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FACULTY OF ENGINEERING SCIENCE

Chair of Ceramic Materials Engineering – Prof. Dr.-Ing. Stefan Schafföner

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Component size-dependent scaling of the strength of ceramic composites

Project description:

Ceramic composites (CMC) are characterized by excellent thermal shock and temperature resistance. In addition, the fiber reinforcement enables a pronounced damage-tolerant failure behavior. These property combinations allow these materials to be used under high thermo-mechanical load requirements in aerospace and friction applications. To expand CMC application potential, reliable predictive models for mechanical properties, including strength evaluation, must be usable.

For CMCs, an effect of component size on strength can be established. Although such size effects of strength are widely observed, the explanatory approaches are still insufficient. Controversial interpretations ranging from the nonexistence of the size effect to the description using the Weakest link approach are available. Moreover, the experimental findings of these size effects on CMCs do not yet allow to derive a possible prediction and attribution of the size effect in relation to the reinforcement concept, the manufacturing conditions, or the material type.

Therefore, the research project intends to elucidate the expression of size-dependent scaling of strength for CMCs and to derive a description, evaluation, categorization and prediction through appropriate models. By understanding these size effects, the knowledge on the material behavior of CMCs will be extended and an important building block for the fracture statistical treatment of composite ceramics will be laid. At the end of the project, a prescription for assigning any CMC material to possible side effects with minimal number of key experiments needed will be established. By understanding the size effect and modeling it appropriately, more reliable predictions of component strengths and failure probabilities should be possible.

To solve the problems, the specimen size under tensile load is varied over several orders of magnitude. Depending on the model approach, the specimen size is defined by the tested volume, the specimen length, or the specimen thickness. As a prerequisite for the reliable evaluation of strength scaling, a concept for tensile testing with automated self-alignment of the specimens has recently been developed and evaluated for valid testing of C/C-SiC by the applicant Stefan Flauder (external link). The scaling of the strength is analyzed and modeled. Here, for the first time for CMCs, the energetic size effect of the quasi-brittle materials is also considered. The microstructure parameters and fracture phenomena are linked to the characteristic quantities of the model approaches. The influences on the size effect and its modeling are investigated by selectively changing the fiber-matrix bonding of a CMC material, evaluating at least two different CMC materials, and tensile testing at different temperatures.

Duration: 09/2022 - 08/2025

Funding body: DFG


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