Fiber Reinforced Metal Matrix Composites Research Articles

Finite element modelling of frictional bridging during fatigue crack growth in fibre-reinforced metal matrix composites

In this study, the influence of thermal expansion mismatch, fibre debonding, interface friction and interface degradation on crack-tip shielding due to crack face bridging is investigated for fatigue of a Ti–6–4 matrix reinforced with unidirectional SiC-fibres using a three-dimensional finite element model of a compact tension specimen. The crack-tip stress intensity factors resulting from the residual stresses due to thermal expansion mismatch are calculated. For several levels of applied external loads, the crack-tip stress intensity factors are calculated for cyclic loading to assess the influence of friction on the build-up of crack-tip shielding during crack growth. The results show that fibre bridging, residual stresses and elastic mismatch produce crack-tip stress intensity factors that are widely different from the externally applied stress intensity factor. The numerical results show that an increase in the friction coefficient leads to a reduction of the crack-tip stress intensity factor, whereas thermal expansion mismatch leads to an increase of the crack-tip load. Finally, the degradation of the interface, modelled by a selective reduction of the friction coefficient, leads to a decrease in crack-tip shielding.

Stress–strain behavior in initial yield stage of short fiber reinforced metal matrix composite

Based on the law of mixture and the large strain axisymmetric elasto-plastic finite element method, the analytical expressions for the stress strain response of short fiber reinforced metal matrix composite were derived and a new method of defining the yield behavior of short fiber reinforced metal matrix composite was proposed. The effects of the material parameters (fiber volume fraction, fiber aspect ratio, fiber end distance and matrix strain hardening coefficient) on the deformation behavior of the composite were also investigated. It was demonstrated that there is a close relationship between the stress strain partition parameter and the deformation behavior of the composite. The effect of the material parameters on the initial yield behavior can be revealed well by this method. The predicted elastic modulus and yield stress are in good agreement with the experiments.

On the role of matrix creep in the high temperature deformation of short fiber reinforced aluminum alloys

The present study investigates creep of short fiber reinforced metal matrix composites (MMCs) which were produced by squeeze casting. Two types of aluminum alloys were used as matrix materials with class I (alloy type) and class II (metal type) creep behavior. The creep behavior of the resulting MMCs is similar both in terms of the shape of the individual creep curves as well as with respect to the stress dependence of the minimum creep rates. Their creep behavior can be rationalized on the basis of a coupling of elementary deformation processes. There is a stress transfer to the fibers by the formation of a work hardened zone. A recovery process in which dislocations move along the fiber to the fiber ends where they shrink and annihilate counteracts this strain hardening process. Once fibers and/or subfibers reach critical stresses they break; this results in multiple fiber breakage. It is the coupling of these elementary processes which governs creep of short fiber reinforced aluminum alloys. This microstructural creep scenario controls creep of short fiber reinforced aluminum alloys irrespective of whether the isolated matrix material shows class I or II creep behavior. The results of this study also show that fiber reinforcing results in a significant increase of creep strength. In this respect the matrix material is important because the creep strengths of the MMCs scale with the creep resistance of the matrix materials.

Rate-dependent constitutive modelling and micro-mechanical analysis of fibre-reinforced metal–matrix composites

A rate-dependent elastic–plastic constitutive model is presented which was implemented into a finite element code. As an illustration, this model is used in a micro-mechanical analysis of a fibre-reinforced metal matrix composite laminate. The matrix (a titanium alloy) is a rate-dependent material while the fibre behaves elastically. The residual stresses due to mismatch of the thermal expansion of the matrix and fibre are determined first. The three-dimensional finite element prediction of the representative volume element, which includes a debonding criterion, is found to be in good agreement with the experimental data in the linear and nonlinear regimes as well as the unloading part of the stress–strain curve. This success is attributed to the accurate representation of the matrix material properties by the rate-dependent constitutive model, and proper simulation of the fibre/matrix interface debonding.

Investigation of the double shear creep specimens to study the creep behaviour of short fibre metal matrix composites

Double shear creep specimens have been investigated by the transverse anisotropic finite element method, in order to be applied to study the multiaxial creep behaviour of short fibre-reinforced metal matrix composites (MMCs). The creep behaviour was dependent on orientations of the specimens and the transverse anisotropic parameters. The detailed evolution of the stress and strain in the double shear creep specimens shows that there is a steady stress state in the shear zone. The shear stress is near to the applied average shear stress, and independent of orientations of the specimens and the transverse anisotropicic parameters. The double shear specimen can provide a simple multiaxial stress state for MMCs. A procedure has been presented to evolve the parameters for the transversely anisotropic MMC materials, so that the double shear specimens can be tested to obtain the transversely anisotropic material properties, some of which are difficult to get by the usual tensile specimens.

