Tensile Strength and Elastic Moduli

If we consider, for instance, a composite material reinforced with continuous fibers, the directions parallel and perpendicular to the fibers are the major axes. 

Loading parallel to fibers. First, if we applied an overall load, F, in N on the composite along the direction of the fibers, this load is carried either by the fibers and the matrix. Moreover, assuming a good bond between matrix and fibers, both stretch similarly and if the loading is isostrain, i.e., all strains are equals, we have  

Therefore, the total load that the composite must withstand corresponds to the sum of individual loads, i.e., the load carried by fibers and matrix respectively, as follows  

Introducing the compression or tension stresses (o, and aj and the cross sectional areas (Af and A ) of each material, we can replace loads by the product, stress times cross section area, and hence we obtain the simple equation  

Rearranging the above equation by dividing by the total cross sectional area of the composite, we obtain an equation giving the stress of the composite material as a function of the stress and surface area fractions of the matrix and the fibers, i.e., volume fractions, since in this case the length of matrix, fibers, and composite are the same  

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Usually, crystallization of flexible-chain polymers from undeformed solutions and melts involves chain folding. Spherulite structures without a preferred orientation are generally formed. The structure of the sample as a whole is isotropic it is a system with a large number of folded-chain crystals distributed in an amorphous matrix and connected by a small number of tie chains (and an even smaller number of strained chains called loaded chains). In this case, the mechanical properties of polymer materials are determined by the small number of these ties and, hence, the tensile strength and elastic moduli of these polymers are not high. 

An important application of polydimethylsilane is as a source of silicon carbide (SiC) fibres, which are manufactured under the trade-name Nicalon by Nippon Carbon in Japan. Heating in an autoclave under pressure converts polydimethylsilane to spinnable polycarbosilane (-Me2Si-CH2-) with elimination of methane. The spun fibres are then subjected to temperatures of 1200-1400 °C to produce silicon carbide fibres with very high tensile strengths and elastic moduli." As a result of their conductivity, polysilanes have also been used as hole transport layers in electroluminescent devices. In addition, the photoconductivity of polymethylphenylsilane doped with Cgo has been found to be particularly impressive. ... 

Fig. 5.2-1. Mechanical properties of fibers, T = 27°C. a) Stress-strain curves b) Tensile strength and elasticity moduli...

Figure 6-38. Tensile strengths and specific moduli of elasticity of different types of composite plastics including whisker fibers, as compared to aluminum, titanium, and steel.

Figure 10.15. Compai isoii of tensile strengths (a), elastic moduli (b) and percent elongations (c) for 130 pm fibers of PP, SWNT/PP and f-SWNT/PP [50]...

The fibers are manufactured in bundles of several 100 filaments and exhibit tensile strengths of 1.4 to 2.3 GPa and elasticity moduli of 320 to 380 GPa. Disadvantages of the fibers formerly manufactured by DuPont (types FP and PRD-166, are no longer manufactured) are their high density (3.6 to 4.2 Mg/m ) and the brittleness of the ca. 20 pm in diameter fibers (see Table 5.2-16). Fibers from Mitsui Mining (type ALMAX) with a diameter of 10 pm are commercially available at a price of 1900 DM/kg. They are mainly utilized in the reinforcing of aluminum and magnesium. 

Polyim-phenylene isophthalamide) forms lyotropic liquid crystals. Fibers can therefore be spun at lower concentration from isotropic solutions and at higher concentrations from nematic solutions. Because of their high degree of chain segment orientation, fibers spun from nematic solutions show, as expected, higher moduli of elasticity, tensile strengths and lower elongation at rupture values than those spun from isotropic solutions. 

Filaments with very high tensile strengths and moduli of elasticity in the fiber direction can be obtained by drawing, annealing, and fixing (see also Table 38-1). For example, highly drawn poly(ethylene) fibers can be obtained... 

In the textile industry, tensile strenths and moduli of elasticity are mostly incorrectly given as mass/titer instead of correctly in terms of mechanical stresses. Consequently, the tensile strengths of the textile industry actually represent breaking lengths (tenacities). In contrast, mechanical stresses as prescribed by the SI system are used for industrial fibers. In this book, tensile strengths and moduli of elasticity will also be given in terms of mechanical stresses so that the mechanical properties of fibers can be compared with those of thermoplasts, elastomers, etc. 

Si-C-N-0 and Si-C-N(O) fibers exhibit linear elastic tensile behavior up to failure. At room temperature HPZ based fibers have tensile strengths ranging from 1.9 GPa [22] to 3.1 GPa [21], and elastic moduli ranging from 200 GPa [22] to 260 GPa [21]. PCSZ based fibers have similar mechanical properties. Oxygen cured Si-C-N-0 ceramic fibers and radiation cured Si-C-N(O) fibers have moduli of 175 GPa and 215 GPa, respectively, and tensile strengths of 1.8 GPa and 2.4 GPa, respectively [10-11]. 

The properties related to the relationships between stress and strain, such as compressive and tensile strengths and moduli, associated with elastic and inelastic reaction to the apphed force. 

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