6 02005 (2010)
EPJ Web of Conferences 6,
DOI:10.1051/epjconf/20100602005
© Owned by the authors, published by EDP Sciences, 2010
Experimental Testing of Single APM Spheres
M. Vesenjak
1
2
1,a
2, b
and L. Krstulovic-Opara
University of Maribor, Faculty of Mechanical Engineering, Maribor, Slovenia
University of Split, Faculty of Elect. Eng., Mech. Eng. and Naval Architect., Split, Croatia
Abstract. Advanced pore morphology (APM) foam, consisting of sphere-like metallic
foam elements, proves to have advantageous mechanical properties and unique
application adjustability. Since the APM foam manufacturing procedure has been
developed recently, the mechanical characterization of these materials is still very
limited. Therefore, the purpose of this research was to determine the behaviour of APM
spheres and its composites when subjected to quasi-static and dynamic compressive
loading. The results of the performed research have shown valuable mechanical
properties of the composite APM foam structures, offering new possibilities for their use
in general engineering applications.
1 Introduction
Development, design, manufacturing, characterisation and application of conventional cellular
structures used in composite materials have been widely studied and reviewed [1-4]. New fabrication
methods have been developed for more convenient and flexible use in different applications to
overcome the technological problems related to the control of structure irregularity, also resulting in
more homogeneous and regular pore distribution [5-8]. Recently, a new type of cellular materials has
been developed, the Advanced Pore Morphology (APM) foam, which consists of sphere-like
metallic foam elements, characterised by advantageous mechanical properties and unique
applicability. Since the APM foam manufacturing procedure has recently been developed, the
mechanical characterization of these materials is very limited. Therefore, the purpose of this research
was to determine the behaviour of the APM foam elements subjected to compressive loading. Single
APM foam elements were experimentally subjected to quasi-static and dynamic compressive
loading, providing the basic properties and knowledge for an efficient application of composite APM
foam structures.
2 Advanced Pore Morphology Foam
The APM foam has been developed at Fraunhofer IFAM, Bremen in Germany, and represents a new
method for production of hybrid cellular structures. The APM foam elements (spheres) consist of
interconnected closed-cell cellular structure (foam), as shown in Fig. 1 [9-10].
a
b
e-mail : m.vesenjak@uni-mb.si
e-mail : lovre.krstulovic-opara@fesb.hr
This is an Open Access article distributed under the terms of the Creative Commons Attribution-Noncommercial License 3.0, which
permits unrestricted use, distribution, and reproduction in any noncommercial medium, provided the original work is properly cited.
Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20100602005
EPJ Web of Conferences
5 mm
Fig. 1. Advance Pore Morphology (APM) foam element:
cross-section (left) and CT scan (right).
The manufacturing procedure consists of powder compaction (by the CONFORM® process) and
rolling to obtain expandable precursor material. The precursor material is then cut into small
volumes (granules) that are in a continuous belt furnace expanded (due to TiH2 foaming agent) into
spherical foam elements [11]. The matrix alloy of the APM foam is AlSi7. Generally, three sizes of
APM foam are manufactured: 5, 10 and 15 mm in diameter. Their density can vary from 500 to 1000
kg/m3 [10]. Their detailed technology concept, production and properties are described in [9, 12].
The APM foam parts exhibit two types of porosity: (i) the inner porosity in single APM foam
elements and (ii) the porosity between the many APM foam elements [13].
As cellular materials, the APM foam elements have a characteristic compressive stress-strain
relationship that can be divided into four main areas: (i) quasi-linear elastic response, (ii) transition
zone, where the materials exhibit buckling, plastic deformation and collapse of intercellular walls,
(iii) stress plateau, where the mechanism of buckling, and collapse becomes even more pronounced,
resulting in large strains at almost constant stress and (iv) densification, where the stiffness increases
and consequently converges towards the stiffness of the base material. The APM foam has a wide
potential application spectrum as energy absorbing structure, stiffening elements (in shell structures),
core layers, damping elements or bonded with a matrix in composite materials. One of their main
advantages is their simple use as filler elements increasing the energy absorption of hollow parts,
e.g. a hollow automotive part can be filled with APM foam elements covered with adhesive, then the
filled part is heated up to join the APM foam elements. Such APM foam filled part has a much
higher energy absorption capabilities with minimal mass increase. It should be also noted that this
procedure is insensitive to hollow part shape and geometry, which can be a challenging problem at
conventional aluminium foams. They are additionally able to undergo large deformations when
subjected to compressive loading.
Since the technology concept and manufacturing procedure of APM foam elements has just recently
been developed there are only a few studies available characterizing this kind of material. The
reference [9], along with the detailed description of the APM technology concepts, represents
fundamental deformation behaviour of single and bonded APM foam elements under quasi-static
compressive loading conditions. Results of uni-axial and hydrostatic compression tests of APM foam
are evaluated in [11], where the authors focus on variation and influence of adhesives and adhesive
coating thicknesses used for bonding the APM foam elements with partial morphology.
Additionally, they provide some distinctive differences between APM and conventional aluminium
foams. A practical example of filling a hollow profile with APM elements and comparing the stressstrain diagrams between the hollow profile, hollow profile filled with APM elements and hollow
profile filled with bonded APM elements is given in [9, 13]. The authors conclude that filling the
profile with APM elements increases the capability of energy absorption which can be further
improved by bonding the APM elements.
02005-p.2
14th International Conference on Experimental Mechanics
3 Experimental testing
The experimental testing was divided into two parts: (i) characterization of single APM foam
elements and (ii) characterization of composite APM foam. The experiments have been performed
on the servo-hydraulic dynamic testing machine INSTRON 8801 under quasi-static and dynamic
uni-axial compressive loading conditions (Fig. 2).
