TY - CONF A1 - Unger, Jörg F. T1 - Multiscale simulation of localization phenomena using XFEM² T2 - ECCOMAS 2012 CY - Vienna, Austria DA - 2012-09-10 PY - 2012 AN - OPUS4-26532 LA - deu AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Unger, Jörg F. T1 - An FE²-X¹ approach for multiscale localization phenomena N2 - In this paper, a new methodology based on the Hill–Mandel lemma in an FE² sense is proposed that is able to deal with localized deformations. This is achieved by decomposing the displacement field of the fine scale model into a homogeneous part, fluctuations, and a cracking part based on additional degrees of freedom (X¹)—the crack opening in normal and tangential directions. Based on this decomposition, the Hill–Mandel lemma is extended to relate coarse and fine scale energies using the assumption of separation of scales such that the fine scale model is not required to have the same size as the corresponding macroscopic integration point. In addition, a procedure is introduced to mimic periodic boundary conditions in the linear elastic range by adding additional shape functions for the boundary nodes that represent the difference between periodic boundary conditions and pure displacement boundary conditions due to the same macroscopic strain. In order to decrease the computational effort, an adaptive strategy is proposed allowing different macroscopic integration points to be resolved in different levels on the fine scale. KW - Fracture KW - Multiscale KW - Finite elements KW - Constitutive behavior KW - Inhomogeneous material PY - 2013 U6 - https://doi.org/10.1016/j.jmps.2012.12.010 SN - 0022-5096 VL - 61 IS - 4 SP - 928 EP - 948 PB - Elsevier AN - OPUS4-35902 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Unger, Jörg F. T1 - Comparison of higher order methods in time and space for the numerical simulation of ultrasonic wave propagation T2 - 11th World Congress on Computational Mechanics (WCCM XI); 5th European Conference on Computational Mechanics (ECCM V); 6th European Conference on Computational Fluid Dynamics (ECFD VI) CY - Barcelona, Spain DA - 2014-07-20 PY - 2014 AN - OPUS4-32901 LA - deu AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Unger, Jörg F. T1 - Hierarchical multiscale models for localization phenomena within the framework of FE2-X1 T2 - Computational Modelling of Concrete and Concrete Structures CY - St. Anton am Arlberg, Austria DA - 2014-03-24 PY - 2014 AN - OPUS4-32902 LA - deu AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Kindrachuk, Vitaliy A1 - Thiele, Marc A1 - Unger, Jörg F. T1 - Constitutive modeling of creep-fatigue interaction for normal strength concrete under compression N2 - Conventional approaches to model fatigue failure are based on a characterization of the lifetime as a function of the loading amplitude. The Wöhler diagram in combination with a linear damage accumulation assumption predicts the lifetime for different loading regimes. Using this phenomenological approach, the evolution of damage and inelastic strains and a redistribution of stresses cannot be modeled. The gradual degration of the material is assumed to not alter the stress state. Using the Palmgren–Miner rule for damage accumulation, order effects resulting from the non-linear response are generally neglected. In this work, a constitutive model for concrete using continuum damage mechanics is developed. The model includes rate-dependent effects and realistically reproduces gradual performance degradation of normal strength concrete under compressive static, creep and cyclic loading in a unified framework. The damage evolution is driven by inelastic deformations and captures strain rate effects observed experimentally. Implementation details are discussed. Finally, the model is validated by comparing simulation and experimental data for creep, fatigue and triaxial compression. KW - Fatigue KW - Compression KW - Constitutive modeling KW - Normal strength concrete PY - 2015 U6 - https://doi.org/10.1016/j.ijfatigue.2015.03.026 SN - 0142-1123 VL - 78 SP - 81 EP - 94 PB - Elsevier CY - Oxford AN - OPUS4-34158 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Titscher, Thomas A1 - Unger, Jörg F. T1 - Application of molecular dynamics simulations for the generation of dense concrete mesoscale geometries N2 - The problem of polydisperse sphere packings is applied to concrete mesoscale geometries in finite sized specimens. Realistic sphere diameter distributions are derived from concrete grading curves. An event-driven molecular dynamics simulation using growing particles is introduced. Compared to the widely used random sequential addition algorithm, it reaches denser aggregate packings and saves computation time at high volume fractions. A minimal distance between particles strongly influences the maximum aggregate content. It is essential to obtain undistorted elements when meshing the geometry for finite element simulations. The algorithm maximizes this value and produces meshable concrete mesostructures with more than 70% aggregate