Investigation of the cyclic behavior of carbon steels at different control modes and strain rates

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The aim of the research project is to extend the state of research on the cyclic behavior of hypoeutectoid carbon steels under different control types and strain rates. Two equally important sub-objectives are being pursued: On the one hand, quantitative models describing the influences of strain rate and control type are improved so that results obtained on ultrasonic fatigue testing equipment can be used to infer force-controlled material behavior at low or moderate frequencies (less than 1 kHz). On the other hand, the mechanism-based understanding of VHCF behavior is improved. For this purpose, the differences between the mechanisms in conventional testing and the mechanisms in ultrasonic testing are characterized across scales. In this project, displacement- and stress-controlled tests on conventional testing machines and displacement-controlled tests on ultrasonic fatigue testing machines will be performed on two heat treatment batches each of 42CrMo4 and 50CrMo4. Based on the resulting data set, the materials scientific method for converting the results of force- and displacement-controlled tests that has been developed in preceding research is evaluated. If successful, validation of the algorithm will allow inference from ultrasonic fatigue testing equipment results to stress-controlled material behavior at high frequencies. Further, the influence of strain rate on fatigue behavior is inferred directly from the results of displacement-controlled tests on conventional testing machines as well as ultrasonic fatigue testing equipment. Compared to the current state of research, this is advantageous because the disturbance variable ‘control type’ is eliminated and thus the uncertainty of the statement is significantly reduced. In parallel to the vibration tests, extensive materials analytical investigations such as (FIB-) SEM, XPS, nanoindentation and EBSD investigations are carried out. These investigations will be adjusted to help elucidate the differences in mechanisms in the ultrasonic and conventional fatigue tests. Improved understanding of the differences in mechanisms will bring improved understanding of mechanisms in general. Emphasis is placed on the following questions: (Why) is the evolution of microstructure in ultrasonic testing different from the evolution of microstructure in conventional testing? (Why) do experiments performed at different frequencies differ with respect to the introduction of oxygen and nitrogen and with respect to the diffusion of carbon? How does the strain rate (variable in single fatigue tests due to the sinusoidal signal) affect the cyclic material behavior? To what mechanisms, if any, are differences in the lifetime and in the development of the microstructure attributed?