Untersuchungen zum Ermüdungsverhalten von hochfesten Stählen im Rahmen von LIFTHIGH

Investigations on the fatigue behaviour of high‐strength steels in the scope of LIFTHIGH. In the research project P 512 of FOSTA, different kinds of longitudinal attachments and five variants of butt welded plates made of high strength steel have been already investigated to enlarge the available recommendations. The obtained results provide a basis for further going investigations within the European ECSC‐project LIFTHIGH – “Efficient Lifting Equipment With Extra High Strength Steel”. In this project longitudinal attachments made of fine grain high strength steel with post weld treatment such as TIG‐dressed welds and ground welds have been investigated. For a more precise evaluation of the different variants of butt welded plates made of S 960 QL, these details have been also produced and tested made of S 690 M an S 1100 QL. In addition transverse attachments have been incorporated in the experimental programme.     

Neue Untersuchungen zum Ermüdungsverhalten von Verbundträgern aus hochfesten Werkstoffen mit Kopfbolzendübeln und Puzzleleiste

New investigations on the fatigue behavior of composite beams made of high strength materials with two different kinds of shear connection. Scope of the research project [1] was the investigation of the fatigue behaviour of different shear connectors (headed studs ∅︁ 22 mm and puzzle‐strip) together with high strength materials (steel S 460 and concrete C 80/95). For both types of shear connectors cyclic push‐out‐tests as well as large scale experiments with composite beams under cyclic loading have been carried out. The headed studs failed after a mean lifetime of 2,3 million cycles, the residual strength after 1 million cycles was reduced by about 25% compared to the static push‐out strength. In the puzzle‐strip cracks were visible, however, the residual strength was not reduced at all. Further, the push‐out tests with headed studs show a slight improvement of the fatigue behaviour using high strongth concrete (HSC). The cyclic beam tests show that the prediction of the fatigue according to EC 4 – as for specimens with normal strength materials – is not satisfactory. A good prediction of the crack propagation (and fatigue) can rather be achieved by using a crack‐slip‐relationship [6]. Also for shear joints with continuous puzzle‐strips a threshold value can be determined by fracture mechanics to which no further crack propagation (theoretically) occurs. However, this depends on the state of the cutting‐induced heat treatment of the surface material and on the surface roughness.     

Zum Ermüdungsverhalten von X‐Knoten aus quadratischen Hohlprofilen

Fatigue behaviour of X‐joints made of rectangular hollow sections. In 1984 Mang and Bucak ([8], [9]) presented their first detailed proposal for FAT‐classes for welded X‐sections made of rectangular hollow sections (RHS) to the committee of regulations for the DIN EN 1993‐1‐9 [1]. The proposal was based on experimental investigations of welded X‐joints of RHS with different width (b/b_o) and thickness (t_o/t) ratios, but were not accepted because the scope of the Eurocode 3 regulation should not be enlarged at that time. There are no FAT‐classes for welded X‐sections made of RHS in the current version of DIN EN 1993‐1‐9 [1], the IIW document [2] and the FKM guideline [4] for a nominal stress approach. There are specifications in the crane construction code DIN EN 13001‐3‐1 [3] but they are very conservative. CIDECT Design Guide No. 8 [12] also does not contain information referring this issue based on the nominal stress concept, but it gives detailed specifications like assessment formulas and graphs for the determination of the stresses (SCF) and strains (SCNF) for welded rectangular and hollow sections on the base of the structural stress concept. At the university of applied sciences Munich and the Labor für Stahl und Leichtmetallbau GmbH, Kissing, further investigations in the research project [13] were executed upon request of several industrial partners with focus on the fatigue behaviour of welded X‐sections made of RHS. Based on the results, a verification and extension of the code proposal of Mang and Bucak ([8], [9]) has been prepared. The main focus of this new investigation was the fatigue behaviour of bigger thickness ratios (t_o/t ≥ 2,0) as well as actual tube grades and welding procedures with high process reliability. This article summarizes the results and the #new code proposal.     

