Resistance welding of carbon fibre reinforced thermoplastic composite using alternative heating element

The focus of this work is the use of a metal mesh as an alternative heating element for the joining of carbon fibre fabric reinforced polyetherimide composite laminate. A more homogeneous temperature distribution was generated by the metal mesh at the bonding surface. Glass fibre fabric reinforced PEI (GF/PEI) was used as an electrical insulator between the heating element and adherend laminates. Experimental results show that the GF/PEI prepreg could effectively prevent current leakage and enlarge the welding area. Welding parameters, such as input power level, welding time and pressure, were optimized according to the results of mechanical and microstructure characterization. Mechanical performance of composite specimens joined using metal mesh, in terms of lap shear strength and Mode I interlaminar fracture toughness, was equivalent to that of compression moulded benchmarks. Fracture surfaces of welded specimens showed mostly cohesive failure or intralaminar failure, indicating that good bonding between the PEI matrix and metal mesh was achieved.     

Fracture toughness of short carbon fibre reinforced thermoplastic polyimide

The fracture toughness testing of short fibre reinforced thermoplastic materials were performed. Materials tested were the polyimide resin and also that reinforced with 20 wt% or 30 wt% short carbon fibre. For introducing the initial crack, the tapping method, the sliding method and the bridge indentation method were examined. Among them, the sliding method was found to be effective for every case. The fracture tests were conducted by the three-point bending test with several loading rates. Stable crack growth was observed for the neat material while unstable fracture occurred for the reinforced materials. The critical values of the stress intensity factor at crack initiation were greater for the reinforced materials than for the neat resin. The fracture toughness of the 30 wt% reinforced material was independent of loading rate while that of 20 wt% reinforced material increased with loading rate. In order to investigate the fracture mechanisms, fractographic observations were also performed.     

Fabrication and mechanical properties of glass fibre reinforced thermoplastic elastomer composite

The paper investigates the effects of fabrication conditions on mechanical properties of glass fibre reinforced thermoplastic elastomer composites. The impregnation time was varied between 5 and 30 min and the cooling conditions were rapid and gradual cooling. Tensile testing was carried out on samples with different fibre orientations. Double Cantilever Beam (DCB) tests were carried out to evaluate the fracture toughness of the composites. The degree of crystallinity and morphology of the composite were studied by using differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Impregnation of matrix resin into glass fibre was found to be complete before 30 min and tensile properties increased with increasing impregnation time. SEM micrographs of fractured surfaces revealed poor adhesion between the matrix and the reinforcing agent. Due to the flexible nature of the composite, the fracture toughness (G_IC) could not be determined because of the formation of ridges on the surface.     

Plasma-sprayed wear-resistant ceramic and cermet coating materials

Some wear-resistant ceramic and cermet coatings, such as Al₂O₃, TiO₂, Al₂O₃-TiO₂, Cr₂O₃, Cr₃C₂-NiCr and cobalt-cladded WC, were prepared by the arc plasma spraying technique. In this paper optimized spraying parameters of various coatings are briefly described. Some physical properties of these coatings materials were measured. Wear tracks and damage forms were studied by microscopy and scanning electron microscopy techniques. Wear damage mechanisms of these coatings are discussed together with the relationship between wear damage of the coatings and their microstructure, physical properties and especially their thermal diffusivity. The higher the thermal diffusivity of a coating, the better is its wear resistance. The wear resistance of coatings has no obvious relationship with many other physical properties. The pore size, shape and distribution as well as the crack texture in the coatings have important bearings on their wear resistance. Approaches to improve wear-resistant coatings are presented.     

Studies on carbon fibre reinforced aluminium composite processed using pre-treated carbon fibres

Carbon fibre reinforced Al-12% Si alloy composite has been fabricated by pre-treating the fibres with K₂ZrF₆ followed by molten alloy infiltration and subsequent hot pressing of the preforms. The infiltration conditions were arrived at based on the measurement of tensile strength of the fibres extracted from the preforms. The fibre volume per cent of 20 was found to result in composite tensile strength of about 240 MPa as compared to tensile strength of 100 MPa for the unreinforced matrix. Characterization of the interface revealed the formation of ZrSi₂ and diffusion of potassium and aluminium into the fibre. The interfacial bonding was strong as is evinced by the absence of fibre pull-out on to the fracture surface.     

