Composite fiber/matrix systems

Properties of Composite Fiber/Matrix Systems. The volume fraction of fibers in most filament-wound structures and in laid-up composite layers can exceed 60 percent. For carbon fibers at this volume fraction, the composite has such a high absorption coefficient that it can be treated as opaque [251] however, for glass-epoxy composites, the absorption coefficient is small enough, particularly in some spectral windows, that radiation must be treated as a volumetric effect. The situation is complicated by the high fiber volume fraction, which causes the fiber absorption and scattering to be in the dependent regime however, some data are presented in Ref. 251. 

It should be noted that a mode 11 crack-resistance R-curve may also be obtained for some fiber-matrix systems (Vu-Khanh, 1987). In conjunction with the mode 1 R-curve the additional information of a mode 11 R-curve will be of great use to the composite design engineers. This is increasingly the view of the ESIS task group on delamination crack growth resistance. 

Predicting fiber orientation. Isotropic constitutive models are not valid for injection-molded fiber-reinforced composites. Unless the embedded fibers are randomly oriented, they introduce anisotropy in the thermomechanical properties of the material. The fiber orientation distribution is induced by kinematics of the flow during filling and, to a lesser extent, packing. An extensive literature deals with flow-induced fiber orientation while much other work has been devoted to micromechanical models which estimate anisotropic elastic and thermal properties of the fiber-matrix system from the properties of the constituent fiber and matrix materials based on given microstructures. Comprehensive reviews of both research areas have been given in two recent books edited, respectively, by Advani and by Papathanasiou and Guell where many references can be foimd. 

In regards to the studies on transcrystalhnity in conventional fiber reinforced composites, their number is vast. A number of issues are related to the formation and growth of TCL [81] crystallinity of the matrix, mismatch of thermal coefficients of the fiber and the matrix, epitaxy between the fiber and the matrix, surface toughness, thermal conductivity, treatment of fiber, etc. Processing conditions such as cooling rate, temperature, and interfacial stress were also found to be important. There are indications that the TC phenomenon is probably too specific for each fiber/matrix system. Nevertheless, it has been recognized that the orientation distribution of the polymer chains in the TCL wUl determine the nature and extent of its effect on the mechanical properties of the composite material [84]. 

Applied Sciences, Inc. has, in the past few years, used the fixed catalyst fiber to fabricate and analyze VGCF-reinforced composites which could be candidate materials for thermal management substrates in high density, high power electronic devices and space power system radiator fins and high performance applications such as plasma facing components in experimental nuclear fusion reactors. These composites include carbon/carbon (CC) composites, polymer matrix composites, and metal matrix composites (MMC). Measurements have been made of thermal conductivity, coefficient of thermal expansion (CTE), tensile strength, and tensile modulus. Representative results are described below. 

The discussion of materials selection factors is naturally divided into three parts (1) overall factors pertinent to selection of the composite material itself, (2) factors governing the selection of the fibers, and (3) factors essential to selection of the matrix system. Those three types of selection trade-offs will be described, followed by summary remarks on the process of selecting a suitable composite material. 

Tests by Gatenholm et al. [8,10] on PHB-HV copolymers containing cellulose fibers (for example, the tradenamed Biopol) show that the mechanical properties of these systems are determined by the fiber and the fiber matrix interface on the one hand, and on the other hand by the composition of the matrix, that is, of HV proportion in the matrix. At an increased proportion of HV, the stiffness of the composite is reduced up to 30%, whereas elongation at break increases until about 60%. 

Other than in polymer matrix composites, the chemical reaction between elements of constituents takes place in different ways. Reaction occurs to form a new compound(s) at the interface region in MMCs, particularly those manufactured by a molten metal infiltration process. Reaction involves transfer of atoms from one or both of the constituents to the reaction site near the interface and these transfer processes are diffusion controlled. Depending on the composite constituents, the atoms of the fiber surface diffuse through the reaction site, (for example, in the boron fiber-titanium matrix system, this causes a significant volume contraction due to void formation in the center of the fiber or at the fiber-compound interface (Blackburn et al., 1966)), or the matrix atoms diffuse through the reaction product. Continued reaction to form a new compound at the interface region is generally harmful to the mechanical properties of composites. 

Microcomposite tests including fiber pull-out tests are aimed at generating useful information regarding the interface quality in absolute terms, or at least in comparative terms between different composite systems. In this regard, theoretical models should provide a systematic means for data reduction to determine the relevant properties with reasonable accuracy from the experimental results. The data reduction scheme must not rely on the trial and error method. Although there are several methods of micromechanical analysis available, little attempt in the past has been put into providing such a means in a unified format. A systematic procedure is presented here to generate the fiber pull-out parameters and ultimately the relevant fiber-matrix interface properties. 

The descriptions presented in the foregoing sections are concerned mainly with composites containing brittle fibers and brittle matrices. If the composite contains ductile fibers or matrix material, the work of plastic deformation of the composite constituents must also be taken into account in the total fracture toughness equation. If a composite contains a brittle matrix reinforced with ductile libers, such as steel wire-cement matrix systems, the fracture toughness of the composite is derived significantly from the work done in plastically shearing the fiber as it is extracted from the cracked matrix. The work done due to the plastic flow of fiber over a distance on either side of the matrix fracture plane, which is of the order of the fiber diameter d, is given by (Tetelman, 1969)... 

Composite system (fiber/ matrix or coating) a Required interface strength, ol (MPa) Calculated transverse strength, oj (MPa) ... 

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