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A rigorous method for the homogenization of general elastoplastic periodic lattices is applied to 3D cellular solids. Tetrakaidecahedral unit cell problems are solved to determine the overall yield surface of foams. Non-symmetric ...
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A rigorous method for the homogenization of general elastoplastic periodic lattices is applied to 3D cellular solids. Tetrakaidecahedral unit cell problems are solved to determine the overall yield surface of foams. Non-symmetric material distribution is introduced and new results concerning the influence of this type of defect are obtained. They show that non-uniform material distribution increases the overall strength, except in particular loading directions and that non-symmetry has no significant influence on the yield surface. (c) 2005 Springer Science + Business Media, Inc.
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Aluminum foams are currently being considered for use in lightweight structural sandwich panels and in energy absorption devices, In both applications, they may be subject to multiaxial loads. The designer requires a criterion to ...
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Aluminum foams are currently being considered for use in lightweight structural sandwich panels and in energy absorption devices, In both applications, they may be subject to multiaxial loads. The designer requires a criterion to evaluate the combination of multiaxial loads which cause failure. Here we measure the failure of one open- and one closed-cell aluminum foam under biaxial and axisymmetric triaxial loading. The data are compared with three yield criteria for metallic foams. The first criterion is based on analysis of the failure mechanisms of an ideal foam. It overestimates the measured foam yield surface: the discrepancy can be related to imperfections in the foam structure. The other two criteria are phenomenological: one is based on the well-known Drucker-Prager criterion while the other is based on a recently developed yield surface for compaction of powders. Both phenomenological yield surfaces give a good description of the multiaxial failure of the aluminum foams tested in this study. (C) 2000 Elsevier Science Ltd. All rights reserved. [References: 20]
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A detailed numerical investigation of the programmable materials presented in part I is conducted in this study. The influence of defects and exogenous factors on the programmability of samples from two different programmable cell...
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A detailed numerical investigation of the programmable materials presented in part I is conducted in this study. The influence of defects and exogenous factors on the programmability of samples from two different programmable cellular material systems is investigated through finite element simulations. The defects studied here are representative of those present in the samples that were tested in the companion paper. These include non-uniform cell wall thicknesses, missing cell walls, misaligned cell walls and fillets at the cell wall junctions. Exogenous factors such as the sample size, friction at the sample boundaries and the strain rate sensitivity of the base material are also included in this study. We base our studies on the analysis of the change of the effective initial modulus on each programmed configuration of both material systems due to these factors. The investigation demonstrates that even in the presence of defects and exogenous factors, programming remains a robust way of modifying the effective initial modulus of programmable cellular materials over a wide range without requiring reprocessing of the material. However, the programmability and the susceptibility to various contributing factors vary with the base material, type of programmed imperfection and the nature of the original cellular solid. (C) 2016 Elsevier Ltd. All rights reserved.
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The cyclic behaviour of 400 n,m pore size replicated aluminium foam is assessed in tension-tension fatigue with a stress ratio equal to 0.1, keeping the load amplitude constant, for relative density values comprised between 0.175 ...
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The cyclic behaviour of 400 n,m pore size replicated aluminium foam is assessed in tension-tension fatigue with a stress ratio equal to 0.1, keeping the load amplitude constant, for relative density values comprised between 0.175 and 0.220. The number of cycles to failure ranges from 6 x 102 (lowest relative density) to 5 x 106 (highest relative density). The foams display cyclic creep coupled with a strong influence of relative density on their general fatigue performance. Data analysis shows that the foam fatigue behaviour is dominated by cyclic creep, which governs both the deformation and the fatigue life of the cycled specimens, yielding characteristics globally in line with what is expected knowing the metal making the foam.
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When temperature increases, the volume of an object changes. This property was quantified as the coefficient of thermal expansion only a few hundred years ago. Part of the reason is that the change of volume due to the variation o...
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When temperature increases, the volume of an object changes. This property was quantified as the coefficient of thermal expansion only a few hundred years ago. Part of the reason is that the change of volume due to the variation of temperature is in general extremely small and imperceptible. Here, abnormal giant linear thermal expansions in different types of two-ingredient microstructured hierarchical and self-similar cellular materials are reported. The cellular materials can be 2D or 3D, and isotropic or anisotropic, with a positive or negative thermal expansion due to the convex or/and concave shape in their representative volume elements respectively. The magnitude of the thermal expansion coefficient can be several times larger than the highest value reported in the literature. This study suggests an innovative approach to develop temperature-sensitive functional materials and devices.
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A cellular metallic material with interconnected porosity of controlled size of n order of 10 μm has been developed by electrochemical dissolution of tungsten grains in a w-Ni-Fe heavy alloy. The nickel superalloy with sponge str...
