The mechanical behaviour of thin sheets can not be deduced from that of plates with larger thicknesses. The reasons originate from statistical and technological size effects, the different part of the surface, as well as the fact that thickness and microstructural lengths are of similar size. The aim of this project is measuring and modelling the thickness effects under mechanical loads.

The present investigations within the framework of this project are resulting in the microsheets (d < 200 µm) edges seeming to have great influence on the mechanical characteristic values. The adjoining picture exemplarely shows the scatter of elongation at fracture of AlSc microsheets within the same manufacturing batch. Consequently an enhanced edge quality of the semi-finished product is expected to have great influence to the application of micro components, especially fatigue strength. To verify this hypothesis the following separating techniques will be investigated:

• shear cutting
• chemical laser cutting
• laser ablation
• spark erosion

Characterisation of the cutting edges created will take place measuring the cutting edges roughness by means of laser scanning microscopy or scanning electron microscopy.

The demand for intelligent lightweight design concepts in the transport industry increases. These concepts offer the opportunity to enhance the payloads, to improve the environmental compatibility and to increase safety as well as comfort. Innovative materials, like aluminium foams, can contribute to reach this target. The suitability of aluminium foams has already been proved at different prototypes in the automotive industry. However, missing layout criteria prevent the application of aluminium foam components on an industrial scale. Statistically secured mechanical characteristic values as well as a fundamental knowledge of the failure mechanisms are required. The aim of this project is to create a database for the structural design, the economic evaluation and the production-orientated planning of aluminium foam parts in the automotive industry. To achieve this, all relevant material properties of an AFS (aluminium foam sandwich) panel, which type is chosen in cooperation with the project advisory board, will be determined experimentally. Those data will be used to create parameter sets, which are required for the simulation of the structural behaviour with FEM software. A demonstrator component will be tested to verify that the gained knowledge is applicable to design parts.

In many areas of engineering, laser cladding gains in importance as a repairing method for high quality steel components. Shaft shaped parts used in the propulsion technology, which are exposed to fatigue loading in operation, are a typical example of such an application. Compared to conventional deposition welding, laser cladding has the advantage of a locally limited heat input. Thus, distortion of the cladded parts is minimal and only a reduced subsequent machining work is required. However, the influence of the laser cladding on the endurance limit of coated components is unknown today.

The aim of this project is to understand and to control the interrelation of coating properties and endurance limits. In order to achieve this, the local properties of cladded cylindrical specimens, namely surface roughness, microstructure, hardness and residual stresses will be characterised. Furthermore, the endurance limit of these specimens will be determined experimentally and their fracture behaviour will be analysed. By means of these data, a modelling of the endurance limit based on the weakest-link concept is planned.

In this project the quenched and tempered steel SAE4140 (DIN 42CrMoS4) and the austenitic steel SAE30304 (DIN X5CrNi18-10) are used as base materials. They are coated in a single stage laser-cladding process. That means the coating material is fed directly as powder into the fusion zone. For both base materials, the influence of three different coating compositions on the mechanical properties is under investigation.

Roller bearings consist of rolling elements which roll on the inner and outer ring, and a cage that keeps the rolling elements at a distance from each other. In operation, mechanical stresses are generated due to the Hertzian pressure in the areas where rolling elements are in contact with the inner and outer ring. Under ideal conditions these stresses are limited by oil lubrication. Contamination of the oil is possible and leads to a massive increase of material stresses when particles get into the contact area. In consequence, the fatigue life of the bearing will be significantly reduced. For such demands, various material conditions containing retained austenite have been proved, although the role and the optimal composition of the retained austenite have not been systematically studied.

This project strives to eliminate these uncertainties through comparative studies of different martensitic and bainitic material states. The influence of retained austenite on the fatigue life under contaminated rolling contact and the crack propagation behaviour respectively, are being comparatively examined with inner bearing rings and tubular specimens. Accompanying metallurgical studies will contribute to the understanding of this influence. In particular, hardness, toughness, spatial distribution and stability of the retained austenite and the influence of the matrix are examined and correlated to damage mechanisms and fatigue life.

The intended research results contribute to the development of material selection and heat treatment on roller bearings. Through the examination and evaluation of the retained austenite they facilitate the use of slow transforming bearing steels that have excellent rolling strength which however are previously been used only rarely due to their time-intensive and therefore expensive heat treatment.

The aim of this project is to investigate in how far surface-near nitrogen is able to increase the endurance limit of two carburised steels with different hardenability. For both steels, the influence two different nitrogen-levels is studied. In the first step samples with a lower nitrogen content between 0.2 and 0.3 % combined with a carbon content of about 0.7 % will be investigated. Afterthat the nitrogen and carbon content will be increased significantly. The influence of the increased nitrogen content on the endurance-limit will be determined under bending condition. In addition, characteristic parameters like residual stresses, austenite contents, hardness and roughness will be determined. The elastic-plastic behavior under bending condition will also be investigated. With the knowledge of mechanical behavior and the characteristic parameters the endurance-limit will be calculated with different calculation methods like the weakest link concept or a fracture-mechanics concept.

The knowledge of the influence of nitrogen on mechanical behavior of case-hardened steels will help to ensure this heat treatment method for a more widespread use by component-manufacturing companies. They will be able to use this method to produce components with higher fatigue strength. In addition to that, the study on the predictability of endurance limit will help to estimate the scattering of the component lifetime in better way.

This project aims at

1. retrieval of multi-mechnisms models as option for classical modelling of material behaviour
2. development of tools for describing transformation plasticity,  cyclic plasticity and ratchetting in steels
3. development of models for calculation of displacements and temperature
4. evaluation of the models by experimental verification

Highly stressed high strength steels are subject to fatigue which is marked by the formation and subsequent propagation of cracks ending in fracture. The region up to 10⁷ load cycles is well researched. The so called region of VHCF (very high cycle fatigue) beyond this number of cycles is of increasing technical relevance though insufficiently researched.  In this region crack initiation may occur at inclusions, at the surface and at so called “non-defect” sites. The latter type of crack initiation cannot yet be explained. Each crack initiation type has its own functional relationship between stress amplitude and fatigue life, represented in a diagram called Woehler-line: The single-flaw-Woehler-line. However, laboratory experiments only reveal multiple-flaw-Woehler-lines because of the competing crack initiation mechanisms. The first objective of this project is the isolation of single-flaw S-N curves from multiple-flaw S-N curves. The second objective resulting from the achievement of the first is to influence the single-flaw S-N curves by a selective variation of the material to get an increased fatigue life for that steel group. The final objective of this project is the prediction of cracking modes, single-flaw S-N curves and multiple-flaw S-N curves based on the presence of material defect populations.

In the first phase of the community initiative HiPerComp material based models will be developed from new material and heat treatment concepts. In this project these models will be applied to roller bearings and their applicability will be verified. Herewith, the potential of homogenous material should lead to lower fatigue life variations, thus enabling a more economical design of structures. Furthermore, these concepts will provide a modified cyclic deformation behaviour for each material whereby stress peaks at discontinuities in the material matrix particularly at non-metallic inclusions can be reduced better. Since under non-critical lubrication conditions these discontinuities are usually the cause of failure an increase of the fatigue life is expected. Because of the very specific stress state caused by rolling contact the test results are used for comparison with the evaluations determined under other test conditions.