Optimization of advanced composite wing box structures subjected to combined loads and impact

A detail design of Solar powered UAV was completed, under coordination of first author, to obtain an High-Altitude Very-Long Endurance Solar–Powered Autonomous Aircraft (VESPAA-UAV) flying at stratospheric altitudes of 17–25 km and with an endurance of more than six months. Two different configurations are under investigation in order to understand the best solution that completely fulfil the a priori imposed constraints. The main goal is to obtain a structure as light as possible that ensures the capability to resist at loads evaluated from the flight envelope without catastrophic failures. A genetic algorithm has been developed to optimize composite laminates subjected to mechanical, thermal and hygroscopic loads. A set of penalty function has been defined in order to guarantee deformations less than the allowable limit and to avoid local or global buckling. A Progressive Ply Failure Analysis (PPFA) code has been developed in order to evaluate the Last Ply Failure (LPF) Loads of composite laminates; Tsai-Wu criterion has been chosen as a failure criterion. After a generic first ply failure (FPF), stiffness matrix has been updated considering a degradation factor of zero. After FPF, loads increments are proportional to the initial applied loads. The deformation induced from previous loads have been taken into account, in order to restore the linearity between applied loads and rupture index. The routine continues iteratively until Last Ply Failure. To validate the PPFA code a series of composite laminates have been studied and results have been compared with both experimental and FE results; a very good agreement is obtained and results and pertinent conclusion are reported. Genetic Algorithm and PPFA have been coupled in order to optimize the lay-up of each panel that ensure the minimum wing box weight, a proper safety margin from FPF to LPF and respects of the penalty function. In particular, no failure are allowed at limit load while, at ultimate load, each panel must stay under LPF level. Furthermore, since impact damage is the most restriction parameter to be considered, several panels with damage from impact were tested to shear fatigue loads in order to evaluate the delamination propagation. An approach to reliability based design shall also be presented in order to ensure a given probability of failure.