Thomas Ummenhofer - Profile on Academia.edu (original) (raw)
Papers by Thomas Ummenhofer
Materials research proceedings, Sep 11, 2018
Previous and new simulations of welding residual stresses with the finite element method are revi... more Previous and new simulations of welding residual stresses with the finite element method are reviewed in the present study. The influence of modelling mechanical boundary conditions, erroneous prediction of the weld heat source coefficient and the influence of microstructural changes in aluminum welds are investigated. The results are analyzed so that concrete suggestions regarding the investigated factors, acting as guidance to the practitioner, can be presented.
MATEC web of conferences, 2019
Tensile fatigue specimen of G20Mn5 and G22NiMoCr5-6 were tested to quantify the influence of inte... more Tensile fatigue specimen of G20Mn5 and G22NiMoCr5-6 were tested to quantify the influence of internal defects on the fatigue resistance of cast steel components. Defects with varying sizes, geometric shapes and distribution were enforced by influencing the solidification and recorded by computer tomography (CT). Besides the characteristics of the detected cavities, the surrounding fungoid microstructure is classified and evaluated. Later the specimens were tested under cyclic tension and S/N-curves are derived. These data form the basis for extensive numerical simulations of the damage process and the crack growth of every individual specimen. Both processes are affected by the local multiaxial stress states and have their origin in the inside of the specimen. For validation, knowledge of the crack initiation time and propagation properties are essential. Therefore, all specimens respectively the properties of the internal defects are monitored during testing with three different state-of-the-art non-destructive testing (NDT) methods. Background and application of these NDT techniques are described within this paper. Finally, fracture surface analyses show different failure modes and provide further information for model validation.
In seismic design a variety of methods can be used in order to assess the response of the structu... more In seismic design a variety of methods can be used in order to assess the response of the structure. This methods are discussed with respect to their impact of the accuracy of the results and the possibility to verify the findings. When using harmonic base excitation usually the natural frequency of the structure is used as driving frequency. This might lead to unsafe results in case when damping or nonlinearities such as plastic actions of the structure are present. This paper focuses on a study on the appropriate driving frequency.
For the development of a new hybrid construction method combining the materials steel and timber ... more For the development of a new hybrid construction method combining the materials steel and timber for linear load-carrying elements subjected to bending loads, different joining methods were investigated. The joining methods were 1) dowel-type fasteners, partly in combination with higher friction in the shear plane and 2) continuous joints, mainly by bonding with adhesives. Tests were carried out to investigate the maximum load-carrying capacity as well as the stiffness of the joints, aiming to transfer the results to the linear load-carrying elements. Where applicable, nominal shear stresses were evaluated. The tests with dowel-type fasteners were mainly carried out to create reference values to compare to the other joining methods. The results with the modified surfaces to increase friction in the shear plane were quite promising regarding the initial stiffness, however, further research of the surface modification has to be carried out. The specimens with continuous joints can be regarded analytically as fully bonded. Bonded specimens with additional bolts combine high stiffness with a residual load-carrying capacity after shear failure of the timber of 30-40% of the maximum load.
Early Detection of Fatigue Damage in Notched and Welded Steel Structures Using Active Thermography
Structural Health Monitoring-an International Journal, 2011
Kleben im Bauwesen - Innovationen durch Technologietransfer?
