Types of Reinforcement

There are three kinds of metal-matrix composites distinguished by type of reinforcement particle-reinforced MMCs, short fiber- or whisker-reinforced MMCs, and continuous fiber- or sheet-reinforced MMCs. Table 1 provides examples of some important reinforcements used in metal-matrix composites as well as their aspect (length/diameter) ratios and diameters. 

Modified Bitumen Membranes. These membranes were developed in Europe during the late 1950s and have been used in the United States since the late 1970s. There are two basic types of modified asphalts and two types of reinforcement used in the membranes. The two polymeric modifiers used are atactic polypropylene (APP) and styrene—butadiene—styrene (SBS). APP is a thermoplastic polymer, whereas SBS is an elastomer (see Elastomers, thermoplastic elastomers). These modified asphalts have very different physical properties that affect the reinforcements used. 

The toughness induced in ceramic matrices reinforced with the various types of reinforcements, that is, particles, platelets, whiskers, or fibers, derives from two phenomena crack deflection and crack-tip shielding. These phenomena usually operate in synergism in composite systems to give the resultant toughness and noncatastrophic mode of failure. 

There are, of course, many more aspects of composite hardware design that differ from metallic bonded structure but do not necessarily involve adhesive bonding. For instance there are many types of reinforcement tape and fabric to choose from, the orientation of the plies must be chosen, the ply stackups must be balanced to avoid part warping after cure, a minimum number of plies must be used to prevent non-visible impact damage that significantly affects the load carrying capability of the part, etc. 

The choice of manufacturing technology for the fabrication of fiber-reinforced plastics or composite materials is intimately related to the performance, economics, and application of the materials. It also depends upon a number of factors, such as component numbers required, item complexity, number of molded surfaces, and type of reinforcement. 

Fig. 7-2 An example of a range in tensile strength, modulus of elasticity, and elongation of some TPs with and without chopped glass fibers by weight and type of reinforcement.

A composite material is defined as a material consisting of two or more distinct constituents or phases, which are insoluble in one another. The main types of reinforcement are particles, discontinuous fibers, continuous fibers (or filaments) and flakes. 

In such cases, it is recommended that the design be conservative and that consideration be given to the use of tee fittings or complete encirclement types of reinforcement. 

Figure 3. Elongation after heat aging as a function of type of reinforcement...

Hand lay-up it is possible to use all other types of reinforcements such as honeycombs or foams. All shapes, sizes and thicknesses are feasible. 

Table 6.2 (p. 787) displays some examples of properties obtained with various matrices for the same type of reinforcement. 

The micro-residual stresses arise from the differential CTE of the fiber and matrix, and the temperature difference. Table 7.4 gives the linear CTE values for various types of reinforcing fibers and matrix materials used widely for composite fabrication. The CTEs of most fibers and ceramic matrices are relatively lower... 

FIG. 10-171 Types of reinforcement for branch connections. From Kellogg, Design of Piping Systems, Wiley, New York, 1965.)... 

In Section 1.4.2, we described several classification schemes for composites, including one that is based upon the distribution of the constituents. For reinforced composites, this scheme is quite useful, as shown in Figure 1.75. In reinforced composites, the reinforcement is the structural constituent and determines the internal structure of the composite. The reinforcement may take on the form of particulates, flakes, lamina, or fibers or may be generally referred to as filler. Fibers are the most common type of reinforcement, resulting in fiber-matrix composites (FMCs). Let us examine some of these reinforcement constituents in more detail. 

The compaction behavior of the preform differs a lot depending on the preforming method and the type of reinforcement that has been used. A typical compaction curve for a carbon-fiber-woven fabric is shown in Figure 12.4. The three curves correspond to... 

Thermoforming is the most commonly used method for volume production today [10]. In this method a special type of reinforcement that is already impregnated with a preforming powder is heated before it is clamped in a cold preforming tool [11], The most common type of reinforcement is continuous strand mat which is manufactured by e.g. Vetrotex and Owens Coming. 

The best way to use the Kozeny-Carman model and other permeability models (e.g. the anisotropic model by Gebart) [18], is to use them as interpolation formulas for intermediate volume fractions between known values. Extrapolation should be done with extreme caution because the models are developed for idealized reinforcements. Typical values for the permeability of different types of reinforcement are given in Table 12.1. 

The performance characteristics of a composite material depend on the type of reinforcing fiber (its strength and stiffness), its length, fiber volume fraction in the matrix, and the strength of the fiber-matrix interface. The presence of voids and the nature of the matrix are additional but minor factors. 

Although few applications have so far been found for ceramic matrix composites, they have shown considerable promise for certain military applications, especially in the manufacture of armor for personnel protection and military vehicles. Historically, monolithic ("pure") ceramics such as aluminum oxide (Al203), boron carbide (B4C), silicon carbide (SiC), tungsten carbide (WC), and titanium diboride (TiB2) have been used as basic components of armor systems. Research has now shown that embedding some type of reinforcement, such as silicon boride (SiBg) or silicon carbide (SiC), can improve the mechanical properties of any of these ceramics. 

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