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Research project (§ 26 & § 27)
Duration : 2017-01-01 - 2019-12-31

In this project a new type of x-ray microscope with an x-ray “color” camera shall be procured and set up. It will allow for the first time the combination of full-field imaging of the chemical composition and 3D crystallographic analysis of materials with micrometer resolution. This method is of special interest for structurally and chemically inhomogeneous material. The areas of application range from biological systems (biomineralized tissues, self-assembly processes), the non-destructive study of art objects and thin film technology to industrial materials science (nanocomposite materials, nanocrystalline metal alloys etc.)
Research project (§ 26 & § 27)
Duration : 2017-05-01 - 2018-04-30

Increasing the strength of materials typically leads to a loss of toughness. Managing steels are one of the few materials that show very high strength and acceptable toughness. This extraordinary combination of mechanical properties makes this of class materials interesting for load bearing components. Especially applications where a high cyclic strength over long periods of usage is required are fields of applications of maraging steels. Very high cycle fatigue properties of a new developed maraging steel is investigated in the present project. The harmful influence of small secondary phase particles in the material should be experimentally quantified and described in a theoretical model.
Research project (§ 26 & § 27)
Duration : 2017-04-01 - 2020-03-31

In the last decades, some knowledge about the mechanisms leading to very high cycle fatigue (VHCF) failure has been accumulated. Experiments in the VHCF regime are predominantly performed under cyclic tension-compression or rotating bending loading, and fewer studies investigate the cyclic material properties in presence of mean stresses. However, the loading condition of several components in technical practice is not adequately represented by experiments performed under uniaxial loading. Coil springs or bearings, for example, are subjected to VHCF loading mainly under cyclic shear superimposed on a static mean shear load. But investigations on VHCF properties under cyclic torsional loading are very rare. It is of great technical as well as scientific interest, what damage mechanisms occur under cyclic shear loading in the VHCF regime, what is the influence of a mean shear load and what are possible differences and similarities to cyclic axial loading. The main aim of the proposed project is to investigate the VHCF properties of two steels under cyclic torsion loading at different load ratios. Experiments will be performed with a high and an intermediate strength steel associated with a different ductility. Most of the experimental work will be performed with a recently developed ultrasonic torsion fatigue testing setup. Some additional tests using a servo-hydraulic torsion fatigue test frame should allow observing possible influences of the testing technique, notably frequency influences, on the measured cyclic properties. Models for the progress of damage will be applied with the observed crack initiation and propagation behaviour at artificial defects as input. Additionally, fatigue cracks propagating under shear stress in sliding and tearing mode (Mode II & III) at very low crack growth rates will be observed. It will be attempted to find physical explanations for failure or for infinite fatigue life by identifying propagating or non-propagating conditions of short fatigue cracks subjected to cyclic shear loading. The process of fatigue damage and influences of the load ratio found under cyclic shear will be compared to cyclic axial loading. This will be done considering the mode of crack growth, i.e. crack opening mode or shear mode, which will probably depend on the magnitude of loading, the crack length and the material.

Supervised Theses and Dissertations