Tension parallel

Fig. 4. The immediate effect of temperature on the modulus of elasticity of clear wood, relative to the value at 20°C. The plot is a composite of studies on the modulus as measured in hen ding, in tension parallel to grain, and in compression parallel to grain. VariabiUty in reported results is illustrated by the...

Extreme fiber in bendii Tension parallel to grain Compression perpendicular Compression parallel ... 

Surface and Interfacial Tension. Some properties of liquid surfaces are suggestive of a skin that exercises a contracting force or tension parallel to the surface. Mathematical models based on this effect have been used in explanation of surface phenomena, such as capillary rise. The terms surface tension (gas—liquid or gas—solid interface) and interfacial tension (liquid—liquid or liquid—solid) relate to these models which do not reflect the actual behavior of molecules and ions at interfaces. Surface tension is the force per unit length required to create a new unit area of gas—liquid surface (mN/m (= dyn/cm)). It is numerically equal to the free-surface energy. Similady, interfacial tension is the force per unit length required to create a new unit area of liquid—liquid interface and is numerically equal to the interfacial free energy. 

There is no known method for determining directly the surface tension of solids against gases. However, we can determine the difference adhesion tension in Eq (11-2) by resolving the surface tension parallel to the solid surface. Thus, for equilibrium... 

Figure 6.1 shows a typical stress-strain curve for a unidirectional SiCf/CAS composite loaded in uniaxial tension parallel to the fibers. The features of this curve are represenative of many ceramic matrix composites. In order to distinguish between the various damage states that a composite undergoes, it is convenient to divide the stress-strain curve into several sequential parts. 

It is emphasized that defines the state of stress and should not be confused with the orientation angle p. For example, — 45° and p = 45° represents equal biaxial tension acting on a sheet whose ellipse is at 45° to one of the chosen external principal stress axes. This is not equivalent to — 0 and ft = 0, which represents uniaxial tension parallel to the elliptic major axis (which is equivalent to = 90° and p — 90°). Because of symmetry considerations, all possible combinations of elliptic... 

Tension Parallel to the Grain. A tension parallel to the grain (T j) stress is a force trying to elongate the wood cells, or straws in... 

Creep. One of the most remarkable aspects of the deformation of polydiacetylenes is that it is not possible to measure any time-dependent deformation or creep when crystals are deformed in tension parallel to the chain direction (14,24). This behviour is demonstrated in Figure 3 for a polyDCHD crystal held at constant stress at room temperature and the indications are that creep does not take place at temperatures of up to at least 100 C (24). Creep and time-dependent deformation are normally a serious draw-back in the use of conventional high-modulus polymer fibres such as polyethylenes (28). Defects such as loops and chain-ends allow the translation of molecules parallel to the chain direction in polyethylene fibres. In contrast since polydiacetylene single crystal fibres contain perfectly-aligned long polymer molecules (cf Figure lb) there is no mechanism whereby creep can take place even at high temperatures. 

Fig. 22. The stress dependence of the isochronal creep moduli of oriented PET, tested in tension parallel to the initial draw (after CrolP ).

Belt take-up A belt pulley, generally under a conveyor belt and in the drive pulley, kept under strong tension parallel to the belt line. Its purpose is to automatically compensate for any slack in the belting created by start-up. 

We neglect here the viscoelasticity of the pofymer matrix, and treat fibres and matrix as being linearly elastic. Under tension parallel to axis 1, Hooke s law then relates stresses to strains for fibres, matrix, and composite as a whole ... 

Schematic sketches of other failure modes in the aligned fibre composite (a) tension parallel to axis 2 and (b) shear parallel to axis 1. 

The injection-moulded bar described in Example 6.9 is tested to failure in tension parallel to the axis of the bar. Predict the mean fibre stress when the bar fails, and the tensile strength of the bar. Take the tensile strengths of the carbon fibres and the nylon to be 3200 MPa and 70 MPa, respectively, and assume the shear strength of the carbon fibre-i lon interface to be 32 MPa. 

In practice, there must be some transverse stress. Since Poisson s ratios of matrix and fibres normally differ, in the absence of transverse stress there would be incompatible transverse strains—fibres would no longer fit in their holes in the matrix Often, Poisson s ratio of the fibres is less than that of the matrix, e.g. for glass fibres VfsO.2, and for most plastics v -= 0.3-0.4. This has an advantageous result. Under tension parallel to the fibres, the matrix exerts a pressure on the fibres which assists in maintaining adhesion between them. 

For each of the following short-fibre composite materials, tested in tension parallel to the fibres, predict the tensUe modulus. (Percentages given are fibre volume firactions.)... 

Similar results may be obtained when the critical volume fraction of fibres is considered. It may be easily defined for aligned long or even continuous fibres in an element subjected to tension parallel to the fibre direction. This reinforcement may carry on the load after cracking of the matrix. Therefore, the value of Vf is derived from the simple equivalence of forces carried on by a composite element of cross-section area A and by the fibres A E = A (Tf , here = , eJ(Tf , and the product represents the cracking tensile stress in the matrix and is the ultimate tensile stress in the fibres. 

Goebel et al. [88] have found that the particle concentration decreases with conversion and the decrease is inversely proportional to emulsifier concentration. The surface tension parallels the number of particles and both abruptly increase with conversion up to 10% conversion. The higher the emulsifier concentration, the higher the number of particles which are observed. Keeping a constant concentration of emulsifier, the number of particles decreases with increasing monomer concentration. When the monomer pressure decreased by one third, the number of particles increased twice. This was ascribed to variation of the emulsifier adsorption activity with the monomer pressure. The particle concentration remains constant if the small amount of inhibitor is added. The growth of polymer particles was reported to proceed by the propagation of monomer as well as by the association of polymer particles. 

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