Geklebte Stahl-Stahlguss-Verbindungen am Beispiel von Fachwerkknoten von Rundhohlprofilkonstruktionen
Schweißsimulation von „Die-Less-Hydroforming“-Platinen mit DynaWeld und LS-DYNA
Geklebte Stahl-Stahlguss-Verbindungen am Beispiel von Fachwerkknoten von Rundhohlprofil-Konstruktionen : IGF-Nr. 17199 N = Adhesive bonded tubular cast steel-steel joints in framework strucktures
Adhesive bonded tubular cast steel - steel joints in steel construction
Bauingenieur, 2013
Sustainability assessment of bridges - Relevance of External Costs
Zum Kleben von CFK-Lamellen im Stahlbau
ABSTRACT Während ihrer Nutzung unterliegen Stahlkonstruktionen oft unterschiedlichen, sich ändern... more ABSTRACT Während ihrer Nutzung unterliegen Stahlkonstruktionen oft unterschiedlichen, sich ändernden Anforderungen. Dies betrifft sowohl Konstruktionen im Bauwesen, wie Hoch- oder Brückenbauten, als auch Konstruktionen des Maschinenbaus, wie Krane und andere dynamisch belastete Bauteile. Beispiele für eine geänderte Anforderung sind eine erhöhte Belastung von Brücken infolge eines gestiegenen Verkehrsaufkommens oder eine aus wirtschaftlichen Erwägungen über die geplante Lebensdauer hinaus anstehende Nutzung von Maschinenbauteilen (Krankonstruktionen) oder Brückenbauwerken. Infolge der Anforderungs- oder Nutzungsänderungen ist ohne konstruktive Verstärkungsmaßnahmen in vielen Fällen keine ausreichende Tragfähigkeit und/oder Performance im Grenzzustand der Gebrauchstauglichkeit mehr gewährleistet. Bisher übliche Maßnahmen zur konstruktiven Verstärkung erfolgen zum einen durch den Ausbau einzelner Teile einer Konstruktion und den Ersatz durch tragfähigere Bauteile, zum anderen durch Verstärkung einzelner Bauteile, z.B. durch Anschweißen oder Anschrauben von Stahllaschen, oder durch Modifikation des statischen Systems z.B. durch den Einbau von (Hilfs-) Stützen bei Brückenbauwerken. Eine weitere Möglichkeit bietet die nachträgliche Verstärkung von Stahlkonstruktionen durch aufgeklebte und ggf. vorgespannte [1] CFK-Lamellen. Diese Verstärkungsmaßnahme weist im Vergleich zu den zuvor genannten Methoden insbesondere Vorteile hinsichtlich des Ermüdungsverhaltens (verringerte Kerbwirkung), der Fertigung (bessere Handhabbarkeit), des geringeren Gewichtes und der Korrosionsbeständigkeit auf.
Advances in Structural Engineering, Dec 1, 2018
Investigation of influence parameters for crack formation on hot-dip metal coated steel
ce/papers, Sep 1, 2017
New design methodologies are being developed that permit the full plastic condition and strain ha... more New design methodologies are being developed that permit the full plastic condition and strain hardening properties of steel members and structures to be exploited. However, it has long been recognised that even the full plastic moment of a cross-section cannot be attained assuming only an ideal elastic-plastic material law, since infinite curvatures would be required to do so. Further, tests on structural members routinely show that, due to strain hardening, the full plastic condition is exceeded at finite slendernesses where one might expect stability to affect the result. This indicates that strain hardening should be a key consideration in design. Unfortunately, the strain hardening properties of carbon steels are not guaranteed by any standard or steel manufacturer, and no reliable depository of information is known to exist. Similarly, the proliferation of powerful modern finite element software that can handle significant geometric and material complexities is at odds with the paucity of information available to the analyst on the reliable choice of post-yield material properties for carbon steels. This paper presents a brief overview of the results presented in recent publications by the authors that attempt to tackle this problem. The intention is to alert the structural engineering community to the wealth of information contained in a typical tensile stress-strain curve relationship beyond that which is commonly reported, and to show the tentative progress that has been made in characterising the post-yield properties of some of the most common structural steels. A specialised algebraic formulation is used to process a database of 225 stress-strain curves of various grades, allowing an accurate assessment of their post-yield material properties. These include the length and gradient of the yield plateau, the initial tangent strain hardening modulus of the post-plateau region, and preliminary statistical bounds for these for the most common grades. The paper additionally flags the novel notion that the yield plateau itself exhibits a small but finite and statistically significant positive gradient, which is potentially important in structural resistance calculations.
