Effects of the Surface Contact on the Uncertainty in Indentation Yield Strength: Surface Roughness and Angular Misalignment

We suggest a method for accurately estimating the uncertainty of indentation yield strength determined from the modified Meyer relation as a mathematical function of the measurement, taking into account Type A and Type B uncertainty. Using this method, we quantitatively compared the expanded uncertainty level of the yield strength as measured by instrumented indentation testing (IIT) and uniaxial tensile testing, and propose a dominant measurand that affects the final expanded uncertainty of the indentation yield strength. To interpret the difference in uncertainty between IIT and uniaxial tensile testing, we investigated the effect of the major sources of uncertainty in the IIT system: sample surface roughness and angular misalignment between the surface normal of the sample and the symmetric axis of the indenter. The surface roughness was controlled using 400-, 1000- and 2000-grit paper and the misalignment angle ranged over 0 degrees, 1 degrees and 2 degrees. Acceptable surface roughness and standard uncertainty of misalignment angle are proposed that give the IIT similar uncertainty to uniaxial tensile testing.