Published: June 29, 2017
Veruska D. Malave, Jason P. Killgore, Edward J. Garboczi, John Berger
Indentation-based studies of inhomogeneous structural materials depend on indenting a surface by loading with a small indenter. The load versus displacement results are used to infer the local mechanical properties of the surface under the indenter tip. One complication in interpreting the measurements in a composite material is the competition between surface topography and material heterogeneity. Typically, the specimen surface is prepared to be as smooth and flat as possible, yet there are always limits to how flat a sample surface will be. Moreover, when a compositional interface is sensed mechanically, any changes in surface topography near the interface are combined with the phase changes to influence the total mechanical measurement. Hence, a long-range goal is to develop a standard method to decouple these two phenomena. This paper presents an elastic nonlinear finite-element elastic model, using a rigid spherical indenter, to suggest such decoupling methods. Three axisymmetric models are presented: (1) a flat surface with a lateral material interface extending in the direction of indentation, (2) convex and concave surfaces with material uniformity, and (3) convex and concave surfaces with a similar material interface to that of (1). Using the exact Hertzian formulae and an empirical relation, the apparent elastic Youngs modulus of the material is computed assuming that the models have flat surfaces and are elastically uniform, which would be the usual experimental assumption when lacking more specific information. The results of (1) and (2), which have only material heterogeneity or a non-flat surface topography, are used to interpret (3), which has both. It is found that in the composite material models, the contact area and the material interface location significantly contribute to the calculated indentation modulus. Findings show that the flat-surface assumption can yield significant errors when extracting the modulus in solids with curved surfaces.
Citation: Journal of Applied Physics
Pub Type: Journals
indentation, material heterogeneity, surface topography, Hertz, composite materials, contact radius, maximum pressure, displacement, finite element
Created June 29, 2017, Updated September 19, 2017