Scientific objectives
In detail the scientific objectives of the project are outlined below:
RO1: To identify the mechanical properties of the as-manufactured aluminium foam in SAS and the steel hollow spheres foam in SFS. Compression and energy absorption tests were undertaken using ASTM-C365M-11 for SAS and SFS. BS ISO 13314:2011 was also used on SFS specimens and shear properties were identified using ASTM C393M-06 and ASTM 7250-06.
RO2: To identify the onset of interactive failure and its propagation in single density (SAS/SFS) and graded core (SFS) tested in edgewise compression. A variation of ASTM C364-07 was used for testing.
RO3: 4-point bending fatigue testing is undertaken for SAS panels of different geometries and SFS panels with different core densities (homogeneous/graded) using ASTM C394-00(08). Furthermore at least two types of metal foam sandwiches were tested after lab based ageing BS EN ISO 11130:2008 using North Sea marine environment solution based on ASTM D1141-98(2003).
RO4: To explore and select the most appropriate type of integrated low power and/or energy scavenging sensor that would require minimum or no energy at all for charging and its embedded position in a sandwich panel, by leveraging biomimetic principles. (Mitcheson et al, 2008)
RO5: In light of the SFSP joint development, to identify and investigate the potential application of lightweight SFSPs with integrated sensing as a novel solution to offshore and marine structures, by quantifying their merits compared to conventional plated structures (Ashby et al, 2000).
RO1: To identify the mechanical properties of the as-manufactured aluminium foam in SAS and the steel hollow spheres foam in SFS. Compression and energy absorption tests were undertaken using ASTM-C365M-11 for SAS and SFS. BS ISO 13314:2011 was also used on SFS specimens and shear properties were identified using ASTM C393M-06 and ASTM 7250-06.
RO2: To identify the onset of interactive failure and its propagation in single density (SAS/SFS) and graded core (SFS) tested in edgewise compression. A variation of ASTM C364-07 was used for testing.
RO3: 4-point bending fatigue testing is undertaken for SAS panels of different geometries and SFS panels with different core densities (homogeneous/graded) using ASTM C394-00(08). Furthermore at least two types of metal foam sandwiches were tested after lab based ageing BS EN ISO 11130:2008 using North Sea marine environment solution based on ASTM D1141-98(2003).
RO4: To explore and select the most appropriate type of integrated low power and/or energy scavenging sensor that would require minimum or no energy at all for charging and its embedded position in a sandwich panel, by leveraging biomimetic principles. (Mitcheson et al, 2008)
RO5: In light of the SFSP joint development, to identify and investigate the potential application of lightweight SFSPs with integrated sensing as a novel solution to offshore and marine structures, by quantifying their merits compared to conventional plated structures (Ashby et al, 2000).
metal foam
Metal foam is a porous type of metal that can combine usual metallic structural properties like plasticity and heat resistance with low weight and enhanced energy dissipation. 15 years ago there was increased activity in Europe, North America and Japan on the development of different metal foams, the majority of which were based on aluminium alloys, although foams based on steel, zinc and copper alloys were also investigated and developed (Banhart, 2001). This was followed by an effort of characterization and determination of the properties of metal foams which is documented in Ashby et al (2000). Since then, commercialization has been slow and mostly seen for aluminium foams, which has a relatively low foaming temperature, low weight and moderately uniform properties achieved by controlling the temperature stages in manufacturing. There has also been plenty of work in the optimization on metal foam properties and manufacturing processes, but little on their commercialization as structural components. Some current structural applications for metal are presented in Lefebvre and Banhart (2008) and Smith et al (2012a). The initial consideration for metal foams to perform as lightweight cold formed plate components, was soon dismissed due to its poor strength in tension (Smith et al, 2012b). A more appropriate function for metal foam as metal component was highlighted as core in sandwich structures or infill in hollow sections, exhibiting merits in buckling mitigation (Szyniszewski et al, 2012) and energy dissipation (Hanssen, 2001). Despite the slow uptake of the material from the industry, a few European manufacturers started appearing which will lead to serial production and decrease in the cost. Scientists, manufacturers and engineers meet at the Metal Foam International Conference series, which runs every two years and aims at bringing metal foam professionals together.
MANUFACTURING PROCESSES
There are two types of metal foams in production as shown in the figures below, closed cell and open cell, which refer to the permeability of each type.
The majority of metal foams are produced through a powder metallurgical method, where alloy fillings are mixed and then compressed with a foaming agent (for example titanium hydride) to form a precursor. The solid mixture is then foamed to a specified density or volume and cooled in stages. Other manufacturing processes include syntactic metal foams by metal powder injection moulding and casting for open-cell foams using organic placeholders, as described in Banhart (2001). Open cell metal foams are less stiff, have higher porosity and see applications in process engineering for heat exchangers and as hosts of catalytic reactions (Boomsma et al, 2003). Taking a different philosophical approach in metal foam manufacturing, the Fraunhofer IFAM Institute in Dresden focussed on developing the unit cell of the metal foam, as a hollow steel sphere---technology that was then taken up by Hollomet GmbH, a spin-off of the Institute. Polymeric spheres are coated with steel powder and heated for the metal to cure and the polymer to evaporate (Lhuissier, 2009). Once assembled, these hollow spheres can be sintered together leading to a syntactic structure of very high porosity or joined together with a resin. Fraunhofer IFAM in Bremen took on a similar approach to develop its Advanced Pore Morphology product (APM). With this method, the aluminium powder precursor is cut to small pellets that are then foamed to a sphere shape of controlled volume and thus bulk density. These are then coated with a polymeric adhesive that can be cured at a much lower temperatute to create a hybrid polymer-metal foam. A recent work by Hohe et al (2012) outlines the manufacturing process and applications of both hollow spheres and APM in aerospace applications.
METAL FOAM SANDWICH PANELS for INSIST
The project has two distinct targets. The first one is to investigate the structural stability and integrity of steel foam sandwich panels with homogeneous and graded cores. The second is to propose metal foam structural components for energy infrastructure. The assembly of the steel hollow spheres could be checked for both the sintered case and the resin matrix. Past experience from the NSF Steelfoam project highlight the difficulty to control the tension response of sintered steel hollow sphere and to machine specimens. It would be interesting to see whether different joining processes and gradients in the core can change that. More pertinent to the second target for offshore energy applications is also worth to consider SAS (Steel-Aluminium foam-Steel) panels, which can be a potential replacement for thick steel plates, eliminating the need for stringers (stiffeners) or secondary beams.
FINAL REPORT
Download the executive summary here.
|
|