Isotropic Engineering Plastics

Because the fibers in mat are randomly oriented, mat-reinforced materials have essentially the same strength and elastic properties in all directions in the plane of the plate, that is, they are essentially isotropic in the plane. Consequently, the usual engineering theories and design methods employed for isotropic engineering materials may be applied. It is only necessary to know strength, modulus of elasticity, shearing modulus, and Poisson s ratio of the combined mat and resin. These can be obtained from standard stress-strain measurements made on specimens of the particular combination of fiber and plastic under consideration. 

Polymer composites are plastics within which fibres are embedded. The plastic is known as the matrix (resin) and the fibres dispersed witbin it are known as the reinforcement Thermosetting matrix materials include polyester, vinyl ester and epoxy resins. For higher temperature and extreme environments, bismaleimlde, polyimide and phenolic resins are used. Composites can be used to replace metal parts but care must be taken during design. Most engineering materials have similar properties in any direction (called isotropic) where composites have not This can however be offset by arranging the reinforcement layers in varying directions. 

In practice of rock engineering projects, the generalized HB criterion is suited to a homogeneous, isotropic and massive rock with few discontinuities or to a heavily jointed rock mass. When a cavern or cavern group is constructed in the heavily jointed rock mass, it has been observed that the surrounding rock failure depends on mutual separation and slip of discontinuities. For this case in the elasto-plastic analysis, it can be accepted that the three parameters of cr, m- and GSI m the generalized HB criterion are fixed to be constants, but D is treated as a variable related to the confining pressure... 

Chu, T.Y. and Hashin, Z. (1971) Plastic behavior of composites and porous media under isotropic stress. International Journal of Engineering Science, 9, 971-994. 

Fig. 151. Dependence of the storage modulus rf a 1 wt% PBLG solution in ben alcohol on (decreasing) tonperatuie (co = I rad/s, strain amplitode = 1%) from the isotropic solution into the gel phase. Reproduced finm Polym Eng Sd [Ref. 410] by the courts of the authors and (rfThe Society frf Plastics Engineers, Brodkfiekl, UK...

As reviewed viscoelastic behavior relates to deformations that are dependent on time under load and the temperatiu e. Therefore, when structural components are to be designed using plastics it must be remembered that the standard engineering equations that are available (Figs 2.31 and 2.32) have been derived under the assumptions that (1) the strains are small, (2) the modulus is constant, (3) the strains are independent of the loading rate or history and are immediately reversible, (4) the material is isotropic, and (5) the material behaves in the same way in tension and compression. 

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