Simple model for consolidation of metal matrix coated SiC fibre composites

Abstract

A simple approach to modelling the consolidation of matrix coated fibre composites is presented. It employs an existing porous material constitutive model for monolithic materials. It is argued that in the consolidation of metal coated SiC fibres, the deformation primarily occurs in an outer layer of the fibre coating, and the internal core remains undeformed, largely because of the generally hydrostatic compressive loading, and because of the incompressible nature of the material in creep. The consolidation process is therefore not vastly different to that occurs for monolithic metal fibres, and similar equations can therefore be used for the composite consolidation. The constitutive equations have been implemented into general purpose non-linear finite element software within a large deformation formulation by means of two different user subroutines, one providing a general implementation, and the other a cpu time efficient approach. The manufacture and testing of SiC continuous fibre, Ti-6Al-4V metal matrix composite specimens is described and the results of the tests compared with the model calculations, showing that good agreement can be achieved with a simple model. The dependence of volume fraction of fibres and temperature can be introduced empirically through the specification of just two material constants. The model is therefore useful in the development of consolidation processes.     

Transverse tensile behaviour of fibre reinforced titanium metal matrix composites

Four Ti MMCs have been tested in transverse tension, at ambient temperature and 600 °C. Generally, mechanical properties are reduced compared to monolithic Ti alloys. Transverse Young's modulus is, however, higher than in monolithic alloys, as a result of constraint of the matrix by the fibres.MMC proportional limits are associated with the onset of interfacial failure. Fibre coating cracking and longitudinal fibre splitting may also contribute to MMC yield and the associated acoustic emission peak. The fibre/matrix interface in IMI 834/SM1140+ appears to be weaker than in the other MMCs, resulting in a lower proportional limit and less acoustic emission. Final failure of the MMCs is generally via ductile shearing of matrix ligaments. The exception to this is IMI 834/SM1140+ in which the matrix fails in a brittle manner. This causes poor transverse tensile strength and failure strain in this MMC.A model to predict the MMC proportional limit, previously proposed by Jansson et al., has been modified to take account of the tensile strength of the fibre/matrix interface. The model previously used by Jansson et al. to predict the transverse tensile strength is acceptably accurate provided that the area fraction of matrix appearing on fracture surfaces is accurately determined.     

Fatigue damage evolution in SiC fiber-reinforced Ti-15-3 alloy matrix composite

Tension-tension fatigue damage behavior of an unnotched SiC (SCS-6) fiber-reinforced Ti-15-3 alloy matrix composite at room temperature was examined, applying maximum stresses of 450, 670 and 880 MPa with R = 0.1. The change in stress-strain hysteresis curves was measured. Fiber fracture behavior and matrix cracking behavior were observed in situ and the results were compared with the change of unloading modulus obtained from the hysteresis curves. The fiber fracture behavior inside the specimen was also determined by dissolving the Ti alloy matrix. The results showed abrupt reductions in the unloading modulus of the composite at stresses of 450, 670 and 880 MPa; the normalized unloading modulus decreased by 8%, 12% and 17%, respectively, in the initial stage (N 10 cycles). This reduction was caused by the multiple fiber fragmentation. Thereafter, the unloading modulus maintained a nearly constant value; and non-propagating matrix cracks were initiated adjacent to the end of fractured fiber. The propagation of the matrix crack again led to a rapid reduction of the unloading modulus, and the composite then failed. With higher applied stress, the fatigue life was reduced. The fracture behavior of the composite was discussed with special attention to the fiber fracture behavior and its effect on the modulus of the composite.     

Fatigue crack growth behaviour of Ti-6 Al-4 V metal matrix/continuous SiC and B4C/B fibre composites

The fatigue crack growth (FCG) behaviour of SiC and B₄C/B reinforced Ti-6 Al-4 V metal matrix composites loaded in the transverse direction as a function of modifications of the interface between the fibre and matrix was studied. The interface chemistry, modified by sulphur diffusion during thermal cycling treatment, changed the FCG in air, dry nitrogen and hydrogen environments when compared with the as-received specimens. The FCG rates tend to be higher in a humid environment. The SEM fractrography indicates that the FCG in humid air was by an increased amount of fibre splitting. The FCG in dry nitrogen environment was more often by interface debonding with some fibre splitting and fiber fracture. The FCG rates in dry hydrogen for both as-received and heat-treated specimens were intermediate between the observed rates for dry nitrogen and humid air. During FCG in laboratory air, the sulphur-enriched interface of the specimens thermal cycled in a sulphur environment reacts with the humidity in air to degrade the interface cohesion, resulting in complete separation of the interface from the matrix and the fibre at low strains. This inability of the interface to sustain any strain further increases the FCG rates in the matrix. The results show that the interface does transfer load during fatigue cycling either in an inert environment or if the interface has a minimal amount of impurities.     

