Evaluation of the structural behaviour of annealed nylon 6 fibres from density measurements

Nylon 6 fibres were annealed in the temperatures range 80–185 °C for times from 1–10 h, and the density of the annealed fibres was measured by a system based on the theory of vibrating strings. The fibre diameter was also determined, using the laser forward diffraction technique. Differential thermal analysis (DTA) measurements were used to determine the glass transition temperature and the melting point of nylon 6. Some annealed samples were subjected to X-ray diffraction to clarify the variation of crystallinity with the annealing conditions. The mechanism of structural variations for nylon 6 fibres due to the annealing process is discussed, and structural details for crystalline and non-crystalline phases of a polymer are suggested. The behaviour of the number of oriented chains, number of crystallized nuclei, relative amount of recrystallized material and the shrinkage ratio with annealing time is proposed to explain the thermal structural variations.     

Birefringence evaluation of helically wound optical fibres

Abstract

A simple, non-destructive method is presented for the characterization of the birefringence properties of a helically wound mono-mode optical fibre. It is shown that the Jones matrix model developed by J.N. Ross can be applied to describe them, if the polarization evolution of light is measured with respect to a local reference frame (Serret-Frenet frame). In this case the Ross model is equal to the matrix model developed by R.C. Jones for a twisted crystal. In order to use a fixed reference frame (laboratory reference frame) it is necessary to consider the rotation of the reference frame at the fibre input face produced by the fibre topology. The orientation of the helical fibre axes at the input is defined following the geometrical criteria proposed by Ross. The use of Poincaré's method and Mueller calculus to rewrite the Ross model simplifies the physical interpretation of the results. The method presented provides an easy way to measure the total linear retardation induced by the fibre curvature, and to separate the circular retardation associated with the photo-elastic response of the fibre, from the topological rotation of the reference frame defined only by the geometry of the helically wound fibre.     

Superplastic behaviour of annealed AA 8090 Al-Li alloy

Abstract

High temperature flow behaviour and microstructural evolution were investigated in an annealed AA 8090 Al - Li alloy over the temperature range 623 - 803 K and strain rate range ~ 6 × 10⁶ - 3 × 10² s^-1. Stress - strain rate data, obtained using a differential strain rate test technique and plotted in log - log scale, exhibited three regions I, II, and III, with increasing strain rate. In these regions, the values of strain rate sensitivity index m and the activation energy for deformation were determined to be 0.17, 0.43, and 0.17; and 758.8, 93.3, and 184.3 kJ mol^-1, respectively. The stress - strain curves obtained from constant strain rate tests exhibited flow hardening at lower strain rates and higher temperatures whereas flow softening occurred at higher strain rates and lower temperatures. The microstructural evolution revealed the dominance of grain growth under the former conditions and dynamic recrystallisation under the latter conditions. Ductility and m were found to increase with temperature, with the maximum values of 402% and 0.55, respectively, at a temperature 803 K and strain rate 2 × 10^-4 s^-1.     

Compressional behaviour of carbon fibres

The axial compressive strain to failure of various types of PAN-based carbon fibres was measured by applying small and defined compressive loads to single filaments which have been bonded to a rectangular polymer cantilever beam and parallel to its long edge. By monitoring the Raman frequencies along the fibre with the 2 m laser probe of a Raman microscope, the critical compressive strain required for first fibre failure could be assessed and the residual load that each type of fibre supported after first failure, could be measured. Estimates of the compressive moduli for all fibres could, also, be obtained by considering the dependence of the Raman frequency upon compressive strain in the elastic region. The critical compressive strain to failure was found to decrease with fibre modulus and its value was, approximately, equal to 50% of the tensile fracture strain. However, for some low-modulus fibres the compressive strain to failure was found to approach the tensile fracture strain. The initial compressive moduli of high-modulus fibres were estimated to decrease up to a maximum of 10% with respect to their tensile moduli, whereas more significant reductions were found in the case of intermediate and low-modulus fibres.     

