An alternative to the determination of the effective zero point in instrumented indentation: Use of the slope of the indentation curve at indentation load values

In instrumented indentation testing, especially in the nano and micro scales, low scale artefacts such as sensor sensitivity, surface roughness, imperfect indenter tip geometry and material heterogeneity can create significant deviations of the measured indentation load-depth curve from the ideal curve corresponding to the bulk material.This study assesses the influence of an offset of the measured penetration depth on the identified bulk mechanical parameter values when using a reverse analysis model based on spherical indentation. It is shown through numerical examples that significant errors arise if the identification procedure is based on the load values recorded at penetration depth values. In over to overcome this effect, an alternative exploitation of the reverse analysis model is proposed, which is based on the use of the slope of the indentation curve at indentation load values. In addition, it is proposed to neglect the data at low load values in order to avoid the effect of low scale artefacts on the shape of the indentation curve at low load values.The proposed approach is applied to the numerical examples, and then an experimental case of a blunted indenter tip is presented. In all cases, the proposed approach provides accurate identification results which are not affected by low scale artefacts. The proposed approach can be significantly useful when analysing indentation curves obtained in the nano and micro scales, as well as indentation curves obtained in industrial conditions.

► A slight offset of measured penetration depth causes high errors on the material parameters identified by reverse analysis.
► The use of the slope of the curve at load values as the cost functional overcomes the problem.
► The proposed approach is validated experimentally in the case of an indenter tip defect.