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A cellular metallic material with interconnected porosity of controlled size of n order of 10 μm has been developed by electrochemical dissolution of tungsten grains in a w-Ni-Fe heavy alloy. The nickel superalloy with sponge structure and high surface/volume ratio can also be processed recycling chips form heavy metal machining (Patent n°P9700191, 1997). Applications for the new material could be found as support for catalysts, high temperature filters for corrosive fluids, burners, etc.
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Despite the recognition of the enormous potential of periodic trusses for use in a broad range of technologies, there are no widely-accepted descriptors of their structure. The terminology has been based loosely either on geometry...
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Despite the recognition of the enormous potential of periodic trusses for use in a broad range of technologies, there are no widely-accepted descriptors of their structure. The terminology has been based loosely either on geometry of polyhedra or of point lattices: neither of which, on its own, has an appropriate structure to fully define periodic trusses. The present article lays out a system for classification of truss structure types. The system employs concepts from crystallography and geometry to describe nodal locations and connectivity of struts. Through a series of illustrative examples of progressively increasing complexity, a rational taxonomy of truss structure is developed. Its conceptual evolution begins with elementary cubic trusses, increasing in complexity with non-cubic and compound trusses as well as supertrusses, and, finally, with complex trusses. The conventions and terminology adopted to define truss structure yield concise yet unambiguous descriptions of structure types and of specific (finite) trusses. The utility of the taxonomy is demonstrated by bringing into alignment a disparate set of ad hoc and incomplete truss designations previously employed in a broad range of science and engineering fields. Additionally, the merits of a particular compound truss (comprising two interpenetrating elementary trusses) is shown to be superior to the octet truss for applications requiring high stiffness and elastic isotropy. By systematically stepping through and analyzing the finite number of structure types identified through the present classification system, optimal structures for prescribed mechanical and functional requirements are expected to be ascertained in an expeditious manner.
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Damping in porous structures is enhanced by localized stresses in comparison with corresponding dense materials. An analytical model for mechanical damping in porous materials is suggested for the 'microplastic' range of deformati...
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Damping in porous structures is enhanced by localized stresses in comparison with corresponding dense materials. An analytical model for mechanical damping in porous materials is suggested for the 'microplastic' range of deformation on the basis of the statistical mechanics of micro-heterogeneous materials. Two types of heterogeneity are considered: one-level porosity (directly sintered powder steels) and two-level porosity (larger pores in a carcass with smaller pores) on one hand, and fatigue-induced cracks on the other. Special emphasis is placed on the crack-related contribution to damping using a Dugdale approach to crack-induced microplasticity.
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The mechanical behaviour of polymeric foams depends on several parameters such as temperature, material density and strain rate. This last point implies that compression tests on conventional testing machines are not sufficient. S...
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The mechanical behaviour of polymeric foams depends on several parameters such as temperature, material density and strain rate. This last point implies that compression tests on conventional testing machines are not sufficient. Study of the behaviour in practical situations requires special apparatus like fly wheels, drop towers or Hopkinson bars, allowing high compression speeds. The polypropylene foams studied are multi-scale materials; agglomerated beads (2-3 mm in diameter), visible to the naked eye, are composed of microscopic closed cells (a few tens of microns). The response of the material to a shock consists of three regions: an elastic phase, a plastic phase and densification. The plastic phase is of prime interest since a great part of the shock energy is dissipated there. Microtomography was used in order to better understand damage mechanisms during the stress plateau of the plastic phase. The final objective of this work is to determine the strain field of porous materials at several levels of shock. As tomography is not fast enough to directly follow the impact deformation, interrupted impact tests were carried out by controlling the levels of sample deformation. Between each impact step, a microtomographic analysis offers insight on the progressive deformation of the sample. The results of these impact tests completed by a microtomographic visualisation in 2D are presented and commented in this paper.
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During compression of brittle or ductile solid foams deformation is, on the scale of the cellular microstructure, characterized by local collapse of cells and the development of strain localization bands ('crushing bands' or 'coll...
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During compression of brittle or ductile solid foams deformation is, on the scale of the cellular microstructure, characterized by local collapse of cells and the development of strain localization bands ('crushing bands' or 'collapse bands'). On the macroscopic scale, one may either observe the formation and expansion of a macroscopic collapse band which accommodates the imposed compressive strain, or more diffuse accumulation of damage with macroscopically homogeneous deformation. We use a compressive strain gradient plasticity framework to study the nucleation and spreading of collapse bands in solid foams with regular and disordered microstructures. It is demonstrated that foams with ordered microstructure may exhibit a macroscopically inhomogeneous deformation mode which is characterized by a yield point followed by nucleation and spreading of a single collapse band. With increasing disorder we find a transition to macroscopically homogeneous deformation behavior characterized by apparently monotonic hardening and the diffuse nucleation of multiple, non-spreading bands.
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