Short Fiber Reinforced Metal Matrix Research Articles

Modeling the temperature-dependent Young’s modulus of short fiber reinforced metal matrix composites and its particle hybrid composites

Modeling the temperature-dependent Young’s modulus of short fiber reinforced metal matrix composites and its particle hybrid composites

Temperature dependent ultimate tensile strength model for short fiber reinforced metal matrix composites

In this paper, considering the combined influences of fiber strength and various strengthening mechanisms including load transmitting, dispersion strengthening, residual thermal stress and dislocation strengthening, and their evolution with temperature, a temperature dependent ultimate tensile strength theoretical model of short fiber reinforced metal matrix composites is established. The model is confirmed by comparing with the available experimental results of six different short fiber reinforced metal matrix composites. Then, based on the model we developed, the contribution of various strengthening mechanisms to the ultimate tensile strength of the composites with the evolution of temperature is analyzed. Besides, the quantitative influences of Young’s modulus of fiber and matrix on the ultimate tensile strength evolution with temperature are discussed. Several opinions on the design of short fiber reinforced metal matrix composites are put forward, especially at high temperatures.

Cavitation instabilities between fibres in a metal matrix composite

Short fibre reinforced metal matrix composites (MMC) are studied here to investigate the possibility that a cavitation instability can develop in the metal matrix. The high stress levels needed for a cavitation instability may occur in metal–ceramic systems due to the constraint on plastic flow induced by bonding to the ceramics that only show elastic deformation. In an MMC the stress state in the metal matrix is highly non-uniform, varying between regions where shear stresses are dominant and regions where hydrostatic tension is strong. An Al–SiC whisker composite with a periodic pattern of transversely staggered fibres is here modelled by using an axisymmetric cell model analysis. First the critical stress level is determined for a cavitation instability in an infinite solid made of the Al matrix material. By studying composites with different distributions and aspect ratios of the fibres it is shown that regions between fibre ends may develop hydrostatic tensile stresses high enough to exceed the critical level for a cavitation instability. For cases where a void is located in such regions it is shown that unstable cavity growth develops when the void is initially much smaller than the highly stressed region of the material.

Modified shear lag theory based fatigue crack growth life prediction model for short-fiber reinforced metal matrix composites

Closed form expressions for the low cycle and high cycle fatigue crack growth lives have been derived for the randomly-planar oriented short-fiber reinforced metal matrix composites under the total strain-controlled conditions. The modeling was based on fatigue-fracture mechanics theory under both the small scale and the large scale yielding conditions. The modified shear lag theory was considered to describe the effect of yielding strength. The present model is essentially a crack growth model because crack initiation period in short fiber reinforced metal matrix composite is much shorter; hence, not assumed to play a dominant role in the calculation of fatigue crack growth life. The effects of short-fiber volume fraction (Vf), cyclic strain hardening exponent (n′) and cyclic strain hardening coefficient (K′) on the fatigue crack propagation life are analyzed for aluminum based SFMMCs at different levels of cyclic plastic strain values. It is observed that the influence of fatigue crack growth resistance increases with increase in cyclic strain hardening exponent (n′) and decreases when volume fraction (Vf) or cyclic strain hardening coefficient (K′) increases. The present MSL theory based fatigue crack growth life prediction model is an alternative of modified rule of mixture and strengthening factor models. The predicted fatigue life for SFMMC shows good agreement with the experimental data for the low cycle and high cycle fatigue applications.

Fatigue life modeling of short fiber reinforced metal matrix composites using mechanical and acoustic emission responses

The cyclic fatigue behavior of short fiber reinforced metal matrix composites was studied. Three fatigue life prediction models were developed by monitoring the resultant maximum strain and counts of acoustic emission during cyclic fatigue. Their increasing trends with the number of fatigue cycles were studied using the assumption that they can be expressed as a power-law function of the fatigue cycle number. Post-fatigue static tension tests were conducted to examine the degradation of the damaged materials. The acoustic emission count accumulated during tension testing decreased as the applied fatigue cycles increased; this change was also used to formulate life prediction models. All the new models show a better agreement to experimental data than do the existing equations.

