PSI - Issue 19

Jennifer Hrabowski et al. / Procedia Structural Integrity 19 (2019) 259–266 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

260

2

1. Introduction

Higher, wider, further... so are the requirements for structural engineering of the present time. To achieve these objectives in modern steel lightweight construction, the development of high-strength steels plays a crucial role. Steel grades with yield strengths up to 1300 N/mm² provide the necessary high strength. Combined with good toughness and weldability they enable higher load capacity and at the same time a lower weight of the construction. So, the efficiency of the structure can be increased using high-strength steels. But for the fatigue design the higher static strength of very or ultra-high strength steels cannot be used according to current standards and regulations. Particularly in crane construction, fatigue is governed by the load cycle range below 50,000 and high single loads. This raises the question of whether the higher static strength of ultra-high strength steels in the low-cycle fatigue range comes into play and how far this can be accounted for. The stress-based S-N-curve is the basis for quasi-elastic design rules, as given in the EN1993-1-9 (2010) or the Fatigue Design Recommendations of the International Institute of Welding IIW (Hobbacher 2016). Herein, the nominal stress approach is the most common and simple way of fatigue design and is done by the assignment of welded details in FAT-classes. The FAT-classes are defined at 2·10 6 load cycles in the stress-based S-N-curves. The constant amplitude fatigue limit of the linear S-N-curve is defined at 5·10 6 load cycles together with a change of inverse slope from m = 3.0 to m = 5.0 according to EN 1993-1-9 (2010). Hobbacher (2016) sees this turning-point at 10 million load cycles with a change to m = 22. From 10 8 load cycles on, it is assumed that the stress amplitudes are so low that they do not lead to damage of the component, even with infinite repetition. This cut-off limit is characterized by a horizontal S-N curve. While the high cycle fatigue limit is well defined, the limit to low-cycle fatigue is not clearly regulated. The fatigue strength curves usually apply from 10 000 load cycles on. But depending on the FAT-class, especially for mild steels this may lead to an overestimation. For this reason, a threshold based on the yield strength is recommended, which leads to a more precise dimensioning, as it is shown in the following. 2. General The existing design rules apply for steels up to yield strengths of 700 MPa in EN 1993-1-9 (2010) or 960 MPa in Hobbacher (2016) accordingly. As mobile crane construction usually uses high-strength and ultra-high-strength fine-grained structural steels, it is first necessary to show whether the existing design rules are also applicable to higher strengths steel grades, such as S960 and S1100. In addition, we are interested in the low cycle fatigue range between 1 000 and 10 000 load cycles and want to know if quasi-elastic design methods are sufficient or plastic design methods have to be used. The fatigue tests are carried out on butt welded plates of the same thickness (see section 3.4) and fillet welds in the form of a transverse stiffener welded on both sides of the plate (see section 3.5). An overview of the test series is given in Table 1. The first material to be used is the S960QL - a structural steel with a minimum yield strength of 960 MPa in tempered condition. Representative for thermomechanical rolled structural steels, the S960M is being investigated. It also has a nominal yield strength of 960 MPa. In addition, the ultra-high strength structural steel S1100QL with higher minimum yield strength of 1100 MPa is examined. In the manufacturing process, the S1100QL complies with the S960QL, but is not listed in EN 10025-6 (2011). 3.2. Test Program 3. Experimental Investigations 3.1. Aim