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Strength of ceramic matrix-metal fibre composites
Abstract
A failure model appropriate to ceramic matrix composites is described. The ultimate strain of the fibre is greater than that of the matrix. The contribution of interface phenomena to the effective surface energy of the composite is assumed to be zero: the interface just causes a crack in the matrix to stop and go around a fibre. Theoretical evaluation and experimental results obtained for composites with oxide matrices and molybdenum fibres show that microcracks in the matrix propagate in an unstable manner at low fibre volume fraction and in a stable manner at high fibre volume fraction. The strength of a ceramic matrix composite with a strong interface can in some cases be higher than that of the matrix.
... If the fibre is metallic, however, then there is the potential for energy absorption, not only from fibre pull-out, but also as a result of plastic deformation of the fibre (assuming that fracture occurs such that fibres bridge the crack plane, which in general will require a certain minimum fibre aspect ratio and also at least some fibre debonding). This potential for energy absorption via fibre plasticity has been highlighted by several researchers [20][21][22][23][24][25] with particular attention having been drawn by Ashby and coworkers [20] to the concept of the constraint imposed by the surrounding matrix having an influence on the volume of fibre in which plastic deformation can occur, and hence on its effective ductility (and on the total plastic work). This is clearly dependent on the interfacial bond strength, which thus affects both fibre pull-out and fibre plasticity. ...
... There have been various observations, over an extended period [16,18,20,21,[24][25][26][27][28][29][30][31][32], concerning the extent and nature of fibre pull-out and plastic deformation in MFCs. In general, it is recognised [20][21][22]24] that strong interfacial bonding is likely to impose greater constraint on fibre plasticity, and hence limit the plastic work done. On the other hand, it is by no means universally accepted that fibre plastic deformation is likely to play a significant role in energy absorption during fracture of MFCs, with several researchers [16,[33][34][35][36] proposing or assuming that pull-out is entirely dominant. ...
A model is presented for prediction of the fracture energy of ceramic–matrix composites containing dispersed metallic fibres. It is assumed that the work of fracture comes entirely from pull-out and/or plastic deformation of fibres bridging the crack plane. Comparisons are presented between these predictions and experimental measurements made on a commercially-available composite material of this type, containing stainless steel (304) fibres in a matrix predominantly comprising alumina and alumino-silicate phases. Good agreement is observed, and it’s noted that there is scope for the fracture energy levels to be high (∼20 kJ m−2). Higher toughness levels are both predicted and observed for coarser fibres, up to a practical limit for the fibre diameter of the order of 0.5 mm. Other deductions are also made concerning strategies for optimisation of the toughness of this type of material.
... As is known, propulsion engines of new generation require new structural low-density materials operating at temperatures above 1100°C, i.e. higher than those typical of most Ni-based superalloys [1][2][3][4][5][6]. Our analysis of the properties of new refractory materials based on refractory metals (Mo, Nb, etc.) [7][8][9][10][11], intermetallic compounds [12][13][14][15][16][17], ceramic and composite materials [18][19][20][21][22][23][24] has shown that the most promising candidates for replacement of Ni-based superalloys seem to be the aluminides of transition metals with a higher melting point and lower density. ...
NiAl-based electrodes of a required size were fabricated by combined use of centrifugal SHS casting and induction remelting in an inert atmosphere and characterized by modern analytical methods. Thus produced electrode materials exhibiting high chemical purity and low content of impurity gases (0.005 wt % O, 0.0001 wt % N) can be recommended for use in centrifugal plasma sputtering of micro granules via the plasma rotating electrode process (PREP).
The developments in the type of metal fiber reinforcements, manufacturing processes, properties and applications of metal fiber-reinforced composites are described in this Chapter. Metal fibers are mainly used as reinforcement in tyres, concrete and also in reinforcing of metals or metal foams. Incorporation of metal fibers into conventional fiber-reinforced polymer (FRP) composites creates a strong, ductile, hybrid fiber composite that reduces material cost and weight in comparison to metal based structures. Metal fibers are used for electromagnetic and electrostatic shielding, signal transmission, heat exchange, abrasive medium, filtration, and design elements. Unidirectional steel reinforcement is also used in hoisting and timing belts. In the automobile industry metal fibers are used in sound absorbers or brake covering. Press molding, thermoforming, and tape winding were successfully applied on steel cord reinforced materials. Metal fibers are commonly available as filament yarns, rovings, mats, and wovens, and their common merit is their high temperature resistance. Except for steel, metal fibers of aluminum, magnesium, copper, molybdenum, and tungsten are made for particular applications. Metal fibers are produced of different pure metals and alloys in various diameters. The metal fibers are used as such for various applications and it is further incorporated into several matrices to form metal fiber reinforced composites.
