Study of the ratcheting by the indentation fatigue method with a flat cylindrical indenter: Part I. Experimental study

Generally, 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 (P_max), indentation load variance (ΔP = P_max - P_min) 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 (P_max) and indentation load variance (ΔP = P_max - P_min) 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.