Compressional behaviour of carbon fibres

Spectroscopic-mechanical studies have been conducted on a range of carbon fibres by bonding single filaments on the top surface of a cantilever beam. Such a loading configuration allows the acquisition of the Raman spectrum of carbon fibres and the derivation of the Raman frequency strain dependence in tension and compression. Strain hardening phenomena in tension and strain softening phenomena in compression were closely observed. The differences in the slopes of the Raman frequency versus applied strain curves in tension and compression respectively, have been used to obtain good estimates of the compression moduli. A method of converting the fibre Raman frequency versus strain data into stress-strain curves in both tension and compression, is demonstrated. Values of fibre stress and fibre modulus at failure in compression compare exceptionally well with corresponding estimates deduced from full composite data. The mode of failure in compression has been found to depend upon the carbon fibre structure. It is demonstrated that certain modifications in the manufacturing technology of PAN-based fibres can lead to fibres which show resistance to catastrophic compressive failure without significant losses in the fibre compressive modulus.     

Fracture behaviour of cross-linked collagen fibres

Mechanical properties such as stress - strain, fracture behaviour and thermal behaviour were studied for cross-linked collagen fibres. The shrinkage temperature of cross-linked fibres shows increase in temperature when compared to the control. The results on measurements of breaking strength and strain show significant change when compared with that of the control. The morphological features of the fractured ends of cross-linked fibres were indicative of certain specific patterns. A critical observation of these patterns indicate the role played by the nature of cross-linking agents on the mechanism of failure of these fibres.     

Pull-out behaviour of hooked steel fibres

A programme of research is described that has investigated the fracture of steel fibre reinforced sprayed concrete under flexural load, with the aim of developing a stress-profile model to predict flexural behaviour in the form of a load-deflection response. This paper reports the work associated with establishing the pull-out characteristics of hooked end fibres. The effects of matrix strength, fibre embedment length and orientation are described, together with the interaction of these parameters. The relationships established can be used to model the tensile response of a beam at varying crack width and hence form a key part of the stress-profile for predicting residual flexural strength, which is an essential requirement of a much needed design rationale for steel fibre concrete.

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Résumé Le programme de recherche décrit porte sur la rupture du béton projeté renforcé de fibres d'acier sous charge en flexion. Il a pour objectif de développer un modèle de profil de contrainte capable de prévoir le comportement en flexion sous la forme d'une réponse charge-déflexion. Cet article présente un travail lié à l'établissement des caractéristiques d'arrachement de fibres aux extrémités crochetées. Les effets de la résistance de la matrice, de la longueur d'enfoncement et l'orientation des fibres sont décrits, ainsi que l'interaction de ces paramètres. Les relations établies peuvent être utilisées pour modéliser la réponse en tension d'une poutre à une largeur de fissuration variable, et donc pour former la majeure partie du profil de contrainte visant à prévoir la résistance à la flexion résiduelle, qui est l'une des exigences essentielles d'une argumentation très attendue sur l'étude des bétons renforcés de fibres.

     

The oxidation behaviour of carbon fibres

The oxidation behaviour of carbon fibres has been studied in air at different temperatures from 550 to 860 °C. A linear relationship has been observed between the carbon fibre size and oxidation time. Experimental results show that the oxidation process is in a mixed control zone, i.e. controlled both by diffusion of the gaseous reactant and product within the boundary layer and by the chemical reaction. The activation energy of carbon burn-off is 140±5 kJ mol^–1. The uniform nature of the surface oxidation makes it possible to re-size the carbon fibres to smaller diameters.     

Tensile fatigue behaviour of PBO fibres

Poly(p-phenylene benzobisoxazole) (PBO) fibres are finding increasing applications on account of their exceptional stiffness and strength. This article presents results from tests on single PBO filaments, to characterize their intrinsic behaviour under quasi-static and cyclic tensile loading. Scanning electron microscopy is used to show the fibrillation mechanism leading to failure. Results are compared to data for polyester, aramid and high modulus polyethylene fibres. PBO fibres show shorter fatigue lifetimes than the other fibres when maximum stress is expressed as a percentage of quasi-static break load, but the absolute stress values applied are much higher for an equivalent lifetime.     

Effect of annealing on crystal size in pure Mysore silk fibres

Wide angle X-ray diffraction studies of pure Mysore silk fibres, annealed at various temperatures at different periods of time were carried out to evaluate crystal size and lattice distortion parameters as these determine the properties of silk fibres.     

