Influence of carbide formation on the strength of carbon fibres on which silicon and titanium have been deposited

Silicon or titanium was deposited on the filaments of carbon fibres by chemical vapour depositions and the reactions between the deposited silicon or titanium and the carbon fibres were investigated below 1300° C. Between the silicon and the carbon fibres, -SiC layers formed at rates of 1.5 to 3 nm in 3 h at 1300° C. These rates were 10^–4 times that of the TiC formation by the reaction of titanium with carbon fibre. Furthermore, the effect of the reaction on fibre strength was investigated. By reaction with silicon, the carbon fibre at a carbonized stage decreased in strength at the beginning of the reaction, but afterwards it recovered to the original level. The carbon fibre at a graphitized stage maintained its original strength after heat treatment for several hours at 1300° C. With the TiC-coated carbon fibres, the carbon fibres decreased in strength following the relation _m d ^–1/2, where d is the thickness of the TiC layer.     

Investigation of the strength of surface-treated carbon fibres embedded in resin, by means of model composite tests

The strength of single fibres of carbon embedded in epoxy resin has been estimated by continuous monitoring of fragmentation tests on single embedded fibres. These fibres had previously been treated to enhance their adhesion to the resin, and the effect of this treatment on the fibre strength is studied. The data generated have been analysed by using an adapted Weibull model which includes the influence of the ‘ineffective’ length at each fibre break. Weibull parameters determined from this analysis have been compared with those from tests carried out on fibres in free air. Similar Weibull moduli are obtained and, although absolute values of strength differ, similar trends are observed and a peak in strength is observed at an intermediate level of treatment in both sets of results.     

A statistical model and experimental study of the strain-rate dependence of the strength of fibres

In the present paper, statistical models of the strain-rate dependence of the strength of fibres and fibre bundles are established and their stress/strain equations are deduced. According to the models, an experimental method of determining mechanical parameters in the stress/strain equations by means of tensile impact loading of the fibre bundles is set up. In order to examine the validity of the models and the experimental method, tensile tests of the E-glass fibre bundles were performed at strain rates ranging from 10⁻⁴ to 10³ s⁻¹. The test results are in good agreement with the models. Hence, the models are reliable and the test method is feasible. The statistical models successfully explain the rate-dependent behaviour of the E-glass fibre.     

A new modified Weibull distribution function for the evaluation of the strength of silicon carbide and alumina fibres

The strength distributions of silicon carbide and alumina fibres have been evaluated using a modified Weibull distribution function. The function provides an upper and lower strength limit and is characterized by two shape and location parameters. The sum of squares was used as a measure of fit between the distribution function and the data. The result showed good agreement between the two. In addition, the strength distribution and the average value at a different gauge length were extrapolated from the parameters estimated at the original gauge length. In this case also, the proposed function accurately predicted the data points.     

Thermal cycles effects on interlaminar shear strength (ILSS) and impact behaviour of carbon/PEI composites

In this paper, the effect of thermal cycle on the interlaminate shear strength (ILSS) and impact behaviour of unidirectional carbon fibre reinforced polyetherimide (PEI) matrix composites were studied. Samples were subjected to 100 thermal cycles (by immersing from boiling water (100°C) to ice water (0°C). The effects of thermal cycles were characterized by short beam shear and instrumented impact testers. Also Fractographic investigations were done using a scanning electron microscope (SEM). It is observed that the plastic deformations at the fibre/matrix and interlaminar interface as well as residual stresses lower the ILSS and flexural modulus of the material proportional with the number of thermal cycles. Up to the first 20 thermal cycles the material shows a brittle fracture with lower fracture energy, but after the 20th thermal cycles it is possible to observe that the material fractures with higher fracture energy at longer fracture time. A remarkable difference in the fracture morphology between the thermal cycled and un-treated materials has been observed. It is found that thermal cycles strictly affect the fracture morphology.     

Effect of the bimodal size distribution on the optical properties of self-assembled Ge/Si(001) quantum dots

In situ reflection high-energy electron diffraction, atomic force microscopy and photoluminescence spectroscopy have been combined to investigate the effects of a bimodal size distribution and of the pyramid/dome transition on the optical properties of related Ge/Si layers. It is shown that the wetting layers are inhomogeneous in thickness due to the lateral diffusion of Ge from 2D layers towards islands, while no change is observed in the island-related photoluminescence. These results obtained indicate that 3D islands are, in their early nucleation stages, formed by consuming Ge from 2D layers, and that island luminescence energy is not sensitive to the vertical confinement inside islands.     

