Architected materials: recent developments and scientific challenges - EUROMECH Colloquium N° 623

Scientific scope of the Colloquium: The development of additive manufacturing has allowed for the engineering of a new class of materials that obtain their effective mechanical attributes by their inner topological design rather than from their chemical composition, and generate exceptional material performances far beyond those of the base material. Artificial materials of the kind have been named architected materials or metamaterials, thus «materials after materials», as of their potential to yield effective static and dynamic attributes that can well extend the design space of the base material used. The quest for artificial materials that exceed the mechanical performance of conventional materials is driven by both weight and stiffness specification. Lightweight designs with prescribed stiffness attributes are primal objectives in the structural design of mechanical components in different engineering fields, amongst others in morphing and composite engineering. Within the context of isotropy the mechanical parameters are directly related and restrained within certain limits, so that anisotropic material designs are required to extend the range of accessible mechanical parameters. During the last decades, anisotropic material architectures have been developed with ultra-soft or ultra-stiff effective material behaviors that cannot be reached with conventional materials. Exceptional mechanical properties are sought in various applications: the development of lightweight, shear stiff inner material architectures have allowed for the construction of fillers of increased mechanical efficiency for polymer nanocomposites; the relative normal to shear stiffness properties are decisive for the engineering of seismic isolation structures. Carefully designed architectures are used to achieve any com¬bination of linear elastic coefficients to reach novel anisotropy classes. These designs become increasingly important for morphing applications, or in civil / mechanical engineering and biomechanical applications, as for the design of tendons and ligament biosubstitutes that exhibit Poisson's ratio values well above the isotropic limits (so-called anti-auxetic behaviors). For the effective mechanical properties of architected materials to be determined, a link between their architecture and their equivalent continuum scale behavior needs to be established. The objective of the Colloquium is to bring together researchers from mechanics and materials science working in the area of architected materials. The Colloquium aims to promote the development of methods to perform material design for target macroscopic properties and to explore a diversity of applications. Experimental, modeling and simulation, and manufacturing aspects will be discussed. The scientific challenges raised by architected materials that the Colloquium will address are: • The elaboration of suitable homogenization schemes to construct adequate mechanical models for geometrically complex inner architectures. Especially, generalized continuum models may be required to account for the specific microstructural deformation modes and scale effects exhibited by these materials. Relating effective properties or architected materials to their microstructural features enables to tune microstructural anisotropy (by adjusting the inner architecture) to exceed the isotropic limits of Poisson's ratio values ([-1, 0.5[), especially to achieve non-conventional mechanical behaviors like auxetics or ultra-soft or ultra-stiff in lightweight designs. • Multiscale models and computational methods to access the static and dynamic properties of architected materials: plasticity, rupture, vibration, damping, shock and impact, acoustic properties, especially the control of bandgap width via the inner architecture. • Large deformation aspects: slender elements of architected materials allow for large deformations, leading to geometric nonlinearities; they are thus prone to elastic instabili¬ties such as buckling, folding and snapping, and exhibit floppy modes and mecha¬nisms due to the presence of hinges. The follow up of such unstable behaviors and their consequences at the macroscopic scale requires dedicated numerical methods. • Most of network materials have substantial porosity that reduces their mechanical performances, like resistance to impact. There is accordingly a need for new designs to achieve a good compromise between sufficiently high mechanical performance and lightweight structures. Improved design tools of novel 3D architectures such as topology optimization methods can be employed in combination with multiscale strategies to find more systematically new designs and reach ultra-stiff or ultra-soft designs. Hierarchical modeling recently emerged as a strategy to achieve a good compromise between high enough properties and lightweight structures. • Full-field measurements: the powerfulness of field measurement methods during in situ testing allows relating the overall mechanical response and the local response of architected materials. They provide a more detailed quantitative understanding of the microscopic deformation mechanisms responsible for the observed mechanical performances at the upper scales. • In the same spirit, the elaboration of measurement methods of mechanical properties of network materials and the identification of effective model parameters - especially higher order moduli - is a challenging aspect. • Since architected materials exhibits defects during manufacturing, the effect of topological disorder on their mechanical properties is a topic of high interest. Challenges from well-defined applications are selected to illustrate modeling and experimental approaches: - Programmable and morphing metamaterials, so materials that change properties under specific external stimuli; - Network materials in biomechanics: scaffolds with very large Poisson's ratio in soft biological tissue engineering; vascular stents; - Biomimetic architectured materials: the main features of natural materials, the presence of microstructures and scale hierarchy, are responsible for their fantastic properties, and they accordingly offer a great inspiration for the design of architected materials with high strength, toughness and low weight. For instance, woods, bone teeth, seashells and bamboo offer great inspiration for the design of architected materials. The Colloquium also aims to educate the next generation of researchers interested in architected materials. The Colloquium is intended for specialists in the field, but also for non-specialists who are interested in an overview in the field, PhD students, post-doctoral researchers, industrial researchers and engineers, and scientists interested in the development of methods of analysis dedicated to this type of materials.