Date of Award
Master of Science
Materials Science and Engineering
George M. Pharr
Warren C. Oliver, Carl J. McHargue
Over the past 10 years, a number of investigators have proposed methods to measure the yield strength of metals using instrumented indentation experiments performed with a sphere [1-6]. Among the proposed methods that are easy to implement experimentally and do not require any additional novel characterization techniques or proprietary software analysis are those of Field and Swain, Yu and Blanchard, Ma et al., Cao and Lu, Kogut and Komvopoulos, and Lee, Lee, and Pharr. However, these methods have yet to be rigorously verified experimentally. The objectives of this work are twofold: first, identify the basic principles, predictions, data analysis routine, and potential experimental obstacles of each proposed method, and second, contribute to the experimental verification of four of the six methods by testing their ability to accurately predict the yield strength of the aluminum alloy 6061-T6. Tensile and indentation samples were taken from the same 3.175 mm thick sheet and the surface of the indentation sample was given the best possible mechanical polish. The indentation experiments were performed using a 90 degree diamond cone with a mechanically polished radius of 385 nm. Field and Swain’s procedure overestimated the tensile flow curve by roughly 40% which precluded obtaining a meaningful estimate of the yield strength. Yu and Blanchard’s model overestimated the yield strength by approximately 55%. The procedures proposed by Ma et al., and Cao and Lu were inconsistent with the experimental observations and could not be implemented. Among the most likely explanations for these surprisingly poor results are the effects of roughness and contaminants on the surface and an indentation size effect.
Herbert, Erik G., "On the Measurement of Yield Strength by Spherical Indentation. " Master's Thesis, University of Tennessee, 2006.