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No description available.

No description available.

No description available.

Once the requirements for a new aircraft have been defined, the Conceptual design phase is launched. During this phase one or more designers have the goal of defining and investigating a number of alternative solutions. Through discussion with industry it has become apparent that optimisation tools are seldom used, even though these could greatly enhance the work of the designers. The objective of the work carried forward has been of identifying, comparing and where necessary improving the most suitable techniques, as well as schemes for their integration, in order to perform effectively Multidisciplinary Design and Optimisation (MDO) in the Conceptual phase of the aircraft design. The techniques that have been investigated include: multi-objective optimisation algorithms, MDO algorithms for treating non-hierarchically decomposable systems and Automatic Differentiation (AD). As a result an algorithm for performing multiobjective MDO has been developed. Given a complete model for a complex non-hierarchically decomposable system and given a number of objectives and constraints, the algorithm is capable of determining a set of well distributed solutions, representative of both local and global Pareto frontiers. A number of test cases have been used for evaluating the alternative methodologies and the proposed algorithm. These include a set of complex algebraic test cases typically used for evaluating global optimisation algorithms and a simplified aircraft conceptual design model, which was provided by industry. The results demonstrate the unique capability of the algorithm of determining well distributed solutions on the global and local Pareto frontiers for global multiobjective continuous nonlinear constrained optimisation problems. The results also show this capability when the algorithm is applied to non-hierarchically decomposable systems, as typically encountered when performing MDO. Further work could extend the approach in order to handle mixed discrete/continuous variables.

This thesis documents the findings of a three year experimental investigation into the impact damage resistance and damage tolerance of composite honeycomb sandwich panels. The primary area of work focuses on the performance of sandwich panels under quasi-static and low-velocity impact loading with hemispherical and flat-ended indenters. The damage resistance is characterised in terms of damage mechanisms and energy absorption. The effects of varying the skin and core materials, skin thickness, core density, panel boundary conditions and indenter shape on the transverse strength and energy absorption of a sandwich panel have been examined. Damage mechanisms are found to include delamination of the impacted skin, core crushing, limited skin-core de bonding and top skin fibre fracture at high loads. In terms of panel construction the skin thickness is found to dominate the panel strength and energy absorption with core density having a lesser influence. Of the external factors considered the indenter noseshape has the largest effect on both failure load and associated damage area. An overview of existing analytical prediction methods is also included and the most significant theories applied and compared with the experimental results from this study. The secondary area of work expands the understanding obtained from the damage resistance study and assesses the ability of a sandwich panel to withstand in-plane compressive loading after sustaining low-velocity impact damage. The importance of the core material is investigated by comparing the compression-after-impact strength of both monolithic carbon-fibre laminates and sandwich panels with either an aluminium or nomex honeycomb core. The in-plane compressive strength of an 8 ply skinned honeycomb sandwich panel is found to be double that of a 16 ply monolithic laminate, with the type of honeycomb also influencing the compressive failure mechanisms and residual compressive strength. It is concluded that under in-plane loading the stabilising effect of the core opposes the de-stabilising effect of any impact damage.