Recemat International
Master Thesis Applied Physics
Research Group: Materials Science
Supervisors: Dr. Ir. P.R. Onck;
Prof. Dr. J.Th.M. de Hosson
Period: August 2001 - August 2002
The fracture behaviour under tensile loading of closed and open cell foams is characterized by the cellular structure (connectivity) of the foam (struts and cell walls) and the microstructure (incl. grain size) of the material from which the foam is made. The structure can be characterized by the relative density, cell size, cell wall thickness (only in closed cell) and strut thickness. The micro structure can be related to the production process. The specific micro structure of the examined closed cell foams was found to lead to quite brittle fracture behaviour. This can also be seen from the stress-strain curves extracted from the in-situ SEM experiments. The crack propagation during the tensile tests was in agreement with the literature. In-situ SEM single strut (extracted from Duocel open cell foam) tests were performed under tensile and shear loading. These test show successive steps in the fracture process. First the strut starts to bend elastically. Second plastic deformation develops followed by necking. Third the neck is extended and a shear band is formed. Finally void formation indicates that complete failure is near.
Analytical and numerical simulations were performed. The analytical results show the importance of implementing the geometry of the strut (non-uniformity along the longitudinal axis in addition to a triangular cross-section) into the numerical models. This has not been done thus far in the literature (only straight beams with circular cross-section). Using the finite element method a single strut model was developed taking into account the geometry of the strut. This model is able to simulate elastic, plastic and fracture behaviour of single struts. The model is calibrated with the in-situ SEM single strut tensile test. The comparison between the in-situ SEM single strut shear test and the numerical model based on this experiment revealed a great difference. The causes for this are not yet well understood. More tests are needed to reveal this.
Master Thesis Applied Physics
Department of Applied Physics
Research Group: Micromechanics
Materials Science
Supervisors: Prof. Dr. J.Th.M. de Hosson
Dr. Ir. P.R. Onck;
Period: September 2002 - May 2003
The research project consists of two interconnected parts. The first part research has been done on the base material and the mechanical properties of nickel-chromium foams. EDS measurements indicated that the nickel concentration was higher in the interior of the strut walls. Comparison of these results with the literature and information provided by the production company Recemat® suggest that the base material is in a ϒ′-phase.
Images obtained during in-situ tensile tests characterize the fracture process. In ex-situ tests a strong anisotropy was observed for one of the foam materials. An attempt was made to investigate a possible relationship between the cell size and the strength of nickel-chromium foams, but none specific relation was found.
The second part focuses on modeling a foam by means of finite elements. Procedures have been developed to test single struts of aluminum foams and images have been taken from the process. The results can be used to obtain input parameters for modeling a foam in the Abaqus computer code with so called 'fracture elements'. This element was characterized and calibrated with respect to the experiments. A parameter study has been performed to scan the overall strut behaviour as a function of several material properties.
Preliminary attempts have been made to model larger foam structures, using 3D Voronoi diagrams.