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.     

Analytical Model and Experimental Validation of Friction Laws for Composites Under Low Loads

In order to account for interfacial friction of composite materials, an analytical model based on contact geometry and local friction is proposed. A contact area includes several types of microcontacts depending on reinforcement materials and their shape. A proportion between these areas is defined by in-plane contact geometry. The model applied to a fibre-reinforced composite results in the dependence of friction on surface fibre fraction and local friction coefficients. To validate this analytical model, an experimental study on carbon fibre-reinforced epoxy composites under low normal pressure was performed. The effects of fibre volume fraction and fibre orientation were studied, discussed and compared with analytical model results.     

Lubrication of carbon fibre-reinforced polymers part I—Water and aqueous solutions

An examination has been made of the friction and wear properties of carbon fibre-reinforced polymers sliding against metals in water, sea water and other aqueous solutions. The conditions of sliding were chosen to minimize hydrodynamic effects. All the fluids inhibit the formation of transfer films of carbon/polymer debris on the counterface and the wear rates are generally greater than those obtained in dry conditions. The topography and type of counterface play a major role in the wear process and corrosion by the fluid is also important. Type II fibre composites generally exhibit lower wear than those containing Type I fibre, and compare favourably with existing materials. Additions of small amounts of finely divided abrasive to carbon fibre composites are very effective in reducing wear in water. Wear rates in sea water and other aqueous solutions on some counterfaces can be significantly lower than in pure water.     

Lubrication of carbon fibre-reinforced polymers : Part II—Organic fluids

A variety of organic fluids, including mineral oils and synthetic lubricants, are shown to be effective boundary lubricants for carbon fibre-reinforced polymers sliding against stainless steel. The wear rates of the composites in some fluids are extremely low, and scanning electron microscopy and pyrolysis-gas chromatography show that low wear is associated with the development of films on the steel counterface. These films appear to be mixtures of lubricant and resin from the composite matrix. The wear rates of epoxy/Type II carbon fibre composites in aircraft hydraulic fluids are generally much less than those of many metal combinations, and the composites therefore show promise as replacements for metals in lubricated systems.     

Thermal imaging of composites

Active thermal imaging techniques and their applications to composite materials are reviewed. The techniques included are transient thermography, scanning thermal microscopy and scanning thermal probe microscopy. The factors that affect the images produced by both pulsed and periodic forms of active heating are considered. For pulsed heating, experimental and numerical modelling results for carbon fibre-reinforced plastic are used to show how the resolution of subsurface features depends on their size and depth and on the anisotropy in thermal materials properties common in such composites. For periodic heating, thermal wave characteristics are introduced to show how the resolution of subsurface features also depends on modulation frequency and focal spot radius. Examples are given of the applications of scanning thermal microscopy and scanning thermal probe microscopy that illustrate the potential of these techniques for the imaging of composite materials.     

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.     

Polymer composites in 2000: structure, performance, cost and compromise

Polymer matrix composites are based on the combination of stiff, strong reinforcing fibres with either thermosetting or thermoplastic polymer matrices. Since their introduction in the early 1940s, the world market has increased to some 5 million tonnes per annum, and some composites may now be considered commodity materials. The spectrum of fibre-reinforced plastics ranges from very high-performance speciality materials costing more than $1000/kg to these commodity composites, with more modest properties, at less than $10/kg. The performance of composites is determined by the properties of the fibre, the fraction of fibre in the composite and the structure, or fibre architecture. Processing technologies have been developed which maximise fibre content and precisely control the fibre architecture allowing for the manufacture of components with mechanical properties tailored to service requirements. Many composites offer significant advantages in specific stiffness and/or specific strength over metals. This makes them attractive for applications where high mechanical performance and minimum weight are important. However, the wider acceptance of composites is based on their ability to offer a more cost-effective alternative. In particular, composites also allow a dramatic reduction in the parts count in many applications, which leads to significant manufacturing advantages and greater economy.     