Effects of interfacial debonding on the transverse loading behaviour of continuous fibre-reinforced metal matrix composites

The current study aims at investigating two issues seriously affecting the transverse loading behaviour of unidirectionally fibre-reinforced metal matrix composites: (1) the effect of thermally induced residual stresses due to a cooling down from processing temperature and (2) the interaction between inelastic matrix deformation and interfacial debonding at the fibre–matrix interface. The unit cell method in combination with the nonlinear finite element technique enhanced by a sophisticated viscoplastic material model and a cohesive zone model for the interface is employed to investigate these influences.

Scale-dependent bounds on effective elastoplastic response of random composites

We consider the mechanical response of random, heterogeneous materials, where each phase is elastic–plastic with an associated flow rule, and the microstructure's statistics is homogeneous and ergodic. Under proportional monotonic loading, the effective (in the macroscopic sense, or overall) elastoplastic response is shown to be bounded from above and below by those obtained, respectively, from displacement and traction boundary conditions applied to finite size domains (square shaped windows). A scale dependent hierarchy of these bounds is obtained by extending the methods used earlier for the elastic moduli estimation: the larger the scale relative to the heterogeneity, the closer are the bounds. A fiber reinforced metal matrix composite is employed to illustrate the theoretical results. Its constitutive response and plastic strain field are investigated by computational micromechanics for different window sizes under both types of boundary conditions; it is found here that the displacement conditions result in denser and more uniformly distributed slip band patterns, while the traction conditions lead to more localized fields. We also investigate a mixed boundary condition, under which the mechanical response of composite is found to fall between those under displacement and traction controlled boundary conditions.

Elevated Temperature Fatigue of Continuous Fiber Reinforced Ti-alloy Matrix Composites SCS-6/SP700(Composite 1)

Continuous fiber reinforced metal matrix composites (MMCs) have been researched and developed because of both high specific strength and high temperature capability and various trial high temperature structural components have been successfully fabricated in the field of aeronautics, aerospace and power generation industries. However, there is the limited number of research for long-term durability performance at practical use temperature ranges in comparison with the static strength characteristics. The final goal of this research is to establish the fatigue damage tolerance design concepts for continuous fiber partially reinforced Ti-alloy matrix composite (TMCs) rotating parts in aircraft engines. This paper summarizes the elevated temperature low cycle fatigue behavior for Ti-alloy matrix composites.

Cellular arrays of alumina fibres

In conventional short fibre reinforced metal matrix composites, the quest is for a method of processing that will provide a homogeneous and preferably random arrangement of fibres. In contrast, recently developed contiguity models for multiphase composites on the one hand, and finite element modelling of structures on the other, independently predict that the modulus enhancement provided by short-fibre reinforcement can be improved if the fibres are arranged in a cellular structure. Furthermore, provided the metallic phase is continuous, the toughness of the composite may also thereby be enhanced. This paper, which is part of an attempt to explore the question of reinforcement arrangements, presents a method for making ceramic preforms for MMCs in which a polymeric foam is used to position the fibres in cellular array. The polymer is then removed by pyrolysis and the preform of fibres is strengthened by sintering. During high temperature sintering, phase changes and grain growth degraded the fibre. Methods of increasing the compressive strength of the preform by incorporation of alumina particles and by subsequent infiltration are described and compared.

A 3-d micromechanical model of cyclic plasticity in a fiber-reinforced metal matrix composite

A 3-d micromechanical model of cyclic plasticity in a fiber-reinforced metal matrix composite

Single crystal oxide and oxide/oxide eutectic fibres for high temperature composites

The utilisation of fibre-reinforced metal, intermetallic and ceramic matrix composites (CMCs) in gas turbine engines offers the potential of improved fuel efficiency, higher operating temperature and greater thrust to weight ratio. The development of ceramic fibres with high strength, excellent strength retention at elevated temperatures and good creep resistance is essential to the successful implementation of composites in various high temperature components. Several single crystal oxide and oxide/oxide eutectic fibres have been developed to meet the demanding performance requirements. Recent progress made on developing these fibers will be discussed.

