Mechanical properties and failure of etched UHMW-PE fibres

The structural hierarchy of fibrillar ultra-high molecular weight polyethylene (UHMW-PE) fibres is investigated and related to fibre mechanical properties. Chemical etching has been used to change the surface properties of these UHMW-PE fibres through the removal of a skin layer and UHMW-PE oxidation. The physical and chemical changes to the fibre surface introduced by etching affect single-fibre mechanical properties. The effects of etchant and etching time on failure properties and mechanisms is discussed. The decrease in failure strain and strength with etching is associated with the change from an energy-absorbing fibril delamination failure to brittle fracture.     

AFM,SEM and XPS characterization of PAN-based carbon fibres etched in oxygen plasmas

Atomic force microscopy (AFM), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) have been used to investigate changes in topography and surface chemical functionality on PAN-based carbon fibres exposed to low-temperature, lowpower, oxygen plasmas. Unsized, type II, Cellion 6000 carbon fibres were treated in oxygen plasmas for 2–60 min at a power of 25 W. Increasing treatment time caused an increase in oxidation from surface alcohol(ether) to carbonyl and carboxyl species, but the total amount of oxidized carbon near the surface remained constant. SEM confirmed that treatments longer than 15 min resulted in pitting on the fibre surface, but even treatments of 60 min did not significantly reduce the overall fibre diameter. AFM showed surface morphology changes after oxygen plasma treatments for 2 and 15 min. 1 m×1 m AFM scans of untreated fibres showed processing grooves with a distribution of depths. Enlarged images along these grooves revealed that their walls were smooth. Oxygen plasma treatments of 2 min roughened fibre surfaces and created holes of the order of 50 nm evenly distributed with a spacing of 150 nm along the bottoms of the grooves. Treatment for 15 min smoothed the overall topography and resulted in smaller holes, of the order of 5–10 nm, with a spacing of < 50 nm. Calculated RMS roughnesses from the AFM data showed an initial increase in roughness with treatment, followed by a decrease to final values lower than those for untreated fibres.     

萃取剂对UHMWPE微孔膜结构与性能的影响

采用热致相分离(TIPS)法制备超高分子量聚乙烯(UHMWPE)微孔膜,研究了分别以正己烷、乙醇和1,2-二氯乙烷作为萃取剂对超高分子量聚乙烯微孔膜结构及其性能的影响.实验表明,当萃取剂依次是正己烷、乙醇和1,2-二氯乙烷时,UHMWPE微孔膜的孔径和孔隙率逐渐减小,但力学性能却逐渐提高.DSC法和WAXD法计算的微孔...     

Optothermal properties of fibres

Studies of the mechanical and optical properties of undrawn polypropylene fibres by annealing and drawing were performed. The optical properties and strain produced in polypropylene fibres at different conditions were measured interferometrically at room temperature. It was found that as the draw ratio of the fibre increased, its birefringence, Δn_a, increased at a constant rate, and then nearly levelled off. The refractive index, n ^∥, and polarizability, p ^∥, increased with different draw ratios; but for fibres annealed at 70 and 100°C, there were no acceptable variations. For fibres annealed at 130°C, n ^∥ and p ^∥ increased compared to those fibres annealed at 70 and 100°C. An empirical formula has been suggested to explain the relationship between the cross-sectional area of polypropylene fibres with the draw ratio, and the constants of this formula have been determined. The effect of annealing on the refractive index profile of undrawn polypropylene fibres, before and after thermal treatment, was studied. The strain optical coefficient and the Poisson's ratio were calculated over different draw ratios. The results obtained clarify the effect of annealing time and temperature with different draw ratios on the optical behaviour of polypropylene fibres. Microinterferograms are given for illustration.     

Structure and properties of coir fibres

In this paper, early research on the structure and properties of coir fibres has been critically reviewed. Gaps in the scientific information on the structure and properties of coir fibre have been identified. Attempts made to fill some of these gaps include the evaluation of mechanical properties (as functions of the retting process, fibre diameter and gauge lengths of fibre, as well as of the strain rates) and fracture mechanisms using optical and scanning electron microscopy. The deformation mechanism of coir fibre resulting in certain observed properties has been discussed with the existing knowledge of the structure of plant fibres as a basis. It is concluded that more refined models need to be developed for explaining the observed mechanical properties of coir fibres. Some of the suggestions for further work include relating properties of fibres to factors like the chemical composition of the fibre and the size and number of cells, size of lumen, variation in micro-fibril angle within each cell and between different cells of the same fibre, and understanding the deformation of the whole fibre in terms of deformation of individual micro-components. Further work is required on the effects of mechanical, thermal and thermomechanical, chemical treatments to modify the structure and mechanical properties of these fibres in such a way as to make them more suitable as reinforcements in polymer, clay and cement matrices.     

