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.     

Characteristics of a continuous Si-Ti-C-O fibre with low oxygen content using an organometallic polymer precursor

A continuous Si-Ti-C-O fibre with 12 wt% oxygen content, which is lower than the usual 18 wt% found in the normal fibres, was synthesized by using polytitanocarbosilane which has fewer Si-Si bonds than the usual precursor polymer. The density, tensile strength, tensile modulus and thermal conductivity were found to be 2.37 g cm^–3, 3.4±0.3 GPa, 190±10 GPa and 1.40 W m^–1 K^–1, respectively. Amongst these properties, the tensile modulus was improved by 20 GPa and the thermal conductivity had a higher value in comparison with that of the ordinary Si-Ti-C-O fibre with 18 wt% oxygen content. The Si-Ti-C-O fibre with a 12 wt% oxygen content has a better heat resistance above 1400 °C in an argon atmosphere and 1300 °C in air, than the usual fibre. About 60 and 40% of its tensile strength at room temperature were retained in air at respectively, 1500 and 1600 °C. This improved ceramic fibre is considered to be useful as a reinforcing material for advanced composites such as high-temperature ceramic matrix composites and metal matrix composites.     

The strength of glass fibre reinforcement after exposure to elevated composite processing temperatures

The results of a study on the properties of glass fibres after thermal conditioning at typical engineering thermoplastic processing temperatures are presented. The mechanical performance of rovings and single fibres of well-defined silane- and water-sized E-glass fibre samples was investigated at room temperature after thermal conditioning at temperatures up to 400 °C. Thermal conditioning for 15 min led to strength degradation of >50 % at higher temperatures. The tensile strength of silane-coated fibres was relatively stable up to 300 °C but exhibited a precipitous drop at higher conditioning temperatures. The water-sized fibres exhibited an approximately linear decrease in strength with increasing conditioning temperature. The strength distribution of the water-sized fibres could be well represented by a unimodal three-parameter Weibull distribution. The strength distributions of the sized fibres were more complicated and required the use of a bimodal Weibull distribution. The results are discussed in terms of the changes in surface coating and bulk glass structure during heat conditioning.     

Characteristics of several carbon fibrereinforced aluminium composites prepared by a hybridization method

The properties and microstructures of several high-strength and high-modulus carbon fibrereinforced aluminium or aluminium alloy matrix composites (abbreviated as HSCF/Al and HMCF/Al, respectively, for the two types of fibre) have been characterized. The composites evaluated were fabricated by pressure casting based on a hybridization method. It was found that the strength degradation of high-modulus carbon fibres after infiltration of aluminium matrices was not marked and depended upon the type of aluminium matrix. However, the strength of high-strength carbon fibres was greatly degraded by aluminium infiltration and the degradation seemed to be independent of the type of aluminium matrix. The longitudinal tensile strength (LTS) of CF/Al composites was very different between HMCF/Al and HSCF/Al composites. The HMCF/Al composites had LTS values above 800 MPa, but the HSCF/Al composites had only about 400 MPa. In contrast, the transverse tensile strength of the HSCF/Al composites, above 60 MPa, was much higher than that of the HMCF/Al composites, about 16 MPa. Chemical reactions were evident to the interface of high-strength carbon fibres and aluminium matrices. There was no evidence of chemical products arising between high-modulus carbon fibres and Al-Si alloy and 6061 alloy matrices. However, it was considered that some interfacial reactions took place in pure aluminium matrix composites. Fracture morphology observation indicated that the good LTS of CF/Al composites corresponded to an intermediate fibre pull-out, whereas a planar fracture pattern related to a very poor LTS and fibre strength transfer. The results obtained suggested that interfacial bonding between carbon fibres and aluminium matrices had an important bearing on the mechanical properties of CF/Al composites. An intermediate interfacial bonding is expected to achieve good longitudinal and transverse tensile strengths of CF/Al composites.     

Tensile properties of carbon fibres and carbon fibre–polymer composites in fire

The effect of fire on the tensile properties of carbon fibres is experimentally determined to provide new insights into the tensile performance of carbon fibre–polymer composite materials during fire. Structural tests on carbon–epoxy laminate reveal that thermally-activated weakening of the fibre reinforcement is the dominant softening process which leads to failure in the event of a fire. This process is experimentally investigated by determining the reduction to the tensile properties and identifying the softening mechanism of T700 carbon fibre following exposure to simulated fires of different temperatures (up to 700 °C) and atmospheres (air and inert). The fibre modulus decreases with increasing temperature (above ～500 °C) in air, which is attributed to oxidation of the higher stiffness layer in the near-surface fibre region. The fibre modulus is not affected when heated in an inert (nitrogen) atmosphere due to the absence of surface oxidation, revealing that the stiffness loss of carbon fibre composites in fire is sensitive to the oxygen content. The tensile strength of carbon fibre is reduced by nearly 50% following exposure to temperatures over the range 400–700 °C in an air or inert atmosphere. Unlike the fibre modulus, the reduction in fibre strength is insensitive to the oxygen content of the atmosphere during fire. The reduction in strength is possibly attributable to very small (under ～100 nm) flaws and removal of the sizing caused by high temperature exposure.     

Temperature dependence of the tensile behaviour of aramid/aluminium laminates

Aramid/aluminium laminates (ARALL^® laminates) are a family of new hybrid composites made of alternating layers of thin aluminium alloy sheets with plies of epoxy adhesive prepreg containing unidirectional aramid fibres. The effect of elevated and cryogenic temperatures on these materials is critical to aerospace applications. ARALL 1, 2, 3, and 4 laminates have been tested in tension at temperatures ranging from –300F–400 °F (–184–204 °C) and at room temperature after exposure. This paper summarizes how tensile properties depend on temperature for these four ARALL laminates under the conditions described. At cryogenic temperatures, no degradation of ultimate tensile strengths, tensile yield strengths and moduli were found for either the longitudinal or transverse directions for ARALL 1–4 laminates. Furthermore, the mechanical properties remained the same or increased slightly as the temperature decreased. Longitudinal and transverse ultimate tensile strengths, tensile yield strengths, and moduli of ARALL 1–3 laminates at room temperature remain nearly constant after the laminates were exposed for 1, 10 and 100 h to temperatures up to 250 °F (121 °C), and up to 350 °F (177 °C) for ARALL 4 laminates. However, these properties determined at the elevated temperatures after 1, 10 and 100 h exposure showed a tendency to decrease with increasing temperature. The properties of ARALL laminates are much better in the longitudinal fibre direction than those of conventional monolithic aluminium alloys. Typical failure modes of the test specimens in the high-temperature range were examined using a scanning electron microscope. The discussions are also described in the paper.     

Mechanical properties and microstructure of a P/M aluminium matrix composite with δ-alumina fibres and their relation to extrusion

Saffil short fibre-reinforced aluminium composites have been prepared via a powder metallurgy route. Three different reduction ratios of extrusion were investigated. The tensile mechanical properties at room and elevated temperature and the microstructure, with emphasis on fibre length, were evaluated. The reduction ratio did not influence mean fibre length, implying that during the extrusion the main fibre breakage occurred in the initial compaction stage. The relative strengthening of the unidirectionally reinforced composites at room temperature is low and depends on extrusion reduction ratio. At elevated temperature the strength of the composites in the longitudinal direction is significantly higher compared to that of the base alloy. At 250°C, improvements were obtained of 15%, 24% and 43% for V _f = 0.048, 0.100 and 0.200, respectively. It is suggested that strengthening is possible by the combined effect of a high ductility of the matrix and the resistance to plastic flow exerted by dislocations and stress fields around aligned fibres. All composites contain highly fibre-enriched layers, with bad internal cohesion. They originate from fibre clusters and form severe macroscopic defects during machining operations. Despite that, the tensile properties in the longitudinal direction are reasonably good.     

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.     

High-modulus-high-strength poly-(p-phenylene benzobisthiazole) fibres

Heat-treatment processing of dry-jet wet-spun poly-(p-phenylene benzobisthiazole) fibres was undertaken to enhance fibre tensile mechanical properties. The effects of fibre tension and temperature and time of heat treatment in a nitrogen atmosphere on fibre mechanical properties were systematically investigated. Fibres possessing a tensile modulus as high as 300 GPa along with a tensile strength of 3 GPa have been produced. To attain this level of tensile properties, heat-treatment temperatures of 630 to 680° C for residence times of under one minute were required while applying tensions approaching fibre breaking stress at the elevated temperatures; conditions bordered on fibre degradation. Fibre structural changes associated with heat treatment and enchancement of mechanical properties are discussed in Part 2 of this work.     

