Transmitted light microscopy of a fibre reinforced metal

A method is presented for studying fibre damage in continuous fibre reinforced composites. It is based on contrasting the transmission of light through intact translucent fibres with the light through fractured or dead‐ended fibres. The method is applied in order to detect processing‐induced fibre fractures in aluminium reinforced with continuous alumina fibres.     

Soundproofing effect of nano particle reinforced polymer composites

In this paper, the effects of soundproofing by polymer and carbon-nanotube (CNT) composites were investigated. The specimens for sound insulation measurement were fabricated with Acrylonitrile Butadiene Styrene (ABS)/CNT composites. Tests showed that sound transmission loss of ABS/CNT 15 vol.% composite was higher by 21.7% (4.1 dB) than that of pure ABS specimen at a frequency of 3400 Hz. It was found that the principal factor influencing the improvement of sound insulations of ABS/CNT composites was increased stiffness by CNT additives. To demonstrate the practical applicability of polymer/CNT composites, tests were conducted for the reduction of operational noise from mechanical relay.     

In-situ monitoring of fibre and matrix deformation in fibre/polymer composites

Three methods, namely microphotoelasticimetry, Raman spectroscopy and surface microgrids, are currently used at ONERA on fibre-reinforced polymers to measure the in-situ fibre or matrix deformation. They provide the materials scientist with valuable indications on the early damage growth. Recent results are given and information about the limitations of the methods are indicated.     

Solid particle erosion of unidirectional fibre reinforced thermoplastic composites

In the present study, the solid particle erosion behaviour of neat PEEK matrix and unidirectional glass fibre (GF) and carbon fibre (CF) reinforced polyetheretherketone (PEEK) and polyetherketoneketone (PEKK) composites has been studied. The erosion experiments have been carried out by using silica sand particles (200 ± 50 μm) as an erodent. Steady state erosion rates of these composites have been evaluated at different impact angles and impact velocities. The neat PEEK exhibited peak erosion rate at 30° impingement angle whereas the composites exhibited a semi-ductile behaviour with peak erosion rate at 60° impact angle. The erosion rate of the glass fibre reinforced composites was higher than that of the carbon fibre reinforced composites. The results show that the fibre orientation has a significant influence on erosion rate only at lower impact angles. The erosion rate of the composites was higher when the particles impact perpendicular to the fibre direction than parallel to the fibres. The morphology of eroded surfaces was observed under scanning electron microscope and damage mechanisms were discussed.     

纤维增强铝镁合金中纳米相的研究

讨论了常用碳及氧化铝增强铝及铝合金复合材料中常见纳米相 ,其中包括纤维本身结构的组元及反应产物中的新生纳米相。由于这些相的形成条件、形成区域不同 ,对复合材料性能的影响也不同。     

Identification of weak interfaces in composites using transmission electron microscopy

A new experimental technique was developed to identify crack paths with a resolution of nanometres in fibre-reinforced composites. Cracks were introduced through Vickers indentations on one side of the sample prior to starting the thinning process. Indentations were placed close to the fibres in order to get enough cracks at the fibre/matrix interface in the electron-transparent region of the thinned sample. The technique was used in a Nicalon-fibre Al₂O₃ matrix composite prior to and after a heat treatment at 1200 °C for 1 h. The analysis of the crack paths allowed the identification of the weakest interface in each condition.     

Estimation of the directional distribution of spatial fibre processes using stereology and confocal scanning laser microscopy