Erhöhung der Ermüdungsfestigkeit von geschweißten höherfesten Baustählen durch Anwendung von Nachbehandlungsverfahren

Enhancement of the fatigue strength of welded high strength steels by application of post‐weld treatment methods. According to the present state of the art welded high strength steels have the same fatigue strength as welded standard steels. For an effective application of high strength steels in constructions subjected to fatigue are therefore additional efforts necessary in order to improve the fatigue strength, for example by the application of post‐weld treatment methods. However up to the present it is not possible to apply the positive effects of these methods in the fatigue design of steel structures. This paper shows first results of a research project, that examines the effectiveness of the post‐weld treatment method TIG‐dressing and the relatively new method “Ultrasonic Impact Treatment” (UIT) for the improvement of fatigue strength of welded high strength steels.     

Neue Untersuchungen zum Ermüdungsverhalten von Kopfbolzendübeln

New investigations on the fatigue behaviour of headed shear studs. In current national and international standards the determination of the ultimate load capacity and the fatigue life of headed shear studs subjected to high‐cycle loading take place with separate and independent verifications at the ultimate limit state, serviceability limit state and fatigue limit state, respectively. The effect of predamage due to fatigue loading is not considered. The paper deals with new research based on a comprehensive program of experimental investigations which considers the crack propagation through the stud foot and the local damage of concrete surrounding the stud as relevant consequences of high cycle loading. New design methods to predict the fatigue life and the residual strength after high cycle loading as well as the development of an improved damage accumulation hypothesis are the main subjects of this paper.     

Untersuchungen zum Ermüdungsverhalten von Konstruktionsteilen für Mobilkrane

Fatigue behaviour of component sections of mobile cranes out of high strength steel plates. High‐strength structural steels with 960 and/or 1100 MPa minimum yield strength allow a decrease of the investment and operating costs of mobile cranes through lightweight design. However, there still exists an uncertainity for stress resistance under cyclic loads and bearable stresses concerning fatigue behaviour of important crane components. This paper deals for the first time with the fatigue behaviour of crane components examined under realistic load‐time‐cycles derived from realistic crane operating conditions. The results supply important information on the service life. The service life depends in this case strongly from the tension level considerably. The influence of the here applied load‐time‐cycles appears on the other hand small. Additionally, this paper provides a new tool with which the fatigue behaviour of crane components under most different aspects and involved variable combinations can be estimated, consistent with measured values, through a fracture mechanically based model. There can be made important procedure recommendations and hints can be given for a more intense use of high‐strength structural steels for practical application in mobile crane engineering. Inspection intervals and contents can be newly determined. The usage of steels with more than 1100 MPa minimum yield strength in the mobile crane building is evaluated crucially.     

