Verbesserung der Ermüdungsfestigkeit höherfester Baustähle durch Anwendung der UIT‐Nachbehandlung

Enhancement of the fatigue strength of welded high strength steels by application of Ultrasonic Impact Treatment (UIT). 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. Within this paper an experimental test program studying the effectiveness of the post‐weld treatment method “Ultrasonic Impact Treatment” (UIT) on the critical construction detail of the transverse stiffener is presented. The test program comprises small scale fatigue tests on the standard and high strength steel grades as well as full size girder fatigue tests. Based on experimental and numerical investigations recommendations for the fatigue design of welded structures treated by UIT are given.     

Reduktion der Wanddicken bei Tragtürmen von Windenergieanlagen durch den Einsatz höherfester Stähle

Reduction of plate thickness of wind energy converters made by high strength steels. The design of wind energy converters is significantly defined by the fatigue strength of critical notch details, especially welded joints. The usage of high strength steels leads to no economical benefit because the fatigue strength of welded notch details are defined in design rules independently from the yield strength. The application of post weld treatment methods may increase the fatigue strength of these details significantly. Investigations on high frequency peening methods prove that a doubling of the fatigue strength can be achieved for steel grade S 690. An example of a connection detail is presented where it can be shown that by the application of the investigated methods together with a steel S 690 Q the plate thickness can be reduced to 45%.     

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.     

Detaillösungen für ermüdungsbeanspruchte Tripod‐Gründungsstrukturen

Detail Solutions for Tripod Foundation Structures under Fatigue Loading. In this article the results of the research project P633 – “Detail solutions regarding fatigue and the use of high‐strength steels for towers of offshore wind energy converters” sponsored by FOSTA is presented. In addition to general conclusions on the increase of fatigue strength by using high strength steel combined with post weld treatment, the focus is on the optimisation of the design of a tripod joint.     

Experimentelle und numerische Untersuchungen von Knotenverbindungen für Tripods von Offshore‐Windenergieanlagen

Experimental und numerical investigations for tripod joints of offshore‐structures. This paper summarises fatigue tests on Y‐joints to estimate the influence of a post weld treatment method called Ultrasonic Impact Treatment (UIT). With this method the fatigue resistance could be increased significantly. Furthermore tubular joints of tripods are analysed with numerical simulations to judge these welded joints with the hot‐spot‐concept. The stress concentration factor for the treated weld toe geometry was determined numerically using submodel analysis. Finally a comparison between welded and cast iron joints is carried out for the fatigue limit state under consideration of experimental results.     

Nachweiskonzepte zur Bemessung ermüdungsbeanspruchter Bauteile unter Berücksichtigung von Schweißnahtnachbehandlungsverfahren durch höherfrequentes Hämmern

Die Anwendung von höherfrequenten Hämmerverfahren zur Erhöhung der Ermüdungsfestigkeit von dynamisch beanspruchten Bauteilen gewinnt zunehmend an Bedeutung. Dies hängt zum einen damit zusammen, dass bei Neukonstruktionen durch den Einsatz von höherfesten Stählen die Frage der Materialermüdung verstärkt die Bauteilauslegung bestimmt, und zum anderen, dass bei vielen bestehenden Bauwerken vermehrt Maßnahmen zur Verlängerung der Restlebensdauer erforderlich werden. Die Anwendung von Schweißnahtnachbehandlungsverfahren in Form von höherfrequentem Hämmern stellt hierfür eine geeignete Maßnahme dar, die Ermüdungsfestigkeit von Neukonstruktionen zu erhöhen bzw. die Restlebensdauer bestehender Anlagen zu verlängern. Eine zuverlässige baupraktische Anwendung setzt jedoch die Existenz anerkannter Regeln für die Bemessung voraus, wie auch eine darauf abgestimmte Vorgehensweise für die Ausführung. Der vorliegende Beitrag widmet sich vor allem der Darstellung bisher existierender Nachweiskonzepte für die Bemessung ermüdungsbeanspruchter Bauteile bei Anwendung von Schweißnahtnachbehandlungsverfahren in Form von höherfrequentem Hämmern. Fatigue design concepts for post‐weld treated joints improved by high frequency hammer peening methods. Post‐weld improvement methods have been widely investigated and have in most cases been found to give substantial increases in the fatigue strength. Nowadays, there is an increasing interest in using such methods in order to either improve the fatigue resistance of new structures built e.g. by high strength steel and/or to enlarge the lifetime of existing structures suffering from fatigue problems. With regard to this, post‐weld improvement using high frequency hammer peening methods lately showed very good and promising results in combination with simple operation conditions. However, the successful application of post‐weld improvement methods always requires the existence of profound fatigue design concepts as well as relevant execution and quality standards. The following paper refers predominately to the first aspect, in trying to summarise and compare existing design concepts and their application range for high frequency hammer peening methods.     

