ABSTRACT
The stiffness of unidirectional composites with off-axis ply orientation is important because composite laminates are normally made of off-axis in addition to on-axis plies. This chapter shows how to determine the contribution to the laminate stiffness by each ply or ply group. One of the most important features of composite materials is the variation of properties as the ply orientation changes. The stiffness of an off-axis unidirectional composite is governed by appropriate stress-strain relations as before. The functional relationship remains the same. Although the number of constants has increased from 4 to 6, the number of independent constants remains the same at 4. The concept of the invariants is also very important in composite materials. The magnitude of the components of stiffness and compliance vary as a function of ply orientation, but the area under the transformed components remains constant. This constant value is equal to the particular invariant associated with each component of stiffness or compliance.