Ermüdungsverhalten geschweißter Rohrknoten von Fachwerkverbundbrücken

Im Verbundbrückenbau werden in letzter Zeit anstatt vollwandiger Ausführungsvarianten vermehrt aufgelöste Fachwerkkonstruktionen aus Stahlhohlprofilen eingesetzt, da diese sowohl in Bezug auf gestalterische Grundsätze als auch hinsichtlich konstruktiver Gesichtspunkte Vorteile versprechen. Hierbei wird insbesondere bei Brückenentwürfen, deren Ausgestaltung eine Vielzahl unterschiedlicher Knotengeometrien erfordert, aus wirtschaftlichen Gründen die Verbindung zwischen Füllstab und Gurtrohr bevorzugt mit direkt geschweißten Knoten anstatt mit Stahlgussknoten ausgeführt. Aufgrund der durch die Knotengeometrie bedingten Spannungskonzentration im Bereich der Schweißnahtkerbe wird der Brückenentwurf bei dieser Ausführungsvariante maßgeblich von der dort vorherrschenden Ermüdungsfestigkeit beeinflusst. Nachdem die in den einschlägigen Normen bereitgestellten Nachweisverfahren gegen Ermüdung für die üblichen im Brückenbau verwendeten Abmessungen nicht anwendbar sind, ist die versuchstechnische Untersuchung zur Absicherung eines ausreichenden Ermüdungswiderstandes bisher unabdingbar. Dieser Beitrag bezieht sich auf insgesamt sieben Versuchskörper, die im Rahmen des Neubaus der Autobahnüberführung Suhl–Lichtenfels auf Ermüdung geprüft wurden. Fatigue behaviour of welded pipe intersections in steel‐concrete composite bridges. In the construction of modern steel‐concrete composite bridges, the steel part is increasingly built by circular hollow sections instead of steel plates. This type of bridge design shows advantageous characteristics concerning architectural aspects as well as benefits in the static load transfer. Especially if the design of the truss girder requires a large number of different node geometries, the connection between the braces and the lower and upper chord are for economic reasons preferably build by directly welded nodes instead of cast steel nodes. Due to the local joint discontinuity of the pipe intersections, the stress distribution is enhanced exactly in the area of the welding seams. As a consequence, the limited fatigue behaviour of the pipe intersections represents the critical aspect in the design of tubular truss‐girder bridges. The difficulties in the design of this construction type are due to the fact that the criterions for fatigue verification given in the current codes do not cover the typical dimensions used in bridge design. Therefore until now it is unavoidable to conduct large‐scale tests in order to verify a sufficient fatigue resistance. This article refers to a total of seven specimens tested on fatigue during the construction of the new Highway Overpass Suhl– Lichtenfels.     

Einfluss der Wanddicke auf das Ermüdungsverhalten von geschweißten X‐Knoten aus Rundhohlprofilen

Influence of wall thickness on the fatigue behaviour of welded X‐joints of circular hollow sections. The current version of EN 1993‐1‐9 stipulates different notch classes for the design of fatigue loaded constructions based on the nominal stress concept. Annex B of EN 1993‐1‐9 also outlines the design referring to the structure stress concept, but includes no specifications for the evaluation of the structural stresses (e. g. extrapolation path) nor detailed classification for specific fatigue loaded hollow sections. More extensive specifications for the application of the structure stress concept with detailed calculation formulas graphs and extrapolation points are given in CIDECT‐Design Guide No. 8, for example. For the design of such welded steel constructions for fatigue, there are different methods, that can be applied. In order to handle the size effect, different recommendations can be found in literature to reduce the predicted fatigue life of plates and tubes with increased thickness. For example in the current DIN EN 1993‐1‐9 wall thickness correction factor for plates exceeding t > t₀ = 25mm is given, i. e.: S_R = S_R,D (t_o/t)^0,25. For welded hollow sections, DIN EN 1993‐1‐9 limits the range of validity to wall thicknesses t ≤ 8mm and 12,5mm respectively and the diameters of circular hollow sections to d ≤ 300mm and to edge length of rectangular hollow section b ≤ 200mm. In the scope of the FOSTA research project P 801, the Labor für Stahl‐ und Leichtmetallbau of the University of Applied Sciences Munich, conducted investigations on different notch cases of welded hollow sections with the outcome of assessing the influence parameters of welded hollow sections.     

Einsatz von hochfesten Stählen und Betonen bei Hohlprofil‐Verbundstützen

Composite columns made of high‐strength steel and high‐strength concrete. The paper deals with the use of composite columns made of high‐strength steel and high‐strength concrete. Based on the design methods in Eurocode 4‐1‐1 and DIN 18800‐5 a simplified design method is presented for composite columns with high strength concrete and high strength steel in combination with massive inner steel core‐profiles. The ultimate strength of these special type of columns is significantly influenced by the residual stresses in the core profiles resulting from the cooling process during fabrication. Based on Finite Element studies new models are presented for the determination of the distribution of residual stress taking into account the steel grade, the dimensions of the core profiles and the cooling conditions during fabrication.