Showing papers by "Eberhard Kerscher published in 2019"

Micro hardness determination on a rough surface by using combined indentation and topography measurements

Abstract: Micro hardness determination on rough surfaces is topic of interest e.g. in industrial applications where the component surface is of functional relevance. Even when the indentation depth is large enough to overcome indentation size effects, surface topography signicantly affects the indentation testing result. In most approaches hardness on a rough surface is determined by simply including profile roughness parameters like R a (e.g. [1-5]) to adjust the measured hardness value or to get the minimum indentation depth value where the influence of surface topography is assumed to become negligibly small. In the present study, local surface topography data were used instead to enable precise micro hardness measurements on a sample with arbitrary surface topography. Three samples made of 1.4571 stainless steel with different surface states were face milled by using an end mill with different feed rates. Instrumented indentation tests were performed on these samples as well as on comparative samples with polished surfaces. From the resulting load-indentation depth (Fd)-curves the averaged indentation hardness was calculated for all surface states. Then, a method was applied to manipulate and to average the F-d-curves in order to eliminate large deviations, mainly occurring at the very beginning of indentation. Indentation hardness was calculated from the resulting modied F-d-curves and compared to the indentation hardness from the actually measured F-d-curves of the polished samples with feasible results. Using surface topography measurements is considered to enable deriving more accurate indentation hardness values directly and to put the investigations to another level. Surface topography of the samples was evaluated by confocal microscope measurements before and after the indentation tests. From local surface topography data at the location of indentation, four parameters were calculated: volume, projected contact area and depth of indentation mark, as well as the curvature of the surface topography before indentation. The four parameters were correlated with the hardness value from the respective indentation and compared to the hardness of the polished sample. The results of the present study are the basis for combining optical imaging techniques like confocal microscopy or white light interferometry and indentation testing equipment to broaden the field of application.

Crack growth and microstructural changes in AISI 52100: white etching cracks (WEC) and fine granular area (FGA)

Abstract: The service life of rolling element bearings can usually be predicted with good accuracy. However, the damage phenomenon „white etching cracks” (WEC) can lead to premature and unforeseeable failures. The formation of zones with strongly decreased grain sizes at the crack faces is a characteristic of this type of failure. An earlier study showed that it is possible to reproduce this type of failure by multiaxial fatigue experiments with superposed cyclic compression and torsion. The present study shows the analysis of the stress state of those multiaxial fatigue tests with stress-based critical plane criteria. Using the critical plane results as a basis for a fracture mechanical analysis, a correlation is found between the crack length, the associated plastic zone size, and the extent of the transformed microstructure. This correlation is very similar to our results of the study of fine granular areas (FGA), a very high cycle fatigue phenomenon in bearing steels. It is argued that a plasticity-driven mechanism similar to the one proposed for FGA formation is responsible for WEC formation, too. Additional factors that are often cited as promoting WEC formation could explain the shift from very high cycle fatigue in the case of FGA to the early failures due to WEC.