Symmetric laminate with generally orthotropic

Table 4-3 Symmetric Laminate with Five Generally Orthotropic Layers...

P(3) Laminates that are subject to bending-stretching-torsion coupling are defined as generally orthotropic. Typically, generally orthotropic laminates have a symmetrical and balanced stacking sequence of plies, with an appreciable amount (more than about 20%) of reinforcement orientated at other than 0° and 90° (e.g. +45° or -45°). 

For many composite applications the laminate is not orthotropic, but anisotropic, though this can exist in many particular forms, one of which occurs when an orthotropic laminate is loaded in a direction which does not coincide with one of the principal axes, or when the lay-up is symmetric but not balanced about a principal reference axis. A [0°/+45°/90°]s laminate is balanced and quasi-isotropic. As a general rule, all laminates should be symmetrically laid up about their mid-plane to avoid stretching bending coupling. 

Because of the analytical complications involving the stiffnesses Ai6, A26, D g, and D26, a laminate is sometimes desired that does not have these stiffnesses. Laminates can be made with orthotropic layers that have principal material directions aligned with the laminate axes. If the thicknesses, locations, and material properties of the laminae are symmetric about the middle surface of the laminate, there is no coupling between bending and extension. A general example is shown in Table 4-2. Note that the material property symmetry requires equal [Q j], of the two layers that are placed at the same distance above and below the middle surface. Thus, both the orthotropic material properties, [Qjjlk. of the layers and the angle of the principal material directions to the laminate axes (i.e., the orientation of each layer) must be identical. 

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