Indentation behavior of polycrystalline copper under fatigue-peak overloading
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e indentation behavior of polycrystalline copper at room temperature was investigated with a flat cylindrical indenter applied to fatigue-peak overloading. The experimental results showed that fatigue-peak overloading can retard the indentation depth (d) propagation rate. After the period of overloading, the d propagation rate arrived at a new steady-state value again. The greater the amplitude of the peak overloading, the more the number of cycles that were needed to remove the effect of overloading. The observations of indentation cross-sectional microstructures revealed that the mechanism of d propagation was the nucleation, formation, and propagation of cracks around the indentation. The experimental results and their interpretation implied that there were some similarities between the indentation fatigue-depth propagation and the conventional fatigue-crack propagation under fatigue-peak overloading.
I. INTRODUCTION
Material mechanical properties, which are usually obtained by a uniaxial test such as round bar specimens, are the basis for the application of materials and structures. With the development of miniature structures such as microelectromechanical systems, it is necessary to establish the small-specimen testing technology to obtain the mechanical properties for limited material volumes. The most attractive technique from the viewpoint of specimen miniaturization is the indentation test. The indentation test provides a simple and nondestructive method of investigating the mechanical properties of materials. Considerable effort and significant progress have been made in extracting mechanical properties with the indentation method in the last 2 decades.1–8 Most research has been focused on indentation behavior under monotonic loading conditions. Alternatively, cyclic indentation has recently attracted some researchers to accurately assess the plastic characterization beneath the indenter, 9 phase transformation under indentation stress,10,11 the failure of coatings,12,13 and others.14,15 However, all this cyclic indentation testing was carried out by repeating several indentations.9–15 Compared to the simple repeated indentation, in 1979 Li and Chu16 completed indention testing on a -tin single-crystal sample with a flat cylindrical indenter applied to a sinu-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0201 J. Mater. Res., Vol. 22, No. 6, Jun 2007
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soidal load. It was reported that d could continue to increase with increasing numbers of cycles (N).16,17 In 1998, Kaszynski et al.18 studied the indentation response of 316L stainless steel specimens under fatigue-loading conditions with a Vickers diamond indenter and found that the d varied proportionally with the logarithm of N. Li17 later pointed out that the indentation fatigue with the Vickers indenter was very difficult to characterize because of the change in contact area between the indenter and the sample. Many researchers have indi
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