Structural characterisation of advanced pore morphology (APM) foam elements
Introduction
Advanced pore morphology (APM) foam is a new type of hybrid cellular structure which has been developed at the Fraunhofer IFAM, Bremen in Germany [1], [2]. APM foam elements (Fig. 1) consist of sphere-like interconnected closed-cell porous structure with thin solid outer skin. The manufacturing procedure consists of powder compaction (by the CONFORM® process [3]) and rolling of AlSi7 alloy with TiH2 foaming agent to obtain expandable precursor material. The precursor material is cut into small volumes (granules), which are then expanded into spherical foam elements due to heat reaction of the TiH2 foaming agent in a continuous belt furnace [4]. The density of APM foam elements varies from 500 to 1000 kg/m3 depending on their size, i.e. 5, 10 or 15 mm in diameter [1].
The detailed description of technology concept, production and some properties can be found in [2], [5], [6]. APM foam elements have a characteristic compressive stress–strain relationship, similar to the other cellular materials [6], [7]. Due to their mechanical and thermal properties APM foam elements have a wide range of potential applications, e.g. as stiffening elements (in shell structures), core layers, energy absorbing structural elements, damping elements or components of a composite material. One of their main advantages is their simple use as a filler element of complex hollow parts to increase the stiffness and impact energy absorption capability. As an example, a hollow automotive part can be filled with APM foam elements bonded with adhesive regardless of its shape, which is usually a very challenging problem when using conventional foam materials. Any structure consisting of APM foam elements exhibits two types of porosity: (i) the inner porosity of a single APM foam element: pi=0.63–0.82 and (ii) the outer porosity in the interstitial space between APM foam elements, which varies with different APM foam element sizes and is typically in range po=0.4–0.5 [8], [9]. The reference [2] contains some fundamental description of deformation behaviour of single and bonded APM foam elements under quasi-static compressive loading conditions. In previous research only a minor strain rate sensitivity in the global behaviour of APM foam elements under quasi-static and dynamic compressive loading conditions has been observed [10]. The use of IR thermography has also demonstrated the importance of studying the heat generation due to fast plastic deformation during dynamic loading. It was observed that the yielding starts at the skin of the APM foam element and then propagates through the sphere in a shear band, finally resulting in a fully plastically compressed APM foam element. Additionally, homogenised APM foam element models are described and their mechanical properties were determined using reverse engineering technique [10], [11]. Results of uni-axial and hydrostatic compression tests of structures, based on APM foam elements, are evaluated in [4], [6], [9], where the authors focus on variation and influence of adhesives and adhesive coating thicknesses used for bonding the APM foam elements with partial morphology. Hohe et al. discuss the potential of functionally graded materials as sandwich cores for multifunctional application with particular interest on APM foams and hollow spheres assemblies [12].
Section snippets
Methods and methodology of characterisation
With the improvement of micro computed tomography (μCT), the analysis of materials with highly complex internal structure has become possible [13]. μCT scanning subdivides a three-dimensional geometry into a set of voxels whose grey level corresponds to the average density of the represented subvolume of the original geometry. Different materials can then be identified based on their density through a process called segmentation. The acquisition of μCT data was performed on Micro CT
Results
In order to test the novel approach for the determination of spatial and size distributions of pores, two sets of APM foam element samples were used. The first set was composed of APM foam elements with the outside diameter of 5 mm while the second set included the APM foam elements with the outside diameter of 10 mm. Five samples per set were used for the analysis. The results of the μCT scans evaluation using the novel algorithm are shown in Fig. 3, Fig. 4. Fig. 3 gives the total number of
Conclusions
The novel approach presented in this paper allows for complete characterisation of geometrical properties of cellular materials based on voxel data. The geometric characterisation is demonstrated on the APM foam elements; however, it can be applied to any type of porous structure with closed-cell morphology and will be extended for characterisation of open-cell cellular architecture in the future.
Acknowledgements
The authors gratefully acknowledge the support from the Slovenian Research Agency and the Japan Society for the Promotion of Science under the project BI-JP/12-14-002. Financial support of the Slovenian Research Agency under grant number MU-PROM/12-003 is kindly acknowledged. Computed tomography images have been generously provided by Prof. Kiyotaka Masaki from the Okinawa National College of Technology. T. Fiedler wants to acknowledge financial support by the Australian Research Council's
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