Design Guidelines in Nonconventional Composite Laminate Optimization

Abstract

Nonconventional laminates, as defined in this paper, are laminates where ply angles are not restricted to a finite set: for example 0, 45, −45 −45 and 90 deg. Removing this restriction, structural behavior (for example, postimpact behavior) can be significantly improved. Nonconventional laminates are made of straight or steered fibers leading to constant- or variable-stiffness composites. In traditional composite design, empirical guidelines are imposed, guaranteeing the robustness of the composite. This paper presents a method to take design guidelines into account during nonconventional laminate optimization. The 10% rule is interpreted as a lower bound on the degree of isotropy and formulated as a positive-semidefinite matrix constraint. Other guidelines are interpreted as bounds on ply angle or angle difference, or the number of variables is reduced by fixing variables to user-defined values. Numerical results optimizing a plate under biaxial tension for strength demonstrate the optimizer generates nonconventional laminates obeying all guidelines, performing at least as well as conventional composites. For variable-stiffness laminates, a flat plate under uniaxial compression is optimized for buckling under a stiffness constraint. Applying the 10% rule, almost half of the improvement over conventional laminates is lost compared to the unconstrained case, demonstrating the importance of including design guidelines during nonconventional laminate optimization.