The Influence of Cell Shape Anisotropy on the Tensile Behavior of Open Cell Aluminum Foam

Abstract

There are numerous processes for the fabrication of openand closed cell metal foams and some of these methods result in a strong anisotropy in cell shape. The mechanical properties of these specific foams, such as the stiffness, yield stress and ultimate tensile strength (UTS) (see Fig. 1), also show anisotropy which is often linked to the cell shape. [1,2,3,4,5] Gibson and Ashby (G&A) constructed an analytical beam bending model to calculate the influence of the cell aspect ratio, i.e. the long cell axis divided by the short cell axis, on the stiffness and plastic strength. [6] Descriptions based on continuum approaches have been proposed as well. [7,8,9] Obviously, for an experimental validation an accurate experimental determination is important. In fact, X-ray tomography can be used to measure a large number of cells with high precision. [10,11,12] From the measured aspect ratio in the cell shape the anisotropy in the stiffness in tension was calculated in agreement with experiments. [11] In addition, by accounting for the anisotropy in plastic behavior the peak stress and strain have been predicted based on a critical strain criterion. [9] In these studies, however, the role of damage accumulation during straining has not been incorporated in the modeling, nor has it been quantitatively measured as a function of density.In a previous study we have shown that damage accumulation starts well before the peak stress has been reached and that the onset and rate of damage evolution depends critically on the base material properties and the relative density of the foam. [13] The ability of the foam to accommodate the applied strain by strut bending and reorientation was identified as an important mechanism responsible for the large peak strain at small densities. The objective of this paper is to study the effect of cell shape anisotropy looked upon in the light of these mechanisms. We start by relating the yield stress to the cell orientation at small strains, followed by a study of the damage evolution at large strain by measuring the electrical resistance in-situ.The material used is ERG Duocel open cell aluminum foam (20 PPI, alloy AA6101). The relative densities of the samples are grouped according to the following three ranges: 3-5 % (low), 6-7 % (medium) and 10-13 % (high). The relative density of each sample was calculated by measuring the mass and dividing it by the volume and the mass of pure Al (2.7 g/cm 3 ). The samples were received in the annealed (AN) condition (3 hrs at 412°C, followed by slow cooling to 260°C and maintaining that temperature for 0.5 hr followed by slow cooling to RT). Some of the samples were subjected to an additional precipitation hardening heat treatment (HT) to increase the yield stress. First the samples were exposed to a solid solution heat treatment (527°C for 8 hrs) followed by quenching in RT water. After 16 hrs at RT the samples were artificially aged for 8 hrs at 177°C, followed by cooling inside the furnace. Table 1 shows the mechanical properties in tension of the Al a...

Multiscale modelling of damage and failure in two-dimensional metallic foams

“…The observation that the peak stress in RD is larger and peak strain is lower compared to TD is consistent with a number of experimental observations (Banhart and Baumeister, 1998;Dillard et al, 2006;Amsterdam et al, 2008d). In RD, we found an early damage onset and higher damage accumulation rate, owing to the decreased critical curvature and increased rate of curvature development at the triple points.…”

“…In tension, the redistribution of active strain regions is attributed to the local hardening caused by the stretching of cell walls (Motz and Pippan, 2001). The relative distribution of the material in the plateau borders and cell wall (Warren and Kraynik, 1987;Harders et al, 2005;Al-Hasani, 2002, 2001;Gong et al, 2005) and the anisotropy in cell shape also have a strong influence on the elasto-plastic properties of foams (Silva et al, 1995;McCullough et al, 1999;Nieh et al, 2000;Amsterdam et al, 2008d). Clearly, the mesoscopic structure introduces a characteristic length scale (the cell size), which becomes important especially when dealing with structures having dimensions on the order of the cell size or in case of inhomogeneous deformation at localization bands, cracks, notches or holes.…”

Coarsening of nanoporous Au: Relationship between structure and mechanical properties

“…It is worth noting, indeed, that the model does not account properly neither for the stochastic nature of local morphology, nor for anisotropy effects that have been often observed in porous systems [51][52][53]. Nevertheless, the modeling approach followed here allows relating the mechanical properties to the different characteristic lengths associated with the microstructural features of NP metal foams.…”

Metallic muscles and beyond: nanofoams at work

“…The popularity of macrofoams, i.e., porous materials where pore size is above the scale of tens of microns, stems from the intersection of a variety of desirable properties in industry. For example, aluminum macrofoam boasts of a high stiffness-to-density ratio, high capacity for energy absorption during compression, high-temperature resistance, electrical and thermal conductivities, good machinability, and cheap production costs [20][21][22][23][24][25], see Fig. 1.…”