 dB) in a feedback-free architecture, which is based on a new low-transconductance composite transistor for an accurate control of this gain. Both amplifiers have been realized in a regular 0.35 μm CMOS process and tested in the 4.2-380 K temperature range, exhibiting good agreement between designed and measured characteristics.     

Experimental and theoretical studies on the properties of injection moulded glass fibre reinforced mixed plastics composites

Recycled mixed post-consumer and post-industrial plastic wastes consisting of HDPE, LDPE and PP were injection moulded with short glass fibre (10–30% by weight) to produce a new generation composite materials. Intensive experimental studies were then performed to characterise the tensile, compression and flexural properties of glass fibre reinforced mixed plastics composites. With the addition of 30 wt.% of glass fibre, the strength properties and elastic modulus increased by as much as 141% and 357%, respectively. The best improvement is seen in the flexural properties due to the better orientation of the glass fibres in the longitudinal direction at the outer layers. The randomness and length of the glass fibre were accounted to modify the existing rule of mixture and fibre model analysis to reliably predict the elastic and strength properties of glass fibre reinforced mixed plastics composites.     

Measurement of the in situ ply fracture toughness associated with mode I fibre tensile failure in FRP. Part II: Size and lay-up effects

The in-plane size, thickness and lay-up effects on the measured fracture toughness associated with fibre tensile failure were investigated for a T300/920 laminated carbon/epoxy material system. Compact tension specimens were tested with scaled in-plane size, increased thickness, and having various proportions of plies orientated at 0° and 90° to the loading direction. No in-plane size effects were discovered; however, testing revealed a thickness dependence. It was found that the ply toughness is significantly dependent on the thickness of the 0° layers. Propagation values of toughness were measured to be 132 kJ/m² for specimens made up of [90/0₂] sub-laminates and between 57 and 69 kJ/m² for all other configurations. Investigation of the fracture surfaces using SEM revealed that the increase in measured toughness for specimens with thicker 0° plies was due to an increase in the amount of pulled-out 0° fibres.     

Production of orientation-patterned GaP templates using wafer fusion techniques

Nonlinear optical frequency conversion is an effective technique for generating infrared (IR) and terahertz (THz) wavelengths not readily available from existing laser sources. Birefringent materials such as LiNbO₃ are often used to generate wavelengths where gaps exist, but are unsuitable in the mid-IR, far-IR, and THz regions as these materials are often opaque in these regions. As an alternative, GaAs has been employed for frequency conversion in these regions using quasi-phase-matching (QPM) to overcome the material’s lack of birefringence. QPM has been successfully demonstrated in GaAs using fused stacks of thin alternately oriented layers or inverted orientation patterned (OP) grating templates overgrown with thick columnar GaAs layers. Although GaAs has a high nonlinear coefficient d₁₄ = 170 pm/V at 1.064 μm and good thermal conductivity (52 W/m K), it suffers from strong two-photon absorption below 1.7 μm making it inefficient when pumped with a source less than or equal to this wavelength. GaP also has a high nonlinear coefficient d₁₄ = 71 pm/V at 1.064 μm, better thermal conductivity (110 W/m K) and much lower two-photon absorption in the 1 μm region. Therefore, OPGaP is desirable for NLO applications in the mid-IR and THz that use commercially available pump lasers in the 1.06-1.55 μm wavelength range. In this work the fabrication of OPGaP templates suitable for thick columnar hydride vapor phase epitaxial growth of GaP is reported using a commercially viable wafer fusion technique.     

Analysis of flax fibres viscoelastic behaviour at micro and nano scales

In glass or carbon fibres reinforced plastics, creep or stress relaxation, arise from the polymeric nature of the matrix. Plant fibres, used in bio-composites, are also polymers. Therefore, the issue of their service life requires studying the viscoelastic behaviour of both the matrix and the fibres. In this study, we investigate, at different length scales, the response of elementary flax fibres to tensile tests, as well as to nano-indentation tests on their secondary cell walls. The results of these experiments are then analysed via linear viscoelastic rheological models and identification procedures. The values of the identified parameters (relaxation time, viscosity and elastic stiffness) are discussed in relation to the microstructure of the flax fibre (cellulose microfibrils, hemicelluloses and pectins). The nano-indentation technique provides much more deterministic results than tension tests on an entire fibre. The scale of the secondary wall cell is then relevant to assess the viscoelastic behaviour of the fibres.