Correlation of Metallic Foam Structure to Mechanical Properties Under Tensile and Compressive Loading

Metal foams made out of Aluminium will have a big potential for structural engineering applications because of their good specific stiffness and strength. Compared to other engineering materials metal foams show a completely different mechanical behaviour. Material data and material law describing the mechanical behaviour are required before metal foams can be used as new engineering material. State of the art in foam research is that the internal structure of foam and properties of foam material are known as key features in determining the response of foam to external mechanical loading. Extreme heterogeneity in solid material distribution and defects in cell material are leading to local stress fields and causing localised deformation. Different failure criteria have to be applied to describe foam behaviour. Main target of this project is to correlate and to analysis defined structural parameters of metallic foam "Alulight" observed under unloaded and loaded conditions with mechanisms of local mechanical behaviour of foam. The modelling of observed local deformation will allow further generalisation so that the application of metal foam as structural material can be exploited. Experimental investigation of foam surface (2 1/2 dim.) and structure (3 dim.) under compression or tension will be the basis for defining structural parameters and identifying deformation mechanisms. Besides typical methods of metallography a new method in material testing by X-ray computer tomography (CT) will be applied for structural analysis. In order to carry out in-situ-compression tests using X-ray CT experimental know-how has to be built up. In order to interpretate tomography experiments focus has to be put on 3 dimensional visualisation and evaluation of tomography data sets. For the purpose of mechanical analysing of deformation behaviour compression and tensile tests have to be carried out in order to get precise material data for modelling. Based on experimental observations local deformation mechanisms and structural parameters will be selected and analysed. The modelling of local deformation mechanisms shall be extended to describe the behaviour of foamed parts ond to other types of metal foams than "ALULIGHT". A co-operation between BAM-Berlin and Department of Material Science and Testing will be established in the field of computer tomography. The close co-operation with the Erich-Schmid Institut for Solid Physics Austrian Society of Science and the Institute für Physik und Meteorologie of BOKU on the topics of characterisation of foam structure and defects increases the scope of the studies essentially.

Metallic foams are light weight materials produced by blowing gas in a metallic melt. This can be done by adding a dissociating foaming agent or by direct blowing of gas. Due to this process metallic foams of 10-20% relative density with different cellular architecture and defect structure are available. Depending on the cellular structure different mechanical behavior is obtained. Inhomogeneities in structure lead to non reliable and non repeatable material characteristic. Because of the high energy absorption capacity of metallic foam when compressed it is essential to understand the interaction of structure and mechanical properties. The project revealed that fluctuations in density cause localized deformation bands. During compressing each individual sample follows its own deformation pattern leading to different stress-strain curves and specific specimen data. The metallic alloy, the foam is made of, has also strong influence on the mechanical behavior. Especially brittle materials like AlSi cast alloys show strengthening and softening cycles within the plateau range of a compression curve. Alloys of the foam, which can not be hardened by heat treatment yield lower plateau stress level. Based on these experimental observation a mesoscopic model was establish, which should prove the dependence of density distribution and material behavior. For that reason the foamed structure was investigated by X-ray computed tomography. Three dimensional local mass density of the three dimensional shaped body was calculated. Each voxel of structure is described by the average density of the adjacent voxels. By this method the structure is transformed into a continuum material, which can be described by constitutive continuum models. Material properties of each density domain are defined by scaling laws and the model of Desphande & Fleck. Simulation of the mechanical behavior of the entirely compressed sample show good agreement with the experimental results. The model has the capacity to calculate the whole stress-strain curve so that a prediction of mechanical properties like stiffness modulus, plateau strength, densification strain and energy absorption can be given. The model was tested for cellular metals having a rather uniform density distribution and regular structure as well as for metallic foams having inhomogeneous density with stochastic structure. The study of the energy absorption of foamed metals, when subjected to an impact test with a spherical indenter, help to find out the optimum density distribution of a flat absorber panel. This simulation is similar to an head impact test for determination of car passengers safety. The calculation showed the way the panel was deformed and so weak points in construction can be detected. After some further refinements the model shall be capable to be used for shape and density optimization of structural parts, especially for energy absorption. Due to the optimization of density new light weight solutions can be developed. Engineers and producers of metallic foams might be interested in the meso model for optimization of foamed parts, so that the acceptance of the new group of cellular material can be inproved.