ABSTRACT
This chapter describes the stiffness of multidirectional laminates consisting of plies and ply groups with arbitrary orientations. It shows that the stiffness properties of bidirectional laminates can vary significantly and be fundamentally different from conventional materials. The chapter examines some commonly encountered symmetric laminates and determines the values of their in-plane stiffness and compliance. Moreover, with symmetric laminates one must double the number of ply groups within a quasi-isotropic laminate. The in-plane stiffness of a laminated composite can be obtained directly by applying the rule of mixtures equation to the stiffness of the unidirectional composite. The in-plane compliance is simply the inverse of the in-plane stiffness. Finally, if engineering constants are desired, they can be obtained from the components of the in-plane compliance. The relationship between engineering constants of the constituent plies to those of a laminate is very complicated. In the process, all the components of the stiffness participate in the determination of each component of compliance.
