Adhesive properties and failure of etched UHMW-PE fibres

The effect of chemical etching on the surface of ultra-high molecular weight polyethylene (UHMW-PE) fibres with emphasis on the adhesion of epoxy to the fibres was studied. The presence of an oxygen-rich weak boundary layer on the non-polar UHMW-PE fibre yields poor adhesion for the as-received fibre and for fibres etched with the weaker etchants. A significant improvement in adhesion resulted when the weak boundary layer was removed and the UHMW-PE oxidized through etching with chromic acid, a stronger etchant. This significant improvement in adhesion was reflected not only in a higher interfacial shear strength but also in the presence of epoxy cohesive failure. The debonding of droplet microbonds was found to be a suitable technique for the characterization of adhesion in the UHMW-PE/epoxy system.     

Mechanical properties of glass fibres containing aluminium dispersoids

Glass fibres containing metallic aluminium dispersoids up to 7.5 at% AI have been made using ceramic bushings. The metallic granules have diameters ranging from 5 to 40 nm. A new technique based on strength-strain regression analysis has been used to determine the Young's moduli of the glass fibres. The Weibull parameters have been evaluated by both the graphical regression (GRE) and maximum likelihood (MLE) techniques. Fracture studies have also been carried out. The presence of aluminium particles increases the Young's modulus of the fibres but reduces the strength. The latter arises due to the metallic particles acting as stress concentrators within the glass matrix.     

Mechanical properties of glassy carbon fibres derived from phenolic resin

Mechanical properties of glassy carbon fibres produced from a phenolic resin were determined by static tensile testing. These specimens are of special interest because they consist of an isotropic core surrounded by a sheath of oriented material of varying relative thickness. The chemistry of pyrolysis of the resin is summarized and the changes in mechanical properties of the fibres are discussed in terms of the pyrolysis mechanisms. The results are compared with hardness measurements made on discs produced from the same starting material. Scanning electron microscope studies revealed that the fibres have various types of flaws both in the surface and in the core. The effect of these flaws on the fibre strength is discussed by applying Griffith crack theory.     

Mechanical properties of high strength concrete reinforced with metallic and non-metallic fibres

This paper focuses on the experimental investigation carried out on high strength concrete reinforced with hybrid fibres (combination of hooked steel and a non-metallic fibre) up to a volume fraction of 0.5%. The mechanical properties, namely, compressive strength, split tensile strength, flexural strength and flexural toughness were studied for concrete prepared using different hybrid fibre combinations – steel–polypropylene, steel–polyester and steel–glass. The flexural properties were studied using four point bending tests on beam specimens as per Japanese Concrete Institute (JCI) recommendations. Fibre addition was seen to enhance the pre-peak as well as post-peak region of the load–deflection curve, causing an increase in flexural strength and toughness, respectively. Addition of steel fibres generally contributed towards the energy absorbing mechanism (bridging action) whereas, the non-metallic fibres resulted in delaying the formation of micro-cracks. Compared to other hybrid fibre reinforced concretes, the flexural toughness of steel–polypropylene hybrid fibre concretes was comparable to steel fibre concrete. Increased fibre availability in the hybrid fibre systems (due to the lower densities of non-metallic fibres), in addition to the ability of non-metallic fibres to bridge smaller micro cracks, are suggested as the reasons for the enhancement in mechanical properties.     

Mechanical properties of alkali treated plant fibres and their potential as reinforcement materials II. Sisal fibres

The tensile strength and Young’s modulus of sisal fibre bundles were determined following alkalisation. The results were then analysed with respect to the diameter and internal structure such as cellulose content, crystallinity index and micro-fibril angle. The tensile strength and stiffness were found to vary with varying concentration of caustic soda, which also had a varying effect on the cell wall morphological structure such as the primary wall and secondary wall. The optimum tensile strength and Young’s modulus were obtained at 0.16% NaOH by weight. The stiffness of the sisal fibre bundles obtained using the cellulose content also referred to as the micro-fibril content was compared with the stiffness determined using the crystallinity index. The stiffness obtained using the crystallinity index was found to be higher than that obtained using the cellulose content however, the difference was insignificant. Alkalisation was found to change the internal structure of sisal fibres that exhibited specific stiffness that was approximately the same as that of steel. These results indicates that the structure of sisal fibre can be chemically modified to attain properties that will make the fibre useful as a replacement for synthetic fibres where high stiffness requirement is not a pre-requisite and that it can be used as a reinforcement for the manufacture of composite materials.     