Effect of the deposition and thickness parameters of titanium nitride (TiN) coatings on the fatigue strength

The effect of vacuum-plasma coatings of titanium nitride (TiN) on the fatigue strength of titanium VT1-0 is investigated in terms of the coating deposition and thickness parameters. The analysis of the obtained results is performed with the goal of improving the required fatigue strength of the base material–coating composition. It is shown that in all the investigated cases, TiN coatings enhance the fatigue strength. The optimum range of coating thicknesses is established, in which the change in the deposition conditions influences slightly the fatigue limit. A satisfactory agreement is noted between the experimental and calculated values of fatigue limits obtained using the calculation model earlier proposed by the authors. The principal possibility of essential increasing the fatigue strength has been discussed and confirmed in the case of formation of ultra-thin coatings under well-defined conditions of their deposition.     

A 3D multilevel model of damage and strength of wood: Analysis of microstructural effects

A 3D hierarchical computational model of damage and strength of wood is developed. The model takes into account the four scale microstructures of wood, including the microfibril reinforced structure at nanoscale, multilayered cell walls at microscale, hexagon-shape-tube cellular structure at mesoscale and annual rings at the macroscale. With the use of the developed hierarchical model, the influence of the microstructure, including microfibril angle (MFA), the cell shape and the wood density (annual ring structure), differences between earlywood and latewood as well as microstructural arrangements and cellulose strength distributions on the tensile strength of wood is studied numerically. Good agreement of the theoretical results with experimental data has been obtained.     

A model with two micro-scales for the effects of geometrical distribution of material inhomogeneity

Summary A general method is proposed to account for the geometrical distribution of material inhomogeneity. It consists in a two-step micro-macro transition. The randomly inhomogeneous material is envisaged as made of cells, each one with a definite distribution of material inhomogeneity, i.e. with a definite state. Only a finite number of different states is considered for the cells. In other words, many identical cells exist, though at random places in the material. In the first step, the behavior of each set of identical cells, i.e., the constitutive relation that relates the average stress in that set of cells to the average strain-rate in the same set, is obtained by periodic homogenization. In the second step, a variational model is used to get the macroscopic behavior of the material, i.e. to appropriately average over the different sets of cells. The method is theoretically justified. It is then applied to a fibre-reinforced mortar. It is efficient in predicting the mechanically measured reinforcement. The influences of the fibres arrangement and the friction coefficient are investigated: an arrangement is found to reinforce more effectively than does another one.     

A model calculation of the tip field distribution for a carbon nanotube array and the optimum intertube distance

In a previous paper, the model of a floating top segment was proposed and used successfully to calculate the field enhancement factor as well as the field distribution on the top surface for a single carbon nanotube (CNT). In this paper, the model is extended to a CNT array. A set of modified linear charge equations has been derived for the CNT array using an approximation of multipole potential expansion. Fictitious charges inside a specific segment can be solved from these equations to calculate the field around the tips. Some numerical calculations of the field enhancement factor have been carried out, quantitatively accounting for the reduction of the factor with decreasing intertube distance. The relative field strength distribution on the top hemisphere of a CNT array is calculated too; it is observed to be close to that of a single CNT. Using the Fowler-Nordheim formula, the field emission intensity of a CNT array is maximized with varying intertube distance. The resulting optimum intertube distance to height ratio decreases gradually with increasing height to radius ratio, and is close to 2 only for height to radius ratio around 3 × 10(2).     

Estimation of the (n,m) concentration distribution of single-walled carbon nanotubes from photoabsorption spectra

Deconvolution of the absorption spectrum of single-walled carbon nanotubes (SWNTs) into distinct (n,m) contributions is complicated because transition energies are closely spaced. The algorithm presented in this work attempts to simplify the problem by grouping nanotubes with similar transition energies and assigning weights to their spectral contributions. Voigt line shapes were used to fit absorption spectra of sodium dodecyl sulfate suspended HiPco SWNT and CoMoCat SWNT. Line widths for the metallic (93.42 meV) and two semiconducting regions (57.96 and 29.86 meV) were obtained from the absorption spectra of DNA-wrapped SWNT fractionated by ion-exchange chromatography. The method is used to describe the reaction kinetics of certain HiPco SWNTs upon reaction with 4-chlorobenzene diazonium and 4-hydroxybenzene diazonium salts. The code for deconvolution has been provided as open source in the Supporting Information for future modifications.     

