R. RaveendranVerhalten von Mikrorissen in Zugproben mit statistisch verteilten InklusionenProject Thesis, 2021
X. HouElastische Anisotropie von zweiphasigen Stählen mit verschiedenen MartensitanteilenProject Thesis, 2020
M. HarnischNumerical modeling of cyclic plasticity by using the Armstrong-Frederick kinematic hardening combined with ductile damageBachelor Thesis, 2019
A. MöglichImplementierung einer thermo-mechanisch gekoppelten FE-Formulierung in FEAPProject Thesis, 2019
M. HarnischModeling of dislocation density in a context of rate-independent plasticity and isotropic hardeningProject Thesis, 2019
Hydrogels, a significant group of highly hydrated polymers, represent the best choice for the potential application to bone fracture regeneration, which goes back to their bioactivity, affinity for biologically active proteins and compatibility with the bone tissue. However, this kind of materials also shows a serious disadvantage, namely, it loses its mechanical strength through swelling. This makes its straightforward usage difficult and motivates the development of different enhancement procedures. One of the most modern techniques for this purpose is calcification or, in a more general sense, mineralization. This method is inspired by the natural process of the bone growth where the enzyme alkaline phosphatase causes mineralization of the bone by cleavage of the phosphate from organic molecules. An analogous process induces homogeneous mineralization of a hydrogel and increases its mechanical strength. Recently, optical and electron microscopy has revealed that calcification yields different types of microstructure dependent on the type of the underlying polymer, and thus has clearly indicated that computational modeling can significantly contribute to the targeted investigation of effective behavior and material parameters. Fracture energy and diffusivity are two particularly important aspects in this context. The former is taken as the main measure of material ductility and represents a weak point of calcified hydrogels. In order to solve this challenging problem, inspiration once more comes from natural materials and their hierarchical microstructure. The study of diffusion in macromolecular solutions is motivated by many biomedical applications as well as by its key role for protein assembly and interstitial transport. The project furthermore studies the design of the mineralization process which includes two essential steps: the understanding of the mechanisms governing the microstructure development and subsequently their optimization. The investigation of the diffusivity and of mineralization requires a profound knowledge on the processes on the nanoscale. This of course strongly substantiates computer simulations, since this kind of processes is yet non-accessible even by the most modern microscopy techniques. The spectrum of applicable methods encompasses the multiscale finite element method, the phase field method, the model reduction strategy and the finite difference method.
The present project treats a polymer affected by the strain induced crystallization (SIC) as a heterogeneous medium consisting of regions with the different degree of network regularity. Such a concept allows depicting the nucleation and the growth of crystalline regions as well as the change of effective material parameters depending on the level of the strain applied. The model proposed is thermodynamically consistent. It is based on the assumptions for the free Helmholtz energy and dissipation. Both of them primarily include bulk- and surface terms due to the deformation and crystallization. The external variables are deformations and temperature, whereas the inelastic deformations and degree of the network regularity are internal variables. Their evolution equations are derived according to the principle of maximum of dissipation. The influences of latent heat and of temperature change are implemented in order to simulate thermal effects. The explained framework is advantageous for several reasons. First, it is suitable to answer the crucial question of which process predominantly influences SIC: the nucleation of new crystalline regions or the growth of already existing ones. Secondly, the proposed model is ideal for a direct implementation within the standard multiscale finite element concept. This numerical homogenization procedure is compatible with the theory of finite strains and is applicable for modeling the cases where the ratio of characteristic lengths of scales tends to zero. Both of these features are necessary for the effective modeling of SIC. The project also includes a study of stochastic aspects of the process, where a distribution function for the observable variables is introduced to express the expectation value of relevant quantities. The necessary evolution equation is derived by considering the effective energy of a control volume. The main goals here are to study nucleation and to evaluate the average size of the regions with different regularities of the network. The solution of the tasks itemized will make it possible to achieve the final project goal: the advanced simulations of SIC which can significantly contribute to the more efficient designing and usage of polymers. This is especially motivated by the fact that SIC has to be understood as a kind of reinforcement already successfully applied for some rubber materials. The proposed concepts are of general nature and can be taken as a basis for the modeling of similar processes involving the evolution of the internal microstructure.