Impact of stacking sequence on tribological wear performance of woven jute-glass hybrid epoxy composites

Natural fibre-reinforced plastic (FRP) composites have gained much interest because of their environment friendliness and cost-effectiveness compared to synthetic fibre-reinforced composites. The availability of natural fibre and ease of manufacturing have tempted researchers worldwide to develop a locally available low-cost fibre and study their feasibility for reinforcement purposes and to what extent they can satisfy the required specifications of well-reinforced polymer composite for tribological application. FRP composites have various applications in the automobile, aerospace and marine fields. They are applied to inlet cone, fan exit guide vanes and other parts of structures in a turbofan engine for lightening an engine. The erosion characteristics of the FRP composites are of vital importance due to the operational requirements in dusty environments. In this present work, the impact of stacking sequence on erosion wear behaviour of untreated woven jute and glass fabric-reinforced epoxy hybrid composites has been investigated experimentally. The orientation of glass and jute fabric was kept at (0°–90°) and (45°–45°) for all stacking sequences. All the laminates were prepared using four plies, and, the number and position of glass layers were varied so as to obtain four different stacking sequences. The erosion rate of these composites were evaluated at different impingement angles (30°–90°) at three different impact velocities (V = 48, 70, 82 m/s). Silica sand was used as the erodent. Our results showed that the impingement angle had a significant influence on the erosion rate. The composite materials showed semi-ductile behaviour with the maximum erosion at an impingement angle of 60°. The morphologies of the eroded surface were observed by a scanning electron microscope, and the possible erosion mechanisms were discussed.     

Mapping micro-mechanical properties of carbon-filled polymer composites by TPM

This paper presents a new approach to the characterisation of carbon-filled polymer composites by using a novel multi-function tribological probe microscope (TPM). The TPM is capable of measuring four functions in a single scan to provide area mappings of topography, friction, Young's modulus and nanohardness. The measurement is based on point-by-point scanning so values of the four measured functions are linked in space and in time. The specimens are PA6.6 and PA12 filled with carbon black or carbon fibre, specially prepared at the Institute for Materials Research III of Karlsruhe Research Centre. The four-in-one measurement of TPM enables us to identify the material difference on the surface in order to estimate the distribution of a particular material within the composite. For each specimen, mappings of topography, hardness and Young's modulus were obtained, and from the latter two, it is easy to see the existence of two different materials. Comparing the topography and hardness mappings, we are able to pick up the areas where carbons are located.     

Investigation on drilling-grinding of CFRP

It is difficult to machine polymer matrix composites reinforced by carbon fibre, and the holemaking process is the most necessary machining process for composite plate products. Conventional drills have a very short life in the drilling of this kind of composites and the quality of the hole is very poor. In this paper, the cemented or plated diamond core tools are tested to make holes in carbon fibre/epoxy composite plates. The effects of machining parameters, cooling and chip removal on the tool life, and the hole quality are investigated. The results indicate that the material removal mechanism of the two kinds of diamond tools is not like the cutting effect of the conventional drilling but similar to that of grinding. Satisfactory effects in making holes in the composites are obtained—quite acceptable machined hole quality, low costs, and long wear-resistant endurance.     

Free vibration analysis of functionally graded CNT-reinforced nanocomposite beam using Eshelby-Mori-Tanaka approach

This work deals with the effect of agglomeration and distribution of carbon nanotube on the free vibration characteristics of a functionally graded nanocomposite beams reinforced by single-walled carbon nanotubes (SWCNTs) by employing an equivalent fiber based on the Eshelby-Mori-Tanaka approach. Different SWCNTs distributions in the thickness directions are introduced to improve fundamental natural frequency of polymer composite beam. The micromechanics models used in the study include a two parameter model of agglomeration. An embedded carbon nanotube in a polymer matrix and its surrounding inter-phase is replaced with an equivalent fiber for predicting the mechanical properties of the carbon nanotube/polymer composite. The system of equations of motion is derived by using the principle of virtual work under the assumptions of the Euler-Bernoulli beam theory. The finite element method is employed to obtain a numerical approximation of the motion equation. Numerical results are presented in both tabular and graphical forms to figure out the effects of nanotube agglomeration, CNTs distribution and boundary conditions on the dynamic characteristics of the beam. The above mentioned effects play very important role on the dynamic behavior of the beam.     

A method for estimating the fibre length in fibre‐PLA composites

Wood pulp fibres are an important component of environmentally sound and renewable fibre‐reinforced composite materials. The high aspect ratio of pulp fibres is an essential property with respect to the mechanical properties a given composite material can achieve. The length of pulp fibres is affected by composite processing operations. This thus emphasizes the importance of assessing the pulp fibre length and how this may be affected by a given process for manufacturing composites. In this work a new method for measuring the length distribution of fibres and fibre fragments has been developed. The method is based on; (i) dissolving the composites, (ii) preparing the fibres for image acquisition and (iii) image analysis of the resulting fibre structures. The image analysis part is relatively simple to implement and is based on images acquired with a desktop scanner and a new ImageJ plugin. The quantification of fibre length has demonstrated the fibre shortening effect because of an extrusion process and subsequent injection moulding. Fibres with original lengths of >1 mm where shortened to fibre fragments with length of <200 μm. The shortening seems to be affected by the number of times the fibres have passed through the extruder, the amount of chain extender and the fraction of fibres in the polymer matrix.     

