Nanocrystalline electrodeposition can be used to reinforce conventional metallic micro-truss materials and conventional metal foams, creating new types of metal/metal cellular hybrids in which the mechanical performance is controlled by an interconnected network of nanocrystalline tubes. This approach takes advantage of the large strength increase that can be obtained by grain size reduction to the nm-scale and the fact that the electrodeposited material is optimally positioned away from the neutral bending axis of the composite cellular struts or ligaments. This article presents an overview of the potential for structural reinforcement of bending-dominated and stretching-dominated cellular architectures by nanocrystalline electrodeposition.

Low density Nitinol shape memory alloy honeycombs were fabricated using a new Nb-based brazing method, which demonstrated enhanced shape memory and superelastic properties under in-plane compression. Adaptive, light-weight cellular structures present interesting possibilities for design of new architectures and novel applications. This paper presents an overview of ongoing work to address the multi-scale stability of superelastic, thin-walled, SMA honeycombs and the need for design and simulation tools.

Many different sandwich panels are used for aeronautical applications. Open and closed cell structured foam, balsa wood or honeycomb are often used as core materials. When the core material contains closed cells, water accumulation into the cell has to be taken into account. This phenomenon occurs when in service conditions lead to operate in humidity atmosphere. Then, water vapor from air naturally condenses on cold surfaces when the sandwich panel temperature decreases. This water accumulation might increase significantly the weight of the core material. Core with a ventilated structure helps to prevent this phenomenon. Periodic cellular metal (PCM) has been motivated by potential multifunctional applications that exploit their open architecture as well as their apparent superior strength and stiffness: pyramidal, lattice, Kagome truss or woven. One of the drawbacks of these materials is the expensive cost of the manufacturing. Recently, a novel type of sandwich has been developed with bonded metallic fibers as core material. This material presents attractive combination of properties like high specific stiffness, good damping capacity and energy absorption. Metal fibers bonded with a polymeric adhesive or fabricated in a mat-like form consolidated by solid state sintering. Entangled cross-linked carbon fibers have been also studied for using as core material by Laurent Mezeix. In the present study, ventilated core materials are elaborated from networks fibers. The simplicity of elaboration is one of the main advantages of this material. Multifunctional properties are given by mixing different sorts of fibers, by example adding fibers with good electrical conduction to give electrical conductivity properties. In this study network fibers as core material are elaborated using carbon fibers, glass fibers and stainless steel fibers. In aeronautical skins of sandwich panels used are often carbon/epoxy prepreg, so epoxy resin was used to cross-link fibers. The core thickness was chosen at 30 mm and fibers length was chosen at 40 mm. Entanglement, separation of filaments and cross-linking are obtained in a specific blower room. Fibers are introduced in the blower room, compressed air is applied and in same time epoxy resin is sprayed. Indeed one of the sandwich core material properties required is low density, so yarns size need to be decreased by separating filaments. Network fibers are introduced in a specific mould and then are compressed. The density obtained before epoxy spaying is 150 kg/m3. Finally samples are polymerized at 80°C for 2 hours in a furnace under laboratory air. Compressive behavior is study to determinate the influence of fibers natures and the effect of cross-linking. Reproducibility is also checked.

Planar assemblies of interlocked cubic blocs have been tested in indentation. Experiments are performed on blocs made of plaster. Influence of key parameters such as the surface roughness, the compression stress and the number of blocs are investigated. A numerical modeling is then proposed based on discrete element method. Each bloc is represented by its centre coordinates. Constitutive equations obtained by finite element simulations are introduced to model the contact between the blocs. The numerical tool is then applied to the case of indentation loading. It is found that the model reproduces all the experimental tendencies.