S. Aygün, T. Wiegold and S. KlingeCoupling of the phase field approach to the Armstrong-Frederick model for the simulation of ductile damage under cyclic loadInternational Journal of Plasticity,143, 103021, 2021
S. Aygün and S. KlingeThermomechanical Modeling of Microstructure Evolution Caused by Strain-Induced CrystallizationPolymers,12, 11, 2575, 2020
S. Aygün and S. KlingeContinuum mechanical modeling of strain-induced crystallization in polymersInternational Journal of Solids and Structures,196-197, 129-139, 2020
S. Klinge, S. Aygün, R. P. Gilbert and G. A. HolzapfelMultiscale FEM simulations of cross-linked actin network embedded in cytosol with the focus on the filament orientationInternational Journal for Numerical Methods in Biomedical Engineering,34, 7, e2993, 2018
S. Klinge, T. Wiegold, S. Aygün, R. P. Gilbert, and G. A. HolzapfelOn the mechanical modeling of cell componentsPAMM,20, 1, e202000129, 2021
S. Aygün and S. KlingeStudy of stochastic aspects in the modeling of the strain-induced crystallization in unfilled polymersPAMM,20, 1, e202000031, 2021
S. Aygün and S. KlingeCoupled thermomechanical model for strain-induced crystallization in polymersPAMM,19, 1, e201900342, 2019
S. Aygün and S. KlingeModeling the thermomechanical behavior of strain-induced crystallization in unfilled polymersProceedings of the 8th GACM Colloquium on Computational Mechanics,151-154, 2019
S. Klinge, S. Aygün and M. BambachExtended Simulations of the Roll Bonding ProcessPAMM,18, 1, e201800257, 2018
S. Aygün and S. KlingeStudy of the microstructure evolution caused by the strain‐induced crystallization in polymersPAMM,18, 1, e201800224, 2018
T. Wiegold , S. Klinge, S. Aygün, R. P. Gilbert and G. A. HolzapfelViscoelasticity of cross‐linked actin network embedded in cytosolPAMM,18, 1, e201800151, 2018
S. Aygün, S. Klinge and S. GovindjeeContinuum Mechanical Modeling of Strain-Induced Crystallization in PolymersProceedings of the 7th GACM Colloquium on Computational Mechanics,579-582, 2017
S. Aygün and S. KlingeMechanical Modeling of the Strain-Induced-Crystallization in PolymersPAMM,17, 1, 389-390, 2017
S. Klinge, S. Aygün, J. Mosler and G. A. HolzapfelCross-linked actin networks: Micro- and macroscopic effectsPAMM,16, 1, 93-94, 2016
Thermomechanical and multiscale modeling of polymeric materials with complex microstructures91st GAMM Annual Meeting, Kassel, Germany, March 15-19, 2021
Modeling the thermomechanical behavior of strain-induced crystallization in unfilled polymers8th GACM Colloquium on Computational Mechanics, Kassel, Germany, August 28-30, 2019
Coupled thermomechanical model for strain-induced crystallization in polymersYoung Researchers' Minisymposia, 90th GAMM Annual Meeting, Vienna, Austria, February 18-22, 2019
Study of the microstructure evolution caused by the strain-induced crystallization in polymers89th GAMM Annual Meeting, Munich, Germany, March 19-23, 2018
Continuum Mechanical Modeling of Strain-Induced Crystallization in Polymers7th GACM Colloquium on Computational Mechanics, Stuttgart, Germany, October 11-13, 2017
Multiscale Modeling of Strain-Induced Crystallization in Polymers88th GAMM Annual Meeting, Weimar, Germany, March 6-10, 2017