Fatigue behaviour of continuous glass fibre reinforced composites

A study has been undertaken of fatigue in glass fibre reinforced composites. Two matrix resins were tested: a standard polyester and a polyurethane-vinyl-ester, which was designed to have a higher toughness. Three different types of glass fibre fabrics were used for reinforcement: a conventional woven roving and two stitch-bonded cloths. The glass cloths were combined into various lay-ups, in order to consider the effects of matrix, cloth and lay-up on the fatigue strength. Additionally, a study was undertaken to evaluate the micromechanisms that occurred during fatigue and how damage accumulated throughout the sample lifetime. This involved measuring stiffness changes during fatigue cycling, followed by microscopic study of the samples. It was found that similar damage micromechanisms occurred in each lay-up regardless of resin and cloth type, and these included matrix cracking, delamination and fibre breakage. However, differences were observed in the extent, location and rate of damage, and these were consistent with the variations seen in the fatigue strengths.     

Fatigue performance of alumina reinforced metal matrix composites

Abstract

The room temperature fatigue performance of two Saffil reinforced metal matrix composites manufactured by squeeze forming is assessed. For the composite with an LM 13 matrix, introduction of Saffil does not result in an increase in the ultimate tensile strength, and the fatigue performance is inferior to the unreinforced alloy. By contrast, the composite with a 6082 type matrix exhibits a markedly superior ultimate tensile strength and stiffness compared with the unreinforced equivalent and this is coupled with an improved overall fatigue performance.

MST/767     

Investigation of impact behaviour of aluminium based SiC particle reinforced metal–matrix composites

In this paper, the impact behaviour of aluminium and silicon carbide (SiC) particle reinforced aluminium matrix composites under different temperature conditions was determined. Charpy impact tests were performed on as extruded and heat treated specimen at temperatures varying from −176 to 300 °C. Composite specimens based on aluminium alloys of 2124, 5083 and 6063 and reinforced by SiC particles were manufactured. Two different SiC sizes of 157 μm and 511 μm and two different extrusion ratios of 13.63:1 and 19.63:1 were used. The results of instrumented impact tests were compared with the microstructural and fractographic observations. The failure mechanisms and deformation behaviour of unreinforced alloys and composites were assessed. The impact behaviour of composites was affected by clustering of particles, particle cracking and weak matrix-reinforcement bonding. Agglomeration of particles reduced the impact strength of Al 2124 and 6063 based composites. Alumınum 6063 alloys and composites showed a better impact strength. The impact strength of 6063 composites increased with particle size and extrusion ratio. The effects of the test temperature on the impact behaviour of all materials were not very significant.     

Fatigue behaviour in hybrid hollow microspheres/fibre reinforced composites

This article presents the results of a current study concerning the influence of the addition of short fibres on the fatigue behaviour of syntactic foams. The material was obtained by vacuum-assisted resin transfer moulding adding hollow glass microspheres to an epoxy resin acting as binding matrix. Specimens with microsphere contents up to 50% and fibre reinforcement up to 1.2% in volume were tested at three-point bending at room temperature. Foams show significantly lower static and fatigue strength than an epoxy matrix. A significant decrease in the absolute strength with filler increase was observed, and even specific strength decreases for low filler contents and is nearly constant for the higher filler contents. Fatigue strength also decreases with the increase in filler content. The addition of glass fibre reinforcement produces only a slight improvement in flexure strength, while the addition of carbon fibres promotes an important improvement; a hybrid composite containing 0.9% carbon fibre is about 30% stronger than unreinforced foams. An improvement in fatigue strength more than 30% was obtained by the addition of small percentages of glass or carbon fibre.     

Interfacial characterization of Nicalon SiC fibre reinforced oxynitride glass matrix composites

Nicalon SiC and Hi-Nicalon SiC fibre oxynitride glass and glass–ceramic composites were prepared and the interface between the fibres and matrix characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) spectroscopy. It was found that the formation and thicknesses of interfacial layers were primarily determined by the type of fibre reinforcement, but the role of these interfaces in influencing composite properties was dependent on the thermal properties of the matrix. For Nicalon SiC composites, the carbon-rich layer did not promote fibre debonding and toughening unless the matrix had a smaller thermal expansion coefficient than the fibres. For Hi-Nicalon SiC composites, the absence of oxygen in the fibre significantly encouraged chemical reaction between fibre and matrix, resulting in no strengthening or toughening. This revised version was published online in November 2006 with corrections to the Cover Date.     

