TY - CONF A1 - Keller, Christian T1 - Plastic Instability of Rate-Dependent Materials - Consideration of Isothermal and Adiabatic Conditions in Dynamic Tensile Tests - N2 - During dynamic processes, a certain range of strain rates is often observed along loaded structures and components. For precise numerical simulations, it is necessary to determine rate-dependent properties in dynamic tests and to describe the material behavior correctly within an appropriate domain of strain rates including adiabatic heating effects at higher strain rates, typically higher than 10 1/s. In principle, numerical simulations are compared to experimental results to verify the applied material models. For dynamic tensile tests considering ductile materials and large plastic deformation beyond uniform elongation, it is challenging to obtain comparable results due to plastic instability and necking of the specimen, e.g.. Based on the strain gradient in a general tensile specimen, a theoretical criterion was derived describing the plastic instability in rate-dependent materials under isothermal conditions in. It was applied to different multiplicative and additive constitutive relations and the analytical onset of necking was compared to results from numerical calculations of quasi-static and dynamic tensile tests. The simulations of a sheet-metal specimen with rectangular cross-section were carried out using the Finite Element Method and it was found that the numerical calculated and the theoretical predicted onset of plastic instability agree very good. The analytical criterion for instability holds even for specimens without geometrical or material imperfections and confirms that the onset of plastic instability must be considered a material characteristic. However, real dynamic problems with higher strain rates are not isothermal, the heat generated by plastic work is not dissipated to the surrounding and the temperature of the material increases significantly. Adiabatic heating and thermal softening must be considered within the constitutive relations of rate-dependent materials and the discussion of plastic instability. In this paper, an enhanced and more generalized approach for the description of the condition for stability is discussed and applied to phenomenological as well as more physical constitutive relations from the literature. This allows an individual assessment of the accuracy and verification of rate-dependent material models with respect to plastic instability. T2 - 13th World Congress on Computational Mechanics CY - New York, USA DA - 22.07.2018 KW - Plastic Instability KW - FEM KW - Rate-dependent Materials KW - Dynamic Tensile Test PY - 2018 AN - OPUS4-48923 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Keller, Christian T1 - Plastic Instability of Rate-Dependent Materials - A Theoretical Approach in Comparison to FE-Analyses - N2 - The condition for plastic instability is a material characteristic and defines the onset of necking in tensile tests. In large deformation problems of ductile materials it is fundamental to determine the strain at which necking starts as well as the post-necking behaviour in the instability region properly. For verification purposes of material models, usually results of numerical analyses are compared to experimental outcomes. For tensile tests with ductile materials under dynamic loading, it is challenging to obtain comparable experimental and numerical results in terms of the onset of necking and the post-critical deformation behaviour. This paper focuses on the derivation of a theoretical criterion describing the plastic instability in rate-dependent materials based on the time variation of the strain gradient in a tensile specimen under isothermal conditions. We examine the influence of various constitutive equations on the theoretical stability condition predicted by different multiplicative as well as additive approaches. For multiplicative relations, the results indicate that the onset of necking is, in principle, independent of the strain rate, whereas for the considered additive relation, the dynamic necking strain must decrease with increasing strain rate. In conclusion, the theoretical stability condition is related to results from finite element simulations of dynamic tensile tests with various loading rates. It is shown that the simulated and the theoretical predicted onset of plastic instability agree reasonably. T2 - 11th European LS-DYNA Conference CY - Salzburg, Austria DA - 09.05.2017 KW - FEM KW - Plastic Instability KW - Dynamic Tensile Test PY - 2017 AN - OPUS4-40662 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - THES A1 - Keller, Christian T1 - Beitrag zur numerischen Beanspruchungsanalyse an dünnwandigen Stahlblechcontainern im Fallversuch - Schädigungsmechanische Materialmodellierung duktiler Werkstoffe unter Berücksichtigung der Dehnratenabhängigkeit der plastischen Instabilität N2 - Gegenstand der vorliegenden Arbeit ist die Bereitstellung expliziter numerischer Simulationen von Fallversuchen mit dünnwandigen Stahlblechcontainern, die als Endlagerbehälter für radioaktive Abfälle mit vernachlässigbarer Wärmeentwicklung, d. h. schwach- und mittelradioaktive Abfälle, für die Verbringung in das Endlager Konrad vorgesehen sind. Die Betrachtung von potentiellen Absturzszenarien ist ein grundlegender und zentraler Bestandteil der Sicherheitsnachweise zur Auslegung der verschiedenen endlagergängigen Behälter- und Containertypen gegen mechanische Störfallbelastungen. Für dünnwandige Stahlblechcontainer existierten bislang keine sicherheitstechnischen Untersuchungen, die auf numerischen Bewertungsmethoden basieren und eine Bewertung der vorhandenen Sicherheitsreserven hinsichtlich der mechanischen Integrität ermöglichen. Daraus resultierte die Motivation, ein verifiziertes Finite-Elemente-Modell eines repräsentativen Referenzcontainers zu entwickeln, das das dynamische Verformungs- sowie duktile Schädigungs- und Versagensverhalten in numerischen Simulationen von dynamischen Lastfällen physikalisch mit der erforderlichen Güte beschreibt. Duktile Schädigungs- und Versagensvorgänge metallischer Werkstoffe gehen grundsätzlich mit großen plastischen Dehnungen einher und führen somit unweigerlich zu der Notwendigkeit der Diskussion von plastischer Instabilität und damit verknüpften Lokalisierungseffekten, die einen wesentlichen Einfluss auf die Entwicklung des lokalen Beanspruchungszustands haben. Für die schädigungsmechanische Materialmodellierung folgt hieraus das grundsätzliche Erfordernis der korrekten Berücksichtigung des Instabilitätsbeginns, der in der Regel dehnratenunabhängig angenommen wird. Für dynamische Randbedingungen ist der werkstoffspezifische Einfluss der Dehnrate auf das Einsetzen der Lokalisierung jedoch nicht uneingeschränkt vernachlässigbar, so dass im Rahmen dieser Arbeit ein theoretischer Ansatz ausgearbeitet wird, der die plastische Instabilität dehnratenabhängiger Werkstoffe anhand des zeit- und ortsabhängigen Dehnungsgradienten im einachsigen Zugversuch diskutiert. Darüber hinaus ist für eine hinreichend genaue Materialmodellierung neben der korrekten Prognose des Instabilitätsbeginns auch die korrekte Beschreibung des Verformungsverhaltens jenseits der Gleichmaßdehnung notwendig. Zu diesem Zweck wird in dieser Arbeit eine konstitutive Beziehung entwickelt, die sowohl unter Berücksichtigung der plastischen Instabilität als auch unter der Beachtung von ver- sowie entfestigenden Einflussfaktoren der dynamischen Plastizität formuliert werden kann. Insgesamt wird eine praxistaugliche Bewertungsmethode der mechanischen Integrität dünnwandiger Stahlblechcontainer unter Störfallbedingungen auf Basis expliziter dynamischer Simulationen erarbeitet, die die komplexen duktilen Schädigungs- und Versagensmechanismen dehnratenabhängiger Werkstoffe berücksichtigt. Mit der zur Simulation von Fallversuchen entwickelten numerischen Modellbildung und den zur Verifizierung entsprechend mit Testcontainern im Originalmaßstab durchgeführten Fallversuchen wird nachgewiesen, dass anhand des Finite-Elemente-Modells eines repräsentativen Referenzcontainers zum einen eine quantitativ belegte Bestimmung ungünstiger Fallorientierungen hinsichtlich maximaler Verformungen und zum anderen eine realistische Abbildung des duktilen Versagensverhaltens ermöglicht wird. Damit können tatsächlich vorhandene Sicherheitsreserven bei potentiell zu unterstellenden Behälterabstürzen quantitativ aufgezeigt und zukünftig eine verbesserte sicherheitstechnische Beurteilung von Stahlblechcontainern im Fallversuch bereitgestellt werden. Damit leistet die vorliegende Arbeit einen wichtigen Beitrag für die Bewertung von Sicherheitsnachweisen von Endlagerbehältern für radioaktive Abfälle und trägt dazu bei, den langfristigen Schutz von Mensch und Umwelt vor Schäden durch radioaktive Stoffe zu gewährleisten. N2 - The subject of the present work is the provision of explicit numerical simulations of drop tests with thin-walled sheet steel containers, which are intended as repository containers for radioactive waste with negligible heat generation, i.e. low and intermediate level waste, for shipment to the Konrad repository. The consideration of potential crash scenarios is a fundamental and central component of the safety analyses for the design of the various types of casks and containers suitable for final disposal against mechanical accident loads. For thin-walled sheet steel containers, no safety investigations based on numerical assessment methods existed so far that would allow an evaluation of the existing safety reserves with regard to mechanical integrity. This resulted in the motivation to develop a verified finite element model of a representative reference container that physically describes the dynamic deformation and ductile damage and failure behaviour in numerical simulations of dynamic load cases with the required quality. Ductile damage and failure processes of metallic materials are fundamentally accompanied by large plastic strains and thus inevitably lead to the need to discuss plastic instability and associated localisation effects, which have a significant influence on the development of the local stress state. For damage-mechanical material modelling this results in the fundamental requirement of correctly considering the onset of instability, which is usually assumed to be strain-rate independent. For dynamic boundary conditions, however, the material-specific influence of the strain rate on the onset of localisation cannot be neglected without restriction, so that a theoretical approach is elaborated within the scope of this work that discusses the plastic instability of strain-rate-dependent materials on the basis of the time- and location-dependent strain gradient in the uniaxial tensile test. Furthermore, for a sufficiently accurate material modelling, besides the correct prediction of the onset of instability, the correct description of the deformation behaviour in the postcritical region, i.e., beyond the uniform strain, is necessary. For this purpose, a constitutive relationship is developed in this work, which can be formulated both under consideration of the plastic instability as well as under consideration of hardening as well as softening influencing factors of the dynamic plasticity. Overall, a practical assessment method of the mechanical integrity of thin-walled sheet steel containers under accident conditions is developed based on explicit dynamic simulations, which takes into account the complex ductile damage and failure mechanisms of strain rate-dependent materials. With the numerical modelling developed for the simulation of drop tests and the drop tests carried out accordingly with full-scale test containers for verification, it is demonstrated that, on the basis of the finite element model of a representative reference container, on the one hand a quantitatively proven determination of unfavourable drop orientations with regard to maximum deformations and, on the other hand, a realistic representation of the ductile failure behaviour is made possible. This allows it to quantitatively demonstrate actually existing safety reserves in case of potential container crashes and to provide an improved safety assessment of sheet steel containers in drop tests in the future. This work thus makes an important contribution to the evaluation of safety verifications of final storage containers for radioactive waste and helps to ensure the long-term protection of humans and the environment from damage caused by radioactive substances. KW - FEM KW - Dynamische Plastizität KW - Plastische Instabilität KW - Duktile Schädigung KW - Endlager Konrad PY - 2024 DO - https://doi.org/10.14279/depositonce-21874 SP - 1 EP - 188 PB - DepositOnce CY - Berlin AN - OPUS4-61581 LA - deu AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Keller, Christian A1 - Herbrich, Uwe T1 - Plastic Instability of Rate-Dependent Materials - A Theoretical Approach in Comparison to FE-Analyses - N2 - The condition for plastic instability is a material characteristic and defines the onset of necking in tensile tests. In large deformation problems of ductile materials it is fundamental to determine the strain at which necking starts as well as the post-necking behaviour in the instability region properly. For verification purposes of material models, usually results of numerical analyses are compared to experimental outcomes. For tensile tests with ductile materials under dynamic loading, it is challenging to obtain comparable experimental and numerical results in terms of the onset of necking and the post-critical deformation behaviour. This paper focuses on the derivation of a theoretical criterion describing the plastic instability in rate-dependent materials based on the time variation of the strain gradient in a tensile specimen under isothermal conditions. We examine the influence of various constitutive equations on the theoretical stability condition predicted by different multiplicative as well as additive approaches. For multiplicative relations, the results indicate that the onset of necking is, in principle, independent of the strain rate, whereas for the considered additive relation, the dynamic necking strain must decrease with increasing strain rate. In conclusion, the theoretical stability condition is related to results from finite element simulations of dynamic tensile tests with various loading rates. It is shown that the simulated and the theoretical predicted onset of plastic instability agree reasonably. T2 - 11th European LS-DYNA Conference CY - Salzburg, Austria DA - 09.05.2017 KW - FEM KW - Plastic Instability KW - Dynamic Tensile Test PY - 2017 UR - http://www.dynalook.com/11th-european-ls-dyna-conference/crash-metal-failure/plastic-instability-of-rate-dependent-materials-a-theoretical-approach-in-comparison-to-fe-analyses SP - 1 EP - 10 AN - OPUS4-40660 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Nagelschmidt, Sven A1 - Herbrich, Uwe A1 - Keller, Christian A1 - Qiao, Linan T1 - Re-evaluation of tensile and creep rupture data of metals using a modified Larson–Miller approach N2 - More than 70 years ago, in 1952 Larson and Miller adapted an existing relationship whichevaluates the relative effects of time and temperature on creep rupture behavior of various alloys by using existing data and by constructing so-called master curves. Since that time, the formulated Larson–Miller relation, commonly known as Larson–Miller parameter, is a widely used