Effect of Short Fiber Reinforcement on the Fracture Toughness of Metal Matrix Composites

Evaluation of fracture toughness of short fiber reinforced metal matrix composites (MMCs) becomes important for their application as structural materials. Therefore, in this study static and dynamic fracture toughness of MMCs manufactured by squeeze casting process were investigated. A number of MMCs were tested with various matrix alloy, volume fractions and specifically types of reinforcements. It was found that static and dynamic fracture toughness of MMCs was remarkably decreased by the addition of ceramic reinforcements. Compared with static fracture toughness, the mean cause of the decrease difference of dynamic fracture toughness and notch dynamic fracture toughness is due to the effect of dynamic velocity under impact loading. The toughness of ceramic reinforced MMCs is controlled by a complex interaction between the matrix alloy and reinforcement. Important properties which influence toughness include the type of reinforcement (appearance, size), volume fraction and combination of reinforcement and the matrix alloy. Notch fracture toughness of MMCs for simple evaluation was also discussed.

In Situ Measurements of Local Strain in Heterogeneous Materials

In this study, the local deformation of an Al-7wt.% Si alloy, a 10 vol.% particle reinforced and 20 vol% short fibre reinforced metal matrix composites is studied. The strain field at the surface of the materials during tensile testing inside a scanning electron microscope is determined by image analysis and compared with FEM simulations.

Effect of interfacial bonding on uniaxial ratchetting of SiC P/6061Al composites: Finite element analysis with 2-D and 3-D unit cells

The effect of interfacial bonding state between particulate and metal matrix on the ratchetting of SiC particulate reinforced 6061Al alloy composites (i.e., SiC P/6061Al composites) in uniaxial cyclic stressing at room temperature was discussed by numerical simulation using a finite element code ABAQUS. In simulation, axi-symmetrical two-dimensional (i.e., 2-D) mono-particle and three-dimensional (i.e., 3-D) multi-particle unit cells were employed, respectively. The ratchetting of un-reinforced matrix alloy was addressed by an elasto-plastic constitutive model with nonlinear kinematic hardening rule, and the particulates were assumed as elastic. Two kinds of interfacial layers were inserted into the unit cells between particulate and matrix, i.e., elastic and elasto-plastic interfacial layers previously employed by Kang et al. [G.Z. Kang, Q. Gao, Composites A 33 (2002) 657–667] in the numerical simulation for monotonic tensile behavior of short fiber reinforced metal matrix composites. Finally, the ratchetting of T6-treated SiC p/6061Al composites was simulated by using 3-D multi-particle unit cell and suitable interfacial property parameters. It is shown that the simulated results are closer to the corresponding experiments than those obtained with assumption of perfect interface. Simultaneously, some microscopic deformation features in the composites and their evolution rules were also revealed by the numerical simulation. Some significant conclusions are obtained, which are useful to establish a constitutive model to describe the ratchetting of the composites.

Load transfer in short fibre reinforced metal matrix composites

The internal load transfer and the deformation behaviour of aluminium–matrix composites reinforced with 2D-random alumina (Saffil ®) short fibres was studied for different loading modes. The evolution of stress in the metallic matrix was measured by neutron diffraction during in situ uniaxial deformation tests. Tensile and compressive tests were performed with loading axis parallel or perpendicular to the 2D-reinforcement plane. The fibre stresses were computed based on force equilibrium considerations. The results are discussed in light of a model recently established by the co-authors for composites with visco-plastic matrix behaviour and extended to the case of plastic deformation in the present study. Based on that model, the evolution of internal stresses and the macroscopic stress–strain were simulated. Comparison between the experimental and computational results shows a qualitative agreement in all relevant aspects.

Analytical study on the elastoplastic behavior in metal matrix composites

An analytical derivation of composite mechanics to investigate elastoplastic behavior is carried out to predict fiber stresses and fiber/matrix interfacial shear stresses. The model is based on the theoretical development considering stress concentration effects and its formulation is extended to investigate the elastoplastic behavior. The procedure is performed to investigate the stress transfer mechanism in a short fiber reinforced metal matrix composite. For the region of matrix plasticity, slip mechanisms between fiber and matrix which normally take place at the interface are considered for the derivation. Results of predicted stresses for the small scale matrix yielding as well as the large scale matrix yielding are described based on closed form solution. It is found that the proposed model shows the capability to correctly predict the values of fiber stresses and fiber/matrix interfacial shear stresses.

Residual stresses in deformed random-planar aluminium/Saffil ® short-fibre composites

The evolution of residual stress with plastic pre-strain was studied on a squeeze-cast, 2D-random short-fibre reinforced metal matrix composite (aluminium/Saffil ®). Specimens of the composite and of unreinforced reference material were subjected to uniaxial compression tests and deformed to different pre-strains in the range 0.2–9.1%. The mean deviatoric residual stresses in the aluminium matrices of the composite specimens were analysed using a neutron diffraction technique. Results show that the dependence of residual von Mises matrix stress on the pre-strain matches the stress–strain curve of the unreinforced aluminium reference material. This behaviour is explained in light of an Eshelby model.

Fiber Orientation Control and Evaluation of Characteristics of Short Fiber Reinforced Metal Matrix Composite Fabricated by Low-Pressure Infiltration Process

Fiber Orientation Control and Evaluation of Characteristics of Short Fiber Reinforced Metal Matrix Composite Fabricated by Low-Pressure Infiltration Process

Fibre breakage in short fibre reinforced metal matrix composites during creep and constant strain rate compression testing

The evolution of fibre breakage during compressive deformation of AlZn11Mg0.2 (wt.%) reinforced with “Saffil” short fibres was characterised. Fibre damage is described in terms of the changes in the distribution of fibre lengths as a function of accumulated strain. Cylindrical specimens with different fibre orientations were deformed under constant compressive load at 573 K and constant strain rate at 573 K and room temperature. No significant difference in the damage evolution for specimens tested under different conditions was observed. A final mean fibre length near 40 μm was observed in all cases.

Characterization of Fracture Toughness of Hybrid MMCs the Short Fiber Reinforced Metal Matrix Composites

Evaluation of fracture toughness of short fiber reinforced metal matrix composites (MMCs) becomes important for the application as structural materials. Therefore, in this study static and dynamic fracture toughness of MMCs manufactured by squeeze casting process were investigated. A number of MMCs have been tested with various matrix alloys, volume fractions, and specifically types of reinforcements. It was found that static and dynamic fracture toughness of metal matrix composites was remarkably decreased by the addition of ceramic reinforcements. Dynamic fracture toughness slightly decreased compared with static fracture toughness because of the effect of dynamic velocity under impact loading. The toughness of ceramic reinforced MMCs is controlled by a complexity interaction between the matrix alloy and reinforcement. Important properties which influence toughness include the type of reinforcement (its physical form, size), volume fraction and combination of reinforcement, and the matrix alloy. And notch fracture toughness of MMCs for simple evaluation was also discussed.

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.

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.

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.

Dislocation fiber interactions in short fiber reinforced metal matrix composites during creep and during thermal cycling

Short fiber reinforced metal matrix composites (SFR MMCs) are attractive engineering materials because they exhibit increased strength and wear resistance as compared to the fiber free matrix materials. For example, an aluminum alloy containing 15 volume percent of Al{sub 2}O{sub 3} fibers with average dimensions of 200 {micro}m length and 3 {micro}m diameter exhibits an improved creep strength with respect to the fiber free matrix. In addition to extended periods of isothermal and static creep loading high temperature components are subjected to temperature changes which are associated with thermal stresses. Thermal cycles can be due to start up and shut down events and can also be a consequence of anisothermal operating conditions. In short fiber reinforced aluminum alloys, in the stress and temperature range of interest, dislocation creep governs the deformation behavior of the MMC`s metallic matrix. It is therefore interesting to discuss the role of dislocations during creep and during thermal cycling of SFR MMCs. In the present paper the authors describe some basic dislocation mechanisms near the fiber/matrix-interface (FMI) of SFR MMCs. They first consider dislocation structures which are associated with the processing of SFR MMCs. Then dislocation processes which are associated with (1) static isothermal creep andmore » (2) thermal cycling are discussed. Common and distinct features of the associated dislocation structures in the matrix zone near the FMI are highlighted. The authors then use the insight they have gained to qualitatively understand the role of dislocations in the macroscopic response of a SFR MMC under more complex load profiles.« less

Prediction of the mechanical behaviour of short fiber reinforced MMCs by combined cell models

The flow behaviour for metal matrix composites reinforced with 2D (planar) and 3D randomly oriented short Al 2O 3 fibers is investigated by combined cell models in conjunction with the finite element method. The mechanical behaviour of short fiber reinforced metal matrix composites (MMCs) with a given fiber orientation can be simulated numerically by averaging results derived from different cell models. These cell models involve two 2D models and two 3D models representing a single fiber in three principal orthogonal planes in the composite. Stress-strain curves have been calculated for MMCs reinforced with 2D randomly planar and 3D randomly oriented short fibers by an appropriate integration of results of all fiber orientations. The numerical results are compared with experimental data of a fiber reinforced aluminium alloy composite obtained in uniaxial tension and compression tests. Good agreement is obtained between experimental results and the predictions of the model in the regimes where no microdamage is observed experimentally. Finally, the effects of residual stresses have been estimated using the model. Both, in tension and in compression, Young's modulus is found to be lower while the yield stresses are increased compared to the case when residual stresses are absent.