Size effect of strength for Ceramic Matrix Composites - Evaluation and classification of models and their relationship to microstructure

• Applicant: Dr.-Ing. Stefan Flauder
• Subject Area: Glass, Ceramics and Derived Composites
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 503759358

Ceramic matrix composites (CMC) have outstanding thermo-mechanical properties including thermal shock and temperature resistance. The fiber reinforcement leads to a distinctive damage tolerant material behavior. Those material properties enable this class of material for applications in aeronautics and tribology under high thermal and mechanical loads. To further broaden the application potentials of CMCs, reliable models must be available to predict the mechanical properties with special emphasis on material strength assessments.It was found that the component size of CMCs can have an influence on the material strength. Despite the multiple observations of a size effect of strength on CMCs, the existing explanations of this phenomenon are still insufficient. There are controversial interpretations of the size effect for CMCs in the range from non-existence to descriptions with the simple weakest-link approach. Furthermore, it is not possible up to now to have a prediction or classification of the possible size effect for a CMC material regarding its concept of reinforcement, processing or even the class of material.The research project aims to elucidate the size effect of strength for CMCs. Hence, the nature of possible size effects for CMCs will be investigated, described, evaluated and finally classified and predicted with derivations of appropriate models. The understanding of the size effect for CMCs will lead to important contributions to statistical fracture mechanics of CMCs and to a broader knowledge of the mechanical material behavior.Additionally, within the project a code of practice shall be developed to correlate an arbitrary CMC material to the suitable possible size effect with a minimum number of necessary experiments. With the elucidation of the size effect and its adequate modelling, more reliable predictions of component strength and failure probability can be derived.The sample size under tensile load will be varied within several orders of magnitude to investigate the size effect. The sample size is defined as tested volume, length, or thickness of the sample depending on the model approach. As a requirement for a reliable evaluation of strength scaling during tensile testing, the applicant recently developed a testing device for automatic self-alignment of tensile samples, which proved to be valid for the testing of C/C-SiC. The size effect of strength will be analyzed and modeled. The modeling will consider the energetic size effect of quasi-brittle materials for the evaluation of CMCs. The microstructure and fracture pattern will be correlated to the characteristic lengths or sizes of the corresponding models. To further investigate the principal behavior of the size effect and its modeling, the fiber-matrix-bonding of one CMC material will be changed, a minimum of two different CMC materials will be evaluated, and the tensile tests will be performed at different temperatures.

• DFG Programme: Research Grants