Tensile properties of jute fibres

Abstract

One hundred tensile tests were undertaken at each of five distinct fibre lengths (6, 10, 20, 30 and 50 mm) on a single batch of jute fibres from South Asia. The Young's moduli were found to be independent of length. The ultimate stress (fracture strength) and fracture strains were found to decrease with increasing fibre length. The variation in mechanical properties at each fibre length was characterised using Weibull statistics based on a maximum likelihood estimate; referred to as point estimates. Two empirical based models (a linear and a natural logarithmic interpolation model) have been developed to estimate the fracture properties at any length between 6 and 50 mm. These two interpolation models were also developed based on maximum likelihood estimates. The point estimates were used to benchmark the performance of the two models. The natural logarithmic model was found to be superior to the linear model.     

Structure and properties of some vegetable fibres

The stress-strain curve for sisal fibres has been experimentally determined. Ultimate tensile strength (UTS), initial modulus (YM), average modulus (AM) and per cent elongation at break of fibres have been measured as function of fibre diameter, test length and test speed. UTS, YM, AM and per cent elongation lie in the range 530 to 630 MN m^–2, 17 to 22 GN m^–2, 9.8 to 16.5 GN m^–2 and 3.64 to 5.12 respectively for fibres of diameters ranging between 100 and 300m. No significant variation of mechanical properties with change in diameter of the fibres was observed. However, with increase in test length of the fibres, the UTS and per cent elongation are found to decrease while YM and AM increased in the test length ranging from 15 to 65 mm. With the increase in speed of testing from 1 to 50 mm min^–1, YM and UTS are found to increase whereas per cent elongation and AM do not show any significant variation. At a test speed of 500 mm min^–1 the UTS value decreases sharply. The above results are explained in terms of the internal structure of the fibre such as the cell structure, microfibrillar angle, defects, etc. Scanning electron microscope (SEM) studies of the fractured tips of the sisal fibres reveal that the failure of the fibre is due to the uncoiling of microfibrils accompanied by decohesion and finally tearing of cell walls. The tendency of uncoiling seems to decrease with increasing speed of testing.     

Structure and properties of some vegetable fibres

The stress-strain curves for pineapple leaf fibre have been analysed. Ultimate tensile strength (UTS), initial modulus (YM), average modulus (AM) and elongation of fibres have been calculated as functions of fibre diameter test length and test speed. UTS, YM, and elongation lie in the range of 362 to 748 MN m^–2, 25 to 36 GN m^–2, and 2.0 to 2.8%, respectively for fibres of diameters ranging from 45 to 205m. UTS Was found to decrease with increasing test lengths in the range 15 to 65 mm. Various mechanical parameters show marginal changes with change in speed of testing in the range of 1 to 50 mm min^–1. The above results are explained on the basis of structural variables of the fibre. Scanning electron microscope studies of the fibres reveal that the failure of the fibres is mainly due to large defect content of the fibre bo1h along the fibre and through the cross-section, The crack is always initiated by the defective cells and further aggravated by the weak bonding material between the cells.     

Analysis of fatigue failure of optical glass fibres

Journal of Materials Science Letters -     

Relationship between surface properties and adhesion for etched ultra-high-molecular-weight polyethylene fibers

Chemical etching is an established and popular method of increasing the adhesion to such materials as polyethylene. Ultra-high-molecular-weight polyethylene (UHMWPE) fibers are exceptional candidates for composite materials except for their poor adhesion. In this research, the bulk, surface and adhesive properties of as-received and chromic acid etched UHMWPE fibers have been examined. The fiber tensile properties, surface chemistry and wettability have been characterized. The adhesion of epoxy has been characterized by the interfacial shear strength of a droplet microbond. The more than six-fold increase in interfacial shear strength observed in this work is related to the etching process. The removal of an oxygen-rich weak boundary layer, surface roughening and oxidation of the UHMWPE contribute to the enhanced adhesion.     