content. KW - Polydisperse sphere packing KW - Mesoscale geometry KW - Concrete mesostructure KW - Molecular dynamics PY - 2015 U6 - https://doi.org/10.1016/j.compstruc.2015.06.008 SN - 0045-7949 SN - 0366-7138 VL - 158 SP - 274 EP - 284 PB - Pergamon Press CY - Oxford AN - OPUS4-34190 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Unger, Jörg F. ED - Idelsohn, S. ED - Sonzogni, V. ED - Coutinho, A. ED - Cruchaga, M. ED - Lew, A. ED - Cerrolaza, M. T1 - Numerical simulation of ultrasonic wave propagation using higher order methods in space and time N2 - The paper discusses the efficient simulation of ultrasonic wave propagation. It is demonstrated that a combination of higher methods in space and time leads to a significant performance boost. Higher order spectral elements are used for the spatial discretization. A comparison with standard finite elements shows the advantages when using explicit time integration schemes. For the temporal discretization, an efficient explicit fourth order Nyström method is presented. Its computational efficiency for wave propagation problems is compared to a second order Velocity Verlet integration. T2 - 1st Pan-American Congress on Computational Mechanics - PANACM 2015 CY - Buenos Aires, Argentina DA - 27.04.2015 KW - Elastic wave propagation KW - Spectral element method KW - Nyström methods PY - 2015 SN - 978-84-943928-2-5 SP - 1041 EP - 1052 PB - International Center for Numerical Methods in Engineering (CIMNE) CY - Barcelona, Spain AN - OPUS4-38645 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Unger, Jörg F. T1 - Numerical simulation of ultrasonic wave propagation using higher order methods in space and time N2 - The paper discusses the efficient simulation of ultrasonic wave propagation. It is demonstrated that a combination of higher methods in space and time leads to a significant performance boost. Higher order spectral elements are used for the spatial discretization. A comparison with standard finite elements shows the advantages when using explicit time integration schemes. For the temporal discretization, an efficient explicit fourth order Nyström method is presented. Its computational efficiency for wave propagation problems is compared to a second order Velocity Verlet integration. T2 - 1st Pan-American Congress on Computational Mechanics - PANACM 2015 CY - Buenos Aires, Argentina DA - 27.04.2015 KW - Elastic wave propagation KW - Spectral element method KW - Nyström methods PY - 2015 AN - OPUS4-38646 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Unger, Jörg F. T1 - Mechanics group at Federal Institute for Materials Research and Testing N2 - The poster presents the work of the mechanics group in BAM, department 7. The current projects deal with the simulation of concrete on different spatial and temporal scales - ranging from the creation of mesoscale geometries up to fatigue and high strain rate impact phenomena. T2 - COST Meeting 1404 CY - Ljubljana, Slovenia DA - 16.05.2015 KW - Mechanics KW - Contact KW - Multiscale KW - Concrete KW - Constitutive modelling PY - 2015 AN - OPUS4-38647 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Unger, Jörg F. T1 - Multiscale modeling of concrete - from mesoscale to macroscale N2 - Concrete is one of the most attractive building materials consumed by humans more than any other material, except water. The particular importance of concrete for a sustainable, energy-efficient economy is highlighted by the fact that about 5% of worldwide CO2 emissions are created from the cement industry. Concrete is a very complex material. Its properties are time dependent, which includes the solidification after casting or creep and shrinkage. In addition, concrete is a quasi-brittle material which requires to model the softening behavior including the challenge of appropriate regularization strategies. Many characteristic features are strongly related to its complex heterogeneous structure, including particles and mortar on the mesoscale or the CSH-phases on the micro scale. At first, a short introduction to the generation of mesoscale geometries as a three phase composite including particles, mortar matrix and the interfacial transition zone is given. Afterwards, the numerical model including meshing (XFEM and aligned meshes) as well as regularized material models for the mortar phase are presented. The focus of the presentation is the discussion of multiscale approaches to combine mesoscale models with realistic macroscale models. This includes a concurrent approach using an adaptive transition between mesoscale and macroscale models which are coupled using the mortar method. A second hierarchical approach is based on the concept of FE², which is extended to incorporate softening by solving a fine scale boundary value problem for each macroscopic integration point. T2 - COST Action TU 1404 – 2nd WORKSHOP CY - Wien, Austria DA - 19.09.2015 KW - Multiscale KW - Localization KW - Damage KW - FE² KW - Mesoscale PY - 2015 AN - OPUS4-38650 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -