Effects of surface and sizing treatments on axial compressive strength of carbon fibres

Attempts have been made to estimate the fibre axial compressive strength of pitch-based graphitized fibres, and the effects of surface- and size-treatment on compressive strength was investigated. The estimated compressive strength of fibres decreases with increasing temperature. This decrease in compressive strength may be accounted for by a decrease in the radial compression force owing to a decrease in the residual thermal stress and a decrease in Young's modulus of the resin matrix. There is a linear relationship between the estimated compressive strength and radial compression force in a temperature range from room temperature to 80 °C. The real compressive strength of the fibres, determined by extrapolating this straight line until the radial compression force is zero, increases with increasing shear yield strength at the fibre-matrix interphase. In order to obtain reinforcing fibres with a higher compressive strength, it will be necessary to surface- and size-treat the fibres.     

Niobium coating effects on the tensile strength of sapphire fibres

Using a modified Weibull analysis to incorporate censored fracture data from fibres containing multiple flaw populations, this study has examined the effect of niobium coatings on the room temperature tensile strength of sapphire fibres. Fibre strengths were limited by failures which occurred predominantly from a combination of surface flaws, which are abrasion-induced, and internal voids, which form during the fibre growth process. Heat treatment of as-sputtered coated fibres at 1375°C caused a significant (∼36%) strength degradation. Unexpectedly, the cause of the strength degradation was traced to internal void growth which occurred only in the coated specimens.     

Tensile properties of heat-treated Nicalon and Hi-Nicalon fibres

In this study we compare the tensile properties of two types of Nicalon fibres, one with high oxygen content and the other with low oxygen content. Both types of fibre were coated with a carbon layer during manufacture. The fibres were tested at room temperature in the as-received and desized conditions and after heat treatment at 800 and 1200°C in flowing air and argon. Nicalon-607 and Hi-Nicalon fibres exhibited brittle behaviour and a decrease in tensile strength after heat treatment at 1200°C. It was found that Hi-Nicalon fibres had generally higher tensile properties than Nicalon-607 fibres. It was also observed that the high-oxygen-content fibres had more surface defects than the fibres with low oxygen content.     

Influence of preform surface treatments on the strength of fluorozirconate fibres

Zirconium fluoride (ZrF₄)-based glass preforms made by the rotational casting process were treated prior to fibre drawing to eliminate pre-existing defects. Emphasis was given to the removal of external defects by different surface preparation methods which include mechanical polishing, and chemical etching in 0.2m H₃BO₃ solution as well as microwave-assisted plasma fluorination. The strength of the fibres was measured using both bending and dynamic loading tests. Fluorine exposure increased the median tensile strength by an additional 20%. As expected, the Weibull distribution for bending tests was shifted to the higher strength range, and was generally a factor of two more than in the dynamic tensile measurements.     

Statistical tensile strength of NextelTM 610 and NextelTM 720 fibres

The properties of fibre-reinforced composites are dependent not only on the strength of the reinforcement fibre but also the distribution of fibre strength. In this study, the single filament strength of several lots of Nextel^TM 610 and Nextel^TM 720 ceramic fibres was measured. Fracture statistics were correlated with the effects of gauge length and diameter variation, and the Weibull modulus was calculated using several different techniques. It was found that the measured Weibull modulus at a single gauge length did not accurately predict either the gauge length or diameter dependence of tensile strength. This revised version was published online in November 2006 with corrections to the Cover Date.     

Inclined tensile strength of steel fibres in a cement-based composite

An investigation of the behaviour of steel reinforcing fibres in a cement-based composite inclined to the direction of tensile load was carried out. Steel fibres with widely differing fibre parameters were tested and a new fibre characteristic, the inclined tensile strength (its), was identified. It was observed that the value of the its decreased with increasing inclination angle but the decrease varied according to the type of fibre. The ultimate tensile force sustained by the inclined fibres depended also on the magnitude of the strain. It indicated that for some types of fibre the properties of a randomly reinforced fibre composite after the first crack depended much more on the inclined fibres and their its than on the fibres aligned to the principal stress.     

The effect of the environment on the tensile strength of fluorozirconate glass fibres

Fluorozirconate glass fibres were drawn from rod preforms without a protective polymer coating and under a nitrogen atmosphere. Fibres which had been subsequently stored and tested under an inert atmosphere had strengths ranging between 0.12 and 0.32GPa. However, the strengths of fibres exposed to the atmosphere and under a relative humidity of 80% deteriorated to a limiting strength of about 0.09GPa after 4 days.     

Effect of thermochemical treatments on the strength and microstructure of SiC fibres

The room-temperature strength of commercially available polymer-derived SiC fibres degrades during the typical high-temperature thermal cycle used in ceramic matrix composite fabrication. Substantial improvements in retained room-temperature strength for two different commercially available fibres were observed after annealing in carbon powder at temperatures up to 1600 °C. Further improvements in strength were observed for both fibres when heat treated in CO atmospheres. X-ray diffraction, TEM, SEM, auger electron spectroscopy, and optical microscopy were used to characterize the microstructure and chemistry of these heattreated fibres in order to understand better the degradation mechanisms of the fibres as well as their improved strength retention.     

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.     

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.     

On the estimation of the tensile strength of carbon fibres at short lengths

Generally, to determine the fibre-matrix interfacial properties in fibre reinforced plastics, it is necessary to know the tensile strength of the fibre at very short lengths, for which direct measurements are not possible. Accordingly, in this study, the determination of the tensile strength of high strength carbon fibres and their gauge length dependence are analysed by means of the Weibull model. The influence of the estimator chosen and of the sample size on the calculated value of the tensile strength of the fibre are first determined. Secondly, the accuracy of the three- and the two-parameter Weibull distributions is examined. Finally, it is shown that the most appropriate extrapolation at short length is performed by means of a linear logarithmic dependence on gauge length of the tensile strength. This method seems to be valid for untreated as well as for surface-treated high strength carbon fibres.     

Effects of intermediate principal stress on tensile strength of rocks

This paper investigated the mechanism of fracture in concrete due to the deflagration phenomenon. For this purpose, the electric discharge impulse crushing method was selected, with liquid nitromethane (NM) taken as the deflagration agent. Employing this technique, NM is set inside charge holes and initiated by electric discharge. The pressure generated by the deflagration of NM in a steel chamber was modeled using the Jones–Wilkins–Lee equation of state. The modeled and measured pressures agreed well and the applicability of the pressure model was validated. Then, assuming controlled splitting along the expected fracture surface in concrete, dynamic fracture process analysis (DFPA) based on two-dimensional dynamic finite element method was conducted. The results showed that fracture patterns predicted in the DFPAs agreed well with those obtained from experiments. The mechanism of fracture in concrete due to deflagration was then discussed in terms of the fracture process in the controlled splitting. Owing to stress interference from each charge hole, compressive stress zones (CSZs) formed above and below the middle regions between charge holes where maximum and minimum principle stresses were both in compression. The CSZs was found to be important in obtaining a flatter fracture surface in the case of controlled splitting. In conclusion, the proposed method was shown to be useful for the investigation of the fracture mechanism in the case of the use of deflagration agents and could be useful for the design optimization of such controlled splitting.     

不同初始静载混凝土轴向拉伸试验研究

为研究混凝土材料的动态性能,利用MTS-810NEW液压伺服试验机对尺寸为100 mm×100 mm×510 mm棱柱体混凝土材料试样进行了初始静态荷载为0~20 k N的动态轴向拉伸试验,研究了混凝土材料经历不同初始静态荷载后的动态拉伸破坏特征、应力应变关系和动态抗拉强度。结果表明:荷载值由静态过渡到动态荷载时,混凝土材料的动弹性模量发生较大变化,且随着初始静态荷载值的增加,混凝土材料动弹性模量有增大趋势;混凝土材料动态应力应变关系曲线中,峰值应力所对应的应变值与初始静态荷载值无关;随着初始静态荷载的增加,混凝土材料动态拉伸破坏断面面积逐渐增大,且粗骨料被拉断的数目随着初始静态荷载的增加而先增加,后趋于平稳;随着初始预加静态荷载值的增加,混凝土材料的动态轴向拉伸强度先增加,然后趋于稳定。     

Effects of defects on the tensile strength of short-fibre composite materials

Heterogeneous materials tend to fail at the weakest cross-section, where the presence of microstructural heterogeneities or defects controls the tensile strength. Short-fibre composites are an example of heterogeneous materials, where unwanted fibre agglomerates are likely to initiate tensile failure. In this study, the dimensions and orientation of fibre agglomerates have been analysed from three-dimensional images obtained by X-ray microtomography. The geometry of the specific agglomerate responsible for failure initiation has been identified and correlated with the strength. At the plane of fracture, a defect in the form of a large fibre agglomerate was almost inevitably found. These new experimental findings highlight a problem of some existing strength criteria, which are principally based on a rule of mixture of the strengths of constituent phases, and not on the weakest link. Only a weak correlation was found between stress concentration induced by the critical agglomerate and the strength. A strong correlation was however found between the stress intensity and the strength, which underlines the importance of the size of largest defects in formulation of improved failure criteria for short-fibre composites. The increased use of three-dimensional imaging will facilitate the quantification of dimensions of the critical flaws.