Application of digital speckle correlation method to investigate a size effect in micro-indentations

An evaluation technique, which digital speckle image correlation method (DSCM) for displacement measure on the surface of the tested object pressed by an indenter, is proposed for the first time in this paper. The size effect in micro-indentation is discussed in detail based on calculating surface plastic deformation modes around indentation by DSCM, comparing hardness measured using three micro-indentation apparatus, and simulating piling-up and sinking-in of 45 carbon steel around conical indentation in finite element. We draw a conclusion that the indentation size effect relates to these factors such as load, test method, geometrical similarity of indentations, geometrical form of indenter tip as well as friction between indenter tip and surface of tested object. The finite element simulation result for 45 carbon steel revealed that the sinking-in occurs at the edge of imprints after unloading, no matter how the magnitude of the load is. In fact, the result obtained based on calculating surface plastic deformation modes around indentation by DSCM indicates that the property of deformation relates to load. When the load decreases from 1 N to 0.5 N, the surface deformation mode around indentation changes along with the load, contrary to the present research which neglects the load when considering deformation around indentation in finite element simulation. DSCM provides the technical support for probing into the essence of the indentation size effects.