A mechanism of fatigue failure in nylon fibres

The use of an apparatus which is able to fatigue test fibres in a new and original manner has revealed that a fatigue mechanism does exist in nylon fibres. This mechanism is revealed by a distinctive fracture morphology, as seen with the aid of a scanning electron microscope, and failure when the fibre is cycled to loads which, under steady conditions, would not result in fracture. It is shown that a necessary condition for fatigue failure is the cycling of the fibre to a zero minimum load. An explanation for the history and development of such a fatigue break is given.     

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.     

Carbon fibres: structure and mechanical properties

The microstructure of six types of acrylic-based and hydrated cellulose-based carbon fibres of strengths from 1650 to 6120 MPa and elastic moduli from 97 to 228 GPa were studied by scanning electron microscopy (SEM). A ‘microcomposite’ structure of carbon fibres studied consisting of quasi-amorphous (‘matrix’) and orientated fibrillar carbon was revealed. This led to a new model of the fibre structure. The analysis of results of testing different carbon fibres defines the elastic modulus of ‘matrix’ carbon, and shows plastic drawing of fibrils. The model describes the properties of fibres and predicts ways to improve the fibre properties.     

Compressive properties of single-filament carbon fibres

Compressive properties of mesophase pitch-based carbon fibres (NT-20, NT-40 and NT-60) were measured using the tensile recoil test and the elastica loop test. The NT-40 fibre with a 400 GPa tensile modulus showed a smaller loop compressive yield strain and a larger recoil compressive strength compared to these values obtained from the longitudinal compression test on its unidirectional composites. Further, the recoil compressive strength of this fibre was higher than that of PAN-based carbon fibre with a corresponding modulus. Under the ideal conditions in the tensile recoil test, the strain energy was conserved before and after recoil, and the initial tensile stress and the recoil compressive stress do not coincide when fibre stress-strain behaviour is non-linear, and the non-linearity in compression and in tension is different. The difference between the composite compressive strength and the recoil compressive strength of NT-40 was quantitatively explained by taking account of the fibre compressive stress-strain non-linear relation. The difference between the loop compressive yield strain and the composite compressive strain to failure was also explained by this non-linearity.     

Crystallization and properties of Li-Al-B-Ti-Zn-silicate system glass-ceramic fibres

Phase separation, nucleation, crystallization and micro-crack extension and their affects on the tensile strength and alkaline resistance of TiO₂ nucleating Li₂O-Al₂O₃-B₂O₃-ZnO-SiO₂ system glass-ceramic fibres are studied by DTA, XRD, TEM and SEM. Phase separation, temperature range of nucleation, T _g, sequence and kinetics of crystallization, sizes of microstructure and surface microcracks, tensile strength and weight loss of alkali corrosion of glass-ceramic fibres are also studied. The mechanism of crystallization and the process of microcrack extension during the preparation of glass-ceramic fibres are discussed in detail. The glass prepared for glass-ceramic fibres should be characterized by the temperature of phase separation, nucleation and crystallization of the glass ought to be low, as near T _g as possible, corresponding to its basic properties and the rate of dense bulk crystallization must be closely controlled. The microstructure of small and concentrated crystallites, about, 25 nm in size while the diameter of the glass-ceramic fibres is 16 m, produced in the glass-ceramic fibres increases its tensile strength and alkaline resistance. A suitable coupling agent covering the surface of the glass-ceramic fibres and tensile stress exerted on them during heat treatment benefit its mechanical and chemical properties.     

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.     

Fabrication,structure and properties of quasi-carbon fibres

Partially carbonized fibres, termed quasi-carbon fibres (QCF), with good thermal stability and acceptable mechanical properties were developed from a polyacrylonitrile-based precursor. Heat treatment temperature (HTT), in the 400–950 °C range, played a major role in determining the thermal, mechanical and electrical properties of the QCF. The thermal stability of the QCF was increased by increasing the HTT. An appreciable amount of the graphite-like structure in QCF began to develop at ca. 650 °C. The Young modulus magnitudes of QCF scaled almost linearly with the pyrolysis temperature. In contrast, the QCF exhibited a decreasing trend in both tensile strength and failure strain up to a HTT of 650 °C, above which both the tensile strength and failure strain of the QCF increased with the HTT. Electrical resistivity values of the QCF covered a very wide range from 10⁷to 10^–2 cm. QCF showed semiconducting behaviours with activation energies falling between 0.690 and 0.0052 eV when the pyrolysis temperature was in the 400–850 °C range.