Composite isotropic properties

Particle or discontinuously reinforced MMCs have become important because they are inexpensive compared to continuous fiber-reinforced composites and they have relatively isotropic properties compared to the fiber-reinforced composites. Figures la and b show typical microstmctures of continuous alumina fiber/Mg and siUcon carbide particle/Al composites, respectively. 

Composites fabricated with the smaller floating catalyst fiber are most likely to be used for applications where near-isotropic orientation is favored. Such isotropic properties would be acceptable in carbon/carbon composites for pistons, brake pads, and heat sink applications, and the low cost of fiber synthesis could permit these price-sensitive apphcations to be developed economically. A random orientation of fibers will give a balance of thermal properties in all axes, which can be important in brake and electronic heat sink applications. 

The Z-direction is perpendicular to the page. For simplicity the material is assumed to be isotropic, ie same properties in all directions. However, in some cases for plastics and almost always for fibre composites, the properties will be anisotropic. Thus E and v will have different values in the x, y and z direction. Also, it should also be remembered that only at short times can E and v be assumed to be constants. They will both change with time and so for long-term loading, appropriate values should be used. 

Two simple invariants, U, and U5, were shown in the previous subsubsection to be the basic indicators of average laminate stiffnesses. For isotropic materials, these invariants reduce to U. =Qi. and U5 = Qqq, the extensional stiffness and shear stiffness. Accordingly, Tsai and Pagano suggested the orthotopic invariants U., and U5 be called the isotropic stiffness and isotropic shear rigidity, respectively [7-16 and 7-17]. They observed that these isotropic properties are a realistic measure of the minimum stiffness capability of composite laminates. These isotropic properties can be compared directly to properties of isotropic materials as well as to properties of other orthotropic laminates. Obviously, the comparison criterion is more complex than for isotropic materials because now we have two measures, and U5, instead of the usual isotropic stiffness or E. Comparison of values of U., alone is not fair because of the degrading influence of the usually low values of U5 for composite materials. 

HIPing is considerably more complex and expensive than hot-pressing but for monolithic silicon nitride it normally produces a superior product with near-isotropic properties at low additive contents. However, with silicon nitride whisker composites, whisker bridging and the formation of whisker nests can be more of a problem.5... 

For a single-channel problem, only one term in the sum in Eq. (56) is to be considered. On the other hand, it is not unusual for multichannel chemical reactions to have a multiexponential form of reaction probability. Reduction to a single exponential form can depend on individual cases under study. For our cluster system, in which various isotropic properties dominate due to the identical particle composition (see Ref. 39), it is quite likely that there are very many critical points that are topologically equivalent or energetically similar to each other. In these cases, averaging should work. In the two familiar means over variables (X i = 1,2,..., M ... 

In fibre-reinforced glass matrix composites, 3-D parts offer greater potential than uni-directionally reinforced composites (in 1-D or 2-D fibre lay-up), discussed in Chapter 19, because of their potentially better interlaminar shear strength and isotropic properties. A... 

Aluminosilicate glass matrix composite reinforced by 3-D chopped Tyranno fi fibre network Isotropic properties Young s modulus 90 GPa, iTacmrc strength 170 MPa, Compressive strength 320 MPa Aerospace Special machinery Hot glass handling [10]... 

The dental ceramics used in this study are described in Table I. Materials were selected in order to provide varied microstructures a) the glass-ceramic El is expected to have a homogeneous microstructure and isotropic properties b) the glass-ceramic E2 is expected to have anisotropic properties due to alignment of lithium disilicate fibers and c) the glass-infiltrated alumina composite IC, may have particle orientation, which could affect the mechanical properties. 

There are now numerous compositions of liquid crystalline polymers under consideration as fiber spinning and injection molding materials. However, the problems involved in processing these systems are similar. In particular, how can one process these polymers to yield desirable isotropic properties or at least have biaxial orientation how can one achieve the optimum properties from a given composition and how does the chemical composition and structure affect the properties In flexible chain systems one must quench in orientation in a time scale which is faster than the relaxation process of the molecules. Typically there is a distribution of relaxation times in which the longest relaxation time is a matter of a few seconds. This longest relaxation time also governs a number of other flow characteristics. 

Another application of dental composites is orthodontic archwires. One example is a unidirectional pultruded S2-glass-reinforced dimethacrylate thermoset resin. Depending on the yam of glass fiber used, the fiber volume fraction varied from 32 to 74 percent The strength and modulus were comparable with those of titanium wires. Orthodontic brackets were also made from composites with a polyethylene matrix reinforced with ceramic hydroxyapatite particles, resulting in isotropic properties and good adhesion to enamel. 

A different kind of 3-D fibre arrangement is the reinforcement of carbon with short fibres or carbon felt. Composites with isotropic properties result, having flexural strengths ranging from 100 to 170... 

