Improving the energy absorption of closed cell aluminum foams

Closed cell aluminum foams have received much recent attention as energy absorbing materials on account of their ability to undergo extensive deformation at a relatively low stress called the plateau stress. Several studies describe the improvements in energy absorption to be obtained, relative to their empty counterparts, when foam filled tubes are crushed either quasi-statically or dynamically [1–4]. Al foams are also of possible interest for ballistic applications because they present a very large acoustic impedance mismatch with common armor materials, offering the possibility of being able to modify the way in which elastic waves travel through multi-component armor [5]. Two cases of energy absorption are of particular practical interest. The first concerns blast protection, in which loading occurs over a broad area, and the second concerns resistance to penetration by small projectiles, in which severe point loading is experienced. The potential advantages that foam can offer in the first case are most clear if the foam remains in contact with a face sheet during deformation because the face sheet serves to spread the deformation throughout the foam and prevent local perforation. However, in the second case, the tendency is for the foam to perforate locally and little energy is absorbed when impacted by a small object. A key factor in this latter behavior is that metal foam has an approximately zero Poisson’s ratio. Hence, no lateral deformation occurs and the only contribution to energy absorpotion is from material directly ahead of the impacting object. The objective of the present study, therefore, was to investigate whether it is feasible to increase the energy absorption by causing the deformation to spread sideways, e.g., by inserting tubes into the foam (rather than vice versa). Experiments have been performed with novel foam/tube architectures and it has been shown that it is possible to increase the energy absorbed when foam is subject to loading by a small blunt object. While this preliminary series of experiments is not exhaustive, the tests definitely confirm that foam-based architectures can be tailored to increase the energy absorption beyond that available from foams alone. Aluminum foam blocks approximately 45 × 45 × 50 mm were cut from larger foam blocks and visually examined to ensure a relatively consistent pore size and freedom from any major, visible defects. Gentle core drilling was used to prepare holes 7.15 mm in diameter in either a square or hexagonal array, Fig. 1. Al-3003 aluminum tubes, 0.4 mm wall thickness, were then inserted into the holes. Samples were weighed at each stage of preparation so that energy absorption can still be compared on a “per kg” or density basis. Aluminum tubes were chosen so as not to increase the density of the assembly much beyond that of the foam; ideally the weight of tube inserted should equal the weight of foam removed if the density is to be maintained constant. Mechanical testing was carried out by subjecting the assemblies to indentation with a 12.7 mm diameter stainless steel indenter, at a penetration speed of 0.2 mm · s−1. Data acquisition software was used to record load and penetration depth measurements during testing. Force/indentation depth curves for typical samples are shown in Fig. 2 where it is clear that, for the same displacement, the hexagonal tube arrangement requires the greatest force. Although an attempt has been made to “standardize” the tests somewhat, it is clear that there are many factors related to the geometry of the samples, size, number of tubes, spacing of tubes, tube wall thickness, tube diameter, etc., that make direct comparison problematical. The data were also converted to display specific energy absorption as a function of displacement using the equation