A GIS-based green supply chain model for assessing the effects of carbon price uncertainty on plastic recycling

Recycling plastic can abate the environmental pollution as well as CO₂ emissions by saving the carbon-intensive feedstock input. The uncertain carbon price places significant effects on the establishment and operation of the whole supply chain. This study develops a green supply chain model combined with geographic information system (GIS) to account for carbon price uncertainty and evaluate its effects on the closed-loop supply chain (CLSC) of plastic recycling. A two-stage stochastic programming model is constructed, in which the stochastic variable, CO₂ price is modelled as a geometric Brownian motion process. Six scenarios are designed with respect to price expectation and volatility. A case study is performed with the GIS information of the plastic supply chain in Zhejiang province, China. The results illustrate that triggering the establishment of reverse logistics requires a carbon price threshold significantly beyond the current level. Lower price volatility would facilitate the decision-making of investment into the reverse logistics. Mechanisms to alleviate the market variation shall be introduced. A sound market condition is desired to obtain the optimal balance that encourages the CLSC without creating extra pressure on the firms. The proposed modelling framework can be easily applied to other sectors with similar characteristics.     

A study of the effect of surface treatments on the tensile strength of flax fibres: Part I. Application of Gaussian statistics

Natural fibres are considered to be a potential alternative to glass fibres for use in composites applications. However, although natural fibres have many advantages their compatibility with most thermoplastics is rather poor. Surface treatments, although having a negative impact on economics, may be applied to overcome the problem of interfacial adhesion. Unfortunately, natural fibres being very different materials from man-made fibres, are very prone to degradation and/or structure alteration during the application of surface treatments. This study focuses on the effect of two surface treatments (acetylation and stearation) upon the tensile strength of flax fibres. The results are discussed in terms of Gaussian statistics and it was found that the treatments did not significantly change the flax fibre tensile strength. In addition SEM examination of the fractured surfaces revealed that acetylated fibres exhibit a different mode of failure from the other fibres, suggesting that the treatment altered the bulk properties along with the surface properties.     

A study of the effect of surface treatments on the tensile strength of flax fibres: Part II. Application of Weibull statistics

The application of Weibull statistics for describing fibre strength is very popular among material scientists. However, although the weakest link model seems very attractive from a physical point of view, the mathematics involved are not so straight forward for all cases. In this study the effect of surface treatments upon the tensile strength of flax fibres is investigated. In part I the data were discussed in terms of Gaussian statistics, while in this paper (part II) the data are discussed in terms of Weibull statistics. Two different methods (the linear regression and the maximum likelihood) were employed in order to estimate the Weibull modulus (m) and the Weibull characteristic strength (σ₀). The two methods yielded similar values for the Weibull modulus (m) and the Weibull characteristic strength (σ₀). It was found that the surface treatments did not significantly change the tensile strength of flax fibres.     

Influence of various fluids on the interlaminar shear strength (ILSS) and impact behaviour of carbon/pei composites

Composites can fail due to exposure to certain corrosive environments such as water, aqueous acids and bases, and organic solvents. The objective of this study was to determine the interlaminar shear strength (ILSS) and impact property changes on unidirectional carbon fibre reinforced polyetherimide (PEI) composites following exposure to various liquid environments at ambient and elevated temperature. The sample weights were increased for both of the samples with swelling effect. From the detailed investigation of the impact energies, it was observed that the crack initiation energy was decreased while the propagating energy was increased. Liquid environments with higher temperature affect material more remarkable compared to lower temperature. Also, remarkable decrease in ILSS values for all samples subjected to liquid environments was observed. By means of scanning electron microscope (SEM) studies the fracture mechanisms and morphology of the material after chemical corrosion was investigated. Remarkable morphological changes at the cross section of treated materials were observed.