Keywords: metal foam, closed-cell material, orthotropic material properties, indirect methods, micro-CT, voxel models, tetrahedral models.

Mechanical properties of geometrically complex materials can be investigated nondestructively using microfocus computed tomography (micro-CT). This involves development of a finite element (FE) model of a sample of the material and performing a "virtual experiment", i.e. computer simulation of a mechanical test. The results of such a simulation (and the calculated material properties) are dominantly influenced by the quality of the finite element model. In this paper a comprehensive comparison on different techniques of finite element microstructural model generation is given. A block sample of aluminum metal foam Alporas® with 25x25x25 mm dimensions was scanned in a custom micro-CT device. As a result of the very small thickness of the metal foam's walls, high-resolution scanning was needed. The sample was scanned in 0.5° increments using a large-area flat panel X-ray detector. Using the tomographic projections, the binary image data containing only the thin walls were produced using segmentation. Variously detailed models were generated for the set of finite element simulations with different sizes of the voxel (i.e. different numbers of pixels used to define a single voxel) and a different density for the tetrahedral finite element mesh. The effect of spatial resolution, segmentation quality and the use of voxel or tetrahedral elements was studied. A virtual uni-axial unit compression test was simulated in all mutually perpendicular directions to obtain elastic moduli and maximum values of principal stresses as well as to quantify the anisotropy of the material characteristics. The material model used in the finite element simulations was based on the results obtained during an experimental-numerical study of the material used for production of the reference foam [1]. The importance of proper image segmentation and minimal requirements for the resolution of the input images is clearly demonstrated by the results obtained. Derived elastic constants indicate that tetrahedral models have a poor prediction of elastic properties compared to voxel models of comparable numbers of elements or nodes, although the acquired elastic constants from the finite element simulations of the most detailed models are in good agreement with the experimental results and the production characteristics of Alporas.

1

I. Jeon, K. Katou, T. Sonoda, T. Asashina, K.-J. Kang, "Cell wall mechanical properties of closed-cell Al foam", Mechanics of Materials, 41(1), 60-73, 2009. doi:10.1016/j.mechmat.2008.08.002

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