There is a great demand for optimization of high performance composite structures, particularly in the aerospace sector. To avoid vibrational resonance in aerospace and naval structures, laminated plates can be designed using fundamental frequency constraints. Composite panels are particularly attractive because the vibration response can be optimised by tailoring the fibre angles of different layers without incurring weight penalties. A viable alternative is to use circumferential and radial layers for optimizing fundamental frequencies. This work focuses on the application these two types of theoretical layers and investigating its efficiency, for plates with and without cutouts.

In this work a novel modification of the well-known Whitney-Nuismer criteria is proposed with the aim of having a simple and yet accurate fracture criterion for the important problem of estimating the strength of composite laminates with circular open-holes. The novelty is the use of a critical path curve that makes the proposed criteria quite general and provides very good correlation with experimental data for a wide range of laminates and hole sizes even when the characteristic lengths are assumed as material constants. This assumption dramatically reduces the amount of required experimental testing making these criteria very suitable for preliminary design.

A process to efficiently design composite wing boxes is presented. It uses analytical and semi-empirical equations for failure modes such as material strength, plate buckling, stiffener column buckling and stiffener flange or web crippling. Laminate layups for the different components are selected in accordance with basic engineering rules and guidelines and are updated as necessary to meet the local loads. The emphasis is in allowing buckling of skins at any fraction of the ultimate load and allowing local load redistribution from buckled to non-buckled panels to save weight. The design process is automated and the design can be automatically transferred over to a commercial finite-element code for detailed design and validation. The effects on weight of number of spars, ribs, and stiffeners as well as the fraction of ultimate load at which buckling is allowed are examined and insight is gained to which of these the weight is most sensitive to. In addition, the effect of minimum gage on weight was found to be a driver.