Architectured materials are a rising class of materials that bring new possibilities in terms of functional properties, filling the gaps and pushing the limits of Ashby’s materials performance maps . The term architectured materials encompasses any microstructure designed in a thoughtful fashion, such that some of its materials properties have been improved in comparison to those of its constituents, due to both structure and composite effects, which depend on the multiphase morphology, i.e. the relative topological arrangement between each phase [1,2]. There are many examples: particulate and fibrous composites, foams, sandwich structures, woven materials, lattice structures, etc. One can play on many parameters in order to obtain architectured materials, but all of them are related either to the microstructure or the geometry. Parameters related to the microstructure can be optimised for specific needs using a materials-by-design approach.
From a macroscopic viewpoint, parameters related to the geometry have mainly been the responsibility of structural and civil engineers for centuries: to efficiently distribute materials within structures. An obvious example would be the many different strategies available for building bridges. At the millimetre scale, materials can be considered as structures, i.e. one can enhance the bending stiffness of a component by modifying its geometry while keeping the lineic mass (for beams) or surfacic mass (for plates) unchanged . On the other hand, one might need a lower flexural strength for specific applications, with the same lineic and/or surfacic masses. This can be achieved with strand structures, i.e. by creating topological interfaces in the material.
Architectured materials thus lie between the microscale and the macroscale, as shown on Figure 2. This class of materials involves geometrically engineered distributions of microstructural phases at a scale comparable to the scale of the component, thus calling for enriched models of continuum mechanics, i.e. generalized continua theories, in order to describe the behaviour of architectured materials, strain-gradient elasticity , and strain-gradient plasticity for instance. This topic has been especially fruitful these last few years for the French mechanics of materials community [5,6]; this results in the availability of versatile models able to describe the various situations encountered with architectured materials.
The concept of architectured material has been developed through previous collaborative research projects related to architectured materials, conducted in France under the influence of Prof. Y. Bréchet. The MAPO project (2005-2008), coordinated by CNRS and ONERA, aimed at developing architectured porous materials for structural, acoustic and insulation properties [7,8]. Another project was the CPR MAM (2008-2011) (for multifunctional architectured materials), which led to further interesting results related to modelling  and topological optimisation of architectured plates, as well as phase-changing materials for heat storage applications. The MANSART project (2009-2012), funded by ANR, aimed at exploring new tools to develop architectured materials for crashworthiness properties.
Several architectured materials were considered to fulfil industrial requirements: entangled monofilament of perlitic steel , sandwich composite structures , segmented interlocking structures, woven and non-woven textile composites [11,12], as well as on lattice-structures with negative Poisson’s ratio (auxetics) [13-15]. Focus was made mostly on the characterisation and modelling of the materials listed. Optimisation of architectured sandwich materials was also investigated at ONERA . From an industrial viewpoint, architectured materials have also been investigated: for instance, EDF collaborated on a study of metal-polymer multilayers for barrier properties ; Airbus took part in the MANSART project; ArcelorMittal R&D has been very active in the field of architectured materials, especially on the topic of processing and mechanical properties of architectured metallic materials [18-22].
On an international scale, architectured materials have also been an exciting research topic in recent years, especially regarding the processing of architectured metallic foams [23,24] or multi-scale architectured ceramics . For architectured metallic materials, the contribution of Embury’s team in McMaster University is of prime importance [18-20]. Apart from processing, most efforts have been focused on the modelling of architectured materials. In particular, the pioneering works done at Cambridge on truss-structures  and metallic foams led to useful results for the modelling of architectured materials . The mechanical modelling of bio-inspired architectured materials has been pursued successfully by J.W.C. Dunlop’s team at the Max Planck Institute of Colloids and Interfaces in Potsdam, yielding results for multi-layered materials that could be used for modelling architectured metallic sheets [28,29]. Vaziri’s team at Northeastern University demonstrated the interest for designing recursive or nested material architectures to achieve enhanced specific mechanical properties . On this aspect, the work of Daniel Rayneau-Kirkhope is also very interesting : the mechanical behaviour of recursive or fractal truss-beam elements was modelled using analytical approximations, in order to demonstrate the potential of such materials in terms of specific rigidity, as shown on Figure 3 taken from the same paper. The underlying principle is that increasing the order of nesting or hierarchy reduce the length of each elementary beam element, making them more resistant to buckling and failure, while keeping the overall mass constant, i.e. reducing the mass while keeping the mechanical requirements constant.
This path is also explored, at least from a computational viewpoint, in the PhD work of Romain Duballet on multiscale optimisation of truss structures. Elastic instabilities have been considered for shape-generation of architectured materials, as showed by the work of Bertoldi’s group in Harvard [32,33]; using instability analysis is an elegant and original way to design shapes, as it was also done by Carolin Körner’s group for auxetic materials . Finally, a specific subclass of auxetic materials has been explored by materials scientists at Monash University: interlocking materials. Interlocking structures have always been ubiquitous both in natural systems and archaic masonry constructions. The coupling between adequate topology and interfacial friction results in structurally efficient structures which are resilient to damage, as shown on Figure 4 [35-39]. The interfacial friction between bricks or tessellation unit-cells can generate an overall hysteretic dissipative behaviour which can be quite interesting for damping applications.
In summary, architectured materials are subject to active research in coordinated research groups and industrial firms in France, as well as in various research groups abroad. Given mature processing techniques, architectured materials are promised to a bright future in industrial applications due to their enticing customisable and multifunctional specific properties. Nevertheless, in most of the works cited, architectured materials are considered from the modelling and simulation viewpoint alone or, when experiments have been conducted, the materials have been processed with a trial-and-error empirical approach which is not satisfying in order to transfer this technology to the industry. One of our current research project in the group is aiming at tackling this problem by establishing a systematic computational approach for modelling, as well as a deterministically controlled processing route for architectured materials, here based on local laser processing.
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