Deformation and failure behaviour of open cell Al foams under quasistatic and impact loading

Abstract

Metal foams show efficient and almost isotropic energy absorption during compression making them interesting materials for the design and development of impact resistant components. The (mechanical) properties of metal foams are determined (i) by the properties of the alloy they are made of and (ii) by their complex meso- and microstructures. While the dynamic mechanical properties of Al foams have been investigated by several groups, the deformation mechanisms at higher strain rates has been considered to a much lesser extent.To gain a deeper understanding of the relationship between the specific properties of the hierarchical structural elements (e.g. cell, strut, microstructure of the struts) and the macroscopic properties we combine mechanical testing with different imaging methods. Compression tests on open cell Al (A356) foams, which have been produced by investment casting and which have very low relative densities (1.7–2.2%), were performed under quasistatic (dε/dt=5×10−4s−1) and dynamic (10s−1) loading conditions. Video recordings allow us to evaluate the failure mechanisms in the foams on a macroscopic scale, while in-situ testing in the µCT or SEM provides important information on the deformation behaviour as well as the crack formation and development on the microscale. Metallographic sections of specimens deformed to defined strain values are furthermore investigated to gain insight into the role of microstructural features with higher resolution.The deformation behaviour and the mechanical properties of the quasistatically and the dynamically tested specimens are very similar. Higher strain rates result in a more homogeneously distributed deformation, however with a negligible influence on the plateau stress, densification strain and energy absorption characteristics. The highly efficient energy absorption during the plateau phase is due to microstructural mechanisms such as plastic bending of struts (mostly C- and S-curved) and different types of crack formation and growth in the struts. Local differences of the strut microstructure cause locally different mechanical behaviour of struts: while high plastic deformation leads to a more homogeneous macroscopic deformation, we assume brittle fracture of single struts promotes the formation of deformation bands.