Material length scales used in conjunction with gradient theories are obtained in this work from experimental observations of body-centred cubic (BCC) metals. Nanoindentation experiments are performed on iron at different strain rates. A constant value material length scale is not always realistic and different problems under various deformation conditions could require different values. A variable physically based relation for the length scale is therefore introduced in this paper for different strain rates, temperatures, and accumulated plastic strain. The material parameters of this model can be scaled with temperature and rate indentation size effects (TRISE) encountered in nanoindentation experiments. Required numerical analysis is performed using ABAQUS software and a physically based viscoplastic constitutive model.
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