Investigation of the double shear creep specimens to study the creep behaviour of short fibre metal matrix composites

Double shear creep specimens have been investigated by the transverse anisotropic finite element method, in order to be applied to study the multiaxial creep behaviour of short fibre-reinforced metal matrix composites (MMCs). The creep behaviour was dependent on orientations of the specimens and the transverse anisotropic parameters. The detailed evolution of the stress and strain in the double shear creep specimens shows that there is a steady stress state in the shear zone. The shear stress is near to the applied average shear stress, and independent of orientations of the specimens and the transverse anisotropicic parameters. The double shear specimen can provide a simple multiaxial stress state for MMCs. A procedure has been presented to evolve the parameters for the transversely anisotropic MMC materials, so that the double shear specimens can be tested to obtain the transversely anisotropic material properties, some of which are difficult to get by the usual tensile specimens.

Cyclic deformation of tungsten monofilament-reinforced multicrystalline copper composites

In an attempt to understand the cyclic deformation behavior of continuous fiber reinforced metal matrix composites, plastic strain controlled tests have been performed on tungsten monofilament-reinforced, multicrystalline, copper composites. The cyclic hardening response of the composites greatly depends on the fatigue dislocation structures corresponding to the strain amplitude. For example, at high strain amplitude, i.e. 1×10 −3, secondary slip stimulated by the self-stresses of the primary dislocations becomes more active, and secondary hardening even occurs during saturation. At low strains, loop patches form and are associated with fine slip. At intermediate strains, persistent slip bands occur, but their distribution is altered by the presence of the fiber. The paper introduces a simple model to link the cyclic stress–strain response of the multicrystalline composites to those of monolithic single crystals and fibers. This model not only represents the fiber reinforcement by the rule of mixtures, but also adopts the Sachs model for the single crystal–polycrystal conversion factor. The results calculated by the model show very good agreement with the experimental data in all strain amplitudes at which the composites were fatigued. This encouraging outcome suggests that the new model could be applied to high-cycle fatigue of commercial continuous-fiber-reinforced polycrystalline metal matrix composites.

Property gradation for modification of response of rotating MMC discs

Aerospace and industrial gas turbine engine components such as discs, blades, and propeller shafts for which long fibre reinforced metal matrix composites are being made, operate under complex mechanical and thermal loading conditions. A major aim of functionally graded materials application is to optimise component response through appropriately tailored microstructures. This paper explores the influences of property gradation, centrifugal body force loading, and thermal loading on stresses in rotating discs. The discs were modelled as non-homogeneous orthotropic materials such as those obtained through non-uniform reinforcement of a metal matrix by long fibres. The results show how different temperature change distribution patterns and property gradation types correlate with hoop stresses developed in the disc.

A Constitutive Model for Distributed Microcracking In Titanium Matrix Composite Laminae

The ply constitutive relations of an energy-based model for distributed microcracking in continuous fiber reinforced metal matrix composites are presented. Simple models for damage progression are used in conjunction with these damage dependent constitutive relations to allow the prediction of fatigue life under constant amplitude, isothermal conditions. The microcracking model is discussed in the context of fatigue analysis of unnotched titanium matrix composite laminates. Analytical results generated by using this model in a unidirectional laminate analysis algorithm are presented and compared with experimental results. [S0094-4289(00)03104-2]

Deformation behavior of model MMC scarf joints

The mechanical response of metal interlayers constrained between two fiber-reinforced MMC (metal–matrix composites) sub-elements was investigated. The fibers were polycrystalline Al 2O 3, discontinuous at the joint, embedded in Al–4.5Mg, that was continuous through the joint and constituted both the composite matrix and the interlayer material. The specimens were produced by pressurized melt infiltration and comprised interlayers oriented from 0 to 75° relative to the plane normal to the fiber axis. Analytical and finite element models were developed and compared with experimental results in order to elucidate the deformation behavior of these interlayers and to identify the key factors controlling joint performance. Two predominant regimes were identified in this manner. Joints in the intermediate angle range (45±15°) exhibit limit load behavior and develop large strains prior to failure, whereas joints at lower (≤15°) and higher (75°) angles show only modest plasticity. The models suggest that the latter group could, in principle, develop large strains and limit loads, but this behavior is precluded by the intervention of failure owing to debonding in the lower angle joints, and to composite fracture in the highest angle ones. The analysis further reveals that the constitutive behavior of the metal in the interlayer exhibits substantial hardening relative to that of the monolithic matrix processed in the same manner. The reasons for this behavior are discussed. The models set the stage for future work to elucidating failure criteria for interlayers in these joint configurations.

Residual stress measurements in an SiC continuous fiber reinforced Ti matrix composite

During the fabrication of ceramic fiber reinforced metal matrix composites mismatch stresses will be introduced due to differences in thermal expansion coefficients between the matrix and the fibers. Calculations, based on a coaxial cylinder model, [1 and 2] predict that, for a Ti matrix SiC continuous fiber composite, the axial stress in the matrix parallel to the fibers and hoop stress around the fibers are tensile while the radial stress perpendicular to the fiber surface is compressive. Saigal et al. [3] used neutron diffraction to measure residual thermal stresses in an SiC continuous fiber Ti-14Al-21Nb matrix composite. In that paper the two principal matrix stresses in the plane of the cross-section of the fibers are supposed to be approximately equal. X-ray and neutron diffraction measured values for the residual stress parallel to the fibers in the matrix of SiC-Ti alloy matrix composites are reported by James et al. [4]. It was found that for composites with more than 6 plies the axial surface stress measured by X-ray diffraction matches that determined by neutron diffraction and therefore represents the axial stress in the bulk of the matrix. Stresses in the plane of the cross section of the fibers were not measured. The present investigation reports about residual stress measurements on the matrix of a continuous SiC fiber-Ti 1100 matrix composite sheet with a thickness of 1.25 mm, using a high resolution neutron diffraction facility and X-ray diffraction. In this investigation we start from a triaxial stress state in the bulk of the matrix, with three different principal components, as measured with neutron diffraction. The neutron diffraction results are compared with those of the X-ray diffraction measurements with which, because of the low penetration depth of the X-rays, a biaxial stress state at the surface is measured. Also investigated was whether any relation could be observed between the predicted stress distribution around the fibers [1 and 2] and the diffraction line broadening measured with neutron diffraction.

Role of interfacial and matrix creep during thermal cycling of continuous fiber reinforced metal–matrix composites

A uni-dimensional micro-mechanical model for thermal cycling of continuous fiber reinforced metal–matrix composites is developed. The model treats the fiber and matrix as thermo-elastic and thermo-elasto-plastic-creeping solids, respectively, and allows the operation of multiple matrix creep mechanisms at various stages of deformation through the use of unified creep laws. It also incorporates the effect of interfacial sliding by an interface-diffusion-controlled diffusional creep mechanism proposed earlier (Funn and Dutta, Acta mater., 1999, 47, 149). The results of thermal cycling simulations based on a graphite fiber reinforced pure aluminum–matrix composite were compared with experimental data on a P100 graphite–6061 Al composite. The model successfully captured all the important features of the observed strain responses of the composite for different experimental conditions, such as the observed heating/cooling rate dependence, strain hysteresis, residual permanent strain at the end of a cycle, as well as both intrusion and protrusion of the fiber-ends relative to the matrix at the completion of cycling. The analysis showed that the dominant deformation mechanism operative in the matrix changes continually during thermal cycling due to continuous stress and temperature revision. Based on these results, a framework for the construction of a transient deformation mechanism map for thermal excursions of continuous fiber composites is proposed.

Finite element micromechanical modelling of yield and collapse behaviour of metal matrix composites

The initial yield and collapse behaviour of fibre reinforced metal matrix composites (MMCs) have been investigated using finite element micro-mechanical models. Initial yield occurs as the loading on the MMC is increased until the most heavily loaded point within the matrix reaches the yield stress. Collapse occurs when the MMC is unable to support a higher load. The results of this work show that loads to cause collapse of MMCs are higher than those to cause first yield, particularly when the effect of residual stress arising from manufacture is included in the analysis. Initial yield and collapse envelopes have been generated for a Silicon Carbide-Titanium MMC for biaxial and shear loading. These envelopes include the effect of residual stress and also various interface conditions between the fibre and matrix: either perfectly bonded or de-bonded, with and without friction. An analytical micro-mechanical model has been developed using the method of cells to predict the collapse behaviour. The results of the analytical model compare reasonably well with those of the finite element method. Using the analytical model the effect of varying the fibre volume fraction on the collapse behaviour has been studied.

Failure Mechanism of Metal Matrix Composites Subject to Transverse Loading

Mechanical behaviors of uniaxially fiber-reinforced metal matrix composites under transverse loading conditions were studied at room and elevated temperatures. A mono-filament composite was selecte d as a representative analysis model with perfectly bonded fiber/matrix interface assumption. The elastic-plastic and visco-plastic models were investigated by both theoretical and numerical methods. The product of triaxiality factor and effective strain as well as stress components and strain energy was obtained as a function of location to estimate the failure sites in fiber-reinforced metal matrix composite. Results showed that fiber/ matrix interfacial debond plays a key role for local failure at the room temperature, while void creation and growth in addition to the interfacial debond are major concerns at the elevated temperature. It was also shown that there would be an optimal diameter of fiber for the strong fiber-reinforced metal matrix composite.