PSI - Issue 19

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

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This evaluation leads to a fatigue strength at 2 million load cycles with freely calculated inverse slope of m = 3.4 at 95% survival probability is  C,corr = 107.9 N/mm². An evaluation with a lower envelope and a fixed inverse slope of m = 3.0 gives a value for the fatigue strength of  C,corr = 88.0 N/mm².A classification into the FAT class or detail category 80 is given for the highest strength steels. The standard deviation is with S = 0.19 very low and all investigated steel grades lie within the narrow scatter. Within the fatigue tests on transverse stiffeners, maximum stresses within the range of the 0.2 % yield strength R p0,2 are achieved. Considering additional bending stresses, the maximum stresses are thus partially in the range of the tensile strength of the respective material. This shows the advantage of high-strength and ultra-high-strength steel grades. Compared to normal-strength steel, a considerably higher stress level can be achieved. The test results obtained show between 1,000 and 2 million load cycles a linear course of the S-N-curve. Hrabowski (2019) confirms that the investigated high-strength structural steels S960QL, S960M and S1100QL provide enough ductile behavior. Carefully processed according to the recommendations of Hrabowski (2019), they can be used and dimensioned like normal strength structural steels. The nominal stress method and the notch stress concept can be used for the design of butt welded joints and transverse stiffeners of very and ultra-high strength steels. Since the structural stress concept does not cover the local weld geometry, it is not recommended for the design of butt welds. Fatigue design proofs are generally carried out without input of material strength. Nevertheless, the fatigue design cannot be carried out without considering the steel used, since the height of the stress level or the maximum possible stress depends on its strength. As a measure of the maximum possible stress, the tensile strength can be used. In order to presuppose predominantly elastic material behavior, the yield strength R e is decisive, which does not preclude local plasticizing at notches. The fatigue design is usually done with tress ranges  , the difference between maximum stress  max and minimum stress  min . Therefore, the so-called deformation criterion by Gudehus and Zenner (2000) is chosen for the definition of the limit of the low cycle fatigue to the high cycle fatigue with predominantly elastic material behavior. Therein, the stress range ratio R is considered. The deformation R ∗e limit is defined as: ∗ = ∙ (1 − ) (1) The yield strength R e can be replaced by the 0.2% yield strength R p0.2 . The stress range ratio R is determined according to equation (2). = ⁄ = (2) The exact demarcation for the fatigue strength analysis based on nominal stresses or elastic design can be determined based on the number of cycles N LCF as a function of the deformation limit R e * . Within the experimental investigations, usable results in the range of 1,000 to more than 2 million load cycles until fracture N B are achieved. Previous evaluations show a linear S-N-curve in the investigated area. Here, the elastic design applies. One sample with welded transverse stiffener cracked in the base material at 35 load cycles. It is not considered for the evaluation and is therefore marked with an arrow, see Figure 4. It can be assumed that below 1,000 load cycles the S-N-curve reaches a plateau. In order to take this into account, an absolute lower limit of N LCF ≥ 1,000 is introduced, illustrated in the orange broken line in Figure 4. 4.2. Nominal stress approach 4. Design recommendations 4.1. General