The influence of creep effects on the determination of local material parameters

Hot-stage nanoindentation experiments were performed to analyze the temperature dependence of Young´s modulus for two eutectic lead-free solder materials, namely Sn91Zn9 and Sn42Bi58. First results show discrepancies between the measurement results and values published in several references, especially at higher working temperatures when the homologues temperature is >; 0.5. Due to the delayed material response during the unloading process it must be assumed that creep effects lead to an inaccurate automatic data evaluation by the nanoindentation system. Creep generally occurs during the dwell time of the indentation procedure. For this reason the holding times are varied between 30 s and 240 s (at different temperatures) in our experiments in order to investigate their influence on the measured values of Young´s modulus. The second major aspect of this paper is the description of the measured material behavior by means of a simple one-dimensional viscous rheological model which results in a differential equation relating stress and strain. The idea is to solve this differential equation with suitable values for viscous parameters in order to match the strain-data of the measurements. For this purpose the viscous parameters are iteratively adjusted until the strain curve fits the calculated strain stemming from the experiment.