Orthotropic and Monoclinic Behaviour:

One of the important differences of the laminated composite as compared to metals is that they are orthotropic in nature. Further, when these are transformed about an axis then the behaviour of the material changes from orthotropic to monoclinic material. The terms like  and   occur in constitutive (stiffness) relations when an orthotropic material is transformed about an axis. Thus, this makes the design and analysis little complicated and longer. Further, these terms result in unwanted coupling behaviour. The coupling effect is explained in the following.

Coupling Terms in Laminate Constitutive Relations:

The CLPT is widely used in laminate analysis. As we have studied earlier, we know that there are many coupling terms. These coupling terms are given in brief:

 - coupling between inplane normal and shear coupling for membrane action
 - coupling between membrane and bending actions
 - coupling between membrane and twisting actions
 - coupling between membrane shear and twisting actions
 – coupling between bending and twisting actions

The knowledge of these coupling is very much important as one coupling actions may lead to an unwanted action. At the same time it may helpful in design. For example, the extension-twisting coupling through terms can be harnessed in the manufacturing of helicopter blades where a pre-twisting of blade is done.