Subproject of SPP 2020: High resolution electron microscopy of fatigue behavior in high performance concrete and multiscale modelling using a bonded particle model
The project focuses on two topics: firstly, the examination of structural damage caused by fatigue in cementitious high performance materials using high resolution analytical electron microscopy. The correlation with the inhomogeneities within the material, for example the residues of the plasticizers in the hydrated material, is of special interest. It is our working hypothesis, that in high performance concretes without capillary porosity, the initiation of cracks is caused by such inhomogeneities, whereas in normal strength concrete the initiation of cracks is linked to capillary porosity and the pores within the transition zone. Secondly, the aim of the project is the multiscale modelling of fatigue in high performance concrete using a bonded particle model (BPM). The project concentrates on concrete without capillary pores. The analysis of the structural damage requires very small specimens Tension tests on such small specimens will be performed in a transmission electron microscope. The results of these mechanical tests will be linked to the spatial distribution of the phases within the concrete. The examination of the crack propagation is performed on macroscopic specimens damaged by fatigue. Parts of these specimens will be analyzed by FIB-tomography. This method allows a three-dimensional reconstruction of the structure with very high resolution. Therefore, a large portion of the scientific work will be the development of methods for analyzing concrete with high resolution electron microscopy. These methods will then be made available for the whole DFG priority program. In addition, these examinations will produce fundamental results with regard to structural damage, crack formation and crack propagation in the case of fatigue, for the own modelling as well as for the modelling work of other projects within the DFG priority program.The numerical research work is focused on the development of a multiscale BPM-based simulation approach and on the validation of meso- and macro-scale models for the description of fatigue behavior. The new approach should be able to predict the influence of cyclic loading on mechanical properties; consider the influence of mesoscopic concrete structure on macroscopic behavior; describe initiation and coalescence of micro- and growth of macro cracks, which lead to material destruction.The modeling of investigated samples will be performed with the self-developed simulation framework MUSEN, which will be extended with new calculation algorithms and rheological models. The validation of the simulation results and the estimation of the model parameters will be made based on data experimentally obtained for high-performance concrete and its components. Thereby, information not only about macroscopic material behavior will be used, but also data obtained from electron microscopy will be applied to create, adjust and validate the models.