Three-dimensional image restoration and super-resolution in fluorescence confocal microscopy

Confocal scanning laser microscopy (CSLM) provides optical sectioning of a fluorescent sample and improved resolution with respect to conventional optical microscopy. As a result, three-dimensional (3-D) imaging of biological objects becomes possible. A difficulty is that the lateral resolution is better than the axial resolution and, thus, the microscope provides orientation-dependent images. However, a theoretical investigation of the process of image formation in CSLM shows that it must be possible to improve the resolution obtained in practice. We present two methods for achieving such a result in the case of 3-D fluorescent objects. The first method applies to conventional CSLM, where the image is detected only on the optical axis for any scanning position. Since the resulting 3-D image is the convolution of the object with the impulse-response function of the instrument, the problem of image restoration is a deconvolution problem and is affected by numerical instability. A short introduction to the linear methods developed for obtaining stable solutions of these problems (the so-called regularization theory of ill-posed problems) is given and an application to a real image is discussed. The second method applies to a new version of CSLM proposed in recent years. In such a case the full image must be measured by a suitable array of detectors. For each scanning position the data are not single numbers but vectors. Then, in order to recover the object, one must solve a Fredholm integral equation of the first kind. A method for the solution of this equation is presented and the possibility of achieving super-resolution is demonstrated. More precisely, we show that it is possible to improve by about a factor of 2 the resolution of conventional CSLM both in the lateral and axial directions.     

Microfractography of granitic rocks under confocal scanning laser microscopy

Scanning laser microscopy, in the confocal mode (CSLM) has been applied to a granitic rock to characterize its fissure space. The technique provides a unique three-dimensional picture of the rock microfractography. CSLM is unique in observing fine details of the fractographic network (connectivity, tortuosity, etc.), its geometry and its relation to other rock-forming components. The fractographic images with standard fluorescence microscopy are compared with those obtained with CSLM. The examples presented emphasize the advantages of CSLM: three-dimensional visualization of the microfractographic network, crack connectivity, automatic evaluation of direction and slope of fissures. These studies are related to the migration of radionuclides in the geosphere. The relations between potentially water-conducting open fissures, and the rock-forming minerals provide a means of modelling the ‘radionuclide retardation mechanism’, a security factor in their definitive storage in rock masses.     

Automated reconstruction of curvilinear fibres from 3D datasets acquired by X-ray microtomography

The characterization of fibrous structures is important in both composites and textiles research for relating to the bulk properties of the material. However, the microscopic nature of the fibres and their high densities make them very difficult to characterize. Many techniques have been developed for the measurement and characterization of fibrous structures but they tend to be restricted to measurements on the sample surface or within physical cross‐sections. X‐ray microtomography can be used to non‐destructively probe the internal structure of a range of fibrous materials, providing large amounts of 3D data. A technique has been developed for tracing fibres within 3D datasets acquired by X‐ray microtomography and this has been applied to a glass fibre reinforced composite and also a non‐woven textile sample. The 3D fibrous structures of both samples were successfully reconstructed and their fibre orientation distributions calculated. This technique enables novel characterizations, such as the through‐thickness variation of fibre orientation in non‐wovens.     

