Free Vibration, Buckling and Design Optimisation of Composite Pressure Hulls

Composite materials are being extensively used for underwater applications due to their high specific stiffness and strength and superior corrosion resistance. The structural weight of the composite pressure hulls intended to operate under high operating depths should be minimized for increasing payload, speed and operating range. Apart from minimizing the weight, these structures should resist the specified buckling loads and should not transmit vibrations that are nearer to the frequencies generated inside the structure and hence the weight is to be minimized under minimum buckling pressure specified and also should have natural frequencies of vibration much higher than those that are in the excitation range of equipment mounted in side the structure. An attempt is made in the present study to minimize structural weight of isotropic and composite stiffened shells for under water applications under given buckling pressure and frequency constraints using PSO based algorithm. PSO is being developed recently and it is well documented in literature that PSO is fairly accurate for determining global minimum when compared to other evolutionary algorithms Viz Genetic algorithm, simulated annealing etc. Buckling pressure and free vibration natural Frequency of a stiffened composite shell depends on various parameters Viz rib thickness, rib spacing, rib width, shell thickness, winding angles in each layer, rib height and stacking sequence. The effect of above parameters on the Buckling pressure and Frequency is studied in detail prior to optimizing the stiffened composite shell under buckling pressure and Frequency constraints. Love-Kirchhoff equations and Jone's equations are used for computation of Natural Frequency and buckling pressures of stiffened composite hulls. Different composite materials Carbon Epoxy, Boron epoxy, Glass Epoxy are considered for design optimization. Carbon epoxies are used for higher operating depths, while Glass epoxies are used for less operational depths. Carbon epoxy is also 100 times more costly than Glass epoxy and hence cost also need to be considered while selecting the above composite materials. The stresses along the fibers and in the direction is perpendicular to the fiber in the different layers of the composite materials namely Carbon epoxy, Boron epoxy and Glass epoxy (90 and 45) under given external pressure are computed. T Sai Hill and T sai - Wu failure theories are applied to the different layers to confirm that all the plies are safe.