The competitive method for the preparation of molybdenum reinforced alumina has been proposed and investigated as a model for obtaining metal reinforced ceramics. The proposed method consists of infiltration of porous ceramic preform with a solution of metallic precursor followed by sintering combined with the reduction of deposited salt. This method enables formation of the composites that exhibit 36% improvement in compressive strength and no significant change in Vickers hardness in comparison to pure alumina with addition of only 0.22 wt% of molybdenum. Obtained results showed that the deposited metal concentration is highly correlated with the concentration of the precursor solution. In order to precisely determine the amount of molybdenum in the samples unconventional and novel method based on the UV/VIS spectrophotometry has been developed. This method shows a good detection ability that is suitable for determination of small amounts of metal below 0.5 wt% - a quantity difficult to analyse by using classic methods such as energy-dispersive X-ray spectroscopy (EDS).
The oxidation kinetics of a composite material, which consists of an Al2O3–Al5Y3O12 matrix and molybdenum fibers and has a high cracking resistance, is studied. The mass loss of the composite material during oxidation is shown to be several orders of magnitude lower than that of molybdenum. Oxidation in quiet air at 1250°C for several hours weakly changes the strength of the composite material at temperatures from room temperature to 1300°C. It is also shown that the strength of the composite material as a function of the oxide matrix composition (Al: Y ratio) changes nonmonotonically. The maximum strength shifts from the Al2O3–Al5Y3O12 eutectic point toward garnet.
Fabrication schemes of two types of novel oxide fibres being either single crystalline alumina or oxide eutectics are presented. The first one (ICM) is based on crystallization of the melt in channels inside of an auxiliary matrix (internal crystallization method) and then removing the matrix. The second one (MIGL) is crystallizing the melt in a substrate which allows to enhance essentially the crystallization rate and, therefore, to decrease the production cost.
Strength characteristics of the fibres that determine the composite strength are measured by testing specially designed composite specimens and interpreting test results basing on corresponding micromechanical models. The ICM fibres are characterized up to temperatures of 1300°C. The MIGL fibres tested at room temperature reveal at least two defect populations, the population of rough defects determines the fibre strength on large fibre lengths, the second one causes the fracture at stresses higher than 1000 MPa at shorter lengths.
Usage of the fibres in both a nickel superalloy and intermetallic based matrices is discussed. Preliminary results of testing composites at temperatures up to 1100°C (nickel matrix) and 1200°C (intermetallic matrix) including creep rupture testing are presented.
A discussion of a possibility to obtain oxide-fibre/oxide-matrix composites by using the internal crystallization method is based on preliminary experiments carried out on some oxide systems.
The tensile properties of high strength polyacrylonitrile-based (IM600) and high modulus pitch-based (K13D) hybrid carbon fibers-reinforced epoxy matrix composite (CFRP) were investigated. Fiber orientation of the hybrid CFRP specimen was set to [0(IM600)/0(K13D)]2S. The fiber volume fraction of the hybrid CFRP specimen was 55.7 vol% (IM600: 29.3 vol%, K13D: 26.4 vol%). The tensile stress–strain curve of the hybrid CFRP specimen shows a complicated shape (jagged trace). By the high modulus K13D CFRP layers, the hybrid CFRP specimen shows the intermediate modulus in the initial stage of loading. Subsequently, when the K13D CFRP layers begin to fail, the high strength IM600 CFRP layers would hold the load (strength) and the material continues to endure high load without instantaneous failure. Because higher strength fiber can help the load for a certain time after failure occur, the hybrid composite could be considered one example of a material possessing preventing instantaneous failure. The Weibull statistical distributions of the mono (IM600 and K13D) and the hybrid CFRP specimens were also examined. The Weibull modulus for the mono CFRP specimens was calculated to be 22.9 for the IM600 CFRP specimen and 14.4 for the K13D CFRP specimen, respectively. The Weibull modulus for the hybrid CFRP specimen was calculated to be 39.6 for the initial fracture strength and 20.6 for the tensile fracture strength, respectively. The Weibull modulus for the initial fracture strength is higher than that for the K13D CFRP specimen and the Weibull modulus for the tensile fracture strength is almost similar to that for the IM600 CFRP specimen.