Sliding wear behavior of planar random zinc-alloy matrix composites

The friction and wear behavior of planar random zinc-alloy matrix composites reinforced by discontinuous carbon fibres under dry sliding and lubricated sliding conditions has been investigated using a block-on-ring apparatus. The effects of fibre volume fractions and loads on the sliding wear resistance of the zinc-alloy matrix composites were studied. Experiments were performed within a load range of 50–300 N at a constant sliding velocity of 0.8 m s⁻¹. The composites with different volume fractions of carbon fibres (0–30%) were used as the block specimens, and a medium-carbon steel used as the ring specimen. Increasing the carbon fibre volume fraction significantly decreased the coefficient of friction and wear rates of both the composites and the medium-carbon steel under dry sliding conditions. Under lubricated sliding conditions, however, increasing the carbon fibre volume fraction substantially increased the coefficient of friction, and slightly increased the wear of the medium-carbon steel, while reducing the wear of the composite.Under dry sliding conditions, an increasing load increased not only the wear rates of both the composite and the unreinforced zinc alloy, but also those of their corresponding steel rings. However, the rate of increase of wear with increasing load for both the composite and its corresponding steel ring was much smaller than for the unreinforced zinc alloy and its corresponding steel ring. The coefficient of friction under dry sliding conditions appeared to be constant as load increased within a load range of 50–150 N for both the composite and the unreinforced zinc alloy, but increased at the higher loads. Under any load the coefficient of friction of the composite was lower than half that of the unreinforced zinc alloy under dry sliding conditions.     

Characterization of the fibre–matrix interfacial structure in carbon fibre-reinforced polycarbosilane-derived SiC matrix composites using STEM/EELS

This paper presents a characterization study of the microstructural evolution of various carbon fibre-reinforced polycarbosilane (PCS)-derived SiC matrix composites during high temperature heat treatment. Both surface-treated and untreated carbon fibre reinforcements were investigated. The STEM/EELS technique was found to be a particularly useful characterization tool. The results of quantitative EELS linescans have been interpreted in terms of the migration of gaseous SiO and CO, produced by the reaction between the small amount of SiO₂ and excess carbon within the PCS-derived SiC matrix, from the central matrix region towards the fibre–matrix interfaces. Generally, the migration of gaseous SiO and CO results in an enrichment of SiO₂ at the region adjacent to the fibre–matrix interface. However, differing final composite microstructures are formed depending on the strength of the fibre–matrix bonding. In the case of strong fibre-matrix interfacial bonding where few escape channels are present, a distinct Si–C–O layer was identified within the matrix adjacent to the fibre–matrix interface; both crystalline β-SiC and the segregated Si–O–C phase coexist in this microstructure up to at least 1450 °C. In the case of weak fibre–matrix bonding this oxygen segregated interfacial layer is eventually removed at high enough temperatures. The final interfacial microstructure has important consequences for the mechanical properties of the composite material.     

Reliability analysis and micromechanics: A coupled approach for composite failure prediction

This work aims at associating two classical approaches for the design of composite materials: first, reliability methods that allow to account for the various uncertainties involved in the composite materials behaviour and lead to a rational estimation of their reliability level; on the other hand, micromechanics that derive macroscopic constitutive laws from micromechanical features. Such approach relies on the introduction of variabilities defined at the microscale and on the investigation of their consequences on the material macroscopic response through an homogenization scheme. Precisely, we propose here a systematic treatment of variability which involves a strong link between micro- and macroscales and provides a more exhaustive analysis of the influence of uncertainties. The paper intends to explain the main steps of such coupling and demonstrate its interests for material engineering, especially for constitutive modelling and composite materials optimization. An application case is developed throughout on the failure of unidirectional carbon fibre-reinforced composites with a comparative analysis between experimental data and simulation results.     

三维多向编织复合材料承力接头破坏模态

通过单向拉伸试验研究了三维多向整体编织复合材料单耳承力接头的破坏模式。试样选用T700碳纤维和环氧树脂,采用RTM(resin transfer molding树脂传递模塑)工艺制成。试验针对由3种编织工艺(三维四向编织、三维五向编织和三维六向编织)、两种连接孔加工方式(机械钻孔和编织孔)、两种几何外形所组成的十组三维编织复合材料单耳承力接头的破坏机理与破坏载荷进行了初步探索研究,并对其在航空结构中的可应用性做出了评估。     

Surface profile texture characterization of trimmed laminated composite in the stacking sequence direction

The surface texture characterization of laminate composites is complex due to its heterogeneous structure and to the different stratified surface properties. Profile roughness parameters are highly impacted by those different layer properties, and their distributions are relatively spread out. A new filtering technique is proposed to improve the surface roughness parameter efficiency. To present issues from current filtering methods and to highlight this new approach, profile surface roughness taken from trimmed carbon fibre reinforced plastics samples are inspected in the composite stacking sequence using a contact profilometer. For each measurement, several roughness parameters are calculated. The efficiency of different filtering techniques is investigated through the roughness parameters distribution, calculated for up- and down-milling and for different tool wear. The new proposed technique, based on the separate characterization of plateaus and large deep valleys of the composite surface profiles, is found to be particularly efficient on the down-milling face.