The performance of components is dependent on the microstructure of the utilized material. To date there are hardly any quantitative approaches which are able to reliably describe the influence of the characteristic microstructure on the performance of pre-damaged components. There are also only very few qualitative approaches for the design of damage tolerant microstructures available. The specific adjustment of a desired component performance while taking the pre-damage resulting from forming processes into account is currently hardly possible. This is why project B05 is aiming to develop a quantitative method for a generic description of damage and microstructure, which is then used for damage tolerant material design. Thereby this project provides the TRR 188 with an essential tool for enhancing the performance of components that were damaged already by forming processes.

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Statistical volume element for representative reproduction of the microstructure of steel

      Three-dimensional representation of a representative volume model

This project combines the observation of processes, modeling approaches, and experimental characterization methods. It is therefore a connecting link between the three parts of TRR 188.

The assessment of the performance of components produced by forming, is usually done by experimental tests which are corresponding to the expected use of the component. Most of the time static and cyclic strength properties as well as the remaining ductility are assessed. Because of that the tests are either leading to a continuation of already commenced ductile damage or a change of mechanism to cyclic damage. For the first case, the failure criteria would therefore be ductile damage during non-proportional strain paths, while the second case would be failure due to overlapping of ductile and cyclic damage.

In the first funding period the methodical approaches for the generic description of microstructure and damage will be developed, while considering both the dual phase steel and the case hardening steel. The generation of microstructural models relies on a generic description of microstructure and damage, so that by defining appropriate parameters artificial microstructures can be produced and evaluated by their performance. For this purpose, the applicant has already developed a methodology, which produces a generic two-dimensional microstructure based on statistic structure description. These models fulfil a substantial requirement for representative elements and on top of that are statistically representative of the properties of the present material. The construction of the mentioned representative volume elements will be extended to three-dimensional models in the first funding period, since the porosity of the microstructure due to ductile damage (voids) needs to be considered. This void evolution will be characterized by computer tomography (micro-CT). Furthermore the current approach for the generation of microstructural models will be developed insofar that more parameters for the different parts of the microstructure will be available. These include, besides the already included phase fraction, grain size distribution, and grain elongation, especially particle distribution, grain orientation distribution, grain disorientation distribution, banding as well as void size and form distribution. On top of that methodical approaches will be developed to give structured information about overlying scale (phenomenological) models.

Provided the coupling of the macroscopic models is guaranteed, this method can be used for identifying customized materials for defined applications. Therefore, in the second funding period a computer based microstructure optimization concerning a dual phase steel will be applied, that will specifically focus on its resistance against ductile damage under non-proportional strain conditions. In the third funding period the microstructure of the case hardening steel 16MnCrS5 will be optimized for the resistance against overlapping ductile and cyclic damage. This allocation of the material concepts on the respective damage mechanisms corresponds to the individual application.

Project leader
Univ.-Prof. Dr.-Ing. Sebastian Münstermann
Teaching and Research Area for Integrity of Materials and Structures
Steel Institute (IEHK), RWTH Aachen University

Project coordinator
Felix Pütz M. Sc.
Teaching and Research Area for Integrity of Materials and Structures
Steel Institute (IEHK), RWTH Aachen University