Engineered Cellular Materials, with Emphasis on Materials Produced with Additive Layer and Fold Core Manufacturing Processes

Abstract

Structural cellular materials provide an important opportunity for the realisation of lightweight structures for transport applications. These materials have been extensively researched in the past, culminating in the classic text by Gibson and Ashby [1]. Such structural cellular materials include honeycomb and foams, both in metallic and polymeric forms. Typically, in these ‘conventional’ cellular materials, the micro architecture is limited. For example, foam cores are available in different densities and parent materials, whilst honeycombs are available in different cell sizes, wall thicknesses and parent material [1]. A major application for these cellular materials is as cores in twin-skinned sandwich structures, where specific bending stiffness and strength are important structural properties. More recently, there has been extensive theoretical work on the optimisation of specific stiffness and strength of these cellular materials, especially in connection with lattice structures [2, 3]. The focus of research here is the investigation of the effects of change in parameters of the micro architecture, e.g. parent material, micro geometry, micro geometry aspect ratio, and the application to structural cases, e.g. as cores in sandwich panels. This work has focussed on elastic behaviour and on cellular materials that can be fabricated with conventional methods [4] as well as more innovative processes, such as textile metals [5]. As far as research into materials and structures is concerned, issues include the quantification of stiffness and strength at the cellular (micro) scale, the quantification of stiffness and strength at the macro scale (e.g. blocks of material), and the quantification of stiffness and strength at the structural scale (e.g. cores in sandwich panels) [6]. The experimental strain measurement and analysis techniques required for this work include testing at the micro/cell scale [7], testing at the macro scale [8, 9], and developing theories for relating micro performance to macro behaviour [9, 10]. In parallel with the above, there are developments in innovative manufacturing processes for realising sophisticated cellular architectures. A subset of these are additive layer manufacturing techniques such as selective laser melting [6, 11]. In this, metallic powder is selectively melted by a fibre laser, and components are built up in layers of 50 μm. Associated with this is the ability to realise structural features at the micro scale, for example, micro lattice structures [6, 12]. The driver here is application, rather than science. For example, in the current manufacture of airframes for civil aircraft, there is 90% material wastage because of subtractive manufacture, e.g. machining [13]. Given the expensive materials used in airframe manufacture, e.g. aluminium and titanium alloys, there is a severe economic penalty as well as an environmental one. Given the ability to design the cellular material at the micro level, the characterisation of the material at the macro level becomes an issue. The multi axial crush behaviour of cellular materials is complex, displaying multi dimensional elastic effects [14], hydrostatic effects [10], and strain localisation [15]. Full field optical techniques are being applied to cellular materials, to quantify macro deformation fields [16, 17]. It should be noted that during the multi axial crush of cellular materials, multiple micro failure modes occur, e.g. elastic buckling, plastic buckling, elastic and plastic bending, rupture, etc. Hence, there is a need to relate these micro failure modes to macro behaviour. One such analysis has been developed for honeycomb for multi axial elastic plastic yield limits [18]. There is a possibility of realising two-dimensional and three-dimensional graded structures using additive layer and other manufacturing techniques [6, 19]. This gives further possibility in tailoring structural behaviour, e.g. for foreign object impact and for blast loading. Ultimately, hierarchical structures will be possible [20]. Another innovative manufacturing technique is fold core manufacture [21, 22]. In this, flat sheets of composite paper material are folded into complex corrugated shapes and geometries can be adjusted to give tailored elastic and collapse responses. In a similar manner to micro lattice structures realised using selective laser melting, parent material properties need to be derived at the micro level and sophisticated analysis techniques are required to relate micro/cellular behaviour to macro behaviour as cores in sandwich construction. In summary, the material and structural possibilities described earlier give a new series of challenges for the Strain researcher, whether from a manufacturing, materials, experimental, or theoretical point of view. Experimental testing techniques are required at the small scale for parent material and single cell properties. Analysis techniques and associated experimental validation techniques are required at the macro scale, which should take into account complex phenomena such as non-linear, hydrostatic, and localisation effects. Finally, theoretical (and associated experimental) techniques are required to fully exploit the ability of new manufacturing techniques to realise bespoke cellular materials for the given structural application.