Statistical prediction of fatigue failure of fibre reinforced composite materials

A statistical approach is proposed to evaluate the residual strength and life of unidirectional and angle-ply composite laminates subjected to in-plane tensile cyclic stresses. The method is based on the extension of previous static failure criteria describing independently the fibre failure and matrix failure modes, combined with the statistical nature of fatigue failure of fibre-reinforced composites. The static and fatigue strengths of composite laminates at any off-axis angle are evaluated using the fatigue failure functions for the three principal failure modes, which are determined from the fatigue behaviour of unidirectional composites subjected to longitudinal and transverse tension as well as in-plane shear stresses. The evaluations of the fatigue strength of unidirectional E-glass/epoxy laminates under off-axis fatigue loading and angle-ply S-glass/epoxy laminates under in-plane fatigue loading show good agreement between theoretical predictions and experimental results.     

Corrosion protection of carbon fibre reinforced aluminium composite by diamondlike carbon coatings

Abstract

Carbon fibre reinforced aluminium exhibits poor resistance against electrochemical corrosion in 3·5 wt-%NaCl solution. Diamondlike carbon (DLC) coatings provide properties which make them interesting materials for external corrosion protection on metal matrix composites (MMCs). The electrochemical corrosion behaviour of uncoated and DLC coated carbon fibre reinforced aluminium was tested in 3·5 wt-%NaCl solution. It has been found that the pitting potential is shifted significantly in the anodic direction and the corrosion current density is much lower due to the presence of the sealing DLC coating. Additionally, scratch tests and SEM studies were carried out in order to characterise the adhesion of the DLC films on the heterogeneous MMCs. Reliable corrosion protection is connected with sufficient coating durability under loading. In order to ensure sufficient loading capacity of the DLC coating under tribological conditions, wear tests were undertaken which revealed a considerable improvement in wear resistance due to deposition of the DLC coatings.     

Recycled carbon fibre reinforced polymer composite for electromagnetic interference shielding

This paper describes the development of an electromagnetic interference shielding material using recycled carbon fibre. Fibre recycled from a fluidised bed process was transformed into a non-woven veil and was moulded into a glass-fibre reinforced polymer plaque to provide shielding. Factors affecting shielding performance were established using a virgin fibre and the result was compared with veil made of the recycled fibre. Shielding performance increased with veil areal densities. The influence of fibre length on shielding seemed insignificant provided the fibre was distributed evenly. Sandwiching the plaque between fibre veils enhanced the shielding performance. A shielding value of 40 dB was attained from a layer of 80 g/m² recycled fibre veil and it was increased to 70 dB when the plaque was sandwiched between two layers of 30 g/m² recycled fibre veil. The correlation between veil formation and shielding effectiveness was established and found that shielding effectiveness increased with the degree of fibre dispersion.     

Impact damage to thick carbon fibre reinforced plastic composite laminates

Recently, very thick section laminates, up to 20 mm in thickness, have been proposed for the wing skins of large aircraft. Composite components in all aircraft have concerns relating to the presence of accidental damage, but there has been little work to investigate the mechanisms and effects of damage in such thick sections. In this work, carbon fibre composite laminates of up to 12 mm thickness have been subjected to dropped-weight impacts of at most 375 J. Two types of impacts were considered. The first is a central impact where the laminate is completely supported and the second a near edge impact where the laminate is partially supported so that one of its edges is free. The geometry of the damage has been studied using C-scan and deply techniques. The residual strengths of the impact-damaged laminates have been measured in tension and compression. The geometry of damage and level of strength reduction is different for central and edge impacts. Generally, an edge impact causes a greater reduction in compressive strength while a central impact causes more tensile strength reduction.     

Effect of fabrication parameters on carbon fibre reinforced magnesium matrix composite components

Carbon fibre reinforced magnesium matrix (C_f/Mg) composite components were made by liquid–solid extrusion process following vacuum infiltration technique. The effect of varying fabrication parameters (extrusion temperature, infiltration pressure and filling pressure) on the forming quality of C_f/Mg components was investigated. The experimental results showed that the extrusion temperature and the infiltration pressure played important roles, and the filling pressure played a secondary role. The components can be obtained at an extrusion temperature of 590°C, an infiltration pressure of 20?MPa and a filling pressure of 0.2?MPa. The ultimate tensile strength of the C_f/Mg composite components was 290?MPa, which was increased by 81% compared with that of casting AZ91D.     

