Prediction of the Constitutive Equation for Uniaxial Creep of a Power-Law Material through Instrumented Microindentation Testing and Modeling

Indentation creep tests and finite element simulations were performed on a model material to show that the constitutive equation for conventional uniaxial creep can be derived using the instrumented indentation testing technique. When the indentation pressure and the indentation creep rate are maintained at constant values of p(s) and (epsilon)over dot(in(s)), respectively, the contours of the equivalent stress and the equivalent plastic strain rate in the region beneath the conical indenter expand according to the increase in the displacement of the indenter while maintaining geometrical self-similarity. These findings indicate that a pseudo-steady deformation state takes place around the indenter tip. The representative point exhibiting the creep behavior within the limited region, which actually determines the indenter velocity, is defined as the location where the equivalent stress (sigma) over bar (r) equals p(s)/3. The equivalent plastic strain rate (epsilon)radical anion(r) at this point is found to be (epsilon)over dot(in(s))/3.6 in the case when the stress exponent for creep is 3. The stress exponent and the activation energy for creep extracted from the results of Al-5.3 mol%Mg solid-solution alloy indentation tests are in close agreement with those of tensile creep tests reported in the literature. In addition, the values for (sigma) over bar (r), and (epsilon)radical anion(r) agree well with the values for the applied stress and the corresponding creep rate in tensile creep tests at the same temperature. The above results show that the creep characteristics of advanced materials, which are often available in minute quantities or as small-volume specimens, can be obtained from carefully designed indentation creep tests, and furthermore the constitutive equation for tensile creep can be predicted with sufficient accuracy through indentation creep test results.