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    Item type:Publication, Access status: with files ,
    Micromechanical modeling of size-dependent crystal plasticity and deformation twinning
    (2019)
    Husser, Edgar
    ;
    Bargmann, Swantje
    The computational modeling and simulation of the deformation behavior of crystalline materials at the micron scale is the objective of this thesis. Numerous experimental studies have proven that small-scaled metallic probes behave mechanically different from their bulk counterparts, in particular, the yield strength and work hardening behavior is affected in the way that smaller single crystals behave stronger. This characteristic behavior opens up new design possibilities for further improvement of mechanical properties of engineering material systems. In view of high expenses for advanced and complex experimental work along with the inherent testing limitations, numerical methods offer a great option to validate experimental findings, support the development of suitable theories, and extend scientific investigations on a computer-aided basis. A gradient crystal plasticity model is presented in this thesis and applied to selected scientific problems including the mechanical testing of single crystals via simulation of microcompression and microbending using a three-dimensional finite element framework. The model is implemented on an element basis and linked to the commercial finite element software Abaqus via the user subroutine UEL. Two major deformation mechanism are considered by the underlying model. Deformation via crystallographic slip is modeled in a non-local fashion allowing to account for gradient effects. Geometrically necessary dislocation (GND) densities associated with plastic slip gradients are introduced as additional nodal degrees of freedom while the actual plastic slip variables are handled as internal variables. Deformation twinning is accounted for in terms of a simple shear deformation associated with shear and shuffling of atoms. In addition, the twinning-induced reorientation of the crystal lattice is fully considered along with subsequent activation of slip modes within the twinned region. In accordance to the non-local crystal plasticity framework, the gradient of the twin volume fraction is introduced at the nodal level. The actual twin volume fraction is treated as an extended internal variable yielding a coupled system of highly non-linear equations at the local level. The competitive nature between slip deformation and deformation by twinning is addressed by physically motivated interaction relations. The characteristic features of the model are portrayed for a variety of micromechanical problems and in relation to experimental results.
    Publicationtype: Doctoral Thesis
      465  1574
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    Crystal plasticity modeling of fully lamellar titanium aluminide alloys
    (2018)
    Schnabel, Jan Eike
    ;
    Bargmann, Swantje
    In the present thesis, a thermomechanically coupled, defect density based crystal plasticity model is presented. This model accounts for the evolution of dislocation densities and twinned volume fractions on different slip and twinning systems during plastic deformation and thermal recovery. Considering the evolution of dislocation densities and twinned volume fractions allows a physics based formulation of the work hardening model and enables a physically meaningful representation of dissipation and stored energy of cold work in the applied thermomechanical framework. In the course of this thesis, the presented crystal plasticity model was applied to investigate several aspects of the plastic deformation behavior of fully lamellar titanium aluminide alloys. After calibrating the work hardening model to fit experimental results, it was successfully used to relate specifics of the macroscopic stress-strain response of fully lamellar titanium aluminides to the work hardening interactions on the microscale. By combining numerical studies and experimental findings from literature, it was further possible to identify and consequently model the relative contribution of the different coexisting microstructural interfaces to the macroscopic yield strength. With this microstructure sensitive model formulation, the influence of the microstructural parameters on the inhomogeneous microplasticity of fully lamellar titanium aluminides was studied. Due to its defect density based formulation, the model enabled trends in the static recovery behavior to be investigated. Finally, the model was extended in order to account for the anomalous dependence of the yield strength of fully lamellar titanium aluminides on temperature.
    Publicationtype: Doctoral Thesis
      587  1846
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    Multiphysically coupled modelling of polymer-based materials
    (2017)
    Wilmers, Jana
    ;
    Bargmann, Swantje
    The multiphysically coupled modelling and numerical simulation of mass transport in polymeric materials is investigated. Coupled behaviour arises in these materials due to molecular interactions of the polymer chains with the transported particles or fluids. Due to these interactions, mass transport in polymers is often associated with large deformations and significant changes of the mechanical behaviour that, in turn, may lead to cracking and failure. On the other hand, this behaviour can be utilised deliberately, for example for microstructuring or in smart materials. Thus, a detailed understanding of the processes involved is necessary. In this thesis, a model for multiphysically coupled mass transport in the framework of nonlinear continuum mechanics is developed. The governing equations describing two different example cases, namely, anomalous Case II diffusion and electroactive polymers, are derived from fundamental balance principles and equipped with suitable constitutive equations. For the description of Case II diffusion, a novel relation for the diffusion flux is developed that accounts for the delayed kinetics resulting from the molecular interactions and, thus, allows to describe the characteristic transport behaviour during Case II diffusion. To accurately model the processes occurring in composites of nanoporous metals filled with different types of electroactive polymers, the interface elasticity theory is extended to chemoelectromechanical coupling, yielding a framework that allows to describe deformation, electrostatics and charge carrier transport and their interactions in the bulk material and on the metal/polymer interface. These models are subsequently implemented into a finite element code and numerical studies are carried out to elucidate the models’ capabilities and provide insight into the coupled nature of the investigated phenomena.
    Publicationtype: Doctoral Thesis
      543  1474
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    Mechanical characterisation and constitutive modelling of wrought magnesium sheets exhibiting tension-compression asymmetry
    (2017)
    Tian, Xiaowei
    ;
    Bargmann, Swantje
    Wrought magnesium alloys have emerged as promising candidates for highly loaded structural components in the automobile industry recently, due to their high specific strength and low density. In this work, two aspects of wrought magnesium alloys are studied experimentally and computationally: the mechanism-based modelling of plastic deformation and the crashworthiness assessment of simple structures. A constitutive model was developed to capture the plastic behaviour of wrought magnesium sheets under monotonic loading. Wrought magnesium sheets exhibit a strong and evolving tension compression asymmetry in terms of flow stress and r-value (strain anisotropy parameter). A yield function originally proposed by Cazacu, Plunkett and Barlat in 2006 was extended by introducing a new parameter depending on the plastic multiplier to describe this tension-compression asymmetry and its evolution. The model was applied to two different magnesium wrought alloys, AZ31 (Mg+3%Al+1%Zn) and ZE10 (Mg+1%Zn+0.3%Ce), each produced as rolled sheet and as extruded profile. A series of mechanical tests including uniaxial tensile and compression tests along different orientations, through thickness compression tests, shear tests, and notched bar tests was conducted for characterising the mechanical behaviour under various stress states. Experimental results confirm significantly different plastic deformation behaviour of the two alloys, which were resulted from their respective initial textures. For calibration of the material model, identification of material parameters was accomplished with the experimental data based on an error minimisation scheme. The computational results show that the proposed model captures the tension- compression asymmetry of both alloys. The calibrated model was finally applied to a pure bending simulation. The good agreement between the simulated and the semi-analytically calculated bending moment-curvature diagram demonstrates the predictive capability of the present model for the considered magnesium alloys. Experimental study and numerical simulations were conducted to investigate the plastic buckling and crush behaviour of structural components with hollow rectangular profiles fabricated from the two magnesium alloys. The used modelling approach is justified by balancing achievable accuracy and computational efforts. The performance of the components made by magnesium alloys was evaluated in term of the dissipated specific energy in the crush tests and compared with respective aluminium components. It is revealed that the observed high dissipation of magnesium alloys during crushing is related to the work hardening behaviour in compression.
    Publicationtype: Doctoral Thesis
      469  1325