The use of NiAl as a structural material has been hindered by its lack of tensile ductility or toughness at room temperature. The operative flow and fracture mechanisms in monolithic NiAl leading to these poor low temperature properties are analyzed, demonstrating the need for ductile phase toughening. Progress in ductile phase reinforced intermetallics and NiAl-based materials are reviewed and the primary mechanisms involved in the flow and fracture of ductile phase reinforced alloys are clarified by recent investigations of directionally solidified NiAl-based materials. The mechanical behavior of these model alloys (Ni-30Al and Ni-30Fe-20Al (at%)) are discussed. The prospects for developing a ductile phase toughened NiAl-based alloy and the shortcomings presently inherent in these systems are analyzed.
The use of a molybdenum substrate in the shape of a microrope permits an increase in the crystallization rate of oxide fibres, resulting in a significant decrease in the fibre cost. Some variants of the method are now disclosed. As-grown fibres contain both an oxide component, which can be considered as a matrix for the composite fibre, and molybdenum wire as a reinforcement. In order to measure the room-temperature strength of the fibres, a new version of the fibre fragmentation test was developed. Interpretation of the test results is made without any assumptions about the strength distribution function of the fibre. The test results are presented in terms of a dependence of the fibre strength on the remaining effective length of the fibre. High-temperature strength properties of these novel fibres were estimated by testing composites reinforced with them. When making composites with a heat-resistant nickel-based matrix by using a routine liquid-phase fabrication method, a significant portion of the molybdenum wire is dissolved in the matrix alloy. It is therefore mainly the oxide component that serves as a reinforcement in the matrix. Preliminary evaluation of the high-temperature strength of the oxide fibre shows that there is a need to optimize the microstructure and fabrication parameters of the fibres in order to achieve strength values the same as those for monocrystalline and eutectic fibres at temperatures above 1000°C. Nevertheless, even at the present stage of development of the fibre technology, the high-temperature strength of composites with nickel superalloy and Ni3Al-based matrices looks very promising. The analysis of creep rupture properties based on preliminary experiments and corresponding mechanical models offers the prospect of developing heat-resistant composites by using MIGL fibres and advanced alloys as the matrix materials. The operating temperature (i.e. for a creep rupture stress of about 150 MPa) can vary between 1100 and 1200°C depending on the particular application.
This study is a numerical exercise to theoretically analyze toughening in brittle materials consisting of ductile reinforcements, on the basis of crack bridging by ductile particles. The effects of the particle constitute behavior, the shape of the stress-displacement law, the matrix fracture toughness and the overall elastic modulus of the composite on toughness have been illustrated by simple fracture mechanics calculations of self-consistent crack opening displacement profiles and crack bridging stress distributions. An approach to calculate the crack surface displacements, crack bridging stresses and stress intensity factors in any specimen geometry, using weight functions, is presented. Different types of idealized particle stress-displacement responses were used in the calculations. The opening characteristics of a bridge crack and the evolution of toughness for these types have been examined. The conditions for maximum bridging and toughness as well as the conditions at which a fully bridged crack transforms to a partially bridged one have been identified. The role of individual parameters used in constructing the stress-displacement law on composite toughness has been assessed. The toughness has been found to manifest from a complex interaction of the matrix fracture toughness, the composite modulus and the flow behavior of the ductile particle.
Considerable work has been performed on NiAl over the last three decades, with an extremely rapid growth in research on this intermetallic occurring in the last few years due to recent interest in this material for electronic and high temperature structural applications. However, many physical properties and the controlling fracture and deformation mechanisms over certain temperature regimes are still in question. Part of this problem lies in the incomplete characterization of many of the alloys previously investigated. Fragmentary data on processing conditions, chemistry, microstructure and the apparent difficulty in accurately measuring composition has made direct comparison between individual studies sometimes tenuous. Therefore, the purpose of this review is to summarize all available mechanical and pertinent physical properties on NiAl, stressing the most recent investigations, in an attempt to understand the behavior of NiAl and its alloys over a broad temperature range.
The internal crystallization method for making fibrous composites, which consists of the infiltration of channels made in the matrix with fibre material melt and subsequent crystallization of the melt to produce high-strength fibre, is now applied to fabricate ceramic-fibre/ceramic-matrix composites. Composites thus obtained were not optimized with respect to the microstructure of the fibre, matrix and fibre/matrix interface. However, the possibility of making oxide-/oxide composites by using the internal crystallization method has been demonstrated. Speculations about possible ways of improving mechanical properties of the composites are presented.
Carbide-matrix composites with graphite fibres have been fabricated by ordinary powder metallurgy means. Various fibre/matrix interactions were expected and have actually been observed in such composite systems during the fabrication stage. Microstructures of composites obtained by this process have been studied. It appears that the porosity in graphite-fibre/carbide-matrix composites is inhomogeneously distributed within the volume. The stress/strain behaviour of graphite-fibre/carbide-matrix composites has been found to be non-linear, the higher the fibre volume fraction, the larger the non-linearity. The reasons for such non-linearity are not clear. The dependence of the effective Young's modulus of composites upon the fibre volume fraction cannot be approximated by any simple and reliable model. A proper approximation should therefore be used to evaluate this dependence for further use.
Fibrous composites are normally fabricated by inserting premade fibres into a matrix and trying to tailor mechanical or physical properties of the material by a proper choice of fibre arrangement, fibre volume fraction, structure and properties of interface, etc. As a rule, this method satisfies all the needs fairly well. But in many cases, particularly when heat-resistant composites are involved, it leads to complications which cause composite experts to refrain from being involved in technically very attractive projects. So the need for alternative methods of composite fabrication obviously exists. The process described here is an example of such an alternative. It is based on the fibres growing from the melt within the volume of the matrix. The matrix should have prefabricated continuous cylindrical channels to be filled with the melt of the fibre material. The process is described using as a model a composite with a molybdenum matrix and single crystalline sapphire fibres. It is shown that the productivity of oxide fibre fabrication based on the process described can be some orders of magnitude higher than that based on the well known Czochralsky's and Stepanov's methods. The strength of the single-crystalline sapphire fibres obtained has been studied, as well as the high-temperature creep strength of composites containing such fibres. Some of the results of these experiments are reported here.
The use of NiAl as a structural material has been hindered by the fact that this ordered intermetallic does not exhibit significant tensile ductility or toughness at room temperature. A critical review of the operative flow and fracture mechanisms in monolithic NiAl has thus established the need for ductile phase toughening in this order system. Progress in ductile phase reinforced intermetallic systems in general and specifically NiAl-based materials has been reviewed. In addition, further clarification of the primary mechanisms involved in the flow and fracture of ductile phase reinforced alloys has evolved from ongoing investigations of several model NiAl-based materials. The mechanical behavior of these model directionally-solidified alloys (Ni-30Al and Ni-30Fe-20Al) are discussed. Finally, the prospects for developing a ductile phase toughened NiAl-based alloy and the shortcomings presently inherent in these systems are analyzed.
Considerable work has been performed on NiAl over the last three decades, with an extremely rapid growth in research on this intermetallic occurring in the last few years due to recent interest in this material for electronic and high temperature structural applications. However, many physical properties and the controlling fracture and deformation mechanisms over certain temperature regimes are still in question. Part of this problem lies in the incomplete characterization of many of the alloys previously investigated. Fragmentary data on processing conditions, chemistry, microstructure and the apparent difficulty in accurately measuring composition has made direct comparison between individual studies sometimes tenuous. Therefore, the purpose of this review is to summarize all available mechanical and pertinent physical properties on NiAl, stressing the most recent investigations, in an attempt to understand the behavior of NiAl and its alloys over a broad temperature range.
The antiplane problem of microcracks in a medium with circular cylindrical inclusions is considered. In addition to the inhomogeneities associated with the presence of the inclusions, whose shear modulus differs from that of the matrix, there are inhomogeneities of the second kind, due to the statistical distribution of the energy of the fiber/matrix interface. First the solution of the problem of separation of a single inclusion is analyzed: then the results of this analysis are employed to describe the deformation curve of a medium with noninteracting inclusions. For this medium the macrocrack problem is then posed and its solution is obtained analytically for some particular cases and numerically for one more general case. Analysis of the solutions shows that it is possible to enhance the macrocrack resistance in a medium with inhomogeneities of the second kind.
The tensile failure of a brittle matrix fiber composite has been analyzed as a function of fiber strength. Strong fibers remain intact during matrix fracture, resulting in multiple matrix cracks and a post-cracking load sustaining capability. Reduced fiber strength causes fiber failure in the crack wake and catastrophic failure from a single matrix crack. Strength and fracture resistance relations are derived for both fracture mechanisms, and microstructural trends elucidated. Furthermore, the transition between mechanisms is used as the basis for a failure map.
The fiber reinforced ceramics discussed in this chapter are composite materials which consist of a ceramic, glass-ceramic or inorganic glass matrix reinforced with strong stiff fibers. Considerable work has been carried out over the last twenty years in the development of such materials and recent technological advances make this an opportune time for a review of their science and technology.
A literature review of the current state of ceramic matrix composites is presented. The ceramic phases that have been investigated for use as matrices and as reinforcements are discussed in some detail, along with some of the properties found for these systems. Processing and characterization of ceramic matrix composites are briefly examined.
In common with other brittle solids, cements are toughened much more by the incorporation of fibres than by inclusions of other geometries. The largest energies required to break a specimen are found when multiple fracture of the specimen occurs before final failure. Theoretical models of a crack moving normal to a set of parallel fibres will be considered, to show that the crack spacing and first cracking strain should depend on the area of fibre-matrix interface per unit volume of composite. The first cracking strain is shown to increase for all fibre volumes provided that the fibre spacing is less than the critical flaw size according to the Griffith's equation. The theoretical models are compared with experiment and the practical difficulties of defining first cracking strain and interfacial area mentioned. The best practical means of assessing the resistance to failure of the composite is the work done per unit volume of the specimen in separating it into two distinct pieces. The maximum values of toughness attainable - some 106 J m-3 - can decrease with time under external weathering, owing to continuing hydration of the matrix and consequent increase in the critical volume fraction of fibres.
Considered in this work is a heuristic model of pseudo-macrocracking of a fiber reinforced composite with a brittle matrix. By preserving the qualitative feature of the fracture process, crack extension corresponds to the formation of microcracks ahead of the main macrocrack. Critical stress intensity factor can be obtained from a simple energy balance. Pseudo-macrocracking can also be observed in a non-homogeneous solid which does not contain macrocracks. The results can be applied for evaluating the load carrying capacity of fiber-reinforced composites.
The theoretical stress-strain behaviour of a composite with a brittle matrix in which the fibre-matrix bond remains intact after the matrix has cracked, is described. From a consideration of the maximum shear stress at the fibre-matrix interface, the extent of fibre debonding and the crack spacing in a partially debonded composite are derived. The energetics of cracking and the conditions leading to an enhanced matrix failure strain are then discussed and, finally, the crack spacing expected in composites containing fibres isotropically arranged in two or in three dimensions is derived for the case of very thin and hence very flexible fibres.
Matrix fracture in brittle-matrix fiber composites is analyzed for composites that exhibit multiple matrix cracking prior to fiber failure and have purely frictional bonding between the fibers and matrix. The stress for matrix cracking is evaluated using a stress intensity approach, in which the influence of the fibers that bridge the matrix crack is represented by closure tractions at the crack surfaces. Long and short cracks are distinguished. Long cracks approach a steady-state configuration, for which the stress intensity analysis and a previous energy balance analysis are shown to predict identical dependence of matrix cracking stress on material properties. A numerical solution and an approximate analytical solution are obtained for smaller cracks and used to estimate the range of crack sizes over which the steady-state solution applies.
A fiber-reinforced ceramic subject to tensile stress in the fiber direction can undergo extensive matrix cracking normal to the fibers, while the fibers remain intact. In this paper, the critical conditions for the onset of widespread matrix cracking are studied analytically on the basis of fracture mechanics theory. Two distinct situations concerning the fiber-matrix interface are contemplated : (i) unbonded fibers initially held in the matrix by thermal or other strain mismatches, but susceptible to frictional slip, and (ii) fibers that initially are weakly bonded to the matrix, but may be debonded by the stresses near the tip of an advancing matrix crack. The results generalize those of the Aveston-Cooper-Kelly theory for case (i). Optimal thermal strain mismatches for maximum cracking strength are studied, and theoretical results are compared with experimental data for a SiC fiber, lithium-alumina-silicate glass matrix composite.
Strain energy for two and three dimensional crack systems subjected to varying loads, detailing loading and crack geometry effects on fracture criterion
Elasticity Theory ¢~fAnisotropic Solids
- S G Lechnitsky
Lechnitsky, S. G., Elasticity Theory ¢~fAnisotropic Solids. Nauka, Moscow, 1977 (in Russian).
The elastic equilibrium of an infinitee body containing a system of concentrical cracks
- Andreikiv