Cyclic material behavior of high-strength steels used in the fatigue assessment of welded crane structures with a special focus on transient material effects

Paper #:
  • 2017-01-0342

  • 2017-03-28
In case of highly loaded welded structures, the fatigue strength of critical sections is dominated by plastic strains in the low cycle fatigue (LCF) regime. Regarding fatigue, the transition from the weld seam to the base material, which additionally is affected by the heat input due to welding, is the most critical region. Fatigue life assessment by the notch strain concept is based on the evaluation of the local strain at the weld toe or weld root and includes the cyclic material behavior. The choice of the stress-strain relation is connected to the cyclic stress-strain curve and a hardening model. Stress-controlled fatigue test results of butt joints and transverse attachments show a linear extension (in the log-log scale) of the S-N line into the LCF regime. Therefore, the nominal and notch stress concept can be applied up to at least 5,000 cycles to failure. However, for stresses above the material’s yield strength, fatigue life is strain dominated and a transition of the S-N curve towards the tensile strength is the consequence. An improvement of the fatigue life assessment by a more realistic implementation of the local stress-strain hysteresis may therefore be expected. The cyclic material behavior is investigated by strain-controlled testing of 8 mm thick sheet metal specimens and butt joints, manufactured by manually GMAW. The materials used in this investigation are the high-strength structural steels S960M, S960QL and S1100QL. Due to the cyclic softening in combination with a high load level at the initial load cycle, the cyclic stress-strain curve cannot directly be applied for the LCF assessment of welded structures. Therefore, the transient effects have been analyzed in order to describe the time-variant material behavior in a more realistic way and enhance the fatigue life estimation.
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