Characterization methods

The cellular structure of metallic foams requires that special precautions must be taken in characterization and testing. Structure is examined by optical microscopy, scanning electron microscopy and X-ray tomography. The apparent moduli and strength of foam test samples depends on the ratio of the specimen size to the cell size, and can be influenced by the state of the surface and the way in which the specimen is gripped and loaded. This means that specimens must be large (at least seven cell diameters of every dimension) and that surface preparation is necessary. Local plasticity at stresses well below the general yield of the foam requires that moduli be measured from the slope of the unloading curve, rather than the loading curve. In this chapter we summarize reliable methods for characterizing metallic foams in uniaxial compression, uniaxial tension, shear and multiaxial stress states, under conditions of creep and fatigue, and during indentation. An optical technique for measuring the surface displacement field, from which strains can be calculated, is described.

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