Mechanical properties of alkali treated plant fibres and their potential as reinforcement materials. I. hemp fibres

In this study a thorough analysis of physical and fine structure of hemp fibre bundles, namely surface topography, diameter, cellulose content and crystallinity index, have been presented. The fibre bundles have been alkalised and physical and mechanical properties analysed. Alkalisation was found to change the surface topography of fibre bundles and the diameter decreased with increased concentration of caustic soda. Cellulose content increase slightly at lower NaOH concentrations and decrease at higher NaOH concentrations. The crystallinity index decrease with increase in caustic soda concentration up to 0.24% NaOH beyond which, it decreases with increase in NaOH concentration. It was also found that the tensile strength and stiffness increases with increase in the concentration of NaOH up to a limit. Tensile strength and Young’s modulus increase with decrease in cellulose content, while crystalline cellulose decreases slightly but with improved crystalline packing order resulting in increased mechanical properties. Similar observations are elucidated by the crystallinity index. Alkalised hemp fibre bundles were found to exhibit a similar specific stiffness to steel, E-glass and Kevlar 29 fibres. The results also show that crystallinity index obtained following alkalisation has a reverse correlation to the mechanical properties. Stiffer alkalised hemp fibre bundles are suitable candidates as reinforcements to replace synthetic fibres. The improvement in mechanical properties of alkali treated hemp fibre bundles confirms their use as reinforcement materials.     

Mechanical and physical properties of sintered YBCO-metal bulk composites with silver powder and fibres

Silver powder and continuous fibres were used in developing sintered YBa₂Cu₃O_7–x (YBCO)-metal composites because applications require further improvement in mechanical and physical properties of the bulk superconducting elements without affecting the critical current capacity. The weight ratios of silver powder to YBCO and silver fibre to YBCO were varied up to 50% and 5%, respectively, in the beam elements. The effect of silver addition on the density of the composite has been quantified. Stress-strain-critical current properties of bulk YBCO-metal composite elements were investigated in bending at 77 K. The addition of silver powder reduced the sintering temperature, increased the dimensional changes after sintering and also improved the strength, toughness and critical current capacity compared to the monolithic. Silver fibres, (aspect ratios varying between 70 and 110), aligned along the length of the element restricted the changes in dimensions of the composite after sintering and also influenced the stress-strain-current capacity relationship, strength and toughness of the composite to varying degrees. The mixture theory was used to predict the composite flexural strength based on the composition of the composite, constituent properties and porosity.     

Mechanical properties and surface defects of boron fibres prepared in a closed CVD system

The tensile strength of boron fibres, prepared on a tungsten wire substrate suspended in a closed CVD system, has been investigated. The influence of strain-rate, gauge length, and fibre diameter on the tensile fracture stress of the fibres has been evaluated and compared to fracture stress data of fibres produced in continuous CVD processes. Moreover, the E-modulus of the prepared fibres has been measured. Finally the surface defects of the fibres have been examined and classified into fracture stress depressive surface defects and non-fracture stress depressive surface defects.     

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.     

Mechanical properties and failure mechanisms of composite laminates with classical fabric stacking patterns

In the design of composite materials, the properties and failure modes/mechanisms are always of the main concern. In this work, the mechanical properties and failure mechanisms of composite laminates with classical fabric stacking patterns ([(0, 90)]₈ and [(0, 90)/(±?45)]₄) were systematically investigated through mechanical experiments and FEM (finite element method) numerical simulations. The results show that the tensile modulus and bending modulus of the laminates were reduced by 22.2% and 37% after partially changing the stacking angle to?±?45°, but corresponding elongation and bending displacement were increased by 8.8% and 11.7%, respectively. Bending failure mode changes from complete fracture to partial fracture. Meanwhile, the delamination damage and tow peeling from the matrix increase significantly. FEM simulations on tensile and bending processes of the composites indicate that the?±?45° stacking angle leads to the change of the axial stress direction from S_X (0°) to S_Y (±?45°), which is difficult to be observed from mechanical experiments. The FEM simulation provides a cost effective and efficient way for the structural visual optimization design and failure prediction of the actual composite materials.

     

Mechanical properties and shear failure surfaces for two alumina powders in triaxial compression

In the manufacture of ceramic components, near-net-shape parts are commonly formed by uniaxially pressing granulated powders in rigid dies. Density gradients that are introduced into a powder compact during press-forming often increase the cost of manufacturing, and can degrade the performance and reliability of the finished part. Finite element method (FEM) modeling can be used to predict powder compaction response, and can provide insight into the causes of density gradients in green powder compacts; however, accurate numerical simulations require accurate material properties and realistic constitutive laws. To support an effort to implement an advanced cap plasticity model within the finite element framework to realistically simulate powder compaction, we have undertaken a project to directly measure as many of the requisite powder properties for modeling as possible. A soil mechanics approach has been refined and used to measure the pressure dependent properties of ceramic powders up to 68.9 MPa (10,000 psi). Due to the large strains associated with compacting low bulk density ceramic powders, a two-stage process was developed to accurately determine the pressure-density relationship of a ceramic powder in hydrostatic compression, and the properties of that same powder compact under deviatoric loading at the same specific pressures. Using this approach, the seven parameters that are required for application of a modified Drucker-Prager cap plasticity model were determined directly. The details of the experimental techniques used to obtain the modeling parameters and the results for two different granulated alumina powders are presented.     

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.     

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.