Introduction

In the previous lecture we have derived constitutive equation for planar state of stress in a lamina. We have derived these constitutive equations in principal material and global directions. In this lecture, we are going to see a practical application of the planar constitutive equations in industry. Although, the engineering constants in principal material directions are known, it is difficult to comment on the engineering constant for off axis lamina, instantly. When laminae are used for designing a structure, the engineering constants in global directions become very useful for a quick estimate of the behavior of the structure under certain loads. Thus, for practical application purpose, the various lamina engineering constants are obtained and their variation for fibre orientation between  for a range of composite materials is given together. Thus, a designer can use the required lamina with appropriate fibre orientation and material.

Here we are going to obtain engineering constants for any off axis lamina as a function of engineering constants of that lamina in principal material directions and fibre orientation. This can be done with the help of lamina constitutive equation with appropriate one dimensional state of stress.

We have constitutive equation in global directions as given in Equation (4.5)

Thus, for a given state of stress in global directions we can find the strains in global directions from this equation.