TY - JOUR A1 - Vöse, Markus A1 - Otto, F. A1 - Fedelich, Bernard A1 - Eggeler, G. T1 - Micromechanical investigations and modelling of a copper-antimony-alloy under creep conditions N2 - In many practical applications, creep damage is the limiting factor of a component’s lifetime. A micromechanical model of creep induced grain boundary damage is proposed, which allows for the simulation of creep damage in a polycrystal within the framework of finite element analysis. The model considers grain boundary cavitation and sliding according to a micromechanically motivated cohesive zone model while creep deformation of the grains is described following the slip system theory. The model can be applied to idealised polycrystalline structures, such as a Voronoi tessellation or, like demonstrated here, to real grain structures of miniature creep specimens. Creep tests with pure Cu single crystals and with a coarse-grained polycrystalline Cu-1 wt.% Sb alloy at 823 K have been performed and used to calibrate the polycrystal model. The grain structure of the polycrystalline Cu–Sb specimens has been revealed by the EBSD method. Extensive grain boundary sliding and cavitation has been observed in the crept specimens. Grain boundary sliding has been found to promote wedge-type damage at grain boundary triple junctions and to contribute significantly to the total creep strain. Furthermore, the assumed stress sensitivity of the models grain boundary cavity nucleation rate strongly influences the development of wedge-type damage. KW - Creep KW - Damage KW - Copper–Antimony-Alloy KW - Grain boundary cavitation KW - Grain boundary sliding KW - Micromechanical model PY - 2014 U6 - https://doi.org/10.1016/j.mechmat.2013.09.013 SN - 0167-6636 SN - 1872-7743 VL - 69 IS - 1 SP - 41 EP - 62 PB - Elsevier Ltd. CY - Amsterdam AN - OPUS4-29396 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Otto, Peter A1 - De Lorenzis, L. A1 - Unger, Jörg F. T1 - A regularized model for impact in explicit dynamics applied to the split Hopkinson pressure bar N2 - In the numerical simulation of Impact phenomena, artificial oscillations can occur due to an instantaneous change of velocity in the contact area. In this paper, a nonlinear penalty regularization is used to avoid these oscillations. Aparticular focus is the investigation of higher order methods in space and time to increase the computational efficiency. The spatial discretization is realized by higher order spectral element methods that are characterized by a diagonal mass matrix. The time integration scheme is based on half-explicit Runge–Kutta scheme of fourth order. For the conditionally stable scheme, the critical time step is influenced by the penalty regularization. A framework is presented to adjust the penalty stiffness and the time step for a specific mesh to avoid oscillations. The methods presented in this paper are applied to 1D-simulations of a split Hopkinson pressure bar, which is commonly used for the investigation of materials under dynamic loading. KW - Penalty regularization KW - Spectral element method KW - Differential–algebraic equations KW - Split Hopkinson pressure bar KW - Dynamic contact PY - 2016 U6 - https://doi.org/10.1007/s00466-016-1311-1 SP - 1 EP - 15 PB - Springer-Verlag CY - Berlin Heidelberg AN - OPUS4-36967 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Vöse, Markus A1 - Fedelich, Bernard A1 - Otto, F. A1 - Eggeler, G. T1 - Micromechanical modeling of creep damage in a copper-antimony alloy N2 - A micromechanical model of creep induced grain boundary damage is proposed, which allows for the simulation of creep damage in a polycrystal with the finite element method. Grain boundary cavitation and sliding are considered via a micromechanically motivated cohesive zone model, while the grains creep following the slip system theory. The model has been calibrated with creep test data from pure Cu single crystals and a coarse-grained polycrystalline Cu-Sb alloy. The test data includes porosity measurements and estimates of grain boundary sliding. Finally, the model has been applied to Voronoi models of polycrystalline structures. In particular the influence of grain boundary sliding on the overall creep rate is demonstrated. T2 - ECF20 - 20th European conference on fracture CY - Trondheim, Norway DA - 28.06.2014 KW - Creep damage KW - Grain boundary sliding KW - Cohesive zone KW - Micromechanical model KW - Polycrystal PY - 2014 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:b43-309914 SN - 2211-8128 VL - 3 SP - 21 EP - 26 PB - Curran CY - Red Hook, NY, USA AN - OPUS4-30991 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Otto, Peter A1 - de Lorenzis, L. A1 - Unger, Jörg F. T1 - Explicit dynamics in impact simulation using a NURBS contact interface N2 - In this paper, the impact problem and the subsequent wave Propagation are considered. For the contact discretization an intermediate non-uniform rational B-spline (NURBS) layer is added between the contacting finite element bodies, which allows a smooth contact formulation and efficient element-based integration. The impact event is ill-posed and requires a regularization to avoid propagating stress