Directional property quasi-isotropic

Quasi-isotropic laminates have the same ia-plane stiffness properties ia all directions (1), which are defined ia terms of the [A] matrix of the laminate. For the laminate to be quasi-isotropic. 

The term quasi-isotropic iaminate is used to describe laminates that have isotropic extensionai stiffnesses (the same in all directions in the plane of the laminate). As background to the definition, recall that the term isotropy is a material property whereas laminate stiffnesses are a function of both material properties and geometry. Note also that the prefix quasi means in a sense or manner. Thus, a quasi-isotropic laminate must mean a laminate that, in some sense, appears isotropic, but is not actually isotropic in all senses. In this case, a quasi-isotropic... 

Traditional fabric reinforcements have fibers oriented at 0 and 90°. For the sake of completeness, we note that triaxial fabrics, which have fibers at 0°, -1-60°, and -60°, are now commercially available. Composites using a single layer of this type of reinforcement are approximately quasi-isotropic, which means that they have the same in-plane elastic (but not strength) properties in every direction. Their thermal conductivity and CTE are also approximately isotropic in the plane of the fabric. 

As a result of the low transverse strengths of unidirectional laminates, they are rarely used in structural applications. The design engineer selects laminates with layers in several directions to meet requirements for strength, stiffness, buckling, etc. There are an infinite number of laminate geometries that can be selected. For comparative purposes, it is useful to consider quasi-isotropic laminates, which have the same elastic properties in aU directions in the plane. 

It is generally believed that the polarizabilities of monatomic ions and molecules are independent of field direction. For undistorted quasi-spherical molecules (e.g. CH4, CC14, etc.) the same is usually assumed. When two such atoms are held together, as in a diatomic molecule, the new system is not isotropically polarizable. The model discussed by Silberstein (1917) makes this understandable. If a unit field acts along the line of centres A-B it will induce primary moments parallel to itself in both A and B, and likewise if it acts at 90° to A-B. Each primary moment will induce a secondary moment in its neighbour in the first case the secondary moments will add to the primary moments, but in the second they will subtract. Hence b along the line of centres exceeds that across it, and the polarizability of A-B is an anisotropic property. A similar situation is to be expected with the majority of polyatomic ions or molecules (see Table 21). 

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