Entangled fibrous materials have a common point with cellular solids: the architecture is at the millimetric scale. However, they present one extra degree of freedom which is the connectivity on the constituents: while compressing a fibrous structure, the number of contacts between fibers is variable, by contrast to cellular solids where the building lock is the cell. In this respect, fibrous entangled solids can span a whole range of mechanical behavior depending on the possibility offered to fibers to create new contacts: from the felt where they are totally free, to the fully sintered wool where their relative motion is constrained by irreversible contacts. The purpose of this paper is to investigate the mechanical properties of this class of materials, based on micro tomography observations for the number of contacts, and on a physically based model for fiber bending and collective reorganization. The material studied is a stainless steel wool. The properties investigated are the loading curves for sintered and non sintered wools. Qualitative differences introduced by sintering motivate a modification of the classical Toll model, which will be presented together with the experimental results.

In many industrial fields, structural materials play a key role in the increase of performance, but new requirements in terms of energy saving, safety, materials economy… lead to more stringent requirements on materials properties. The two usual strategies –microstructure optimisation and shape optimisation-, which act at two different scales, the micrometer scale and above the centimetre scale, become less and less efficient with this new strong demand for multi-functional properties. The largely unexplored millimetre scale, domain of the so called “structural materials”, is a possible answer. Structural materials benefit of an extra degree of freedom well suited for multi-functionality: they allow using combination of materials from different classes, allow geometrical optimisation, and can be naturally integrated in structures such as sandwiches and various stiffened plate geometries. The price to pay for this extra-richness is the extraordinary wide variety of potential solutions to investigate for a given problem. Hence modelling plays a crucial role for selecting and optimising such innovative materials. This paper is an overview of a project, named MAPO (“Materiaux Poreux”), aiming at designing high-temperature materials with acoustical and structural properties.

Since its introduction in 2001 [1], the concept of topological interlocking has advanced to reasonable maturity, and various research groups have now adopted it as a promising avenue for developing novel structures and materials with unusual mechanical properties. In this paper, we review the known geometries of building blocks and their arrangements that permit topological interlocking. Their properties relating to stiffness, fracture resistance and damping are discussed on the basis of experimental evidence and modeling results. An outlook to prospective engineering applications is also given.

Herein we introduce nanoparticle based periodic multilayers as base materials to create different types of multifunctional coatings that combine optical, mechanical and diffusion properties. The technological potential of these versatile materials is demonstrated by showing applications in the fields of sensing and photovoltaic materials. Due to the porous nature of such structures, liquids and gases can infiltrate or condensate, respectively within the interstices, causing a variation of the refractive index (R.I.)of the layers. This gives rise to clear but gradual changes of the optical responses, either when liquids or the partial pressure of vapors are infiltrated in the structure. Also, photoconducting Bragg mirrors can be built by precise control of the spatial variation of the R.I. of the layers in a pure TiO2 multilayer. Rationally placed within a Dye Sensitized Solar Cell (DSSC), that gives rise to a significant enhancement of the solar to electric power conversion efficiency through the amplification of sunlight absorption. Direct observation of both optical absorption and photocurrent resonances can be seen.

Sandwich structures are especially interesting when multiple functionalities (such as stiffness and thermal insulation) are required. Properties of these structures are strongly dependent on the general geometry of the sandwich, but also on the detailed patterns of matter partitioning within the core. Therefore it seems possible to tailor the core pattern in order to obtain the desired properties. But multi-functional specifications and the infinite number of possible shapes, leads to non-trivial selection and/or optimization problems. In this context of “material by design”, we propose a numerical approach, based on structural optimization techniques, to find the core pattern that leads to the best performances for a given set of conflicting specifications. The distribution of matter is defined thanks to a level-set function, and the convergence toward the optimized pattern is performed through the evolution of this function on a fixed grid. It is shown that the solutions of optimization are strongly dependent on the formulation of the problem, which have to be chosen with respect to the physics. A first application of this approach is presented for the design of sandwich core materials, in order to obtain the best compromise between flexural stiffness and relative density. The influence of both the initialization (starting geometry) and the formulation of the optimization problem are detailed.