Fig. 2. Compressive testing of a single APM foam element.
Two sizes of APM foam elements (ρ ≈ 800 kg/m3) have been used: ∅5 and ∅10 mm. The matrix
alloy of all specimens was AlSi7. Figure 3 shows the deformation behaviour of the APM foam
element.
Fig. 3. Compression phases of a single APM foam element.
The results of the single APM foam elements experiments are shown in Fig. 4-8, showing
characteristic cellular material (foam) behaviour, i.e. the stress plateau followed by the densification.
Due to the spherical shape of the APM foam elements the diagram shows the compressive force vs.
macroscopic (global) engineering compressive strain. Figures 4 and 5 represent the quasi-static and
dynamic compressive behaviour of APM foam elements of 5 mm in diameter. Strain rates up to
33 s-1 have been achieved. From the comparison it can be observed that in the case of quasi-static
loading the stress increase more gradually compared to the dynamically loaded elements. However,
this difference can be observed only up to 15 % of deformation. For the larger deformation, the
response of quasi-statically and dynamically loaded APM elements is similar.
02005-p.3
EPJ Web of Conferences
Fig. 4. Behaviour of single APM foam elements (∅5 mm)
under quasi-static compressive loading conditions.
Fig. 5. Behaviour of single APM foam elements (∅5mm)
under dynamic compressive loading conditions.
02005-p.4
14th International Conference on Experimental Mechanics
Fig. 6. Behaviour of single APM foam elements (∅10 mm)
under quasi-static compressive loading conditions.
Fig. 7. Behaviour of single APM foam elements (∅10 mm)
under dynamic compressive loading conditions.
02005-p.5
EPJ Web of Conferences
Figures 6 and 7 represent the quasi-static and dynamic compressive behaviour of APM foam
elements with 10 mm in diameter. Strain rates up to 20 s-1 have been achieved. From the comparison
it can be observed that their global response is similar. However, higher strain rates might influence
their behaviour.
Fig. 8. Behaviour of single APM foam elements (∅5 and ∅10 mm)
under quasi-static compressive loading conditions.
From the Fig. 8 it can be observed that the plateau (force) region for larger specimens (∅10 mm) is
approximately 4.6x higher in comparison to ∅5 mm APM foam elements. Additionally, it is clear
that larger foam elements experience lower densification strain (approx. ∆ε = 0.15). The
experimental results are in a good agreement with results achieved by Stöbener et al. [11].
Conclusions
The APM foam elements, characterized by efficient manufacture and repeatable production
parameters, are opening new directions in engineering of energy absorption components. The paper
describes behaviour of APM foam subjected to compressive mechanical loading. The single APM
elements showed a characteristic cellular material behaviour. It has been observed that the larger
foam elements experience lower densification strain. Additionally, minor strain rate sensitivity in the
global behaviour has been noted. The APM spheres are mainly considered as basic components,
which together with adhesive form composite materials having enormous potential in various
applications. In our ongoing research an advanced approach to study the topology and morphology
influence on the global behaviour of composite APM structures by means of the CT and thermal
image scanning is planned. Furthermore, different adhesive materials will be tested in order to
increase the application range of the studied composite materials.
02005-p.6
14th International Conference on Experimental Mechanics
Acknowledgments
Support of Dr. Karsten Stöbener and Croatian Ministry of Science, Education and Sports project
“Fatigue strength of constructions and materials” is gratefully acknowledged.
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
M.F. Ashby, A. Evans, N.A. Fleck, L.J. Gibson, J.W. Hutchinson, H.N.G. Wadley. Metal
foams: a design guide. Burlington, Massachusetts: Elsevier Science (2000).
J. Banhart. Manufacture, characterisation and application of cellular metals and metal foams.
Prog Mater Sci, 46, 559 (2001).
L.J. Gibson, M.F. Ashby. Cellular solids: structure and properties. Cambridge: Cambridge
University Press (1997).
M. Vesenjak, L. Krstulović-Opara, Z. Ren, A. Öchsner, Ž. Domazet. Experimental Study of
Open-Cell Cellular Structures with Elastic Filler Material. Exp Mech, 49, 501 (2009).
O. Andersen, U. Waag, L. Schneider, G. Stephani, B. Kieback. Novel Metallic Hollow
Sphere Structures. Adv Eng Mater, 2, 192 (2000).
C. Augustin, W. Hungerbach. Production of hollow spheres (HS) and hollow sphere
structures (HSS). Mater Lett, 63, 1109 (2009).
M. Vesenjak, T. Fiedler, Z. Ren, A. Öchsner. Behaviour of Syntactic and Partial Hollow
Sphere Structures under Dynamic Loading. Adv Eng Mater, 10, 185 (2008).
H. Nakajima. Fabrication, properties and application of porous metals with directional pores.
Prog Mater Sci, 52, 1091 (2007).
K. Stöbener. Advanced Pore Morphology (APM) - Aluminiumschaum. Bremen: University of
Bremen (2007).
Advanced Pore Morphology (APM) Metal Foams. Bremen: Fraunhofer Institut
Fertigungstechnik Materialforschung (IFAM) (2007).
K. Stöbener, D. Lehmhus, M. Avalle, L. Peroni, M. Busse. Aluminum foam-polymer hybrid
structures (APM aluminum foam) in compression testing. Int J Solids Struct, 45, 5627 (2008).
K. Stöbener, G. Rausch. Aluminium foam–polymer composites: processing and
characteristics. Journal of Materials Science, 44, 1506 (2009).
K. Stöbener, J. Baumeister, G. Rausch, M. Rausch. Forming metal foams by simpler methods
for cheaper solutions. Metal Powder Report, 60, 12 (2005).
02005-p.7