Abstract
Hardness is an important design parameter but, in rate-dependent materials, its value depends on the indentation speed and dwell time during measurement. Dimensional analysis for indentation testing provides rigorous descriptions for the load-displacement curves of elastoplastic materials; viscoplastic materials can be treated likewise by neglecting the plastic part of the deformation, which is not accurate for most engineering alloys. This work presents a methodology for constructing model indentation curves taking into account concurrent viscous and plastic strains, as well as corrections for tip roundness, load frame compliance, and the point of first contact. A procedure is presented to calculate the parameters of a single model curve by fitting to multiple experimental curves, incorporating the numerical solutions of the differential equation describing viscoplastic behaviour. The procedure is applied to Vickers indentation in brass and steel calibration blocks and to a SAE783 Al-Sn alloy for journal bearings, where creep at room temperature is observed. The soundness of the approach is demonstrated by the large reduction of statistical uncertainty on the parameters describing the indentation curves. A rate-independent hardness will be found and a brief comment is provided on the comparison between creep analysis by indentation and uniaxial tension.
Highlights
Microindentation is a fairly easy and quick test which is widely used in quality control because it does not require the rigorous sample preparation needed for tensile testing and can be applied to small volumes
The results will be presented as a comparison between the raw measured data with the starting point of the indentation curve as determined by the equipment on the one hand and the corrected data with the fitting curve superimposed (Figure 1)
It was found that if different indentation curves are obtained from the same material and if this material is homogeneous, these curves must be fitted to a single curve, instead of fitting each curve to an individual model and averaging the parameters
Summary
Microindentation is a fairly easy and quick test which is widely used in quality control because it does not require the rigorous sample preparation needed for tensile testing and can be applied to small volumes. Oliver and Pharr [2], using instrumented indentation, demonstrated that, besides hardness, the stiffness of the material can be obtained by careful analysis of the unloading curves. In a follow-up paper [3], they express caution with respect to the determination of further plastic properties from the curves, mainly due to the possible occurrence of material pile-up, which cannot be predicted without previous knowledge of the same properties that are to be determined. The latter problem can be partially solved by the observation of the geometry of the indentation [4].
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