Strength and modulus

Aluminum nitride and silicon nitride, like other refi tory carbides and nitrides, have die ability to deform plastically to some degree above the ductile-to-britde transition temperature. Below that temperature, they are intrinsically brittle (for discussion, see Sec. 4.3 of Ch. 4). 

Hexagonal boron nitride, formed by CVD, is highly anisotropic and the basal planes (ab directions) can slip over one another as temperature increases. Thus brittle fiacture can be avoided. As a result, the strength increases with temperature as shown in Fig. 13.6, while the modulus generally decreases. 

Boron nitride is an unusual material since its properties vary considerably depending on its structure (hexagonal or cubic). For instance, h-BN is a soft and lubricious material wiiile c BN is next to diamond in hardness. Cubic boron nitride maintains its hardness to ISOO CI ira (see Sec. 4.4 of Ch. 4). 

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After the washing, the fiber is dried, and then is heat-drawn in the same manner as in the case of dehydration—coagulation with salt but to a much higher draw ratio. As a result the finished fiber has high strength and modulus and is, without acetalization, sufficientiy resistant to boiling water. Figure 3 shows schematic fiber stmctures (17). 

Density and polymer composition have a large effect on compressive strength and modulus (Fig. 3). The dependence of compressive properties on cell size has been discussed (22). The cell shape or geometry has also been shown important in determining the compressive properties (22,59,60,153,154). In fact, the foam cell stmcture is controlled in some cases to optimize certain physical properties of rigid cellular polymers. 

Tensile strength and modulus of rigid foams have been shown to vary with density in much the same manner as the compressive strength and modulus. General reviews of the tensile properties of rigid foams are available (22,59,60,131,156). 

Those stmctural variables most important to the tensile properties are polymer composition, density, and cell shape. Variation with use temperature has also been characterized (157). Flexural strength and modulus of rigid foams both increase with increasing density in the same manner as the compressive and tensile properties. More specific data on particular foams are available from manufacturers Hterature and in References 22,59,60,131 and 156. Shear strength and modulus of rigid foams depend on the polymer composition and state, density, and cell shape. The shear properties increase with increasing density and with decreasing temperature (157). 

Two approaches have been taken to produce metal-matrix composites (qv) incorporation of fibers into a matrix by mechanical means and in situ preparation of a two-phase fibrous or lamellar material by controlled solidification or heat treatment. The principles of strengthening for alloys prepared by the former technique are well estabUshed (24), primarily because yielding and even fracture of these materials occurs while the reinforcing phase is elastically deformed. Under these conditions both strength and modulus increase linearly with volume fraction of reinforcement. However, the deformation of in situ, ie, eutectic, eutectoid, peritectic, or peritectoid, composites usually involves some plastic deformation of the reinforcing phase, and this presents many complexities in analysis and prediction of properties. 

Polyester. This fiber has several performance advantages versus polypropylene, although it is less economical. Polyester can produce higher tensile strength and modulus fabrics that are dimensionally stable at higher temperatures than polypropylene. This is of importance in selected appHcations such as roofing. Polyester fabrics are easily dyed and printed with conventional equipment which is of extreme importance in apparel and face fabrics although of lesser importance in most spunbonded appHcations (see Fibers, polyester). 

Carbon-Fiber Composites. Cured laminates of phenoHc resins and carbon-fiber reinforcement provide superior flammabiHty resistance and thermal resistance compared to unsaturated polyester and epoxy. Table 15 shows the dependence of flexural strength and modulus on phenoHc—carbon-fiber composites at 30—40% phenoHc resin (91). These composites also exhibit long-term elevated temperature stabiHty up to 230°C. 

Empirical attempts have been made to relate strip and grab test results, particularly for cotton fabrics, so that if one strength is known, the other can be calculated. The relationship is complex, depending on fiber strength and modulus, yam size and crimp, yam-to-yam friction, fabric cover factor, weave, weight, and other factors (19). 

Strength and modulus normali2ed to linear density (Tex = (g-wt)/km), an appropriate basis for materials of similar density however, this breaks down for dense materials (eg, steel) because the basis for tire use is better described by properties per unit cross section (MPa) rather than weight. 

A key feature implicitly included in the use of organic fibers for tire reinforcement is the abiHty to retain strength and modulus characteristics at elevated temperatures (80—120°C for most tires) sufficient to sustain service demands. Thus, a tire which may appear to be overdesigned at room temperature is, in many cases, reflecting the changes in properties experienced at operating temperatures. 

The mechanical properties of wood tend to increase when it is cooled and to decrease when it is heated (6,18). If untreated wood heated in air is not exposed to temperatures of more than - 70° C for more than about 1 year, the decrease in properties with increasing temperature is referred to as immediate or reversible ie, the property would be lower if tested at the higher temperature but would be unchanged if heated and then tested at room temperature. The immediate effect of temperature on strength and modulus of elasticity of clear wood, based on several different loading modes, is illustrated in Figures 4—6 (6). 

Fig. 6. Each of carbonization temperature on PAN-based carbon fiber strength and modulus (31). To convert GPa to psi, multiply by 145,000.

Aramid Fibers. Aromatic polyamide fibers exhibiting a range of mechanical properties are available from several manufacturers, perhaps the best known being Du Pont s proprietary fiber Kevlar. These fibers possess many unique properties, such as high specific tensile strength and modulus (see Fig. 4). Aramid fibers have good chemical resistance to water, hydrocarbons, and solvents. They also show excellent flame retardant characteristics (see High PERFORMANCE fibers Polyamdes). 

Fibrous fillers are often embedded in a laminar form. The fibres used have higher moduli than the resins in which they are embedded so that when the composite of resin plus fibre is strained in the plane of the fibrous layer the bulk of the stress is taken up by the fibre. This results in an enhancement of both strength and modulus when compared with the unfilled resin. 

The absorbed water has a plasticising effect and thus will cause a reduction in tensile strength and modulus, and an increase in impact strength. As has already been mentioned the presence of absorbed water also results in a deterioration of electrical properties. 

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