The normals are presumed to have constant length so that the strain perpendicular to the middle surface is ignored i.e. = 0. Under these assumptions, the strain-displacement relations for a point at a distance z from the middle surface of the laminate are given by

	(16.1)

or,        

(16.2)

where   is the mid plane strain vector and   is the curvature vector; u, v are the tangential mid plane displacements. The constitutive relationship for the k_th layer with respect to the principal material axis is given by

(16.3)

The elements of the matrix Q are expressed in terms of elastic constants as

	(16.4)

Of the 5 engineering elastic constants, only 4 are independent and the fifth is evaluated from the relation

(16.5)