Cyclic Material Behavior of Ultra-High-Strength Fine-Grained Steels and Welds for the Application in Fatigue Assessment Approaches

Ultra-high-strength fine-grained steels are a convenient choice in highly loaded structures in order to realize lightweight design applications. An example is given by the topside of the telescopic boom of truck cranes. Such applications belong to the low cycle fatigue, where the material behaviour predominantly elastic-plastic. Within elastic-plastic approaches, the cyclic material behavior is the basis for a fatigue assessment. Therefore, not just the cyclic material behavior of 8 mm thick sheet metal of two steels, S1100QL and S1300QL, but also their laser beam and laser hybrid welded butt joints is characterized based on strain-controlled fatigue testing. The basis for the cyclic material behavior of welded joints is an integral treatment from base material to base material. On the one hand, the strain-controlled fatigue results allow a comparison of investigated steels in base material and as-welded state. Even though, a longer fatigue life of laser beam welds is found compared to laser hybrid welds, the as-welded condition shows a reduced fatigue life in general. On the other hand, the cyclic material behavior leads to parameters that are applied in elastic-plastic fatigue assessment approaches, e.g. the notch strain approach, and includes information about transient material effects. A cyclic softening of both the base material and the as-welded state is identified. In the future, a functional relation of the parameters of the cyclic stress-strain curve with the damage is capable to describe the cyclic softening.