Fibrous structures like polymers, glass fibres, muscle fibres and capillaries are important components of materials and tissues. A spatial fibre process is the union of smoothly curved or linear one-dimensional features of finite length, arranged in an unbounded three-dimensional reference space according to some random mechanism. Design-based stereology was combined with confocal scanning laser microscopy to study samples of fibre-reinforced composites, which were considered as realizations of not necessarily isotropic fibre processes. The methods enable the unbiased estimation of the intensity and of the directional distribution of spatial fibre processes from arbitrarily directed pairs of registered parallel optical sections a known distance apart. The directions of fibres sampled by a frame of observation on the reference plane are estimated from the coordinates of the intersection points of the fibres with both optical planes using confocal scanning laser microscopy. The true directional distribution of the fibre process is estimated by weighting each measured direction by the reciprocal of its chance of being sampled, which can be inferred from the data. The concept of complete directional randomness for uniformly and independently distributed spatial directions is introduced. The cumulative distribution function of the angular distances between different directions and other exact relations are derived for complete randomness of vectorial and axial directions. A Monte Carlo method is constructed to test spatial fibre processes, whose fibres have negligibly small curvature, for complete directional randomness. Confocal scanning laser microscopy was used to study the angular distribution of glass fibres in a polymer composite which was subjected to increasing hydrostatic extrusion. The hypothesis of complete directional randomness had to be rejected for all samples with 1% probability of error. The directional distribution was of the bipolar type, with the principal axis directed parallel to the axis of extrusion. Progressive stretching of the material increased the degree of anisotropy of the glass fibres. Although presented for an application in polymer physics, the methods are general and may also be applied in biological investigations.     

Tribological properties of high temperature resistant polymer composites with fine particles

The tribological properties of two kinds of high temperature resistant thermoplastic composites, polyetheretherketone (PEEK) and polyetherimide (PEI), reinforced with short carbon fibre (SCF), graphite flakes, and sub-micro particles of TiO₂ and ZnS, were investigated in dry sliding conditions. Friction and wear experiments were conducted on a pin-on-disc apparatus, using composite pins against polished steel counterparts, performed within moderate pv-ranges at room and elevated temperatures (up to 150 °C). It was found that conventional fillers, i.e. SCF and graphite flakes, could effectively enhance both the wear resistance and the load-carrying capacity of the base polymers. With the addition of sub-micro particles, the frictional coefficient and wear rate of the composites were further reduced especially at elevated temperatures. On the basis of microscopic observation of worn surfaces, dominant wear mechanisms are discussed.     

Nanostructure of interlayers in different Nicalon fibre/glass matrix composites and their effect on mechanical properties

Interlayer phenomena, revealed by high-voltage electron microscopy (HVEM) and high-resolution electron microscopy (HREM), are presented as they occur in various SiC(Nicalon) fibre-reinforced Duran glass composites (differing in the specific sol-gel supported production processes). Their dependence on the production parameters and their influence on the materials properties are discussed, taking into account the results of scanning electron microscope (SEM) in situ tensile tests. Besides graphitic carbon, textured to a variable degree and influencing the tensile behaviour, oxycarbide formation is indicated. A reactive matrix additive, such as, e.g. TiO₂, resulted in a decrease in strength and a brittle behaviour, while the addition of ZrO₂ markedly improves the mechanical properties.     

Effect of nanoparticles on the tribological behaviour of short carbon fibre reinforced poly(etherimide) composites

The tribological behaviour of nano-TiO₂ particle filled polyetherimide (PEI) composites, reinforced additionally with short carbon fibre (SCF) and lubricated internally with graphite flakes, was investigated. The wear tests were conducted on a pin-on-disc apparatus, using composite pins against polished steel counterparts under dry sliding conditions, different contact pressures and various sliding velocities. It was found that the conventional fillers, i.e. SCF and graphite flakes, could remarkably improve both the wear resistance and the load-carrying capacity. With the addition of nano-TiO₂, the frictional coefficient and the contact temperature of the composite were further reduced, especially under high pv (the product of the normal pressure, p, and the sliding velocity, v) conditions. Based on microscopic observations of worn surfaces and transfer films on the counterparts, possible wear mechanisms were discussed.     

大口径空间光学遥感器的碳纤维复合材料精密支撑构件的结构稳定性研究

摘要：为了研究应用于大口径空间光学遥感器中由碳纤维复合材料（CFRP）制成的精密支撑构件的结构稳定性,设计并研制了连接在主、次镜间的CFRP连接筒。对该连接筒建立等效力学模型、计算相关参数和确定铺层的合理形式,运用有限元法对连接筒进行分析计算,通过试验验证连接筒的结构稳定性。首先,由给定的主、次镜间角度变化量计算出连接筒前端面的最大挠度,根据实际载荷情况建立等效力学模型,计算连接筒轴向弹性模量,结合复合材料层压板理论,确定碳纤维铺层的合理形式。然后,运用ANSYS软件对有限元模型进行分析,计算主、次镜间的角度变化量和支撑结构的模态分布。最后,通过量级逐增的力学试验,采用光学测量的方法测量主、次镜间角度变化量,验证CFRP连接筒的结构稳定性。试验结果表明：主、次镜间角度变化量小于10″,CFRP支撑构件一阶基频大于75Hz。CFRP支撑构件满足主、次镜间角度变化量要求,具有较好的结构稳定性。     