Investigation of carbon nanotube reinforced aluminum matrix composite materials

We have increased the tensile strength without compromising the elongation of aluminum (Al)–carbon nanotube (CNT) composite by a combination of spark plasma sintering followed by hot-extrusion processes. From the microstructural viewpoint, the average thickness of the boundary layer with relatively low CNT incorporation has been observed by optical, field-emission scanning electron, and high-resolution transmission electron microscopies. Significantly, the Al–CNT composite showed no decrease in elongation despite highly enhanced tensile strength compared to that of pure Al. We believe that the presence of CNTs in the boundary layer affects the mechanical properties, which leads to well-aligned CNTs in the extrusion direction as well as effective stress transfer between the Al matrix and the CNTs due to the generation of aluminum carbide.     

The buckling response of carbon fibre composite panels with reinforced cut-outs

This paper investigates the influence of cut-out diameter and reinforcement type upon the buckling stability of square CFRP panels. The study undertaken at Cranfield University in GB uses NASTRAN Finite Element Analysis (FEA) extensively for this investigation. The FEA results have been compared with results from practical tests and good agreement was found. Diagrams showing the influence of cut-out diameter, reinforcement lay up and thickness on the buckling stability of clamped or simply supported CFRP panels are presented. The results are shown for 2 mm thick ((± 45/0)_s)_s square CFRP panels with 30 mm wide reinforcement rings bonded around the central circular cut-outs. The panels are loaded in pure shear or in compression. The effect of reinforcement rings bonded to one or both sides of the panel has been investigated and shown in diagrams. The results presented can be used to find the optimum reinforcement type and thickness for square CFRP panels with central circular cut-outs.     

碳纤维改性环氧树脂复合材料研究进展

介绍最近几年碳纤维增强环氧树脂复合材料研究的前沿动向,重点叙述了碳纤维表面处理方法以及碳纤维在环氧树脂的应用,综述了环氧树脂/碳纤维复合材料的研究发展。     

Fracture mechanism in short fibre reinforced thermoplastic resin composites

The properties of two types of short carbon fibre (CF) reinforced thermoplastic resin composites (CF-PPS and CF-PES-C), such as strength (_y). Young's modulus (E) and fracture toughness (K _1c), have been determined for various volume fractions (V _f) of CF. The results show that the Young's modulus increases linearly with increasingV _f with a Krenchel efficiency factor of 0.05, whereas _y andK _1c increase at first and then peak at a volume fraction of about 0.25. The experimental results are explained using the characteristics of fibre-matrix adhesion deduced from the load-displacement curves and fractography. By using a crack pinning model, the effective crack tensions (T) have been calculated for both composites and they are 57 kJ m^–1 for CF-PPS and 4.2 kJ m^–1 for CF-PES-C. The results indicate that the main contribution to the crack extension originates from localized plastic deformation of the matrix adjacent to the fibre-matrix interface.     

Improved fracture toughness of carbon fibre/epoxy composite laminates using dissolvable thermoplastic fibres

This paper reports on a novel toughening concept based on dissolvable phenoxy fibres, which are added at the interlaminar region in a carbon fibre/epoxy composite. The composites were prepared by resin infusion of carbon fibre fabric with the phenoxy introduced as a chopped fibre interleaf between the carbon fibre plies. The thermoplastic phenoxy fibre dissolved in the epoxy during curing at elevated temperatures and a phase separated morphology with phenoxy-rich secondary phase was formed upon curing. It was found that the average Mode-I fracture toughness value, G_1c increased tenfold with only 10 wt.% (with regard to the total matrix content) phenoxy fibre added. Other properties such as Young’s modulus, tensile strength and thermal stability were not adversely affected. The mechanical and thermal properties of the neat epoxy–phenoxy blends were also studied for comparison.     

Morphometry of fibre reinforced cementitious materials

A spacing and geometric efficiency concept is developed that deviates from the available formulae by its foundation on geometric probability theory. The solutions can easily be extended to cover non-random structures as well. Two-dimensional structural information is presented by means of sections and projections (X-ray radiography). Counting of features or intersections with a superimposed test-line system can in both cases provide the necessary information for the determination of the fibre spacing. The spacing formulae are also transformed for design purposes, in which case they contain only geometric parameters and the volume fraction. The spacing concept comprises formulae for the mean free spacing and the average nearest neighbour distance in two- and three-dimensions. As such they can be applied to structure-insensitive and structure-sensitive properties (cracking), respectively, of fibre reinforced cementitious materials. A brief survey of experimental results is presented.     

