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An Examination of the Indentation Size Effect and Bi-Linear Behavior of FCC Metals

Published online by Cambridge University Press:  10 February 2012

D.E. Stegall
Affiliation:
Department of Mechanical and Aerospace Engineering, Old Dominion University, Norfolk, VA, 23529, United States
B. Crawford
Affiliation:
Nanomechanics Inc, Oak Ridge, TN, 37830
A.A. Elmustafa
Affiliation:
Department of Mechanical and Aerospace Engineering, Old Dominion University, Norfolk, VA, 23529, United States
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Abstract

We investigated pure FCC metals including Aluminum, Nickel, Silver, and 70/30 Copper Zinc (alpha-brass) alloy for the indentation size effect (ISE) and the bilinear behavior using a single Berkovich indenter tip in a single test machine. The results were consistent with those reported by Elmustafa and Stone, 2003 of the ISE and the bilinear behavior using two separate indenter tips (Berkovich and Vickers) from two separate machines. This behavior is mechanistic in nature and is observed regardless of the type of the self similar indenter tip employed. Furthermore, the research presented in this paper would seem to also validate the conclusions that Elmustafa et al (2004) articulate that the Strain Gradient Plasticity collapses at small scales and that the bilinear behavior of these FCC metals is attributed to the presence of long range shear stresses induced by geometrically necessary dislocations. Also, we observed what has been defined as a “tapping” issue for materials with high E/H ratios when using the CSM. The CSM protocol results in erroneous hardness results at very shallow depths for high E/H ratio soft metals due to the so called “tapping” of the stylus as articulated by Pharr et al. (2009). This method should only be used as a secondary technique to the load control protocol when examining the ISE effect.

Type
Research Article
Copyright
Copyright © Materials Research Society 2012

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