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     

Crystallization behaviour of calcium aluminate glass fibres

Crystallization behaviour of as-spun and fully-nucleated calcium aluminate (CA) glass fibres produced via inviscid melt spinning (IMS), was studied. Differential thermal analysis (DTA) scans on the as-spun and fully-nucleated CA fibres were performed at different heating rates. By applying the Kissinger method to the DTA scan data the activation energy values for crystallization were determined to be 569 and 546 kJ mol–1, respectively for the as-spun and fully-nucleated CA fibres. The Ozawa analysis on the DTA scan data gave the Avrami parameters at 2.7 and 2.4, respectively, for the as-spun and fully-nucleated CA fibres, which indicates high tendency of bulk crystallization mode. The formation of equilibrium phases of Ca12Al14O33 and CaAl2O4 in the crystallized CA fibres was identified by using X-ray diffraction (XRD).     

The fatigue behaviour of Kevlar-29 fibres

Single Kevlar-29 fibres have been subjected to a variety of tensile cyclic and steady loading conditions. The dispersion of tensile strengths of the samples tested was found to be inherent to the fibre due to the distribution of defects in it and not due to variations of diameter between samples. Cyclic loading was found to produce both longer and shorter lifetimes than those recorded under steady loads equal to the maximum cyclic load. Longer lifetimes indicated failure due to creep mechanisms whereas shorter lifetimes, seen with greater load amplitudes, suggest a fatigue mechanism. No difference was seen in the fracture morphologies of Kevlar-29 fibres broken under simple tensile, fatigue and creep conditions because of the complex splitting which occurs in all cases.     

The oxidation behaviour of low-temperature heat-treated carbon fibres

The oxidation behaviour of partially carbonized polyacrylonitrile fibres was studied primarily by both dynamic and isothermal thermogravimetric analysis. These fibres, referred to as quasi-carbon fibres (QCFs), were obtained by pyrolysis of a polyacrylonitrile precursor at a heat-treatment temperature (HTT) ranging from 400–950°C. Results indicated that QC fibres exhibited increased thermal stability with increasing HTT. The oxidation behaviour was strongly related to the microstructure of QC fibres. An empirical model was developed to simulate the real oxidation process. Two different oxidation mechanisms were operational in QC fibres; one with a more moderate oxidation rate and the other with an auto-acceleration effect. Only the QC fibres that were heat treated above 650°C to develop an extended two-dimensional graphite-like structure, exhibited an auto-acceleration effect. This revised version was published online in November 2006 with corrections to the Cover Date.     

Tensile static and fatigue behaviour of sisal fibres

The work describes the quasistatic tensile and fatigue behaviour of as-received sisal fibres. Natural-based reinforced composites are gaining significant interest within the structural community, due to their interesting mechanical properties, recyclability and environmentally-sustainable production and use. Natural fibres such as sisal constitute an excellent reinforcement material, due to the low extraction costs from plants, and high level of recycling involved in their manufacturing process. In this work the diameter, Young’s modulus, strength and strain to failure over 15 different samples are measured and compared against data from open literature. Tensile cyclic fatigue loading at eight loading levels (from 0.6 to 0.95) has been carried out. The maximum forces involved (between 9 N and 22 N) are considerably higher than the ones used previously in open literature, and lead to significant dependence of the hysteresis loops, energy dissipation and S–N behaviour of the sisal fibres versus the cycle and loading ratio levels. The results obtained from this work can be used to predict from a fatigue and structural integrity point of view the behaviour of sisal-based reinforced composites with high load bearing capability, and extend the design envelope of this class of natural-reinforced materials.     

Time-evolution of microstructure and mechanical behaviour of double annealed medium Mn steel

Cold rolled 0.1C-4.7Mn (wt-%) steel was submitted to double annealing. The holding time of second intercritical annealing at 650°C was varied between 3 min and 30?h. Tensile behavior after each treatment was measured and analysed. Microstructure characterisation was performed using field emission gun scanning electron microscope, transmission electron microscope and saturation magnetisation method. Moreover, interrupted tensile tests were done to obtain the kinetics of austenite destabilisation during straining. An important effect of soaking time on the microstructure and associate mechanical properties was revealed and analysed. Considering thermal and mechanical stability of retained austenite, the optimum combination of phases, providing the best strength-ductility balance, was found after 2?h holding.