Assessment of fibre orientation in reinforced concrete using Fourier image transform

In this study, ribbon-shaped amorphous cast-iron fibres were used to reinforce a concrete matrix. X-ray photographs have been taken to detect fibres in situ . Their orientation has been investigated by automatic image analysis methods. However, this measurement should not be influenced by the digitization on the square frame of the analyser. For that purpose, the Fourier transform was used rather than the rose of direction method. This analysis revealed the transverse isotropic nature of the spatial arrangement of these fibres, whose axis of revolution corresponds to the concrete casting axis. Such a morphological characterization of the fibre-reinforced concrete reveals its mechanical behaviour.     

Measurements of friction-induced surface strains in a steel/polymer contact

The paper describes an experimental study combined with analyses and numerical simulations of the surface strains developed in a metal–polymer contact under a variety of loading configurations. Specifically, a steel ball is caused to slide over a poly(methylmethacrylate) flat counterface under a fixed normal load where the imposed motions are small and consist of sliding and rotation and the combination of both. The surface strains have been measured directly using conventional strain gauges in two types of configurations specifically designed to monitor the strains for sliding and rotation. Calculations of frictional forces provide friction coefficients which are self-consistent and the computed ‘friction displacement loops’ correspond closely to those measured. In addition, the surface strain measurements provide a convenient and accurate insight into the stick–slip transitions in fretting contacts.     

Comparison of interfaces in Ti composites reinforced with uncoated and TiB2/C-coated SiC fibres

Interfaces play an important role in determining the mechanical properties of composite materials. The interfaces established between a titanium-alloy matrix (Ti-6Al-4V) and uncoated and TiB₂/C-coated SiC fibres are analysed by scanning electron microscopy, transmission electron microscopy and X-ray techniques. Emphasis is placed upon the interfacial morphology and microstructure, identification of reaction products, and the stability of the coating layer. Complex multi-reaction layers are observed frequently in the interfacial zones. Previous, often contradictory, reports about the interlayers are reviewed. Experimental observation demonstrates that the type and distribution of interlayers vary in a given system, due to prolonged treatment of the samples at temperature. The formation and distribution of the interlayers are discussed further, with respect to these and previous findings. Methods of reducing interfacial reactivity are discussed.     

Sample preparation and image acquisition using optical-reflective microscopy in the measurement of fiber orientation in thermoplastic composites

A complete sample preparation procedure used to determine three-dimensional fiber orientation from optical micrographs of glass fiber-reinforced thermoplastic composites is presented. Considerations for elimination of irregularities in the elliptical footprints, contrast enhancement between fibers and surrounding polymer matrix, controlled-etching that allows the identification of small shadows where fiber recedes into the matrix, and topographical reconstruction of the elliptical footprint are described in the procedure. This procedure has produced high-quality optical micrographs employed to obtain accurate fiber orientation data for thermoplastic composites using the method of ellipses. The optimal definition of the nonelliptical footprints' borders allows an accurate measurement of orientation in small sampling areas.     

A micromechanics analysis of the material removal mechanisms in the cutting of ceramic particle reinforced metal matrix composites

In previous investigations on the cutting of ceramic particle reinforced metal matrix composites using the finite element (FE) method, the particles are usually considered to be rigid. This is inconsistent with the actual situation and thus makes the FE predictions less practical. This paper proposes a micromechanics model to investigate the material removal mechanisms by considering the elasticity and fracture of the particles. It was found that the interaction position of a particle relative to the cutting edge greatly influences the particle and matrix fracture, particle-matrix debonding, surface integrity and cutting forces. There are two particle cracking mechanisms. One is caused by the direct contact of the cutting edge with the particle, and the other is due to the indirect tool-particle interaction through matrix. When the cutting path is below a particle, a smooth surface without subsurface damage can be achieved. When it is passing through a particle, particle fracture mostly occurs. If it is above a particle, subsurface damage is dominated by particle-matrix debonding. Cutting force peaks once the cutting edge is in contact with a particle and the cutting edge is easier to be damaged when the tool is passing through the upper part of the particle.     