time–temperature parameter for various applications, e.g. for rupture life, creep and relaxation analyses of metals and some other materials such as polymers, concrete and ceramics.Nevertheless, the physical meaning of this parameter as well as the determination of master curves are still the subject of major criticism. In this work, both aspects are reviewed and analyzed with data originally considered in thepaper of Larson and Miller based on the following approach: (a) the parameter was modifiedregarding a reference time and a reference temperature according to the time–temperature superposition principle; (b) master curves were generated for five materials investigated originally by Larson and Miller, based on a stretched exponential function type. It has been shown, that: (1) With normalized time and temperature, the Larson–Miller parameter corresponds to real logarithmic rupture time. (2) For the relationship between Creep rupture stress and the modified Larson–Miller parameter, the stretched exponential function is a good choice as a master function. (3) Corresponding model parameters have physical meaning and can be easily determined based on data from creep test data. The investigations provide a better understanding and applicability of Larson–Miller approach. KW - Larson–Miller parameter KW - Creep rupture time KW - Time–temperature equivalence KW - Stretched exponential function KW - Extrapolation of short-term data PY - 2025 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-644486 DO - https://doi.org/10.1016/j.engfracmech.2025.111582 SN - 0013-7944 VL - 329 SP - 1 EP - 16 PB - Elsevier Ltd. AN - OPUS4-64448 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Qiao, Linan A1 - Keller, Christian A1 - Zencker, Uwe A1 - Völzke, Holger T1 - Three-dimensional finite element analysis of O-ring metal seals considering varying material properties and different seal diameters N2 - Metal seals of O-ring form are often used in lid-systems of transport and storage casks for radioactive waste in Germany. To investigate their mechanical behaviour, three dimensional (3D) finite element (FE) models were created using solid elements for all of the seal components. The material behaviour of each component is described with a unified static elastic-plastic material model. The total strain is defined as the sum of linear elastic strain and plastic strain with power-law hardening. The model was carefully validated by comparison of Simulation results with experimental results. The influence of material fluctuation of each seal component due to varying properties and the sensitivity of different seal diameters on the seal force are analysed and discussed. The results show that the material properties of helical spring have major influence on seal force and that the influence of seal diameters is negligible small in the studied range. This is very important to use the test results from seals with small diameter for the assessment of seal behaviour with larger diameter as used in transport and storage cask. KW - Sensitivity analysis KW - Transport and storage cask KW - O-ring metal seal KW - Lid-system KW - Finite element analysis KW - Stochastic variation of material properties PY - 2019 DO - https://doi.org/10.1016/j.ijpvp.2019.103953 VL - 176 SP - 102953 PB - Elsevier Ltd. CY - Amsterdam AN - OPUS4-49263 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Qiao, Linan A1 - Nagelschmidt, Sven A1 - Herbrich, Uwe A1 - Keller, Christian T1 - Introduction of a Power Law Time-Temperature Equivalent Formulation for the Description of Thermorheologically Simple and Complex Behavior N2 - Abstract: In this work, a conceptual framework is suggested for analyzing thermorheologically simple and complex behavior by using just one approach. Therefore, the linear relation between master time and real time which is required in terms of the time-temperature superposition principle was enhanced to a nonlinear equivalent relation. Furthermore, we evaluate whether there is any relation among well-known existing time-temperature equivalent formulations which makes it possible to generalize different existing formulations. For this purpose, as an example, the power law formulation was used for the definition of the master time. The method introduced here also contributes a further framework for a unification of established time-temperature equivalent formulations, for example the time-temperature superposition principle and time-temperature parameter models. Results show, with additional normalization conditions, most of the developed time-temperature parameter models can be treated as special cases of the new formulation. In the aspect of the arrow of time, the new defined master time is a bended arrow of time, which can help to understand the corresponding physical meaning of the suggested method. KW - bended arrow of time KW - time-temperature superposition principle KW - time-temperature equivalent formulation PY - 2022 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-543800 DO - https://doi.org/10.3390/ma15030726 VL - 15 IS - 3 SP - 1 EP - 11 PB - MDPI AN - OPUS4-54380 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -