The challenge of achieving hygrothermal stability in composite laminates with optimal couplings

The necessary and sufficient conditions for hygrothermal curvature stability of composite laminates have been derived previously, and their development is summarized in this work. These conditions are shown to be material independent. A constrained optimization routine is implemented to arrive at hygrothermally stable stacking sequences that are optimal for each of bend-twist and extension-twist coupling. The necessary and sufficient conditions for hygrothermal stability are used as constraints, while the compliance coefficients from classical lamination Theory are used as the objective functions. Both sequential quadratic programming and ant-colony optimization routines are used to ensure global optimality. Laminates consisting of two through ten plies are considered. Laminates consisting of five through ten plies are manufactured and tested to demonstrate the achievable level of extension-twist or bend twist coupling. A geometrically nonlinear model is presented to predict the response of bend-twist-coupled laminates. Comparison with the previous state-of-the-art stacking sequence for achieving extension-twist coupling with hygrothermal stability demonstrates a nearly 80% increase in the level of coupling.