Effect of tool vibration on chip formation and cutting forces in the machining of fiber-reinforced polymer composites

This article investigates the chip formation mechanism and its influence on cutting forces during the elliptic vibration-assisted (EVA) cutting of fiber-reinforced polymer composites. To clarify the effect of the vibration, systematic finite element and experimental studies were performed on both the EVA and the traditional cutting of unidirectional fiber-reinforced polymers with various fiber orientations. The key factors that govern the cutting forces have been taken into account, such as the depth of cut, feed rate, tool vibration frequency and amplitude. The study found that fiber orientation significantly affects the chip formation and cutting forces. Fiber fracture can happen either above or below the trimming path, but that above the path dominates chip formation. When a fiber orientation is less than 90°, chipping is mainly through bending-induced fracture of fibers; when it is beyond 90°, however, chipping is mostly by crushing the fracture of fibers. Compared with a traditional cutting process, the EVA cutting can minimize the fiber orientation effect through localized fiber fracture. A dimensional analysis was then performed to provide a quantitative prediction of the cutting forces.     

Applications of confocal microscopy in industrial solid materials: some examples and a first evaluation

We report here the first results of the application of confocal scanning laser microscopy (CSLM) for the study of the microstructure of solid industrial materials. Glass-fibre-reinforced composites, heterogeneous and conductive polymers, homogeneous as well as heterogeneous catalyst (precursor) specimens and soils were examined. We conclude that both the fluorescence and reflection modes of CSLM can yield valuable information. In particular, the optical sectioning capability of CSLM appears to be of great value as it enables one to access the 3-D organization of the specimen without the need for a difficult and time-consuming specimen preparation procedure. However, local obscuration may be an important factor in confocal image formation, limiting the penetration capabilities of the technique for industrial materials.     

The use of a single fibre test technique to investigate interfacial phenomena in SiC/Ti3Al-based composites

A study of the chemical compatibility of a Ti₃ Al-based alloy (Ti–24Al–10Nb, at.%) with two silicon carbide continuous fibres (SCS-6, SM 1240) has been conducted. Owing to the difficulty in processing intermetallic matrix composites, this type of material has been simulated in the present work by sputtering a thin titanium aluminide layer onto fibres and heat treating at temperatures representative of fabrication conditions. The degradation of the fibre strength due to its interaction with the matrix was correlated with analytical studies of the fibre/matrix interface using a combination of SEM, TEM, EELS and a submicrometre ion probe.     

The microstructure of silicon carbide (Nicalon) fibre reinforced silicon carbide ceramic-matrix composites heat treated in vacuo and in pure oxygen

Silicon carbide (Nicalon) fibre reinforced SiC composites have been heat treated in vacuo and in pure oxygen environments at 1400°C for 100 h. The response of the microstructure and, in particular, of the interface between fibre, carbon interlayer and SiC matrix components has been studied. Microstructural modifications were observed by transmission electron microscopy, using imaging, electron energy-loss spectroscopy and electron diffraction techniques, and fibre stoichiometries were determined using a scanning Auger microprobe. Recrystallization of Nicalon fibres within composites heat treated in vacuo was found to result from decomposition of the metastable silicon oxy-carbide phase found in the fibres. No significant changes to the pyrolytic carbon interlayer were observed. Fibre recrystallization was considered to embrittle the composite. Samples heat treated in oxygen showed no appreciable fibre recrystallization. Study of interlayers in such aged samples often revealed decohesions, holes and narrow silica layers. In the most extreme cases, complete displacement of carbon by SiO₂ was found and such interfaces were identified as silica and α-cristobalite. Interfacial modifications were considered to be responsible for the retention of the small β-SiC grain size in Nicalon fibres and were also considered to be deleterious to the mechanical properties.