Application of model for work hardening behaviour of SiC reinforced magnesium based metal matrix composites

Abstract

An understanding of the work hardening behaviour of particulate reinforced metal matrix composites is crucial in optimising the parameters for deformation processing of these materials. In the present study, SiC reinforced magnesium metal matrix composites were produced using a liquid phase process. The microstructure of the composite was characterised and the mechanical properties were determined. The results of the ambient temperature tensile testing on the extruded Mg and Mg/SiC specimens revealed that an increase in the weight percentage of SiC particulates in pure magnesium increases the elastic modulus, does not affect the 0·2% yield strength, and reduces the ultimate tensile strength and ductility. A modified continuum model was applied to relate the work hardening behaviour of the composites to microstructural parameters and to predict the fracture strain of the composites. The model is shown to predict the fracture strain of the composites quite accurately for all the three weight fractions of reinforcements evaluated in the present study.     

Joining of SiC fibre reinforced borosilicate glass matrix composites to molybdenum by metal and silicate brazing

The brazing of SiC fibre reinforced borosilicate glass matrix composites with Mo plates has been investigated. Molybdenum was chosen as the metallic partner under consideration of system requirements, e.g. thermomechanical stability at temperatures of interest (500–750^∘C), and physical properties, e.g. coefficient of thermal expansion close to that of the glass matrix composite. Two brazing filler materials were investigated: a glass braze (Schott G018-174) and an active filler metal (Incusil ABA, brazing temperature = 740^∘C). When using the glass braze the surface of the metal had to be roughened to ensure a bond of significant strength. Vacuum brazing with the active filler metal resulted in joints with high strength, which allows to fully utilise the mechanical competence of the glass matrix composite when the joint configuration is adapted to the relevant loading conditions. A novel design of a tool for hot glassware handling, made of glass matrix composite/Mo joints, is presented.     

Fatigue behaviour of glass fibre reinforced epoxy composites enhanced with nanoparticles

Nanoparticle reinforcement of the matrix in laminates has been recently explored to improve mechanical properties, particularly the interlaminar strength. This study analyses the fatigue behaviour of nanoclay and multiwalled carbon nanotubes enhanced glass/epoxy laminates. The matrix used was the epoxy resin Biresin® CR120, combined with the hardener CH120-3. Multiwalled carbon nanotubes (MWCNTs) 98% and organo-montmorillonite Nanomer I30 E nanoclay were used. Composites plates were manufactured by moulding in vacuum. Fatigue tests were performed under constant amplitude, both under tension–tension and three points bending loadings. The fatigue results show that composites with small amounts of nanoparticles addition into the matrix have bending fatigue strength similar to the obtained for the neat glass fibre reinforced epoxy matrix composite. On the contrary, for higher percentages of nanoclays or carbon nanotubes addition the fatigue strength tend to decrease caused by poor nanoparticles dispersion and formation of agglomerates. Tensile fatigue strength is only marginally affected by the addition of small amount of particles. The fatigue ratio in tension–tension loading increases with the addition of nanoclays and multi-walled carbon nanotubes, suggesting that both nanoparticles can act as barriers to fatigue crack propagation.     

Generalised constitutive equations for densification of metal matrix coated fibre composites

Abstract

The present paper completes a study of constitutive equations for the consolidation processing of continuous fibre reinforced metal matrix composite materials. It builds on an earlier paper in which physically based constitutive equations were derived for the case of symmetrical, isostatic loading. In the present paper, constitutive equations are developed for in plane, general stress states. The total deformation of the consolidating composite is expressed as the sum of a conventional deviatoric creep term, together with a dilatational term, which was derived using a variational method previously published. The equations contain only two material parameters, which are the conventional creep coefficient and exponent for the fibre coating material (in this case, Ti-6Al-4V). The resulting equations have been implemented into finite element software enabling the simulation of practical consolidation processes. The model has been verified by comparing predicted results with those obtained from independent micromechanical models. A number of experimental tests have been carried out, and the model is used to predict the rates of densification for a range of experimental pressure and temperature histories. Good comparisons have been achieved.     

