Showing papers by "John Banhart published in 2001"

Experimental and numerical analyses of bending of foam-filled sections

Abstract: Numerical simulations and experiments are conducted to study the bending crush behavior of thin-walled columns filled with closed-cell aluminum foam. A nonlinear dynamic finite element code was used to simulate quasi-static three point bending experiments. The aluminum foam filler provides a higher bending resistance by retarding inward fold formation at the compression flange Moreover, the presence of the foam filler changes the crushing mode from a single stationary fold to a multiple propagating fold. The progressive crush prevents the drop in load carrying capacity due to sectional collapse. Henceforth, the aluminum foam filling is very attractive to avoid global failure for a component which undergoes combined bending and axial crushing. This phenomenon is captured from both experiment and numerical simulation. It was found that partially foam-filled beams also still offer, high bending resistance, and the concept of the effective foam length is developed. Potential applications of foam-filled sections for crashworthy structures are suggested.

Metal foam evolution studied by synchrotron radioscopy

Abstract: High-intensity synchrotron x-ray radioscopy was used to obtain real-time images of foaming metals, thus allowing the formation, growth, and decay of such systems to be studied. Bubble generation, foam coalescence and drainage of an aluminum-based alloy foam were investigated. Although the foaming process appears to be very similar to the formation of aqueous foams, the observed rupture behavior of thin metal films suggests that the processes responsible for metal foam stabilization and destabilization must be quite different.

Real-time X-ray Investigation of Aluminum Foam Sandwich Production

Abstract: Sandwich structures with an aluminium foam core and aluminium face sheets were produced by making use of the powder compact melting method. For this, metal powders and a powdered blowing agent were mixed and densified to a foamable precursor material which was then bonded to two aluminium face sheets by roll-cladding in a second step. Finally, the resulting composite was foamed by heating it up to the melting point of the foamable core layer. Foam evolution was monitored in-situ and in real-time by synchrotron x-ray radioscopy with a spatial resolution of 40 μm and a time resolution of 500 ms. The various stages of the sandwich formation process could be identified. Tests at different temperatures allowed for a discussion of technologically relevant process parameters. Finally, the evolution of a crack in the foamable precursor could be investigated.

Development of Advanced Foams under Microgravity

Abstract: The European Space Agency ESA has been funding a project with 5 partners from 4 European countries since the year 2000. The aim of the work in the project is to investigate the foaming process of wet aqueous and metallic foams without any disturbing gravity. As under normal terrestrial conditions many concurring processes take place at the same time in an evolving foam, the situation is quite confusing (see diagramme). Eliminating gravitationally driven drainage would allow us to simplify this scenario. In particular we could study the coalescence behaviour of metallic foams in the absence of drainage, coarsening and flow and develop more stable metallic foams this way. Coarsening gas diffuses through thin f ilms

Study of lead foams under microgravity

Abstract: This article describes an experiment to investigate the influences of physical properties like drainage and surface tension during the production of a metallic foam according to a powder metallurgical method. Foams were produced under microgravity conditions, so that gravity driven drainage was not present during the process. This lead to significant changes in the resulting structures especially of material containing a low amount of oxide. We found evidence that oxides not only influence the surface tension and thus the foamability of a material, but also act as pinning centres in the liquid metal.

Methods and models of metallic foam fabrication

Abstract: The new science of metallic foams is growing rapidly, in both the scientific research community and in industrial applications. Several methods now exist for foaming metals. One of these was invented a few years ago at the Fraunhofer-Institute in Bremen [1, 2]. The foam is fabricated from a metal powder, often aluminium, which is mixed with a blowing agent that is chosen to release gas close to the melting point of the metal, e.g. 99.5% aluminium powder and 0.5% titanium hydride powder. This powder mixture is processed to give a dense precursor material which is then heated up to the melting point of the metal. As the metal starts to melt, the blowing agent releases gas and the mixture expands. The resulting foam is then cooled to freeze the structure, resulting in a solid foam. Figure 1 shows an example of such a foam, which can easily be fabricated inside a mould, leading to the possibility of reduced post-processing. After the expansion phase therefore, the foamed liquid metal undergoes simultaneous liquid drainage and cooling. The liquid drainage, due to gravity, introduces inhomogeneity into the structure, which is generally undesirable in view of the uniform properties required in the solidified structure. If it continues for too long, rupture and collapse of the bubbles will occur. These mechanisms are prevented if the freezing process is rapid enough. Freezing fronts move inwards through the sample, arresting the drainage process. In the model described below we estimate their velocity in relation to the velocity of drainage. Figure 1 shows that uniform foams can currently be fabricated. We wish to define the physical and material parameters which will allow other materials to be foamed with this process.