Internal friction in metallic foams and some related cellular structures

Density, pore size and shape, chemical composition, pre-deformation, number of vibration, and air pressure influence damping of cellular metallic materials (CMM: foams with open and closed-cells, sponges and porous sintered metals). At least three ranges of amplitude domains of vibrations are distinguished with respect to internal friction mechanisms. The thermoelastic damping plays an important role in the range of amplitude-independent damping (Al- and Zn-based foams). The amplitude-dependent damping caused by reversible motion of dislocations is enhanced in porous structures by localised stresses. In case of magnetic CMM, the magnetomechanical effect can be an additional source of damping (Ni-based sponges). Up to a certain amplitude of deformation the damping is stable, i.e. practically does not depend on the number of vibrations. Beyond this limit, irreversible microplastic deformation (mpd) enhances the amplitude dependence; a model for damping in porous materials is discussed for this “microplastic” range. An additional time dependence due to fatigue processes is attributed to growth and movement of cracks.