Flexible armor inspired from natural fish scales and osteoderms

The skins of armadillo, rhinoceros, and fish display interesting combinations of resistance to penetration, flexibility, and light weight. These armors are made of stiff and hard plates of finite size (tiles or scales), embedded in soft tissues. The plates are orders of magnitude harder and stiffer than the soft tissues to resist penetration, but they can also move relative to each other to provide compliance. Here, we have implemented these bio-inspired principles in an artificial system consisting of small hexagonal glass plates resting on a soft silicone substrate. -Compared with a continuous layer of glass, the flexural stresses in our segmented armor were greatly reduced, and as a result the puncture resistance was up to 70% higher. We also duplicated the robustness of natural armor: as opposed to pristine glass which is entirely destroyed by sharp puncture, damage in our segmented armor was confined to only one hexagonal plate. The structure and mechanics of this system can be translated to armor-grade materials for high-performance flexible, light-weight, puncture resistant, and robust protective systems

Overcoming brittleness through bioinspiration and -microarchitecture

The fracture of highly mineralized natural materials such as bone, teeth, or seashells is largely controlled by the interfaces they contain. These interfaces, relatively weak, deflect and guide cracks into configurations which eventually impede their propagation. As a result, weaker interfaces turn brittle minerals into tough materials which can deform and absorb energy from impacts. To explore these concepts in synthetic materials, we used a 3D laser-engraver to carve arrays of microcracks with well-defined geometries and toughness within the bulk of borosilicate glass slides. The microcracks, positioned along specific surfaces within the material, coalesce upon application of an external load and guide large cracks following the concept of “stamp holes”. Using this approach, we engraved curved interfaces with re-entrant features ahead of an incident crack in glass, defining jigsaw-like building blocks. After initial crack propagation along the weak interface, the jigsaw features produced a tremendous amount of toughness by pull out and interlocking, dissipating energy by friction. This powerful nonlinear failure mechanism, inspired by natural nacre, amplified the toughness of glass 100x. Infiltration of the interfaces with polyurethane generatedadditional toughening mechanisms by ligament bridging, similar to the mechanism of proteins in hard biological materials. This additional step led to a material made of ~95% of glass but 200x tougher, as stiff as plain glass but displaying nonlinear deformations and failing at strains in excess of 3%. This bioinspired glass not only shows how the powerful toughening mechanisms observed in nature can be harnessed in synthetic materials, but also showcase a new approach to making bioinspired composite materials by “fabricating” weak interfaces within stiff materials rather than assembling stiff building blocks within soft matrices

Strain rate hardening in biological and biomimetic composites: a critical ingredient to mechanical performance?

Natural materials such as nacre, bone, collagen, or spider silk boast unusual combinations of stiffness, strength, and toughness. Behind this performance is the staggered microstructure, which consists of stiff and elongated inclusions embedded in a softer and more deformable matrix. The micromechanics of deformation and failure associated with this microstructure are now well understood at the “unit cell” level, the smallest representative volume for this type of material. However, these mechanisms only translate to high performance if they propagate throughout large volumes, an important condition which is often overlooked. Here we present, for the first time, a model which captures the conditions for delocalization in staggered composites, and which determines whether the material is brittle or deformable at the macroscale. The macroscopic failure strain for the material is derived as function of the viscoplastic properties of the interfaces and the severity of the defect. As expected, larger strain can be achieved with smaller defects or with more strain hardening at the interface. The model also shows that strain rate hardening is a powerful source of delocalization for the material, a result we confirmed and validated with tensile experiments on glass-PDMS nacre-like staggered composites. An important implication is that natural materials, largely made of rate-dependent materials, may rely on strain rate hardening to tolerate initial defects and damage to maintain their structural integrity. Strain rate hardening should also be harnessed and optimized in bio-inspired composites to maximize their overall performance

Strong and Tough Ceramics Using Architecture and Topological Interlocking

Strong and tough ceramics using architecture and topological interlockin

Density functional calculations of the formation and migration enthalpies of monovacancies in Ni: Comparison of local and nonlocal approaches

