Indentation Size effects

Indentation Size Effects

Size effects in depth sensing indentation test are widely observed in metals and in other materials as well, where the measured hardness turns out to be higher than the bulk hardness (the hardness values measured from experiments such as Vickers, Rockwell hardness tests) at very shallow indentation depths. This is usually observed in nano/micro indentation experiments where the depth of indentation is small. However, with increasing indentation depth the measured hardness approaches the value of the bulk hardness. The hardness measured from these indentation experiments shows an increasing trend with smaller depths of indentation. This is commonly referred as Indentation Size Effect (ISE).

Often, ISE manifests itself only below a certain depth of indentation and it is regarded as the characteristic of the material being tested. Thus a certain length scale, specific to a material, exists below which ISE would show up. Naturally, to capture such behaviour, a length scale parameter must be present in the model being used. Conventional plasticity theory is scale independent and does not contain such length scale parameter. Thus conventional theories are incapable of modelling such behaviour. To this point several theories (crystal plasticity, dislocation dynamics and other continuum based models) have been used to model not only ISE but other size effects associated with non-homogeneous deformation. The non-homogenous deformation is accommodated by Geometrically Necessary Dislocations (GNDs) which has a correlation with the plastic strain gradients in metallic materials. Strain gradient theory is one among those and we have used the same for modeling ISE (you can find an extensive review on such models here Guha et. al, 2015) .