Mechanical properties of squeeze-cast zinc alloy matrix composites containing α-alumina fibres

α-Alumina fibre-reinforced ZA12 alloy matrix composites, with fibre volume fractions ranging from 7.5 to 30%, were manufactured by squeeze casting. The alumina fibres were homogeneously distributed in the matrix and had a planar-random orientation. Mechanical properties of the composites such as hardness, tensile strength, Young's modulus, elongation and wear resistance were measured and the effect of fibre volume fraction on these properties was investigated. At room temperature the hardness, Young's modulus and wear resistance increased with increasing volume fraction of alumina fibres, but the other properties were inferior. At elevated temperature (above 80°C) the tensile strengths of the composites were higher than that of the matrix alloy.     

Tensile mechanical properties and surface chemical sensitivity of technical fibres from date palm fruit branches (Phoenix dactylifera L.)

The paper describes the manufacturing process and the characterization of the tensile mechanical properties of treated and untreated palm dates long technical fibres. The fibres extracted from Fruit Bunch Branch of Palm Date (FBBPD) have been subjected to alkaline treatment with different NaOH concentrations at room temperature. The experimental results show that the chemical technical fibre treatments provide an increase of the mechanical properties (tensile strength and Young’s modulus) under quasi-static tensile loading. A specific treatment leads a threefold increase of the failure stress. An analysis of stress at failure has been performed over a population of 630 samples using Weibull statistics with two and three-parameters, together with a one-way analysis of variance (ANOVA). FBBPD technical fibres show stiffness and strength performance comparable to the ones of agave Americana L fibres, and higher failure at strength than okra fibres.     

Grain growth and strength degradation of SiC monofilaments at high temperatures

The microstructural stability of SCS-6 SiC monofilaments was determined by measuring the average grain size, tensile strength and critical length of the fibres as a function of various annealings in vacuum (0.1 Pa) at different temperatures (1400–1600 °C). The average grain size, calculated from X-ray diffraction line broadening, increased from 23 nm for the as-received fibres to 46 nm for fibres annealed for 2 h at 1600 °C. The corresponding tensile strength measured at room temperature dropped from 3.6 GPa for the as-received fibres to 2 GPa for the treated fibres. Simultaneously, the average critical lengths, measured using the glass-slide technique decreased from 0.37 mm for the as-received fibres to an average of 0.23 mm for the heat-treated fibres. The degradation of the mechanical properties was attributed to a combination of coarsening of the -SiC grains as well as interactions with the annealing environment, namely the vacuum hot-press chamber.     

Influence of environmental relative humidity on the tensile and rotational behaviour of hemp fibres

The aim of this study is to throw new light on the influence of moisture on the mechanical properties of hemp fibres. Indeed, the behaviour of plant-based fibres strongly depends on their humidity. Although this topic has been relatively well treated for the case of wood, the literature on fibre stemming from annual plants is unfortunately poor. This purpose is, however, of great importance, particularly in view of the production of high-performance composites. The influence of environmental conditions on the static and dynamic tensile moduli and the strength of elementary fibres are investigated using a versatile experimental setup. Novel equipment was also designed to measure the rotation of a fibre about its axis when it was subjected to static loading and moisture variations. Water sorption is shown to have a significant influence on the apparent tensile stiffness, strength and fracture mode of such fibres, and is also shown to act like an activator of the stiffening phenomena under cyclic loading. A remarkable increase in the fibre stiffness of up to 250% is measured. Significant longitudinal elongation, reaching a value in excess of 2%, is associated with this increase in stiffness. The absorption and desorption of moisture also lead to substantial rotation of the fibre about its axis. Water sorption certainly involves a modification of the adhesion between cellulose microfibrils and the amorphous matrix. Under cyclic loading, the cellulose microfibrils could be able to creep into the relaxed amorphous matrix, leading to their re-arrangement, with more parallel orientations with respect to the fibre axis.     

Critical fibre length and tensile strength for carbon fibre-epoxy composites

The tensile strength of epoxy resin reinforced with a random-planar orientation of short carbon fibres decreases with increasing temperature. This decrease may be estimated by the strain rate and temperature dependence of both the yield shear strength at the fibre-matrix interphase and the critical fibre length obtained by taking the distribution of fibre strength into consideration. The experimental value at room temperature is smaller than the calculated value. It is inferred that this result is attributed to the stress concentration caused by ineffective fibres produced during preparation which were shorter than the critical fibre length.     

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.     

