P 900 – Enhancement of the local concept of fatigue assessment of welded crane structures of high strength steels in the low cycle fatigue regime
The design of crane structures is linked to the fatigue life of welded high-strength and ultra high-strength fine -grained steels, which are widely used in order to meet requirements such as the carrying of increased loads and the enabling of lightweight design. These requirements arise from the technological goals of material efficiency and reduction of weight. High loads, which are applied to the telescopic boom of truck cranes for example, are related to the low cycle fatigue (LCF) regime.
This research report addresses the question as to whether the higher static strength of the high-strength steels S960QL, S960M and S1100QL will be confirmed under LCF loading in the welded condition. Therefore, the fatigue behaviour of welded high-strength and ultra highstrength steels, with and without post weld treatment, has been characterised and prepared for implementation into numerically based fatigue life approach methods.
The fatigue test results under constant and service-related variable amplitude loading with a stress ratio of Rs = 0.1 show that the high static strength of the base material is an advantage, even for welded structures under cyclic loading in the LCF regime. While the Woehler curve of welded butt joints resulting from constant amplitude loading can be linearly extended up to a nominal stress amplitude of Dsn = 900 MPa, the fatigue of variable amplitude loading shows a bilinear characteristic. Furthermore, the advantages of high-strength steels become apparent for welded transverse stiffeners. Both Woehler and Gassner curves show a linear relation up to the material’s static yield strength. The influence of the welding quality and consequent irregularities of the welded joints have to be considered. For the investigated welded details and tube-cleviscomponents, test results were processed, so that, for nominal stresses, a fatigue life assessment by fatigue (FAT) classes could be carried out. The effect of post weld treatment methods, such as grinding, highfrequency hammer peening and TIG-dressing, is mostly shown by an increased fatigue life in the LCF regime.
Local design concepts were used, through consideration of the elastic-plastic material behaviour by the notch strain concept and of a linear-elastic material behaviour using the notch stress concept. In particular, the notch stress concept, using a reference radius of rRef = 1 mm, is useful for the design of welded components made of high-strength steels in the LCF regime. The numerical models were set up with respect to the global specimen and local weld geometry. The improved structural design, based on consideration of LCF fatigue properties, is an important enabler for the economic application of high-strength steels.
The research project (IGF-Nr. 17102 N) was carried out at Technischen Universität Darmstadt, Fachgebiet Systemzuverlässigkeit und Maschinenakustik SzM und am Karlsruher Institut für Technologie KIT, Versuchsanstalt für Stahl, Holz und Steine. FOSTA has accompanied the research project work and has organized the project funding from the Federal Ministry of Economics and Technology through the AiF as part of the programme for promoting industrial cooperation research (IGF) in accordance with a resolution of the German parliament.
Only available in german language.
T. Melz, B. Möller, J. Baumgartner, T. Ummenhofer, S. Herion, J. Hrabowski, J. Henkel, B. Boos, E. Baier