We examine in this work the potential and the functional to be used in a density functional theory approach in order to describe correctly the formation and migration energies of monovacancies in nickel. As the formation enthalpy is not well-known experimentally at 0 K, we choose in a first step to determine some structural, magnetic, and elastic properties of the bulk, which are well-established experimentally. The comparison between both approaches, i.e., the local spin density approximation LSDA and the generalized gradient approximation GGA exchange-correlation functionals is analyzed. We conclude that the contribution of nonlocal GGA terms in order to describe correctly the electronic density is necessary to determine the formation and migration enthalpies and activation energy of monovacancy. The calculated formation Hv f and migration Hv m enthalpies differ significantly between both approaches. The overestimation of the LSDA approximation is of 0.25 eV for Hv f and of 0.23 eV for Hv m with respect to the GGA one, leading to a gap of 0.48 eV between both methods for the activation energy Q1. We show that the GGA results are comparable with experimental data if the thermal expansion contribution is taken into account through the lattice parameter variation. Finally, it is shown that the activation energy is nearly independent of the thermal expansion effects; thus we can expect that the curvature of the Arrhenius plot of the diffusion factor near the melting point is essentially due to the contribution of divacancies

Threatened flora of Mayotte: heritage value and challenges for conservation

This paper deals with the natural history and the flora of Mayotte by presenting some of its characteristics and providing an assessment of its conservation status. Mayotte is a French islands group belonging to the Comoros archipelago (Indian Ocean), located between Madagascar and East Africa. Its geological history, its geographical position and multiple human influences have shaped some remarkable landscapes composed by unique ecosystems and habitats, with an extraordinary biodiversity for a small territory. Since the first inhabitants, via the colonial centuries to the contemporary era, natural vegetation is restricted to the bare minimum but nevertheless it houses most of the plant biodiversity in Mayotte. As sanctuaries these small natural areas are unfortunately still poorly known and highly threatened. The flora of Mayotte was not well studied in the past because the naturalists’ attention was probably drawn by Madagascar. Most of the discoveries were made during the two main periods of investigation that took place during the second half of the nineteenth century and the late twentieth to date. The vascular flora of Mayotte is composed by 681 species that have been recently evaluated by using the categories and criteria of the IUCN Red List. To illustrate the need and urgency to ensure the conservation of this unique heritage, a "Top fourteen" of the critically endangered species is proposed and suggestions for conservation measures are discussedCet article traite de l’histoire naturelle et de la flore de Mayotte en présentant quelques unes de ses caractéristiques et en proposant un état des connaissances de son statut de conservation. Mayotte est un groupe d’îles françaises de l’archipel des Comores situé dans l’océan Indien entre Madagascar et l’Afrique de l’Est. Son histoire géologique, sa position géographique et les influences humaines ont façonné des paysages singuliers composés de milieux naturels aujourd’hui relictuels qui contiennent une grande diversité biologique pour un territoire de cette taille. Des premiers habitants à la démographie exponentielle contemporaine, en passant par l’agriculture rentière coloniale, la végétation naturelle n’occupe plus aujourd’hui qu’une portion congrue de Mayotte mais abrite l’essentiel de la biodiversité végétale mahoraise. Ces espaces naturels qui constituent assurément des sanctuaires sont malheureusement encore méconnus et grandement menacés. L’attention des naturalistes ayant porté sur Madagascar dans le passé, la flore de Mayotte est longtemps restée peu étudiée. L’essentiel des découvertes a été fait lors des deux principales périodes d’investigation qui ont eu lieu pendant la deuxième moitié du 19ème siècle et de la fin du 20ème à ce jour. Sa flore originale est représentée par 681 espèces de plantes vasculaires qui ont été récemment évaluées selon les catégories et les critères de la Liste Rouge de l’UICN. Pour illustrer la nécessité et l’urgence de garantir la pérennité de ce patrimoine unique, une liste des 14 espèces en danger critique d’extinction est proposée et des suggestions d’actions de conservation sont discutée

Crystallographic disorder and electron scattering on structural two-level systems in ZrAs1.4Se0.5

Single crystals of ZrAs1.4Se0.5 (PbFCl type structure) were grown by chemical vapour transport. While their thermodynamic and transport properties are typical for ordinary metals, the electrical resistivity exhibits a shallow minimum at low temperatures. Application of strong magnetic fields does not influence this anomaly. The minimum of the resistivity in ZrAs1.4Se0.5 apparently originates from interaction between the conduction electrons and structural two-level systems. Significant disorder in the As-Se substructure is inferred from X-ray diffraction and electron microprobe studies