Brian Ellul

An improved analytical model for the elastic constants of auxetic and conventional hexagonal honeycombs

Cellular solids, in particular hexagonal honeycombs have been the subject of numerous studies in the last decades in view of their extensive use in many applications. In particular, there have been various studies aimed at expressing the mechanical properties of honeycombs in terms of the geometrical parameters used to describe the structure of such honeycombs. Despite improvements over the first established model, finite element simulations performed in this work on honeycombs having ribs with a realistic thickness-to-length ratio suggest that the mechanical properties for such systems differ from those predicted by current models, sometimes to a very significant extent. In view of this, we analyse in detail the deformed structures in an attempt to gain insight into how and the extent to which the shape of the ligaments, in particular its thickness and mode of connection affects deformation in conventional and re-entrant hexagonal honeycombs. Based on these observations, we propose a modified version of the previous analytical models that take into consideration the finite thickness of the ligaments.

A Progressive Failure Analysis Applied to Fiber-Reinforced Composite Plates Subject to Out-of-Plane Bending

The ability to predict the structural response of composites offers a significant advantage to design engineers and provides the possibility of identifying structurally efficient composite assemblies. Various analytical and numerical models are possible, but care has to be taken to ensure that the appropriate structural performance and failure criteria are used. In particular, modeling the progressive failure of composite laminas requires robust and validated failure algorithms that are not only computationally efficient, but are also able to predict the load–deformation characteristics and to ultimately establish the failure load appropriately. This study looks into different progressive failure macromechanical algorithms applied to e-glass-fiber-reinforced composite plates subject to out-of-plane bending. The influence of different boundary conditions of the plates, ranging from fully clamped to simply supported ones, on their ultimate failure load is also investigated. The results are validated by experimental data found in the literature and show that boundary conditions have a significant influence on the predicted ultimate failure load. The study also shows that, in this case, the predominant failure mechanism is the failure of matrix, and after the redistribution of stresses, no consecutive failure due to fiber or fiber-matrix failure occurs in the lamina, therefore a sudden-degradation progressive ply failure algorithm based on the failure mode is sufficient to model the structural performance of composite plates subject to out-of-plane bending.

Design-by-Analysis Methods for Asymmetric or Unbalanced Cylindrical Composite Pressure Vessels

The combination of fibre volume fraction, fibre orientation and lay-up sequence in composite materials makes it possible to design a multitude of composite pressure vessels and pipes. Analytical models, based on the classical laminate theory and numerical predictive techniques offer a means to optimize the lay-up sequence in order to maximize the strength to weight / cost ratio. This study looks at the validity of using analytical models prescribed in the design by analysis filament wound composite standards and compares the results with realistic test and numerical models. The results show that the classical laminate theory accurately establishes the design load in symmetric and balanced lay-up laminates when appropriate material properties are assigned. However in the case of asymmetric or unbalanced lay-up sequences, the bending and twisting stiffness geometrically strengthens the pipes such that the classical hoop and axial loading conditions based on isotropic material properties, no longer apply. In such instances the analytical solutions can underestimate the design load by more than 33%. An analytical solution that accurately establishes the loading configuration and magnitude is required. On the other hand numerical models gave good agreement with the experimental test results immaterial of the lay-up sequences, when appropriate end coupling, pressure loading and material properties are applied.Copyright