Einsparpotentiale bei Tragkonstruktionen von Offshore‐Windenergieanlagen

Savings potential for support structures of offshore wind turbines. Due to the high ratio of dynamic loads from wind and wave forces on offshore wind turbines, the fatigue design analysis proves to be the determining factor for the dimensioning of support structures. As a result, the current design practice is limited, particu‐larly with regard to the economic benefits of employing high strength steels. Especially from the aspect of mass production, however, the economic optimization of the components is a sub‐stantial economic advantage for the companies. This paper presents different options for savings in the design and assessment of offshore‐foundation structures and provides the example of a transition piece for a jacket foundation structure. Savings potential is realized through optimal construction design, directionally dependent load determination, a refined methodology of analysis, as well as through modern methods of post weld treatment.     

Lebens‐ und Restlebensdauerverlängerung geschweißter Windenergieanlagentürme und anderer Stahlkonstruktionen durch Schweißnahtnachbehandlung

Life cycle extension of welded wind energy converters and other steel constructions by weld improvement techniques. The design of Wind Energy Converters and other dynamic loaded structures is significantly defined by the fatigue strength of critical notch details, especially welded joints. Weld improvement techniques seem to be promising to increase the fatigue strength and life time of those details. In context of presently conducted research activities by the “Institut für Bauwerkserhaltung und Tragwerk” at the Technical University Carolo‐Wilhelmina Braunschweig shot peening and ultrasonic peening are more detailed analysed. Both methods seem to produce by their combination of geometrical and mechanical effects a clear improvement of the fatigue strength of welded details. First results from fatigue tests confirm this for welded ring‐flange‐details. The ultrasonic peening was further more tested for the use for the rehabilitation of fatigue stressed existing structures. First results show that the use of the method for specimen which reached their calculated life time can improve the fatigue strength to the level of originally treated specimen.     

Schweißnahtnachbehandlung in der Praxis

Weld improvement technique in practice. Due to the post weld treatment the fatigue resistance of steel structures may be enhanced considerable. Within a large research project called REFRESH several partners from different industries examine the application of high frequency hammer peening methods. Beside the creation of classical SN‐curves research concentrates on quality control and design with respect to the high potential increase of fatigue resistance. The present results of the project are quite positive, so that all industrial partners use the post treatment methods HiFIT and UIT for fabrication within pilot projects in practice.     

Ermüdungsverhalten von Mobilkranbauteilen aus hochfesten Baustählen

Fatigue behaviour of component sections of mobile cranes out of high strength steel plates. High‐strength structural steels with 960 and 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 as they indicate that the service life for both steels, even with very high applied stress, is at least 20years. Moreover, it is shown that further improvements of the durability are possible around up to 50% through subsequent treatment of the welds, toughness improvement of the base material and constructive remodelling of the components. Additionally, this paper provides a new design method 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. 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.     

Versuche und Anwendung hochfrequenter Hämmerverfahren als Schweißnahtnachbehandlung

Die Lebensdauer von dynamisch belasteten Bauteilen aus Stahl stellt ein entscheidendes Qualitätsmerkmal von Produkten der Stahlbau Firmen dar. Der Einsatz höherfrequenter Hämmerverfahren, im Besonderen des UIT oder HiFIT‐Verfahrens, als lebensdauerverlängernde Maßnahme von ermüdungsbeanspruchten Schweißverbindungen wurde im Rahmen des REFRESH‐Forschungsprojekts erstmals wissenschaftlich untersucht. Einsatz und Wirksamkeit der Verfahren wurde von den am Projekt beteiligten Firmen an ausgeführten Bauteilen untersucht. Die Erkenntnisse aus Versuchen und praktischer Anwendung der Firmen Schachtbau Nordhausen und Maurer Söhne werden im folgenden Artikel geschildert. Tests and application of high frequency peening as a post weld treatment method. The fatigue life of dynamic loaded steel construction elements is an influential quality characteristic for products of steel fabricators. The application of high frequency peening, especially the UIT (ultrasonic impact treatment) or the HiFIT (high frequency impact treatment) as a method to extend the fatigue life of welded connections, has been investigated in course of the research project called REFRESH for the first time. The practical application and the effectiveness of high frequency peening at finished building components have been examined by building companies which are concerned to the research project. The present paper describes the tests and the practical application of the companies Schachtbau Nordhausen and Maurer Söhne.     

