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.     

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.     

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.     

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.     

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.     

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 structure on the tensile,creep and fatigue properties of polyester fibres

The behaviour of two types of polyester fibres have been compared and the differences observed explained in terms of the differences in molecular structure. One fibre which had a higher crystallinity than the other showed improved creep behaviour however shorter fatigue lifetime. Crack propagation during fatigue suggests a macrofibrillar structure superimposed on the microfibrillar molecular arrangement. Fatigue lifetime is suggested as being greatly influenced by the time necessary for crack initiation which involves modification of the molecular structure. The influence of the minimum cyclic load on fatigue lifetime has been studied for both fibres.     

Modelling tensile properties of jute fibres

Abstract

This short communication extends earlier modelling of the tensile strength and failure strain of jute technical fibres. A maximum likelihood estimate (MLE) model, a linear model and a natural logarithmic interpolation model (NLIM) are compared. The NLIM model is found to give superior predictions.     

Effects of geometry and specimen size on out-of-plane tensile strength of aligned CFRP determined by direct tensile method

The out-of-plane tensile strength of CFRP laminate determined by the direct tensile method varies with specimen geometry and size. This effect was first experimentally observed using aligned CFRP. To explain the geometry and size effects from a mechanical point of view, an analytical model combining Weibull statistics, including the concept of effective volume, and a fracture criterion under multi-axial loading was constructed on the basis of stress distributions calculated using the finite element method. The predicted out-of-plane tensile strength of aligned CFRP was found to be consistent with experimental results. Thus, the present model is useful for reducing experimentally determined out-of-plane tensile strength under complex stress distributions to that under a uniaxial and uniform stress distribution.     

The tensile properties of nickel-coated carbon fibres

The tensile properties of nickel-coated Grafil HT carbon fibres were studied as a function of the coating thickness, taking into account the diameter-dependence of the properties of the fibres themselves. The stress-strain curve exhibited three stages, following from an initially elastic coating which yields and then extends plastically before failure of the fibre. The behaviour could be described by a simple law of mixtures and the grain size of the coatings.     

The tensile strength of hardmetals

In conventional bend tests on hardmetal specimens with a rectangular cross-section the strength values exhibit a wide scatter as a result of fracture being initiated from pores and inclusions. A new bend test-piece geometry has been devised which subjects a relatively small volume of material to a high tensile stress and so reduces the probability of fracture starting from defects. The test gives reproducible results with low scatter, and, by suppressing defect initiated failures, it enables a more accurate assessment to be made of the effect of metallurgical variables, such as grain-size and composition, on strength.     

Compressive and tensile behaviour of carbon fibres

Attempts have been made to discuss the fibre axial tensile and compressive behaviour of several carbon fibres prepared from different precursors, and surface- and/or sizing-treatments. With all fibres, the number of breaks increased with increasing tensile or compressive strain, and remained constant beyond a certain strain. The constant number of breakages based on the precursor differed remarkably in compression, whereas those based on the surface- and/or sizing-treatments differed remarkably in tension. In tension, the PAN-based fibre could be broken with a somewhat greater ease than the pitch-based fibre, while in compression, the pitch-based fibre could be broken with somewhat greater ease than the PAN-based fibre. © 1998 Chapman & Hall