Indentation Curves in Viscoplastic Alloys: Mathematical Model, Fitting Procedures, and Application to the Room-Temperature Creep of an Al-Sn Alloy

Abstract EN

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.