This paper is part of a Thematic Issue on Medium Manganese Steels.     

On non-Hookean behaviour of carbon fibres in bending

The deformation characteristics of a range of single, large diameter pitch-based carbon fibres, and some PAN- and rayon-based fibres, were studied in elastica loop bending experiments. In pitch-based fibres, non-Hookean behaviour was found to occur at lower strains the greater the fibre anisotropy. Only elastic deformation to failure, at strains of 3%, was found for low-modulus, nearly isotropic pitch- and rayon-based fibres. The non-linear elastic behaviour in pitch-based fibres resulted in pronounced hysteresis and discontinuities in the stress-strain plots on both unloading and reloading the loops. These effects are due to the initiation, propagation and possible healing of transverse microcracks on the compression side of the fibres.     

Effects of hydrogen charge on microscopic fatigue behaviour of annealed carbon steels

The effects of hydrogen charge on cyclic stress‐strain properties, slip band morphology and crack behaviour of annealed medium carbon steels (JIS‐S45C) were studied. The total strain range of the stress‐strain hysteresis loop in the hydrogen‐charged specimen was smaller than that in the uncharged specimen. Localized slip bands were observed in the hydrogen‐charged specimen, while the slip bands were widely and uniformly distributed in the uncharged specimen. It is presumed that the decrease in the total strain range of the hysteresis loop is due to the slip localization caused by the hydrogen charge and cyclic loading. The sites of fatigue crack initiation were mostly at grain boundaries in the uncharged specimen. The sites of crack initiation in the hydrogen‐charged specimen were not only at grain boundaries but also at slip bands inside ferrite grains. These results imply that hydrogen enhances dislocation mobility along slip bands and results in slip localization. These slip bands then attract hydrogen. This mechanism of hydrogen ‐ slip band interaction may play an important role in the hydrogen‐ influenced metal fatigue.     

Mechanical behaviour of coir fibres under tensile load

Stress-strain curves of coir fibres have been determined. Mechanical properties including initial modulus, strength and percentage elongation of coir fibres have been evaluated as functions of retting treatment (during retting the coconut husks are soaked in saline water for a period of about six months to facilitate the extraction of fibres presumably due to a bacterial process), fibre diameter, gauge length and strain rate. No significant differences in mechanical properties were observed between retted and unretted fibres. The strength and percentage elongation seem to increase for both retted and unretted fibres up to a fibre diameter of 0.2×10^–3 m whereafter they remain almost constant. On the other hand, moduli seem to decrease with increase in diameter of the fibre. The observed modulus values and percentage elongation have been related to microfibrillar angle. Observed strength values have been explained on the basis of structural changes occurring with an increase in the diameter of the fibre. Scanning electron/microscope studies have indicated that the failure of the fibre is due to the fracture of the cells themselves accompanied by the uncoiling of microfibrills. There is no appreciable variation in strength and percentage elongation with strain rates for any one diameter of the fibre. On the other hand, with increase in gauge length, a decrease in both strength and percentage elongation at break has been observed. These have been attributed to an increase of probability of defects and localized deformation and gentle necking, respectively.     

Compressive behaviour of Kevlar 49 fibres and composites

The low compressive strength of Kevlar 49^® unidirectional composites cannot be satisfactorily explained in terms of current theories which assume that failure is due to the matrix material. For a given matrix, Kevlar 49 composites are considerably weaker in compression than those based on other comparable high strength, high modulus filaments. Fracture is found to occur before any plastic deformation of the matrix is observed.This behaviour can be explained in terms of the very low compressive yield strength of the Kevlar 49 fibres themselves. Elastica loop tests show that non-Hookean deformation of the fibres occurs at quite low stresses corresponding to values of the order of those at which fracture takes place in the composite. This deformation is plastic in nature.Buckled areas on the compression side of the elastica loop can be seen in the optical and scanning electron microscopes. It is suggested that the buckling follows from the separation of microfibrils under compression.