Extracting yield strength and strain-hardening exponent of metals with a double-angle indenter
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OE Key Laboratory for Strength and Vibration, School of Aerospace, Xi’an Jiaotong University, Xian 710049, People’s Republic of China
Kewei Xua) State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China (Received 14 September 2008; accepted 31 October 2008)
A double-angle indenter model is proposed to determine the representative strain in the indentation process, and a new method is then developed aiming at the extraction of the yield strength and strain-hardening exponent from the surface layer of metals, because surface properties, especially in a small region, may differ from bulk ones and are sometimes closer to service properties such as fatigue strength, wear, and corrosion resistance. First, the isotropic metal was analyzed, the elastic modulus of which was fixed at 128 GPa, the yield strength was 50 to 200 MPa, and the strain-hardening exponent was 0.1 to 0.5. By introducing the yield strain to substitute the yield strength in the calculation, it was proved that the model can cover the majority of metals because the introduced weight parameter l is independent of the yield strength and the elastic modulus, although it depends on the strain-hardening exponent to some extent. For the determination of yield strain eY (or yield strength Y), the precision is better for low C/E and low n, whereas for the determination of strain-hardening exponent n, the precision is better for high C/E and low eY. By using the double-angle indenter, the material constitutive relationship at the surface can be evaluated from just one indentation without any other measurements.
I. INTRODUCTION
Many new advanced instruments, such as atomic force microscopy, scanning tunneling microscopy, and nanoindentation have been developed in recent years and brought a revolution in material structural characterization and properties evaluation. Among them, the nanoindentation with various kinds of indenters can be used to determine materials stress-strain behavior in a nondestructive and localized fashion. Moreover, compared with traditional tools such as tension or bending, it does not need precise sample machining and can also be used localized in, e.g., heat-treated or welded or other property-changed microzones. Therefore, nanoindentation becomes one of the most popular methods to investigate the mechanical properties of new-type materials or materials processed with novel technologies for funca)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0203
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http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 5, May 2009 Downloaded: 25 Mar 2015
tioning surface layers. In fact, material properties at the surface layer may differ from those of bulk material and are sometimes closer to service properties such as fatigue strength, wear, and corrosion resistance.1–3 A central question in instrumented indentation technologies is how to extract the uniaxial mechanical properties from the load-displacement curve. There have
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