39 Indentation and hardness testing

Reproducible hardness data require that the indenter (a sphere or a flat-ended cylinder) have a diameter, D, that is large compared with the cell size, d(D/di>i7). Edge effects are avoided if the foam plate is at least two indenter diameters in thickness and if the indentations are at least one indenter diameter away from the edges of the plate. Because they are compressible, the indentation strength of a foam is only slightly larger than its uniaxial compressive strength. By contrast, a fully dense solid, in which volume is conserved during plastic deformation, has a hardness that is about three times its yield strength for shallow indentation and about five times for deep indentation. The indentation strength of foams is slightly larger than the uniaxial compressive strength because of the work done in tearing cell walls around the perimeter of the indenter and because of friction, but these contributions diminish as the indent diameter increases.

0 0

Post a comment