Indentation size effect

The effect has been seen through nanoindentation and microindentation measurements at varying depths.

[3][clarification needed] Materials contain statistically stored dislocations (SSD) which are created by homogeneous strain and are dependent upon the material and processing conditions.

These additional geometrically necessary dislocations (GND) further increase the flow stress in the material and therefore the measured hardness.

For practical purposes this effect means that hardness in the low micro and nano regimes cannot be directly compared if measured using different loads.

Several new plasticity models have been developed using data from indentation size effect studies,[4] which can be applied to high strain gradient situations such as thin films.

Higher hardness values are measured at lower indent depths which correspond to smaller indent areas. The increase measured hardness is believed to be the result of geometrically necessary dislocations. is the hardness due solely to statistically stored dislocations without the impact of geometrically necessary dislocations.
Indenter tip generating geometrically necessary dislocations