Formation of thermally-induced crack patterns in anisotropic, fiber- reinforced composites with a brittle matrix

• Applicants: Privatdozent Dr. Michael Fleck; Professor Dr.-Ing. Walter Krenkel
• Subject Area: Glass, Ceramics and Derived Composites ; Materials in Sintering Processes and Generative Manufacturing Processes ; Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: from 2020 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 431968427

The excellent thermo-mechanical properties of fiber-reinforced composites with a brittle matrix are decisively dependent on production related, thermally induced crack patterns. The cause of these crack patterns are local tensile stresses, which result from the combination of rigid fibers in a thermally induced shrinking matrix. In the planned project the formation of crack patterns is investigated in a combined approach of new fracture mechanics simulations based on the phase field method together with systematic, tailor-made experiments.The special feature of the planned project lies in the close interaction between simulation and experiment, which manifests itself in two ways. On the one hand, the simulation studies as well as the experimental campaign are both concerned with the investigation of the effect of variations of accessible material and process parameters, such as the fiber volume fraction, the heating rate, as well as the fiber matrix bond. On the other hand, the complexity of the underlying microstructure is increased step by step in the experiment as well as in the simulation by variations of the type of reinforcement.In the experiment, the investigation of the formation of crack patterns is carried out in-situ in the heating cell and, on the other hand, by systematic interruption of the pyrolysis process followed by a comprehensive characterization of the respective samples. In an iterative approach, different types of reinforcement are investigated. The geometric complexity of the microstructure is gradually increased: From unreinforced through unidirectional over alternating 0/90° laminates to tissues. The crack patterns occurring in the experiment are quantitatively measured, analyzed and compared with the crack patterns observed in the simulation. As input parameters for the simulations, the anisotropic shrinkage behavior, the stiffnesses and the fracture toughness of different composites as well as pure matrices are determined as a function of the pyrolysis progress.The use of simulations enables the mechanism-oriented investigation of the formation of thermally-induced crack patterns. Two hierarchically nested models are used for the respective description of brittle crack growth on the different length scales. On a microscopic length scale, both the fibers and the matrix are each modeled as separate brittle-cracking phases with different mechanical properties. On the higher mesoscale, whole fiber bundles are then each described as homogeneous, brittle phases with the homogenized material properties of a unidirectional fiber-matrix composite.Upon successful completion of the research project, it is expected that for the increasingly important class of fiber-reinforced composite materials with a brittle matrix (e.g. C/C, C/C-SiC) a comprehensive understanding of the correlation between thermally induced crack patterns and the fiber matrix bond and the resulting composite properties will be present.

• DFG Programme: Research Grants