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    Computational micromechanics of matrix-inclusion composites
    (2019)
    Schneider, Konrad
    ;
    Cyron, Christian J.
    This thesis is concerned with aspects of the computational retrievement of macroscopic material responses and investigations of the microscopic deformation behavior of random matrix-inclusion composites. To this end, the focus lies on the representative volume element (RVE) approach in the scope of finite element analyses and its applicability and integration into engineering practice. The artificial random generation of the microstructural geometry, in a periodic and non-periodic manner, is established by studying and enhancing existing methods such as the random sequential adsorption method or collective rearrangement methods for cubic RVEs. Based on distance computations between two inclusions, the non-overlap requirement, which is characteristic for matrix-inclusion composites, is ensured for ellipsoidal, capsular, cylindrical and convex polyhedral inclusion shapes. To comply with the state-of-the-art approach that utilizes fully periodic RVEs featuring a periodic topology, a periodic discretization and periodic boundary conditions (PBC), a new algorithm that automatically generates periodic meshes is developed. The algorithm systematically combines various meshing tools in an efficient and robust way. Special emphasis is placed on the discretization procedure to maintain a high quality mesh with as few elements as possible, thus, manageable for computer simulations applicable to high volume concentrations, high number of inclusions and complex inclusion geometries. Examples elucidate the ability of the proposed approach to efficiently generate large RVEs with a high number of anisotropic inclusions but still maintaining a high quality mesh and a low number of elements. Since the generation of such a state-of-the-art RVE can significantly reduce the overall efficiency in multiscale finite element simulations, a benchmark study that investigates possible relaxations to this cumbersome task is performed. In particular, the RVE size, periodic and non-periodic RVE topologies, different discretization variants and various types of boundary conditions that either require periodicity or do not require periodicity of the underlying discretization are benchmarked. Approximate periodic boundary conditions are discussed in detail. The benchmark study proves that a fully periodic topology and mesh discretization with periodic boundary conditions is not necessary in order to identify effective macroscopic material parameters for technologically relevant composites. Eventually, the gathered methodologies are applied to model a modern high-tech polymer nanocomposite. By utilizing a sophisticated material model for the polymeric matrix, the entropic and energetic deformation regime can be represented in a physically meaningful way. Furthermore, the explicit consideration of the matrix-inclusion interface failure via a cohesive zone model accounts for the non-negligible surface effects. Comparisons with experiments underline the predictive character of the modeling approach and allow for investigations of local deformation mechanisms.
    Publicationtype: Doctoral Thesis
      668  359
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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