oscillations. A nonlinear mesh-dependent penalty regularization is used, where the stiffness of the penalty regularization increases upon mesh refinement. Explicit time integration methods are well suited for wave propagation problems, but are efficient only for diagonal mass matrices. Using a spectral element discretization in combination with a NURBS contact layer the bulk part of the mass matrix is diagonal. KW - Explicit dynamics KW - Impact simulation KW - Isogeometric analysis KW - mortar method KW - spectral elements PY - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:b43-506041 VL - 121 IS - 6 SP - 1248 EP - 1267 PB - Wiley Online Libary AN - OPUS4-50604 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Otto, Peter A1 - Lorenzis, L. A1 - Unger, Jörg F. T1 - Explicit dynamics in impact simulation using a NURBS contact interface N2 - In this paper, the impact problem and the subsequent wave propagation are considered. For the contact discretization an intermediate NURBS layer is added between the contacting finite element bodies, which allows a smooth contact formulation and efficient element‐based integration. The impact event is ill‐posed and requires a regularization to avoid propagating stress oscillations. A nonlinear mesh dependent penalty regularization is used, where the stiffness of the penalty regularization increases upon mesh refinement. Explicit time integration methods are well suited for wave propagation problems, but are efficient only for diagonal mass matrices. Using a spectral element discretization and the coupled FE‐NURBS approach the bulk part of the mass matrix is diagonal. KW - Impact simulation KW - Explicit dynamics KW - Isogeometric analysis KW - Spectral elements KW - Mortar method PY - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:b43-494947 SP - 1 EP - 21 PB - Wiley Online Libary AN - OPUS4-49494 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Otto, Peter A1 - De Lorenzis, L. A1 - Unger, Jörg F. T1 - Coupling a NURBS contact interface with a higher order finite element discretization for contact problems using the mortar method N2 - In this paper, a contact problem between two bodies, discretized by finite elements, is solved by adding an auxiliary NURBS layer between the bodies. The advantages of a smooth contact formulation in a NURBS approach are combined with simple mesh generation procedures for the bodies discretized with finite elements. Mesh tying conditions are used to couple the NURBS layer with the finite element discretization. The NURBS layer is the master side for contact and mesh tying. Mesh tying is enforced either using pointwise or mortar type approaches. Frictionless 2D and 3D contact problems are considered using small deformations. The contact problem is discretized with the mortar method and a penalty approach is used to enforce the contact constraints. A robust element-based quadrature is applied for mortar tying and contact discretizations, thus avoiding computationally expensive Segmentation. KW - Contact mechanics KW - Isogeometric analysis KW - Mortar methods KW - Spectral element method PY - 2018 U6 - https://doi.org/10.1007/s00466-018-1645-y SN - 0178-7675 SN - 1432-0924 VL - 63 IS - 6 SP - 1203 EP - 1222 PB - Springer AN - OPUS4-46800 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Otto, Peter A1 - Unger, Jörg F. T1 - Contact mechanics in the simulation of the Split-Hopkinson-Pressure-Bar (SHPB) N2 - Explicit time integration scheme with smoothed contact-impact T2 - ICCCM 2015 Hannover CY - Hannover, Germany DA - 27.05.2015 KW - Smoothed contact-impact KW - Higher order time discretization for contact-impact KW - Stress wave propagation KW - Split-Hopkinson-pressure-bar PY - 2015 AN - OPUS4-46239 LA - deu AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Otto, Peter A1 - Unger, Jörg F. T1 - Time integration schemes for normal impact with smoothing N2 - In this work an approach for smoothing the oscillations of normal impact is presented. In addition, a higher order time discretization scheme in association with a higher order spatial discretization, like the spectral element method, is investigated regarding its convergence rates. T2 - 3rd ECCOMAS Young Investigators Conference (YIC) and 3rd AC.CES CY - Aachen, Germany DA - 20.07.2015 KW - Impact KW - Stress wave propagation KW - Split-Hopkinson-Pressure-Bar PY - 2015 AN - OPUS4-46240 LA - deu AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Otto, Peter A1 - Unger, Jörg F. T1 - Contact mechanics in the simulation of the Split-Hopkinson-Pressure-Bar (SHPB) N2 - In this work an approach for smoothing the oscillations of normal contact is presented. On the other side a higher order time discretization scheme in association with a higher order spatial discretization, like the spectral element method, is used. The contact constraints are reformulated in order to get an explicit equation for the motion. T2 - ICCCM 2015 Hannover CY - Hannover, Germany DA - 27.05.2015 KW - Impact KW - Time integration schemes for impact KW - Stress wave propagation KW - Split-Hopkinson- Pressure-Bar PY - 2015 AN - OPUS4-46243 LA - deu AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -