5.1 Basic Assumptions

It is apparent that the overall behavior of a multidirectional laminate is a function of the properties and stacking sequence of the individual layers. The so-called classical lamination theory predicts the behavior of the laminate within the framework of the following assumptions: ^[1], ^[2], ^[3]

1. Each layer (lamina) of the laminate is quasihomogeneous and orthotropic.

2. The laminate is thin with its lateral dimensions much larger than its thickness and is loaded in its plane only, i.e., the laminate and its layers (except for their edges) are in a state of plane stress ( ? _z = ? _xz = ? _yz = 0).

3. All displacements are small compared with the thickness of the laminate ( u, v, w ? h).

4. Displacements are continuous throughout the laminate.

5. In-plane displacements vary linearly through the thickness of the laminate, i.e., u-and v displacements in the x- and y-directions are linear functions of z.

6. Transverse shear strains ? _xz and ? _yz are negligible. This assumption and the preceding one imply that straight lines normal to the middle surface remain straight and normal to that surface after deformation.

7. Strain-displacement and stress strain relations are linear.

8. Normal distances from the middle surface remain constant, i.e., the transverse normal strain ? _z is negligible (compared with the in-plane strains ? _x and ? _y).