An Examination of the Indentation Size Effect and Bi-Linear Behavior of FCC Metals
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An Examination of the Indentation Size Effect and Bi-Linear Behavior of FCC Metals D.E. Stegall¹, B. Crawford², A.A. Elmustafa¹ ¹ Department of Mechanical and Aerospace Engineering, Old Dominion University, Norfolk, VA, 23529, United States ² Nanomechanics Inc, Oak Ridge, TN, 37830 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. INTRODUCTION The hardness of a metal is typically defined as a resistance to permanent or plastic deformation. The hardness of most polycrystalline metals is related to the flow stress by Tabor’s relation, which states that the hardness is generally three times the flow stress[1]. The size dependence has been particularly noted when measuring the indentation hardness of metals and is termed the indentation size effect (ISE). The ISE of hardness for crystalline metals has been the subject of intense research for many years [2, 3]. The exact cause for the ISE has been the subject of much debate and has included theories based on machine error, plastic hinges along the indenter interface, oxide layers and or contamination on the sample surface, and the presence of strain gradients. In recent years all of these theories have been dismissed except for those based on the proposition that the ISE can be described in terms of strain gradient plasticity (SGP). In particular, SGP theories that have been most widely accepted include an intrinsic length scale to replace conventional plasticity models to describe the ISE at small depths of indentation (generally taken to be less than 1 μm) [4-6]. These theories are based on the presumption that the flow stress (hardness) is directly influenced by the density of dislocations in the plastic zone ahead of the indenter and was modeled using the familiar Taylor dislocation hardening (T
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