Hygrothermally stable laminated composites for optimal extension twist coupling in closed cell configuration

An optimization routine utilizing the sequential quadratic programming method is used to develop laminates that are hygrothermally stable in flat-strip configuration and are optimal for extension twist coupling in closed cell configuration. The performance of the laminates obtained from the optimization in terms of the level of extension twist coupling and hygrothermal stability are evaluated and compared to laminates from a previous routine. A parametric study is conducted to illustrate the relationships between the geometric dimensions of a representative closed cell section and the level of extension twist coupling. Results from the parametric study suggested a more practical optimization approach in which the trend between extension twist compliance and stiffness is the optimization goal. This approach produces laminates that are more material efficient and have a better trend between extension twist coupling and stiffness. A case study on a blade section of an XV-15 tilt rotor illustrates the advantage of the new optimization approach.