Compressive properties of Advanced Pore Morphology (APM) foam elements
Introduction
Cellular materials are known for their very high porosity, high specific stiffness, acoustic damping and the ability to absorb a relatively high amount of energy at a low stress level [1], [2]. Their versatile heat insulation property makes them useful for heat exchangers or thermal insulation [3]. A major limitation of conventional cellular materials is their stochastic geometry which can result in unreliable mechanical properties. Another important challenge is the reduction of their high manufacturing cost. Control of the pore structure will allow the varying of the morphology and topology (pore size distribution, pore shape and cell wall geometry), which can be expected to allow a better definition of mechanical properties [4], [5]. Recently, a new concept of metal foam components production was patented by Stöbener et al. [6]. Carrying the name Advanced Pore Morphology (APM, see Fig. 1) foam, this innovative cellular material has been developed on an improvement of the powder metallurgical FOAMINAL® process that was introduced earlier by Baumeister [7].
Several investigations have been conducted on either single or composite APM foam elements with both partial and syntactic morphology. The experiments have been carried out by Lehmhus et al. [8] to investigate the influence of APM foam density for both quasi-static and dynamic compressive loading as well as the effect of varying the bonding agent (an epoxy-based adhesive and polyamide). They reported that the dynamic initial strength of epoxy-bonded APM foam is slightly higher than the quasi-static initial strength. A notable increase in the dynamic initial strength was observed for polyamide-bonded APM foam. Vesenjak et al. [9] investigated the behaviour of a single APM foam element and a composite APM foam with partial and syntactic morphology under compressive loading. Two APM foam element sizes with diameters ∅=5 mm and ∅=10 mm were tested experimentally. The results indicated that the larger APM spheres exhibit a higher energy-absorption capability due to a lower densification strain. They also investigated cylinder-shaped (d=h=30 mm) epoxy samples with embedded APM foam elements. Experimental testing was conducted using free and confined radial boundaries. It was found that syntactic APM composites have energy absorption capacity approximately four times higher than non-bonded or partially bonded APM foam elements. Hohe et al. [10] conducted experimental and numerical tests on graded APM foams for multi-functional aerospace applications. The main focus of their investigation was perforation resistance against bird strike events. In a case study, a sandwich plate with graded APM foam core was compared with a sandwich plate with a conventional foam core. The results indicated that the use of a graded APM foam core increases the perforation resistance performance of the sandwich plate. This was achieved by dissipating the plastic energy over a larger volume. Vesenjak et al. [11] used an infrared thermal imaging camera to enhance the usual data acquisition during compressive experimental testing of APM foam elements. Infrared thermal imaging indicated that plastic yielding occurred predominantly in the outer region of the APM foam element and then propagated inwards in a shear band into the sphere. The present paper combines, for the first time, micro-computed tomography imaging and finite element analysis of APM foam elements. In contrast with earlier investigations, a highly accurate representation of the complex inner foam geometry is obtained and incorporated in the numerical analysis. As a result, detailed information on the internal deformation mechanisms is obtained. For verification purposes, numerical results are compared with experimental data published in the literature.
Section snippets
Manufacturing process of APM foam elements
The FOAMINAL® process is capable of successfully manufacturing near net-shaped parts and three-dimensional (3D) sandwich panels with a foamed core layer. In [10], an AlSi7 aluminium alloy was prepared in powder metallurgy precursor form with TiH2 added as foaming agent. Foaming was activated by the heating of the precursor. This process usually takes place within a mould cavity. During the foaming process, furnace temperatures up to 800 °C are used depending on matrix alloys and the presence of
Model generation
Micro-computed tomography (μCT) imaging is a non-destructive procedure that allows precise measurements on aluminium foams [15], [16], [17] with the advantage of repetitive 3D assessment and computation of micro-structural and micro-mechanical properties. The most outstanding feature of μCT is the ability to image the sample׳s interior with high spatial and contrast resolution. A thorough review of μCT can be found in the monograph [18]. In previous analyses, sintered metallic hollow sphere
Results and discussion
Fig. 5 shows the quasi-static force–displacement response of APM foam elements with ∅=5 mm and ∅=10 mm diameters. Numerical simulations of APM foam elements are denoted by thin continuous lines and their average value is represented by a thick continuous line. The experimental data taken from [9] and the corresponding average are denoted by the thin and thick dotted lines, respectively. It can be observed that absolute values as well as the scattering of numerical and experimental results are
Conclusions
This paper has addressed the compressive properties of spherical APM foam elements. Two different sphere sizes, i.e., ∅=5 mm and ∅=10 mm were investigated. Numerical finite element analyses were conducted for quasi-static and dynamic loading. To account for the complex interior foam geometry, calculation models were directly derived from micro-computed tomography data. Results were compared with experimental measurements conducted on similar samples. Good agreement was found. Single APM foam
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