Numerical investigation of indentation fatigue on polycrystalline copper
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Z.F. Yue School of Mechanics, Civil Engineering and Architecture, Northwestern Polytechnical University, Xi’an 710072, Peoples Republic of China
X. Chen Columbia Nanomechanics Research Center, Department of Civil Engineering and Engineering Mechanics, Columbia University, New York, New York 10027-6699 (Received 5 July 2008; accepted 3 November 2008)
The dynamic indentation response of polycrystalline copper under cyclic fatigue loading is studied with a flat cylindrical indenter. First, a simple analytical model shows that in a purely elastic solid, the indentation depth responds with the same wavelength and frequency as the applied sinusoidal fatigue load. Next, a numerical simulation of an indentation fatigue test on an elastic-plastic solid (polycrystalline copper) is performed. Finite element analyses reveal that the mean indentation depth is controlled by both the mean of the indentation fatigue load and the load amplitude, while the amplitude of the indentation depth is independent of the mean load. Further investigations indicate that with an increased number of cycles, the increment of indentation depth reaches a constant rate. The steady state indentation depth rate is dependent not only on the amplitude of indentation fatigue load but also on the fatigue mean load, which is similar to strain accumulation during a conventional fatigue test. A parallel indentation experiment on annealed polycrystalline copper also confirms the effect of the fatigue mean load, indicating consistency with numerical results.
I. INTRODUCTION
The continued effort toward the applications using small material structures requires advances in the fundamental understanding of the mechanical properties of materials of small volume, which are distinct from their bulk counterparts and often difficult to measure by conventional tension/compression tests. As an alternative technique, a microindentation or nanoindentation test exhibits a strong potential for addressing this challenge thanks to the low requirements for sample preparation, the high resolution, and the short duration of the experiment; it has been routinely used to extract the mechanical properties of small material structures from the measured indentation load–depth curves over the last two decades.1–8 Most previous studies focused on the static indentation test, which can be carried out by a monotonic, quasi-static increase of the indentation load or displacement. However, static indentation may not provide a complete spectrum of the mechanical properties and a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0107 J. Mater. Res., Vol. 24, No. 3, Mar 2009
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indentation mechanisms.9,10 As an example of dynamic indentation, the indentation fatigue test, which is carried out by applying a cyclic fatigue-like load on the indenter, provides a complementary approach and offers a unique advantage to understanding mechanical properties compared to static indentation. For example, F
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