The influence of each of the six different types of morphological imperfection-waviness, non-uniform cell wall thickness, cell-size variations, fractured cell walls, cell-wall misalignments, and missing cells-on the yielding of 2D cellular solids has been studied systematically for biaxial loading. Emphasis is placed on quantifying the knock-down effect of these defects on the hydrostatic yield strength and upon understanding the associated deformation mechanisms. The simulations in the present study indicate that the high hydrostatic strength, characteristic of ideal honeycombs, is reduced to a level comparable with the deviatoric strength by several types of defect. The common source of this large knock-down is a switch in deformation mode from cell wall stretching to cell wall bending under hydrostatic loading. Fractured cell edges produce the largest knock-down effect on the yield strength of 2D foams, followed in order by missing cells, wavy cell edges, cell edge misalignments, г Voronoi cells,θ Voronoi cells, and non-uniform wall thickness. A simple elliptical yield function with two adjustable material parameters successfully ®ts the numerically predicted yield surfaces for the imperfect 2D foams, and shows potential as a phenomenological constitutive law to guide the design of structural components made from metallic foams.

Sandwich panels with two-dimensional metal cores can be used to carry structural load as well as dissipate heat through solid conduction and forced convection. This work attempts to uncover the nature of heat transfer in these lightweight systems, with emphasis on the eects of varying cell morphologies and cell arrangements. The types of cell shape and cell arrangement considered include regular hexagon, square with connectivity 4 or 3, and triangle with connectivity 6 or 4. Two analytical models are developed: corrugated wall and eective medium. The former models the cellular structure in detail whilst, the latter models the fluid saturated porous structure using volume averaging techniques. The overall heat transfer coeffcient and pressure drop are obtained as functions of relative density, cell shape, cell arrangement, fluid properties, and overall dimensions of the heat sink. A two-stage optimization is subsequently carried out to identify cell morphologies that optimize the structural and heat transfer performance at speci

The fluid-flow and heat-transfer features of cellular metal lattice structures made from copper by transient liquid phase (TLP) bonding and brazing of plane weave copper meshes (screens) were experimentally characterized under steady-state forced air convection. Due to the inherent structural anisotropy of this metal textile derived structure, the characterizations were performed for several configurations to identify the preferable orientation for maximizing thermal performance as a heat dissipation medium. Results show that the friction factor of bonded wire screens is not simply a function of porosity as stochastic materials such as open-celled metal foams and packed beds, but also a function of orientation (open area ratio). The overall heat transfer depends on porosity and surface area density, but only weakly on orientation. Comparisons with stochastic metal foams and other heat dissipation media such as packed beds, louvered fins and microtruss lattice cellular materials suggest that wire-screen meshes compete favorably with the best available heat dissipation media. The overall thermal efficiency index of the copper textiles-based media is approximately three times larger than that of stochastic copper foams, principally because of the lower pressure drop encountered during coolant propagation through the periodic wire-screen structure.

The paper explores the use of open-celled metal foams as compact heat exchangers, exploiting convective cooling. An analytical model is developed for model foams with simple cubic unit cells consist-ing of heated slender cylinders, based on existing heat transfer data on convective crossflow through cylin-der banks. A foam-filled channel having constant wall temperatures is analyzed to obtain the temperature distribution inside the channel as a function of foam density, cell size and other pertinent heat transfer par-ameters. Two characteristic length scales of importance to the problem are discussed: the minimum channel length required for heating the fluid to its goal temperature and the thermal entry length beyond which the transfer of heat between fluid and channel wall assumes a constant coecient. The overall heat transfer coecient of the heat exchanging system is calculated, and the pressure drop experienced by the fluid flow obtained. These results are used to analyze and guide the design of optimum foam structures that would maximize heat transfer per unit pumping power. Two examples are given to demonstrate the applicability of the analytical model: heat sinks for high power electronic devices and multi-layered heat exchangers for aeronautical applications. The present model perhaps oversimpli

A minimum-weight flexural actuator is designed. The actuator comprises a triangular corrugated core with shape memory alloy (SMA) faces. It is clamped at one end and free at the other. For design and optimization, the temperature history of the face sheets upon heating and subsequent cooling is 7rst obtained as a function of the cooling e8ciency (Biot number) and the operational frequency deduced. Based upon this response, a phenomenological model is employed to represent the martensite evolution. Thereafter, the end de'ection is calculated as a function of temperature. The minimum weight is calculated subject to the provisos that: (i) the end de'ection attains a speci7ed value; (ii) the power consumed is less than the upper limit of the supply; and failure is averted by (iii) face=core yielding and (iv) face=core buckling; (v) the operational frequency of the panel achieves a speci7ed limit.