ce/papers, Sep 1, 2017
Steel poles of polygonal sections are an economic alternative to circular hollow sections for use... more Steel poles of polygonal sections are an economic alternative to circular hollow sections for use as lighting towers or transmission line pylons. Especially for transmission lines, the design of the poles is intended to become more compact to avoid land usage and to obtain an inconspicuous appearance. If the pole diameter is reduced, the slenderness of the sections needs to be reduced as well to achieve the desired resistance, but an increasing wall thickness disproportionately reduces the cost-effectiveness of the structure. The European design rules for overhead electrical lines [1] do not allow using the plastic behaviour of stocky sections, which reduces the advantages of compact sections. In modern standards for steel structures, such as Eurocode 3 (EC3) [2], activating plastic reserves is generally acknowledged state of the art. According to EC3-1-1, the slenderness of the profile decides on the allocation to cross section class 1 or 2, where plastic design is allowed, class 3, where only elastic behaviour is used or class 4, where local buckling is assumed. When changing from class 2 to class 3 sections, this standard provides a sudden drop in resistance, which has long been recognised to be unjustified. Research has been done to overcome this mismatch for circular hollow sections . In this paper the transition between the plastic bending moment capacity and the limit state of buckling is investigated to identify the ultimate load of polygonal sections. The investigated polygon of 16 edges can be treated as a collection of plate strips using the plate buckling design rules EC3-1-5 for cross sections class 4. Detailed examinations illustrate the inconsistency of EC3 in the bending capacity of compact sections right before the start of stability problems. On the basis of experimental data gained from full-scale bending tests, along with imperfection measurements via laser scanning, numerical investigations have been performed. These results suggest that the bending moment capacity can be increased above the calculated elastic capacity with more compact sections, but might be overestimated when calculating the full plastic capacity according to EC3-1-1.
Technical, ecological and economical assessment of welded joints for offshore wind energy plants
CRC Press eBooks, Nov 18, 2016
Wind energy plants play a decisive role for renewal energy production. Especially offshore constr... more Wind energy plants play a decisive role for renewal energy production. Especially offshore constructions imply an engineering challenge during the design and erection process. Therefore, in this paper aspects describing the technical performance are combined with ecologic and economic assessments. For the foundation structure tubes with diameters from 219 mm up to 914 mm and three different welding methods are compared with regard to their ecologic and economic impacts. The investigations show that a precise modelling of the single production steps is necessary to generate meaningful results. For smaller diameters especially the weld seam preparation is important for the overall results, for larger diameters mainly the welding process of filler layers cause the highest environmental and economic impacts. Since existing regulations do not allow for a consistent specification of the fatigue behavior of the offshore relevant tube diameters, the Wohler curves presented help to design wind energy foundation structures.
Sustainability analyses and assessments of steel bridges
Creep-behaviour of welded structures
Behavior of UDM processed epoxy based TiO2 nano filler composite adhesive joints under fatigue loading
Materials research express, Jul 3, 2019
Materials research proceedings, Sep 11, 2018
Previous and new simulations of welding residual stresses with the finite element method are revi... more Previous and new simulations of welding residual stresses with the finite element method are reviewed in the present study. The influence of modelling mechanical boundary conditions, erroneous prediction of the weld heat source coefficient and the influence of microstructural changes in aluminum welds are investigated. The results are analyzed so that concrete suggestions regarding the investigated factors, acting as guidance to the practitioner, can be presented.
MATEC web of conferences, 2019
Tensile fatigue specimen of G20Mn5 and G22NiMoCr5-6 were tested to quantify the influence of inte... more Tensile fatigue specimen of G20Mn5 and G22NiMoCr5-6 were tested to quantify the influence of internal defects on the fatigue resistance of cast steel components. Defects with varying sizes, geometric shapes and distribution were enforced by influencing the solidification and recorded by computer tomography (CT). Besides the characteristics of the detected cavities, the surrounding fungoid microstructure is classified and evaluated. Later the specimens were tested under cyclic tension and S/N-curves are derived. These data form the basis for extensive numerical simulations of the damage process and the crack growth of every individual specimen. Both processes are affected by the local multiaxial stress states and have their origin in the inside of the specimen. For validation, knowledge of the crack initiation time and propagation properties are essential. Therefore, all specimens respectively the properties of the internal defects are monitored during testing with three different state-of-the-art non-destructive testing (NDT) methods. Background and application of these NDT techniques are described within this paper. Finally, fracture surface analyses show different failure modes and provide further information for model validation.
In seismic design a variety of methods can be used in order to assess the response of the structu... more In seismic design a variety of methods can be used in order to assess the response of the structure. This methods are discussed with respect to their impact of the accuracy of the results and the possibility to verify the findings. When using harmonic base excitation usually the natural frequency of the structure is used as driving frequency. This might lead to unsafe results in case when damping or nonlinearities such as plastic actions of the structure are present. This paper focuses on a study on the appropriate driving frequency.