Silicon carbide (NicalonTM) fibre-reinforced borosilicate glass composites: mechanical properties

The objective of this study was to assess the applicability of an extrinsic carbon coating to tailor the interface in a unidirectional NicalonTM–borosilicate glass composite for maximum strength. Three unidirectional NicalonTM fibre-reinforced borosilicate glass composites were fabricated with different interfaces by using (1) uncoated (2) 25 nm thick carbon-coated and (3) 140 nm thick carbon coated Nicalon fibres. The tensile behaviours of the three systems differed significantly. Damage developments during tensile loading were recorded by a replica technique. Fibre–matrix interfacial frictional stresses were measured. A shear lag model was used to quantitatively relate the interfacial properties, damage and elastic modulus. Tensile specimen design was varied to obtain desirable failure mode. Tensile strengths of NicalonTM fibres in all three types of composites were measured by the fracture mirror method. Weibull analysis of the fibre strength data was performed. Fibre strength data obtained from the fracture mirror method were compared with strength data obtained by single fibre tensile testing of as-received fibres and fibres extracted from the composites. The fibre strength data were used in various composite strength models to predict strengths. Nicalon–borosilicate glass composites with ultimate tensile strength values as high as 585 MPa were produced using extrinsic carbon coatings on the fibres. Fibre strength measurements indicated fibre strength degradation during processing. Fracture mirror analysis gave higher fibre strengths than extracted single fibre tensile testing for all three types of composites. The fibre bundle model gave reasonable composite ultimate tensile strength predictions using fracture mirror based fibre strength data. Characterization and analysis suggest that the full reinforcing potential of the fibres was not realized and the composite strength can be further increased by optimizing the fibre coating thickness and processing parameters. The use of microcrack density measurements, indentation–frictional stress measurements and shear lag modelling have been demonstrated for assessing whether the full reinforcing and toughening potential of the fibres has been realized. This revised version was published online in November 2006 with corrections to the Cover Date.     

一种具有稳定富碳表层的SiC纤维的制备与性能

采用不饱和烃不熔化处理后的聚碳硅烷（PCS）纤维经高温烧成可制得一种新型的SiC纤维,纤维的抗张强度达2.5~2.8GPa,氧含量4wt%~6wt%,电阻率仅为0.5Ω·cm左右,大大低于采用传统空气不熔化方法得到的SiC纤维.研究表明:该纤维表面存在厚度约50nm的富碳层,并且在Ar气中进行高温热处理后,表面富碳层结构无明显变化.与日本通用级SiC纤维Nicalon NL202 相比,纤维的耐热性提高200~300℃.纤维具有低电阻率稳定性,从室温到1600℃,其电阻率始终保持在0.4~0.8Ω·cm.     

Ultimate strengths of fibre-reinforced ceramics and metals

A theory to predict the ultimate tensile strength of fibre-reinforced ceramics (CMCs) is presented. The theory incorporates the statistical nature of the fibre strength and the presence of fibre/matrix sliding, the latter allowing broken fibres to retain some load-carrying capacity, and yields a simple analytic expression for the strength. Comparisons with measurements on a wide range of CMCs indicate that the theory improves considerably on rule-of-mixtures estimates. An extension of the concepts used for CMCs to metal-matrix composites (MMCs) with weak, sliding fibre/matrix interfaces is then proposed, and the resultant predictions for MMC strengths agree well with data on SCS-6/Ti-alloy materials.     

An experimental study of synthetic fibre reinforced cementitious composites

Fibre reinforcement is one of the effective ways of improving the properties of concrete. However, current studios on fibre -reinforced concrete (FRC) have focused mainly on reinforcements with steel and glass fibres. Thin paper reports on an experimental programme on the properties of various synthetic fibre reinforced cementitious composites and the properties of the reinforcing fibres. Acrylic, polyester, and aramid fibres were tested in uniaxial tension, both in their original state as we!! as after ageing in nerO*nL Samples of these fibres were found to lose varying amounts of strength with time, depending on the ageing temperature. Two different test methods were used to measure the fibre-cement interfacial bond strength. The tensile properties of concrete reinforced with acrylic, nylon, and aramid fibres, in the form of random distribution or unioxial alignment, were studied by means of three different tests: compact tension, flexural, and splitting tensile tests. The properties of concrete, particularly that of apparent ductility, were found to be greatly improved by the inclusion of such fibre reinforcement.     

Effects of fibre length on tensile strength of carbon/glass fibre hybrid composites

The tensile strength of epoxy resin reinforced with random-planar orientation of short carbon and glass fibres increased as the length of the reinforcing fibres increased, and the increase in tensile strength remained almost unchanged after the fibre length reached a certain level. The tensile strength of composites at any fibre length could be estimated by taking the strain rate and temperature dependence of both the yield shear strength at the fibre-matrix interphase and the mean critical fibre length into consideration. The tensile strength of the hybrid composite could be estimated by the additive rule of hybrid mixtures, using the tensile strength of both composites.