Low cycle fatigue behaviour of particulate reinforced metal matrix composites

The low cycle fatigue (LCF) resistance of two different 6061 Al/20 vol% alumina particulate metal matrix composites (MMCs) in a peaked-aged condition has been evaluated under fully reversed strain control testing. Test results were combined with scanning electron and optical microscopy investigations to determine the effects of reinforcement particles and strain amplitude on the LCF behaviour of these MMCs. Both materials show three stages of response to LCF: initial fast hardening or softening in the first few cycles; gradual softening for most of the fatigue life; and a rapid drop in the stress carrying capability prior to failure. Both MMCs exhibit short LCF life which follows a Coffin-Manson relationship. All tested specimens demonstrate ductile fracture morphology at final failure. The experimental results are discussed in respect of strain amplitude, matrix composition and reinforcement shape and crack initiation.     

Lifetime behaviour of unidirectionally wire reinforced lightweight metal matrix composites

In the field of lightweight construction for transportation means, hybrid structures composed of high-strength and low-density materials exhibit a high application potential. The current work shows that a spring steel wire reinforcement content of 11.1 vol.% leads to a significant increase in lifetime of the magnesium alloy AZ31 under stress controlled fatigue loading. Based on the knowledge of the quasi-static behaviour of the single components and the fatigue behaviour of the single matrix material and on the understanding of mechanisms and mechanics of constituent deformation and damage evolution a new model is deduced from the quantitative strain response in order to predict the lifetime for different unidirectionally reinforced material systems.     

Fatigue behaviour of aluminium matrix composites reinforced with continuous alumina fibres

Abstract

Alumina (N610) reinforced pure Al (A9) or Al–2 wt-%Cu alloy (AU2) unidirectional (UD) composites which combine a highly ductile matrix with a strong interface bonding, present high static and dynamic mechanical performances. In the present paper, the fatigue behaviour of quasi­UD N610+A9 and N610+AU2 composites is investigated. Notwithstanding the presence of the transverse bundles, the longitudinal behaviour in tension and fatigue of these quasi-UD composites is nearly equivalent to that of the pure UD. Only in the case of the N610+AU2 quasi-UD composite, the fatigue limit is about 30% lower than that of the pure UD. Acoustic emission (AE) monitoring correlated with microfractography and microstuctural examinations has enabled identification of different stages in the evolution of damage and the association of these with damage and failure mechanisms. In fatigue, three main damage mechanisms are activated in a sequential and/or superposed mode during a three stage evolution.     

Fatigue of metal matrix composites

One unidirectional and two laminated 6061-0 A-B composite plates were tested under various cyclic loading conditions. Three types of material response to cyclic loading were identified; No evidence of damage at relatively low cyclic loads, damage accumulation caused primarily by growth of long matrix cracks parallel to the fibers in off-axis layers at higher loads, and sudden localized failure of the fibers. Quantitative analysis of the results shows that the extent of internal damage, demonstrated by a reduction in axial elastic modulus, depends on the applied stress range and is independent of mean stress. The stress range at which damage first starts to appear coincides with the shakedown range of the laminate.

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Résumé On a testé sous des conditions de contrainte cyclique variable des plaques constituées d'une part par des alliages 6061-0 A-B unidirectionnels et d'autre part du même alliage composite bilaminé. On a identifié trois types de réponse du matériau aux contraintes cycliques, à savoir la non-évidence d'un dommage pour des cycles de charge à faible amplitude relative, une accumulation du dommage causé principalement par la croissance de longues fissures matricielles parallèles aux fibres dans les couches éloignées du plan médian à des contraintes plus élevées, et une fissure soudainement localisée des fibres. L'analyse quantitative des résultats montre que 1'extension du dommage interne telle qu'elle est dóntree par une réduction du module axial d'élasticité, dépend de l'amplitude des contraintes appliquées et est indéndante de la contrainte moyenne. L'amplitude de contrainte auquelle un dommage commence à apparaître coíncide avec l'amplitude de l'adaptation plastique du laminé telle qu'établie par le théorème de Melan.

     

The bending fatigue behaviour of unidirectionally reinforced SCS6/Ti-15-3

The fatigue behaviour of a commercial, unidirectionally reinforced metal-matrix composite, Textron SCS6/Ti-15-3, has been investigated. Tests were conducted on plain unnotched specimens in pure bending at room temperature, and the growth of freely initiating cracks was monitored using acetate replication. The mechanisms of crack growth in these materials are outlined and a simple model is presented to explain the failure mechanics of the composite. This is used to estimate the fatigue limit of the composite material from basic fibre and matrix properties.