Morphometry of fibre reinforced cementitious materials

A theoretical and methodological framework, based on geometric probability theory, is developed facilitating the analysis of inhomogeneity with respect to location (segregation) and orientation (anisometry) of non-random fibre structures. This article in particular focusses on partially planar-oriented structures, that can be conceived as being originated by the addition of two sub-sets, one of which is composed of randomly distributed fibres (3-D structure), while the second one only comprises fibres that are disposed randomly in planes (2-D structure). In such a way a simple solution can be obtained providing various procedures to completely and accurately investigate the morphological features of fibre distributions met with in practical situations. For opaque materials like FRC, in principal, sections as well as projections, obtained by X-ray radiography, can be subjected to methods for quantitative image analysis based on either counting of features or counting of intersections with a superimposed system of directed secants. Since methodological arguments militate against applying the directed secants technique to section analysis, the remaining three approaches have only been elaborated. In addition to measures for density and degree of orientation, concepts for orientation efficiency and spacing are presented. Analogously to Part I of this publication, this present one comprises formulae for the mean free spacing and the average nearest neighbour distances in two and three dimensions, applicable for design puroses or for experimental evaluation with either one of the three alternative methods described. Generally, two independent observations suffice to yield a complete solution. The techniques have been applied to an investigation of compaction-induced fibre inhomogeneity in (steel) FRC cubes and beams. In the latter case also fracture surfaces have been analysed. The significance of the results, revealing the occurrence of a considerable amount of segregation and a moderate increase of the anisometry towards the bottom side of the specimens, is extensively discussed. The structural implications of such results for design and materials testing are indicated.

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Résumé On établit un cadre théorique et méthodologique qui repose sur une théorie géométrique et probabiliste dans le but de faciliter l'étude de l'hétérogénéité tant du point de vue de la localisation (ségrégation) que de celui de l'orientation (anisométrie) des structures non aléatoires. Cet article est centré sur les structures partiellement orientées dans un plan que l'on peur concevoir comme résultant de l'addition de deux substructures, l'une composée de fibres à distribution aléatoire (structure 3-D) et l'autre qui ne comprend que des fibres à disposition aléatoire par plan (structure 2-D). De la sorte, une solution simple peut être trouvée qui permette une étude complète et précise par différents procédés des caractéristiques morphologiques des distributions de fibres que l'on rencontre en pratique. Dans le cas de matériaux opaques tels que le béton de fibres, on peut soumettre des sections du matériau aussi bien que des projections obtenues par radiographie X aux méthodes visuelles d'analyse quantitative qui consistent soit dans le comptage des sections des fibres soit dans le comptage des intersections avec un réseau de lignes superposées. étant donné que les arguments méthodologique s'opposent à l'application de la technique des sécantes dirigées à l'analyse par sections, on n'a donc élaboré que les trois autres procédures possibles. Les mesures de densité et de degré d'orientation sont ici complétées par les notions d'orientation effective et d'espacement. De même que dans la première partie de cette étude, on donne ici des formules relatives à la distance libre moyenne et à la distance moyenne à la fibre la plus proche dans des structures à deux et trois dimensions, formula que l'on peut appliquer à des fins constructives ou à une évaluation expérimentale effectuée avec l'une des trois méthodes signalées plus haut. En général, deux observations indépendantes suffisent à aboutir à une solution complète. Les techniques ont été appliquées à l'étude des hétérogénéités de fibres dues au compactage dans des cubes et des poutres de béton renforcé de fibres d'acier. Dans ce dernier cas, on a procédé aussi à l'étude des surfaces de rupture. L'interprétation des résultats, qui rèvèle des ségrégations nombreuses et un accroissement modéré de l'anisométrie dans la direction de la base des spécimens, fait l'objet d'un examen attentif. On indique les conséquences sur le plan structural que ces résultats peuvent avoir. On précise en quoi ces résultats intéressent la construction et les essais des matériaux.

     

Chemically-assisted fracture of thermoplastic PET reinforced with short E-glass fibre

The study of the stress-rupture lifetime of a PET/glass fibre system by means of fracture mechanics methods indicates the degradation of lifetime under an aggressive environment (10% HCl solution). The SEM-EDAX analysis reveals the depletion of calcium and aluminium elements from the fibre, and this is believed to be the cause of multiple fibre fractures. The fracture toughness is decreased because the role of fibre pull-out, which is the significant energy-consuming process, is negligible. A statistical analysis, from which the lifetime behaviour can be predicted, is carried out.