Composites laminar

The hand lay-up or spray-up process, used universally for the production of laminar composites incorporating glass fiber reinforcement, is most efficient for the manufacture of large parts, such as boats, bathtubs, tanks, architectural shapes, and recreational accessories. Resins intended for spray-up processes are usually modified with thixotropic additives, such as fumed siHca (1%), to reduce the risk of drainage when appHed over large vertical mold surfaces. Molds are also made from ERP for short-mn products usually surfaced with a tooling gel coat to provide consistent surface quaHty and appearance. 

Structural composites can be combinations of homogeneous and composite materials. Laminar composites are composed of two-dimensional sheets that generally have a preferred... 

There are many ways to classify composites, including schemes based upon (1) materials combinations, such as metal-matrix, or glass-fiber-reinforced composites (2) bulk-form characteristics, such as laminar composites or matrix composites (3) distribution of constituents, such as continuous or discontinuous or (4) function, like structural or electrical composites. Scheme (2) is the most general, so we will utilize it here. We will see that other classification schemes will be useful in later sections of this chapter. 

As was the case with momentum transport, the heat transport properties of heterogeneous systems are difficult to correlate and virtually impossible to predict. There are two topics worthy of note, however, namely, the heat transport properties of filled composites, and the thermal conductivity of laminar composites. 

Thermal Conductivity of Laminar Composites. In the case of laminar composites or layered materials (cf. Figure 1.74), the thermal conductance can be modeled as heat flow through plane walls in a series, as shown in Figure 4.36. At steady state, the heat flux through each wall in the x direction must be the same, qx, resulting in a different temperature gradient across each wall. Equation (4.2) then becomes... 

Environmental barrier coatings are a type of laminar composite. As with heat transfer, diffusion in laminar composites can be modeled as steady state diffnsion throngh a composite wall, as iUnstrated in Fignre 4.56. Here, hydrogen gas is in contact with solid material A at pressnre Pi and in contact with solid B at pressnre P2. At steady state, the molar flux of hydrogen throngh both walls mnst be the same (i.e., Jh ax = Bj) and Fick s Law [Eq. (4.4)] in the x direction becomes... 

Figure 5.117 Schematic illustration of unidirectional (a) and crossplied (b) laminar composites. Reprinted, by permission, from M. Schwartz, Composite Materials Handbook, 2nd ed., p. 3.71. Copyright 1992 by McGraw-HUl, Inc.

Unlike ductile metals, composite laminates containing fiber-reinforced thermosetting polymers do not exhibit gross ductile yielding. However, they do not behave as classic brittle materials, either. Under a static tensile load, many of these laminates show nonlinear characteristics attributed to sequential ply failures. One of the difficulties, then, in designing with laminar composites is to determine whether the failure of the first ply constitutes material failure, termed first-ply failure (FPF), or if ultimate failure of the composite constitutes failure. In many laminar composites, ultimate failure occurs soon after first ply failure, so that an FPF design approach is justified, as illustrated for two common laminar composites in Table 8.9 (see Section 5.4.3 for information on the notations used for laminar composites). In fact, the FPF approach is used for many aerospace and aircraft applications. 

Table 8.9 First Ply Failure (FPF) and Ultimate Tensile Properties of Two Common Laminar Composites...

Figure 4.6 Thermal conduction in a laminar composite. The macroscopic value of the...

When a more permeable polymer is blended with PVC, the permeability coefficient is expected to increase. The magnitude of the increase, however, will depend on the morphology of the blend. A laminar composite film containing 10% EVA would give an increase in permeability of about 10% compared with a pure PVC film (15, 18). The reported measurements for PVC with 10% EVA give an increase of about 400% (Figures 1, 2, and 3). 

A structural composite is composed of layers of homogenous or composite materials, with properties that depend on the characteristics of the composite and also the geometrical design of various stmctural laminar composites. Figure 6.5a shows a diagram of the layers of a sandwich composite and (b) illustrates a long fiber composite and (c) a laminar fiber composite. 

Laminar composites are combinations of several layers of composite sheets that are bonded together. Polyolefin sandwich structures are a laminar composite. They are composed of two or more layers of polyolefin with a different material in between the layer (long fiber mat or short fiber). Sandwich panel and laminar composites are common methods for thermoset resins and thermoplastic composites. This simple method can be applied to natural fiber-reinforced thermoplastic composites using the hot press method. The addition of a reinforcement material between polyolefin layers does not require extrusion or injection molding and minimizes fiber degradation [35, 36]. 

Figure 6.10. Schematic for a gas line for a CVD reactor. Carrier gases are bubbled through liquid precursors and the concentration of the saturated vapors reduced by a reflux condenser kept at the temperature of the wanted vapor pressure. Laminar composites are made by changing the reactant gas mixture during the deposition.

G. H. Staab, Laminar Composites, Elservier Science Ltd, Amsterdam 1999. 

M. A. Pinto, V.B. Chalivendra, Y.K. Kim, and A.E Lewis, Valuation of surface treatment and fabrication methods for jute fiber/epoxy laminar composites. Polyni. Compos. 35, 310-317 (2014). 

Distinguishing between pores, cavities, and pull-outs can be difficult in the preparation of thermally sprayed coatings, as well. Cavities remains between the solidified particles of the spray material. Occluded gases may also be present, and particles that are only partially bonded to one another may be broken apart by thermal stresses. These laminar composites are discussed in Section 4.4. However, it is only possible to make conditional recommendations for the preparation of the sprayed coating being examined in a given case. Success depends on experience and on the execution of the preparation steps in a manner that is appropriate for the given material. 

Figure 101. Laminar composite of graphite/AgCuTi braze/TZM, unetched, BF. Active braze with 70% silver, 27% copper, and 3% titanium. The braze has penetrated deeply into the porous graphite.

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