Seeking to quench airliners’ unending thirst for lightweight, reliable and more comfortable seating solutions, designers are developing a new generation of slim economy – class seats. Challenge in front of the designers is to carve out additional “living space”, as well as to give a “lie – flat” experience to air travellers with strict adherence to safety regulations. Present research tries to address all these industry needs through an innovative and novel “Sleep Seat”. A generous angle of recline (40 degree), movement of “Seat Pan” along the gradient, fixed outer shell of backrest, and unique single “Forward Beam” design distinguishes “Sleep Seat” form current generation seats. It is an ultralightweight design weighing 8kg (typical seat weight is 11kg). It satisfies “Generic Requirements (GR2)” which ensures “Comfort in Air”. It will be a “16g” seat, means it can sustain the “Emergency landing” loads as specified by “Certification Specifications (CS 25.561 and CS 25.562)”. For present research, only CS 25.561 has been considered. Since, the design of “Sleep Seat” is still in its conceptual phase, it is not possible to build the prototypes and their physical testing, due to costs and time involved. “Finite Element Analysis (FEA)” is a useful tool to predict the response of the structure when subjected to real life loads. Hence, the aim of research being undertaken is to develop a detailed FE model of the complete seat structure, which will help designers to identify potential weak areas and to compare different design concepts virtually, thereby reducing the development cycle time. In order to avoid handling of large number of design variables; major load carrying members (called Primary Load Path) i.e. Forward beam and leg; are designed for the most critical “Forward 9g” loads; using FEA results as a basis. A robust framework to verify the FEA results is developed. “Sequential Model Development Approach”; which builds the final, detailed FE model starting from preliminary model (by continuously updating the FE model by addition of details that are backed up by pilot studies); resulted in a FE model which could predict the stress induced in each of the components for applied CS 25.561 loads along with “Seat Interface Loads”. The “Interface Load” is the force exerted by the seat design on the floor and is one of the main contributing factors in seat design. “Optistruct” is used as a solver for linear static FEA, whereas “Abaqus / Standard” is used for non-linear FEA. Stepwise methodologies for mesh sensitivity study, modelling of bolt-preload, representing bolted joint in FEA, preventing rigid body motion, and obtaining a converged solution for non-linear FEA are developed during this research. Free-Shape Optimisation is used to arrive at a final design of Seat-leg. All the findings and steps taken during this are well documented in this report. Finally, a detailed FE model (involving all the three non-linearities : Contact, material and geometric) of the complete seat structure was analysed for the loads taken from CS 25.561, and it was found that design of “Forward beam” and leg are safe against CS 25.561. Therefore, all the aims and objectives outlined for this research were accomplished. For future work, first area to look for, would be validation of present FEA results by experimental testing. FE model to simulate dynamic loads CS 25.562 can be developed followed by design improvements and optimisation.

It is widely accepted that, for many buckling problems of plates and shells in the plastic range, the flow theory of plasticity either fails to predict buckling or significantly overestimates buckling stresses and strains, while the deformation theory, which fails to capture important aspects of the underlying physics of plastic deformation, provides results that are more in line with experimental findings and is therefore generally recommended for use in practical applications. This thesis aims to contribute further understanding of the reasons behind the seeming differences between the predictions provided by these two theories, and therefore provide some explanation of this so-called ‘plastic buckling paradox’. The study focuses on circular cylindrical shells subjected to either axial compression or non-proportional loading, the latter consisting of combined axial tensile stress and increasing external pressure. In these two cases, geometrically nonlinear finite-element (FE) analyses for perfect and imperfect cylinders are conducted using both the flow and the deformation theories of plasticity, and the numerical results are compared with data from widely cited physical tests and with analytical results. The plastic buckling pressures for cylinders subjected to non-proportional loading, with various combinations of boundary conditions, tensile stresses, material properties and cylinder’s geometries, are also obtained with the help of the differential quadrature method (DQM). These results are compared with those obtained using the code BOSOR5 and with nonlinear FE results obtained using both the flow and deformation theories of plasticity. It is found that, contrary to common belief, by using a geometrically nonlinear FE formulation with carefully determined and validated constitutive laws, very good agreement between numerical and test results can be obtained in the case of the physically more sound flow theory of plasticity. The reason for the ‘plastic buckling paradox’ appears to be the over-constrained kinematics assumed in many analytical and numerical treatments, such as BOSOR5 and NAPAS, whereby a harmonic buckling shape in the circumferential direction is prescribed.