Papers by Author: Ulrich Krupp

Paper TitlePage

Authors: Ulrich Krupp
Abstract: The present paper is about dynamic embrittlement as a generic damage mechanism. It involves grain-boundary diffusion of an embrittling species at elevated temperatures under the influence of mechanical stress. The embrittling species, either coming from the material itself or from the environment, reduces the grain-boundary cohesion and, hence, causes time-dependent intergranular fracture. Evidence of the technical significance of dynamic embrittlement is given by two examples, stress-relief cracking in steels and hold-time cracking during low-cycle-fatigue loading of nickel-base superalloys. There is an obvious relationship between the grain-boundary structure and the local susceptibility to dynamic embrittlement. This was proven by mechanical experiments on bicrystals and grain-boundary-engineering-type-processed specimens.
Authors: Ulrich Krupp, V.B. Trindade, B.Z. Hanjari, S. Yang, Hans Jürgen Christ, U. Buschmann, W. Wiechert
Authors: Ulrich Krupp, R. Orosz, Hans Jürgen Christ, U. Buschmann, W. Wiechert
Authors: Ulrich Krupp, R. Orosz, Hans Jürgen Christ, Daniel Monceau
Abstract: Thermal cycling has been observed to cause a transition from superficial alumina formation to internal oxidation and nitridation, an effect that was shown to depend on the specimen thickness and geometry, which can be described by a spalling-probability model. Once protection by a dense and adherent alumina scale got lost, the internal-corrosion rate is determined by the diffusivity and solubility of nitrogen and oxygen in the alloy. These parameters seem to depend not only on the temperature and the alloy composition but also on the applied mechanical stress. Internal nitridation under a superimposed creep loading was found to follow a higher rate constant than under just isothermal exposure. This effect can probably be attributed to dislocation-pipe diffusion, a mechanism which has been claimed also to be relevant for outward solvent diffusion during internal corrosion, a phenomenon, which was observed as a stress-relief mechanism during various internal-reaction processes
Authors: Liisa Heikinheimo, Klaus Hack, David J. Baxter, Michael Spiegel, Ulrich Krupp, Marko Hämäläinen, Mikko Arponen
Authors: Ulrich Krupp, Helge Knobbe, Olaf Düber, Hans Jürgen Christ, Philipp Köster, Boris Künkler, Claus Peter Fritzen
Abstract: In the present paper examples for propagating and non-propagating conditions of slip bands and short fatigue cracks in a ferritic-austenitic duplex steel are given, which were quantified by means of SEM in combination with automated EBSD. To classify the results within the scope of predicting the service life under HCF- and VHCF-loading conditions a numerical model based on the boundary-element method has been developed, where crack propagation is described by means of partially irreversible dislocation glide on crystallographic slip planes in a polycrystalline model microstructure (Voronoi cells). This concept is capable to account for the strong scattering in fatigue life for very small strain amplitudes and to contribute to the concept of tailored microstructures for improved cyclic-loading behaviour.
Authors: Ulrich Krupp, V.B. Trindade, Peter Schmidt, Hans Jürgen Christ, U. Buschmann, W. Wiechert
Abstract: Even though the oxidation behavior of steels is generally considered as to be widely understood, a closer look reveals some open questions, e.g. regarding the influence of the substrate grain size on the overall oxidation kinetics. At temperatures below 570°C the main constituent of the oxide scale formed on top of low alloy steels is magnetite. As shown by gold marker experiments it grows outward and inward at the same time, the latter exhibiting a gradual transition to the more stable spinel compound FeCr2O4. As indicated by intergranular-oxidation attack below the superficial scale, inward oxide growth seems to be driven by oxygen transport along the grain boundaries serving as fast diffusion paths. This is supported by thermogravimetric oxidation tests in air on low-Cr steels with varying grain size: The smaller the grains the higher the oxidation rate. Recently, a numerical model for the diffusive transport processes based on the finite-difference approach has been developed, which distinguishes between fast grain-boundary diffusion and bulk diffusion. Qualitatively, it is capable to predict the relationship between substrate grain size and inward oxide growth kinetics. Together with the thermodynamic tool ChemApp and in combination with a data set for the Fe-Cr-O system the mechanism-based simulation of the overall oxidation process of low-Cr steels is possible.
Authors: Ulrich Krupp, I. Roth, Hans Jürgen Christ, M. Kübbeler, Claus Peter Fritzen, M. Scharnweber, C.G. Oertel, Werner Skrotzki
Abstract: During high-cycle-fatigue loading of metastable austenitic steel AISI304L, the elastic anisotropy between neighboring grains causes the occurrence of stress peaks at grain boundaries, which again act as crack nucleation sites. This is in particular the case at twin boundaries. Cyclic crack tip plasticity leads to a transformation from  austenite to ´ martensite when different slip bands are activated, alternating during their operation. By means of in-situ fatigue testing in a scanning electron microscope (SEM) in combination with electron back-scattered diffraction (EBSD), the distributions of grain size, geometry, and crystallographic orientation relationship were correlated with the local occurrence of slip, martensite formation and fatigue-crack initiation and propagation. It was shown that the extent of martensite formation ahead of a propagating crack increases with increasing crack length and eventually, due to its higher specific volume, gives rise to transformation-induced crack-closure effects. The variation in the crack-propagation rate depending on the local microstructure was simulated by means of a short crack model, where the displacement fields within the crack, the adjacent plastic zone and the grain boundaries in combination with the martensite volume increase strain are superimposed by means of a boundary-element approach.
Showing 1 to 9 of 9 Paper Titles