Experimental and FEM analysis on three point bending of aluminium foam sandwiches

The present work deals with the formability of metal foams and it is focused on three point bending of Aluminum Foam Sandwich (AFS) panels. Among forming operations, bending can be regarded as one of the simplest processes for both study and application. Besides, bending tests may yield interesting information about material properties. In this study, several tests on both glued and foamed sandwich panels were carried out by varying bending conditions. A universal testing machine was employed for this purpose, collecting data about the deformed geometry and about load-displacement. In any case the sample deformation was related to the occurrence of foam cells failure or collapse. Specimens obtained from foamed panels showed an interesting behaviour during bending and, once the process is terminated, they still retained a significant bending strength; on the contrary, no significant results yielded from glued panels. During bending tests several samples showed an irregular fracture behaviour. In these cases cells fracture often started in a generic part of the specimen and propagated through the foam. The reasons of this problem and the possibility of failure prediction were investigated using different approaches based on X-ray analysis and thickness measurement of the welding layer between foam and cover sheets. In addition, a study based on foam density showed a remarkable data scatter, that can be considered as a characteristic of the state of the art in foamed panels manufacturing. Basing on experimental results, a numerical model of the bending process was developed by using a commercial FEM code. Foam behaviour was simulated by using a compressible (porous) material model. The onset of foam instability was simulated using a damage model based on the foam density parameter. Finally, a comparison between numerical and experimental results allowed to validate the predictive ability of the FEM model. The same model was used to study the consequence of defects inside the sandwich panels; in particular, the defects in bonding between foam and cover sheets were taken into account.