Magnetotactic bacteria are aquatic micro-organisms which have the specific capacity to navigate along the lines of the earth’s magnetic field. This property is related to the formation of chains of magnetic crystals called magnetosomes. All magnetotactic bacteria synthesize nano-sized intracellular magnetosomes that are surrounded by ultra-thin bio-membranes. The magnetosome chains serve as compass for navigation of the magnetotactic bacteria, and the cell flagella are considered as the mechanism for propelling the bacteria forward. This presentation describes various functions of the architectured structure of magnetotactic bacteria as well as their possible applications in biotechnology.

Nature builds materials like an architect to obtain a variety of properties with a limited number of building blocks. In contrast, engineers have access to a wide range of constituent materials to fulfil a variety of requirements. The classical degrees of freedom for controlling the properties of man-made materials are the microstructure, or the macroscopic shape. Only recently, the architecture at the millimetre scale was perceived as an efficient way of expanding the range of properties offered by bulk materials. The aim of this paper is to compare the different strategies and to outline some observations on natural materials which may serve as inspiration to develop engineering architectured materials.

The present study demonstrates that interfacial acoustic waves can be excited at the interface between two phononic crystals. The interfacial wave existing between two phononic crystals is the counterpart of the surface electromagnetic wave existing between two photonic crystals. While past works on phononic crystals exploit the unique bandgap phenomenon in periodic structures, the present work employs the Bloch wave in the stop band to excite interfacial waves that propagate along the interface and decay away from the interface. As a result, the proposed structure can be used as a wave filter as well as a thermal barrier. In wave filter design, for instance, the incident mechanical wave energy can be guided by the interfacial wave to the lateral direction; thus, its propagation into the depth is inhibited. Similarly, in thermal barrier design, incident phonons can be coupled with the interfacial acoustic wave, and the heat will be localized and eventually dissipated at the interface between two phononic crystals. Consequently, the thermal conductivity in the direction normal to the layers can be greatly reduced. The advantage of using two phononic crystals is that the interfacial wave can be excited even at normal incidence, which is critical in many engineering applications. Since the proposed concept is based on a one-dimensional periodic structure, the analysis, design, and fabrication are relatively simple compared to other higher dimensional material designs.

This paper shows different examples where the architecture of cellular materials has been determined exactly using 3D X ray computed tomography. The images were then subsequently used to generate FE meshes reproducing the architecture as exactly as possible. The FE meshes where in turn used to simulate the mechanical (monotonous and fatigue compression) and the thermal (radiative properties) behavior of the studied materials.

Natural materials such as bone, shell, tendon and the attachment system of gecko exhibit multi-scale hierarchical structures. Here we summarize some recent studies on an idealized self-similar hierarchical model of bone and bone-like materials, and discuss mechanical principles of self-similar hierarchy, in particular to show how the characteristic length, aspect ratio and density at each hierarchical level can be selected to achieve flaw tolerance and superior stiffness and toughness across scale. Tel.: (401) 863-2626; Email address:

Bio-inspired architectures, especially metallic foams, have been receiving an increasing interest for the last 10 years due to their unusual mechanical properties. Among commonly dealt foamed metals, like aluminum and steel, titanium possesses a distinctive place because of its high strength-to-weight ratio, excellent corrosion resistance and biocompatibility. In this study, Ti foams were produced by a very simple and common method, sintering under inert atmosphere with fugitive space holder. Removal of the space holder was conducted by dissolution in hot deionized water which makes it possible to minimize contamination of Ti. Sintering of remaining Ti skeleton at 1300 °C offered a wide range of properties and cost savings. The effects of the processing parameters such as sintering temperature and powder characteristics on the 3D foam architecture were investigated by using X-ray microtomography (μ-CT). Use of bimodal Ti powders caused a decrease in final theoretical density when compared to the ones prepared with the same amount of space holder but with monomodal Ti powders. It was also observed that the use of bimodal Ti powders decreased compressive strength, by introducing pores into the cell walls, when compared to the ones having the same theoretical density.

