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Analysis of Indentation Creep

Published online by Cambridge University Press:  01 February 2011

Donald Stone  and
A. A. Elmustafa
Donald Stone
Affiliation:
dsstone@wisc.edu, University of Wisconsin-Madison, Materials Science and Engineering, 1509 University Ave, Madison, WI, 53706, United States, 608-262-8791
A. A. Elmustafa
Affiliation:
AElmusta@odu.edu, Old Dominion University, Department of Mechanical Engineering, Norfolk, VA, 23529, United States
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Abstract

Increasingly, indentation creep experiments are being used to characterize rate-sensitive deformation in specimens that, due to small size or high hardness, are difficult to characterize by more conventional methods like uniaxial loading. In the present work we use finite element analysis to simulate indentation creep in a collection of materials whose properties vary across a wide range of hardness, strain rate sensitivities, and work hardening exponents. Our studies reveal that the commonly held assumption that the strain rate sensitivity of the hardness equals that of the flow stress is violated except for materials with low hardness/modulus ratios like soft metals. Another commonly held assumption is that the area of the indent increases with the square of depth during constant load creep. This latter assumption is used in an analysis where the experimenter estimates the increase in indent area (decrease in hardness) during creep based on the change in depth. This assumption is also strongly violated. Fortunately, both violations are easily explained by noting that the “constants” of proportionality relating 1) hardness to flow stress and 2) area to (depth)2 are actually functions of the hardness/modulus ratio. Based upon knowledge of these functions it is possible to accurately calculate 1) the strain rate sensitivity of the flow stress from a measurement of the strain rate sensitivity of the hardness and 2) the power law exponent relating area to depth during constant load creep.

Keywords

hardnesssimulationstress/strain relationship
Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 1049: Symposium AA – Fundamentals of Nanoindentation and Nanotribology IV , 2007 , 1049-AA10-02
Copyright
Copyright © Materials Research Society 2008

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