Study of the ratcheting by the indentation fatigue method with a flat cylindrical indenter: Part I. Experimental study
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nerally, ratcheting is studied on round specimens under tension–compression tests with a nonzero mean load. This work explored the possibility of studying ratcheting by indentation fatigue with a flat cylindrical indenter. In the experiment, emphasis was concentrated on the influence of maximum indentation load (Pmax.), indentation load variance (⌬P = Pmax − Pmin) and frequency of cycling (f) on the indentation depth–cycle curves. The experimental results showed that the indentation depth–cycle curves are analogous to the conventional strain–cycle curve of uniaxial fatigue testing, which has a primary stage of decaying indentation depth per cycle followed by a secondary stage of nearly constant rate of indentation depth per cycle. It was found that the steady-state indentation depth per cycle is an approximate linear function of maximum indentation load (Pmax) and indentation load variance (⌬P = Pmax − Pmin) in the log–log grid. This relationship can be given with a power-law expression as an analogous equation of steady-state ratcheting rate. Further study showed that the influence of frequency of cycling on the steady state indentation depth per cycle can be ignored when the frequency of cycling exceeds a certain value. Finally, comparison was made between the conventional uniaxial fatigue test and indentation fatigue test for the steady-state stage. It was shown that the conventional uniaxial fatigue parameters can be obtained by the indentation fatigue method.
I. INTRODUCTION
In engineering materials science, uniaxial fatigue testing is the standard technique for obtaining the fatigue properties of materials. However, for some materials or structures, especially to service materials or structures (e.g., turbine blades), there are some differences in the mechanical properties between the specimens and the service materials. The best approach is to obtain the mechanical properties from the service materials. As a localized testing technique, indentation testing has provided an alternative approach for settling this question. Several techniques have been reported in the literature to extract the mechanical properties of homogeneous and composite or coated materials from indentation experiments during the past few decades.1–6 However, these indentation methods are mainly applied to static loading conditions. It is well known that, when an indenter is pressed onto the surface of a specimen by a
Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2006.0222 J. Mater. Res., Vol. 21, No. 7, Jul 2006
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constant load, the indenter sinks into the materials at a certain depth and will stop unless the temperature is high, when there will be indentation creep. For the indentation test under cycle loading conditions, Li et al. found that the indentation depth can continue to allow the indenter to sink further into the sample.7,8 The studied sample was high-purity -tin single crystal. Li et al. a
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