The phase change materials (PCM) are known since years with high thermal storage capacity but with limited applications. The modified PCM mainly paraffin watery suspensions, so called PCM slurry, improve some PCM drawbacks (thermal conductivity) and as paraffin multifunctional fluids can work in both, heat transport and heat storage for cooling technology applications. The structural and thermophysical properties of two types PCM slurry were good basis for comparison of their efficiency: paraffin microcapsule slurry (A) and paraffin emulsion slurry (B), both working in temperature range of phase transition from 2-12 degreesC. The equipments as differential scanning calorimetry (DSC), model NETZSCH DSC 200 PC; scanning electron microscopy (SEM), JOEL model JSM-5510; hot stage optical microscopy, LINKAM model TMS 94 and hand made thermal cycling system operational with Danfoss cooling machine that ensured 2 kW cooling capacity at 40oC; gave accurate results, characterizing completely, from structural and thermal point of view both types of PCM multifunctional structured fluids. Structural stability of the advanced phase change multifunctional fluids was discussed on sample imaging in variable magnifications made by method of Hot Stage microscopy and precise SEM study. Systematization of the DSC results obtained, including temperature range of phase transition and thermal storage capacity, measured before and after repeatable thermal cycling of the PCM multifunctional fluids, allowed selection of the PCM slurry working samples with relatively high thermal capacity applicable to further development in prototypes. Heat absorbed/released, calculated by NETZSCH DSC software, was for PCM slurry A in the range of 80 to 82 kJ/kg, while PCM slurry B showed thermal storage capacity from 56 to 53 kJ/kg. Correlation between the structural properties and thermal storage capacity of the phase change multifunctional fluids led to practical conclusions concerning: homogeneity; crystal growth/conditions; structural compatibility between components; prediction of the heat flow behavior of multifunctional PCM slurries in cooling technology for storage and transport of heat.

The combination of high strength and high radiation damage tolerance in nanolaminate composites can be achieved when the individual layers in these composites are only a few nanometers thick and therefore these materials contain a large volume fraction associated with interfaces. These interfaces act both as obstacles to slip, as well as sinks for radiation-induced defects. The morphological and phase stabilities of these nano-composites under ion irradiation are explored as a function of layer thickness, temperature and interface structure. Using results on model systems such as Cu-Nb, we highlight the critical role of the atomic structure of the incoherent interfaces that exhibit multiple states with nearly degenerate energies in acting as sinks for radiation-induced point defects. Reduced radiation damage also leads to a reduction in the irradiation hardening, particularly at layer thickness of approximately 5 nm and below. The strategies for design of radiation-tolerant structural materials based on the knowledge gained from this work will be discussed.

Composite aerogels (with varying concentrations of silica and poly-dimethylsiloxane) were developed and their acoustic absorption coefficient as a function of composition and average pores size have been measured. The polydimethylsiloxane modified the ceramic structure of the silica aerogels, decreasing the material’s rigidity while maintaining the high porosity of the aerogel structure. The composite aerogels were found to exhibit different modes of acoustic absorption than that of typical porous absorbers such as fiberglass. At some frequencies, the composite aerogels had 40% higher absorption than that of commercial fiberglass. Physical data show that these materials have a large surface area (> 400 m2/g) and varying pore sizes (d ˜ 5 - 20 nm).

Clay aerogel composites have been around for over 50 years but still they represent a relatively under studied class of materials. Clay aerogel composites have been made in our labs that have low densities, 0.05-0.1g/cm3, provide good thermal insulation, k 0.02W/mK, and are created through an environmentally benign process. The mechanical properties of the composites resemble those of typical foamed polymers such as expanded polystyrene and polyurethane, with compressive moduli ranging from 0.5MPa to 40MPa depending on composition. Aqueous solutions of clay and polymer are frozen in cylindrical molds and freezedried to create these foam-like materials. Typically there is no particular orientation to the often layered structure that results, however if frozen in a unidirectional manner, anisotropic materials can be made. In this paper we will discuss the effects of molecular weight on mechanical properties of various composites as well as discussing the orientated layered structure within the anisotropic materials.

The particular plastic behaviour of corrugated metallic strips under tension has been investigated and it has been exploited to propose a new design of structural material using strengthening by plastic corrugated reinforcement. It is reported that the proposed approach is suitable to strongly improve the strain-hardening by this specific architecture.