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Current Projects

DFG Project – Multiscale modeling of calcified polymer hydrogels
(Start: July 2020)
PI: S. Klinge
Coworker: S. Aygün

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.


DAAD bilateral project (PPPP): Development of a new method for determination of microstructure complexity of robot laser hardened materials in different hardening conditions
(January 2021 – December 2022)
PI: Dragan Marinkovic (Germany), Matej Babic (Slovenia)

Heat treatment processes, including hardening and tempering of tool steels to achieve the desired microstructure with improved mechanical properties, is one of the last production steps which determines the final bulk residual stresses and the dimensions of the workpiece. The laser thermal hardening method is one of the most effective and innovative methods for hardening the surface layer of a material. The great advantages of laser hardening are the reduction in additional material processing, no need for a separate cooling process, the possibility of processing inhomogeneous three-dimensional blanks and complex geometries. Robot systems that provide not only heat treatment, but also positioning, automatic feeding of workpieces, measurement of results, etc., are becoming more relevant. Within the framework of this project, materials will be hardened using a robot laser cell, RV60-40 (Reis Robotics Company). Materials will be hardened using different parameters, involving speeds v ∈ [2,5] mm/s with a step of 1 mm/s, temperature T ∈ [1000, 1300] °C with a step of 100 °C and angles ∈ [0, 90]° with a step of 15°. Five different options of robot laser hardening with different angles will be used.

Completed Projects

D-A-CH Project (DFG, FWF) – Computational Modeling of Vesicle-Mediated Cell (CM-TransCell)
(Start: March 2018)
PIs: S. Klinge and G. A. Holzapfel
Coworkers: T. Wiegold and D. Haspinger

The particularly important characteristics of eukaryotic cells are the enormous complexity of their membrane anatomy and the high level of organization of the transport processes. The surprisingly precise manner of the routing of vesicles to various intracellular and extracellular destinations can be illustrated by numerous examples such as the release of neurotransmitters into the presynaptic region of a nerve cell and the export of insulin to the cell surface.
The key idea of the present project is to couple results of biomedical investigations and mechano-mathematical models with the highly efficient engineering software packages in order to simulate this type of processes, in particular the vesicle transport. The results should bridge the theoretical investigations and medical praxis and shift the paradigm in understanding and remedying different diseases, which certainly is the primary and long-term goal of the project. The individual objectives coincide with the modeling of single aspects of the vesicle transport, namely with the simulation of mechanisms by which the vesicles form, find their correct destination, fuse with organelles and deliver their cargo. The application of several different approaches is envisaged for this purpose, but three main strategies build the underlying skeleton: the theory of lipid bilayer membranes, the homogenization method and the diffusion theory. The mentioned approaches will furthermore be combined with the modern numerical techniques such as the finite element method and the multiscale finite element method.
In the final stage, the realization of single objectives will allow the simulation of vesicle transport as a continuous process and the study of the impact of various factors on the whole process. This way, the project will yield a significant shift from "static" bio-computations related to the single cell compartments and substeps of its activities, to the "dynamic" simulation of the real living processes. 


TRR 188 – Damage Controlled Forming Processes
Subproject C04 – Micromechanical modelling of damage in polycrystals on the basis of the extended crystal plasticity
(Start: January 2017)
PIs: S. Klinge and J. Mosler

The objective of subproject C04 is the development of a micromechanical model for polycrystals which shall be able to consistently simulate plastic deformations together with damage. To this end, an extended crystal plasticity model able to simulate the damage within single crystals is proposed. It is furthermore combined with an interface model in order to additionally capture influences of damage on the grain boundaries. Finally, both material models are applied in order to simulate the behavior of an appropriately chosen RVE and to study the influence of initial damage.


DFG Project – Multiscale Modeling of Strain-Induced Crystallization in Polymers (MM-SIC)
PI: S. Klinge
Coworker: S. Aygün

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.

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