Numerische Analyse geschweißter Verbindungen von Duplexstahl und Quarzglas

Numerical analysis of welded joints between duplex steel and quartz glass. Welded joints are a vital element in structural engineering. Originating from conventional carbon steel welding in construction, recent advancements in welding technology now allow the joint of modern high‐strength steel and glass materials. With today's methods, an analysis of the welded joints' structural behaviour can be conducted by experiment, as well as by numerical analysis. Particularly for the numeric analysis, capturing the non‐linear thermal and mechanical properties of the materials is important, in order to allow a realistic determination of temperature, microstructure and residual stresses for different types of joints. Simulations of multi‐layered weld joint on duplex steel show, that a targeted heat treatment during MAG‐welding by variation of the welding parameters achieves a beneficial ratio between ferrite and austenite which, for example, ensures a high resistance of the weld to corrosion. The material quartz glass can generally be welded as butt‐weld with a CO₂‐laser. The simulations of a welded joint of a plate and a pipe show, that an optimization of the welding technology of preheat laser beam and welding laser beam is necessary, in order to reduce the thermal impact during the welding process, as well as residual stress in the joint. At the Department of Steel Structures at the Bauhaus‐Universität Weimar, numerical simulations of welded joints between steel and glass materials are a current and topical research focus.     

Alternativer Berechnungsansatz zum Hochfrequenzhämmern – Versuch und praktische Anwendung in der Windenergietechnik

Die lokal anwendbare Schweißnahtnachbehandlung bietet ein vielversprechendes Potential zur Lebensdauerverlängerung und wirtschaftlicheren Fertigung geschweißter Bauteile. In diesem Beitrag wird ein alternativer Berechnungsansatz auf bruchmechanischer Basis vorgestellt, der es erlaubt, wesentliche Effekte des Hochfrequenzhämmerns, die für die signifikante Verlängerung der Lebensdauer verantwortlich zeichnen, direkt in die Abschätzung der Lebensdauer mit einfließen zu lassen. Ferner werden die Ergebnisse eines Schwingfestigkeitsversuches an dickwandigen, durchgeschweißten Kreuzstößen mit einer Blechdicke von 60 mm präsentiert. Auch aufgrund der sehr positiven Erfahrungen aus der Anwendung des Hochfrequenzhämmerns an verschiedenen Bauteilen von Windenergieanlagen in der Praxis wird diesem Verfahren zukünftig eine hohe Bedeutung zukommen. An alternative calculation approach for high frequency peening – further validation and practical application in wind energy technology. A local application of post weld treatment shows a high potential in increasing fatigue life and more economic manufacturing of welded components. An alternative assessment based on fracture mechanics is presented which takes into account the main effects of high frequency peening. Furthermore results of a fatigue test on a cross‐joint K‐butt weld with 60?mm thickness are presented. Also because of positive experiences in field application of high frequency peening on different components in wind energy converters this kind of post weld treatment will be a very promising technique in future.     

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.     

Höherfrequente Hämmerverfahren – Ursachen der Steigerung der Ermüdungsfestigkeit und Nachweismodell

Der Einsatz von Schweißnahtnachbehandlungsverfahren zur Optimierung wechselnd beanspruchter Stahlkonstruktionen in Hinblick auf deren Lebensdauerverlängerung oder eine Reduzierung des Konstruktionsgewichts wurde in den letzten Jahren verstärkt untersucht. Dabei erwiesen sich höherfrequente Hämmermethoden als besonders wirkungsvoll zur Steigerung der Ermüdungsfestigkeit. Der vorliegende Artikel zeigt die Potentiale der Verfahren auf und belegt deren Einfluss auf das Rissinitiierungs‐ und Rissfortschrittsverhalten von Schweißnähten. Anhand eines entwickelten Bemessungsmodells wird eine Möglichkeit der rechnerischen Erfassung der positiven Wirksamkeit der Verfahren dargelegt. Reasons and calculation of the improved fatigue life of welds treated with high frequency peening methods. The application of post weld treatment methods in order to optimize fatigue loaded steel structures in view of an extension of their service life or a reduction of the construction weight has been studied intensively in the last years. High frequency peening methods proved to be extreme effectively in order to increase the fatigue strength. The following article shows the potentials of the methods and proves their influence on the crack initiation and crack propagation behaviour of welds. With help of a developed design procedure the possibility of respecting the positive effect on the fatigue strength during the design is provided.     