For the development of a new hybrid construction method combining the materials steel and timber ... more For the development of a new hybrid construction method combining the materials steel and timber for linear load-carrying elements subjected to bending loads, different joining methods were investigated. The joining methods were 1) dowel-type fasteners, partly in combination with higher friction in the shear plane and 2) continuous joints, mainly by bonding with adhesives. Tests were carried out to investigate the maximum load-carrying capacity as well as the stiffness of the joints, aiming to transfer the results to the linear load-carrying elements. Where applicable, nominal shear stresses were evaluated. The tests with dowel-type fasteners were mainly carried out to create reference values to compare to the other joining methods. The results with the modified surfaces to increase friction in the shear plane were quite promising regarding the initial stiffness, however, further research of the surface modification has to be carried out. The specimens with continuous joints can be regarded analytically as fully bonded. Bonded specimens with additional bolts combine high stiffness with a residual load-carrying capacity after shear failure of the timber of 30-40% of the maximum load.
Early Detection of Fatigue Damage in Notched and Welded Steel Structures Using Active Thermography
Structural Health Monitoring-an International Journal, 2011
Kleben im Bauwesen - Innovationen durch Technologietransfer?
Geklebte Stahl-Stahlguss-Verbindungen am Beispiel von Fachwerkknoten von Rundhohlprofilkonstruktionen
Schweißsimulation von „Die-Less-Hydroforming“-Platinen mit DynaWeld und LS-DYNA
Geklebte Stahl-Stahlguss-Verbindungen am Beispiel von Fachwerkknoten von Rundhohlprofil-Konstruktionen : IGF-Nr. 17199 N = Adhesive bonded tubular cast steel-steel joints in framework strucktures
Adhesive bonded tubular cast steel - steel joints in steel construction
Bauingenieur, 2013
Sustainability assessment of bridges - Relevance of External Costs
Zum Kleben von CFK-Lamellen im Stahlbau
ABSTRACT Während ihrer Nutzung unterliegen Stahlkonstruktionen oft unterschiedlichen, sich ändern... more ABSTRACT Während ihrer Nutzung unterliegen Stahlkonstruktionen oft unterschiedlichen, sich ändernden Anforderungen. Dies betrifft sowohl Konstruktionen im Bauwesen, wie Hoch- oder Brückenbauten, als auch Konstruktionen des Maschinenbaus, wie Krane und andere dynamisch belastete Bauteile. Beispiele für eine geänderte Anforderung sind eine erhöhte Belastung von Brücken infolge eines gestiegenen Verkehrsaufkommens oder eine aus wirtschaftlichen Erwägungen über die geplante Lebensdauer hinaus anstehende Nutzung von Maschinenbauteilen (Krankonstruktionen) oder Brückenbauwerken. Infolge der Anforderungs- oder Nutzungsänderungen ist ohne konstruktive Verstärkungsmaßnahmen in vielen Fällen keine ausreichende Tragfähigkeit und/oder Performance im Grenzzustand der Gebrauchstauglichkeit mehr gewährleistet. Bisher übliche Maßnahmen zur konstruktiven Verstärkung erfolgen zum einen durch den Ausbau einzelner Teile einer Konstruktion und den Ersatz durch tragfähigere Bauteile, zum anderen durch Verstärkung einzelner Bauteile, z.B. durch Anschweißen oder Anschrauben von Stahllaschen, oder durch Modifikation des statischen Systems z.B. durch den Einbau von (Hilfs-) Stützen bei Brückenbauwerken. Eine weitere Möglichkeit bietet die nachträgliche Verstärkung von Stahlkonstruktionen durch aufgeklebte und ggf. vorgespannte [1] CFK-Lamellen. Diese Verstärkungsmaßnahme weist im Vergleich zu den zuvor genannten Methoden insbesondere Vorteile hinsichtlich des Ermüdungsverhaltens (verringerte Kerbwirkung), der Fertigung (bessere Handhabbarkeit), des geringeren Gewichtes und der Korrosionsbeständigkeit auf.
Advances in Structural Engineering, Dec 1, 2018
Investigation of influence parameters for crack formation on hot-dip metal coated steel
ce/papers, Sep 1, 2017
New design methodologies are being developed that permit the full plastic condition and strain ha... more New design methodologies are being developed that permit the full plastic condition and strain hardening properties of steel members and structures to be exploited. However, it has long been recognised that even the full plastic moment of a cross-section cannot be attained assuming only an ideal elastic-plastic material law, since infinite curvatures would be required to do so. Further, tests on structural members routinely show that, due to strain hardening, the full plastic condition is exceeded at finite slendernesses where one might expect stability to affect the result. This indicates that strain hardening should be a key consideration in design. Unfortunately, the strain hardening properties of carbon steels are not guaranteed by any standard or steel manufacturer, and no reliable depository of information is known to exist. Similarly, the proliferation of powerful modern finite element software that can handle significant geometric and material complexities is at odds with the paucity of information available to the analyst on the reliable choice of post-yield material properties for carbon steels. This paper presents a brief overview of the results presented in recent publications by the authors that attempt to tackle this problem. The intention is to alert the structural engineering community to the wealth of information contained in a typical tensile stress-strain curve relationship beyond that which is commonly reported, and to show the tentative progress that has been made in characterising the post-yield properties of some of the most common structural steels. A specialised algebraic formulation is used to process a database of 225 stress-strain curves of various grades, allowing an accurate assessment of their post-yield material properties. These include the length and gradient of the yield plateau, the initial tangent strain hardening modulus of the post-plateau region, and preliminary statistical bounds for these for the most common grades. The paper additionally flags the novel notion that the yield plateau itself exhibits a small but finite and statistically significant positive gradient, which is potentially important in structural resistance calculations.
ce/papers, Sep 1, 2017
Steel poles of polygonal sections are an economic alternative to circular hollow sections for use... more Steel poles of polygonal sections are an economic alternative to circular hollow sections for use as lighting towers or transmission line pylons. Especially for transmission lines, the design of the poles is intended to become more compact to avoid land usage and to obtain an inconspicuous appearance. If the pole diameter is reduced, the slenderness of the sections needs to be reduced as well to achieve the desired resistance, but an increasing wall thickness disproportionately reduces the cost-effectiveness of the structure. The European design rules for overhead electrical lines [1] do not allow using the plastic behaviour of stocky sections, which reduces the advantages of compact sections. In modern standards for steel structures, such as Eurocode 3 (EC3) [2], activating plastic reserves is generally acknowledged state of the art. According to EC3-1-1, the slenderness of the profile decides on the allocation to cross section class 1 or 2, where plastic design is allowed, class 3, where only elastic behaviour is used or class 4, where local buckling is assumed. When changing from class 2 to class 3 sections, this standard provides a sudden drop in resistance, which has long been recognised to be unjustified. Research has been done to overcome this mismatch for circular hollow sections . In this paper the transition between the plastic bending moment capacity and the limit state of buckling is investigated to identify the ultimate load of polygonal sections. The investigated polygon of 16 edges can be treated as a collection of plate strips using the plate buckling design rules EC3-1-5 for cross sections class 4. Detailed examinations illustrate the inconsistency of EC3 in the bending capacity of compact sections right before the start of stability problems. On the basis of experimental data gained from full-scale bending tests, along with imperfection measurements via laser scanning, numerical investigations have been performed. These results suggest that the bending moment capacity can be increased above the calculated elastic capacity with more compact sections, but might be overestimated when calculating the full plastic capacity according to EC3-1-1.
Technical, ecological and economical assessment of welded joints for offshore wind energy plants
CRC Press eBooks, Nov 18, 2016
Wind energy plants play a decisive role for renewal energy production. Especially offshore constr... more Wind energy plants play a decisive role for renewal energy production. Especially offshore constructions imply an engineering challenge during the design and erection process. Therefore, in this paper aspects describing the technical performance are combined with ecologic and economic assessments. For the foundation structure tubes with diameters from 219 mm up to 914 mm and three different welding methods are compared with regard to their ecologic and economic impacts. The investigations show that a precise modelling of the single production steps is necessary to generate meaningful results. For smaller diameters especially the weld seam preparation is important for the overall results, for larger diameters mainly the welding process of filler layers cause the highest environmental and economic impacts. Since existing regulations do not allow for a consistent specification of the fatigue behavior of the offshore relevant tube diameters, the Wohler curves presented help to design wind energy foundation structures.
Sustainability analyses and assessments of steel bridges
Creep-behaviour of welded structures
Behavior of UDM processed epoxy based TiO2 nano filler composite adhesive joints under fatigue loading
Materials research express, Jul 3, 2019