The strength of materials approach provides fonr of the five elastic properties of transversely isotropic nnidirectional composites. Two properties Ey, V12) are well predicted by this simple approach, i.e. nsing the law of mixtures. The other two ( , G 2) require more accurate micromechanics models. The main reason for this is that E i and V12 are independent of fibre packing while d E and G i2 depend strongly on fibre arrangement. 

For the transverse shear modulus, the approach designated self-consistent was based on the formula obtained by the self-consistent method for the plane-strain bulk modulus (11.61), on the transverse modulus calculated using the Chamis approach (11.49b) and the in-plane Poisson s ratio given by the rule of mixtures. Except when used to predict the axial modulus and the major Poisson s ratio, the rule of mixtures underestimates the remaining composite elastic properties. The Bridging Model proved to be a very effective theory to account for all five elastic properties for unidirectional composites that are transversely isotropic. 

Kriz R. D. and Stinchcomb W. W. (1979) Elastic moduli of transversely isotropic graphite fibres and their composites - Equations used to calculate the complete set of elastic transversely isotropic properties for unidirectional fibre-reinforced materials having transversely isotropic fibres are experimentally verified by using improved ultrasonic techniques. Experimental Mechanics, 19(2), 41 9. 

Second, the growth process allows a variety of routes to the formation of fibers but there are no simple way s to form a dense isotropic plastic. Thus, even essentially isotropic materials will have a fibrous composite microstmcture. Weiner et al. (2000) have argued that many biological structures can be viewed as a search for isotropic properties, or at least orthotropic properties (strong in two dimensions), from fibrous materials. This would reflect the unpredictability of stresses encountered by a structure in a dynamic environment. 

IR reflection/absorption spectroscopy (IRRAS), 65, 67 IR spectra of sol-gel materials composition, 87 properties, 87 structure, 87 IR spectroscopy absorption spectroscopy, 69 advantages, 143 FTIR spectroscopy, 143 significance, 84 IR spectrum types far IR, 66 middle IR, 66 near IR, 66 isostatic pressure, 216 isothermal annealing, 183 isotropic samples amorphous samples, 19 liquids, 19... 

The relationship between chemical composition, structure and properties for the copolymers has been described in det elsewhere [68,69]. In general it is found that the glass transition temperature T, and the Young s modulus increase with hard segment content and heat treatment temperature. It was found that the material with the optimum composition and properties had a value of T, of 80°C and a Young s modulus (isotropic) of 1.7 GPa, both of which are typical of a conventional glassy polymer. 

Composites with uniformly distributed unidirectional fibers aligned with a principal axis additionally possess a plane of isotropy in the transverse direction, where the material behavior is invariant to rotations. For such transversely isotropic properties, the number of constants is reduced to five. When the concerned axis of rotation is oriented in the es-direction,... 

There are three basic types of engineered composites (1) laminates, (2) particle-reinforced composites, and (3) fiber-reinforced composites. In particle-reinforced composites, one can make the distinction between small (submicron) particle composites, where the particles are incorporated in the microstructure, vs. large particle composites, where the particles themselves actually do the work or carry the load. The reinforcing fibers can be discontinuous or continuous. The fibers in discontinuous fiber-reinforced composites can be randomly oriented to provide isotropic properties or aligned to enhance a specific property in a specific direction. Continuous fiber composites are generally designed for their unidirectional properties but can be crisscrossed to obtain multidirectional property enhancement such as in a filament-woimd pressure container. All possible permutations of metal, ceramic, and pol)uner are foimd in the laminated as well as in the reinforced composites. 

Both nature and man have made extensive use of composite materials in which two or more different materials are joined in such a manner that they maintain their identity but work together to add their strengths and decrease their weaknesses. Composites can be classified into three categories (1) Laminates, in which sheets of different materials are laminated together (2) particle-reinforced composites, in which particles of one material are imbedded in a matrix of a second material and (3) fiber-reinforced composites, in which fibers of one material are encapsulated in a matrix of a second material. Particle-reinforced composites can be subdivided into small particle composites, where the particles are incorporated into the microstructure, such as dispersion-hardened alloys, and large particle composites, where the matrix simply supports the particles. Fiber-reinforced composites may have continuous versus discontinuous fibers and aligned versus randomly oriented fibers, which can provide anisotropic versus isotropic properties. Composites combine all combinations of metals, ceramics, and polymers into MMCs, where a metal... 

Vanadium—Cobalt-Iron Alloys. V—Co—Fe permanent-magnet alloys also are ductile. A common commercial ahoy, Vicahoy I, has a nominal composition 10 wt % V, 52 wt % Co, and 38 wt % Fe (Table 10). Hard magnetic properties are developed by quenching from 1200°C for conversion to bcc a-phase foUowed by aging at 600°C (precipitation of fee y-phase). The resulting properties are isotropic, with ca kJ/m ... 

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