高强度钢材疲劳性能研究进展

疲劳破坏是钢结构失效的主要形式,也是工程界与学术界关注的重点。在总结了国内外高强钢母材、焊缝连接和螺栓连接的疲劳性能研究现状基础上,介绍了疲劳寿命理论计算方法,结合大量试验数据分析统计了高强度钢材疲劳寿命发展规律,并基于国内外不同的疲劳设计规范对高强钢母材及连接接头的抗疲劳能力进行了评估。结果表明：随着钢材强度等级的提高,高强钢母材表现出较好的疲劳性能,规范设计曲线偏于保守,明显低估了高强钢母材的抗疲劳性能;受焊接工艺和焊接质量影响,高强钢焊缝连接的疲劳强度提高幅度较小,ANSI/AISC 360-10和EN 1993-1-9规范曲线能够较好地评估Q460D与Q690D焊缝连接的疲劳特性,并具有足够的安全储备;螺栓连接的疲劳强度受预紧力、摩擦面处理、成孔方式等众多因素影响,已有研究表明随着钢材强度的提高,疲劳强度亦有改善,ANSI/AISC 360-10和BS 7608-2014设计曲线适用于Q460螺栓连接疲劳寿命计算,具有足够安全保障,对于Q690钢螺栓连接疲劳性能评估,规范方法偏于保守。随着高强度钢材在实际工程中的应用增多,需开展大量不同等级高强钢母材和连接形式的疲劳试验,补充Q460及以上强度钢材的疲劳设计方法和细部连接构造。     

Fatigue Strength and Root-Deck Crack Propagation for U-Rib to Deck Welded Joint in Steel Box Girder

Fatigue tests and numerical analysis were carried out to evaluate the fatigue performance at the U-rib to deck welded joint in steel box girder. Twenty specimens were tested corresponding to different penetration rates (80 and 100%) under fatigue bending load, and the fatigue strength was investigated based on hot spot stress (HSS) method. The detailed stress distribution at U-rib to deck welded joint was analyzed by the finite element method, as well as the stress intensity factor of weld root. The test results show that the specimens with fully penetration rate have longer crack propagation life due to the welding geometry, resulting in higher fatigue failure strength. The classification of FAT-90 is reasonable for evaluating fatigue strength by HSS method. The penetration rate has effect on crack propagation angle near the surface, and the 1-mm stress below weld toe and root approves to be more suitable for fatigue stress assessment, because of its high sensitivity to weld geometry than HSS.     

Effect of compression overload on fatigue strength improved by ICR treatment

ICR (Impact Crack-closure Retrofit) treatment was developed for extending fatigue life of cracked steel structures by closing the fatigue crack opening. By applying the ICR treatment to as-welded condition, compressive stress remains at weld toe. Therefore, ICR treatment can be used for the improvement of fatigue strength of welded joint at the same level of the other fatigue strength improved techniques. If a compression overload is subjected after ICR treatment, however, the residual compressive stress at weld toe will shift the tensile stress region. Accordingly, improved fatigue strength of welded joints by ICR treatment will be reduced after the compression overloaded. In order to investigate the effect of the compression overload on fatigue strength improved by ICR treatment, fatigue tests of out-of-plane gusset specimens were carried out. As the results, fatigue strength improved by ICR treatment was hardly reduced under the compression overload condition of nominal stress of ?190 MPa.     

Zum Beulverhalten kaltgeformter und geschweißter Rechteckhohlprofile aus hochfestem Stahl

Local Buckling Behaviour of Cold‐formed and Welded Rectangular Hollow Sections made from Extra High Strength Steel (EHSS). New structural steels, such as (liquid) quenched and tempered (QT) steels, with extra high strength, good toughness and weldability have been developed in Europe in recent years and are included in Eurocode 3 Part 1‐12 for strengths up to S 700. However, extra high strength steels (EHSS) with a yield strength of 1100 N/mm² so far have not been taken into account in the present version of Eurocode 3 due to insufficient knowledge on their buckling behaviour and fatigue strength and lack of associated experimental data. In this article results of experimental and numerical investigations concerning the local buckling behaviour of cold‐formed and welded hollow sections made of EHSS are summarised. The investigations have been carried out within the scope of an European research project. Results have been used to check the applicability of existing design rules for local buckling to EHSS. As a consequence the scope of Eurocode 3, Part 1.12 could be expanded to cover stability rules also for such steels.     

高性能桥梁钢典型焊接接头疲劳性能试验研究

针对Q500qE高性能钢两种典型焊接接头,包括对接焊缝和横向角接焊缝,设计疲劳试件。其中对接焊试件包括两种形式,分别为板厚56mm和8mm的对 接焊试件;横向角接焊缝试件的主板厚56mm、附连件厚20mm。对三组试件进行了有限元分析,掌握了试件的应力分布状况及薄弱环节,验证了试件设计的合 理性。随后针对三组试件展开了疲劳试验,掌握了不同类型焊接接头的破坏位置及破坏形式。根据试验结果拟合出了三组试件的S-N曲线,并与普通钢的疲 劳性能进行比较,表明Q500qE高性能钢的这两种典型焊接接头的疲劳强度略高于普通钢。采用目前规范规定的疲劳强度容许值进行抗疲劳设计是合理可行的,并且具有足够的安全余量。