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Research Papers

Experimental and Numerical Modeling of Surface Indentation Response of Plastically Graded Materials

[+] Author and Article Information
Michael A. Klecka

United Technologies Research Center,
East Hartford, CT 06108

Ghatu Subhash

e-mail: subhash@ufl.edu

Nagaraj K. Arakere

Department of Mechanical and Aerospace Engineering,
University of Florida,
Gainesville, FL 32611

1Corresponding author.

Contributed by the Materials Division of ASME for publication in the JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received November 11, 2012; final manuscript received May 18, 2013; published online July 10, 2013. Assoc. Editor: Joost Vlassak.

J. Eng. Mater. Technol 135(4), 041004 (Jul 10, 2013) (9 pages) Paper No: MATS-12-1255; doi: 10.1115/1.4024791 History: Received November 11, 2012; Revised May 18, 2013

Materials with customized spatial gradients in mechanical properties are increasingly used in high performance applications requiring enhanced resistance to contact loads, wear, and fatigue. In many engineering materials, multiple property and microstructural gradients may occur simultaneously with depth. In this manuscript, two case carburized steels are analyzed for their gradient in hardness with depth, emphasizing the resulting variation in surface hardness under increasing indentation loads. A parametric study using finite element analysis is then conducted in order to characterize the influence of individual property gradients on the surface indentation response of graded materials. It is shown that the measured surface hardness value decreases rapidly under increasing surface indentation loads in materials with sharp negative hardness gradients. It is also shown that this trend is independent of the magnitude of the strain hardening exponent of the material, as well as the gradient in strain hardening exponent. Gradients in elastic properties were also shown to have negligible influence on surface hardness trends for a fixed gradient in hardness. Finally, it is revealed that the depth of subsurface plastic deformation increases with sharper gradients in hardness, while being insensitive to changes in strain hardening exponent. For elastically graded materials, a decreasing gradient in elastic modulus limits the depth of plastic deformation.

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Figures

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Fig. 1

Optical micrograph illustrating the variation in carbide volume fraction with depth in case hardened M–50 NiL (darkfield image—carbides appear bright)

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Fig. 2

Complete profiles of the hardness variation with depth in P675 and M50-NiL materials. Inset diagram indicates test sections and local average subsurface hardness gradients considered in this study.

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Fig. 3

Measured surface hardness as a function of increasing indentation load. Hardness gradient for each section is indicated in the legend in units of kg/mm2/mm (GPa/mm).

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Fig. 4

Finite element mesh used for indentation modeling with schematic of stress–strain curves at various depths in a graded material

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Fig. 5

Influence of strain hardening exponent on the relationship between hardness, yield strength, and elastic modulus determined via indentation simulations for homogeneous materials

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Fig. 6

Trends in surface hardness as a function of indentation load for graded materials with starting surface hardness of 900 kg/mm2 (8.83 GPa). Subsurface hardness gradients are indicated next to each set of simulations in units of kg/mm2/mm (GPa/mm).

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Fig. 8

Surface hardness under increasing load for materials with gradients in elastic modulus

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Fig. 7

Surface hardness under increasing load for materials with gradients in strain hardening behavior with depth

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Fig. 11

Subsurface plastic strain and von Mises stress along centerline beneath indents of 50 μm depth on graded materials. Dashed arrows indicate the axis to which the data refers.

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Fig. 10

Comparison of subsurface equivalent plastic strain contours for nongraded and graded materials

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Fig. 9

Normalized surface hardness as a function of increasing surface indentation load for experimental and numerical models. Hardness gradient is indicated next to each plot in units of kg/mm2/mm (GPa/mm).

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