Global and local fatigue concepts

For the assessment of the fatigue strength (fatigue assessment) of steel structures are used in the SKI engineering company following fatigue concepts are used: 

An overview of the fatigue Concepts offers the following representation:

Fatigue concepts (schema based on Radaj)
Fatigue concepts (schema based on Radaj)
Cracking on a specimen after dynamic loading
Cracking on a specimen after dynamic loading

Evaluation of cracked components

The today's wind turbines are highly dynamic stressed structures, which must ensure on the one hand a sufficient stability and on the other the fatigue lifetime.

Cracking can occur in both concrete and steel structures. Damage often occurs at notched transitions or at stiffness joints with different materials. Examples are the base of a tower at the transition to the foundation, ring flange connections of steel sections, transitions between concrete and steel sections in hybrid towers or the tower top connection to the nacelle. 

Classification of verification format
Classification of verification format

Due to the loads can cracks appear in concrete components which do not directly affect the load-bearing capacity of the structure. In this case, the reinforcement can absorb the tensile forces. This can be caused by an extreme load event in which high loads act on the construction and as a result cracks form. An increase in the crack width after this event does not necessarily have to occur or be critical. SKI uses various fatigue concepts based on fracture mechanics, including the crack propagation concept.

If cracks can be detected in the structure, it is important to assess whether the crack width increases with increasing operating time (crack propagation) due to the relatively high number of load cycles. The cracks may have a negative effect on the remaining lifetime of the structure. In this case, it may be necessary to develop suitable repair measures. 

In this context, SKI can provide technical assessments / appraisals for a wide range of cracked components:

Detection levels for the fatigue strength

For dynamically loaded components with a calculated fatigue assessment is to provide for the anticipated operating loads (fatigue analysis). For steel structures, the fatigue analysis is aimed to DIN EN 1993-1-9. Depending on the calculation method used for the internal forces, the calculated fatigue detection at the level of: 

  • Stress time series,
  • Stress collectives or
  • Collective damage equivalents Step

be performed in conjunction with appropriate damage accumulation hypothesis. For assessing the security against material fatigue are among experimental methods listed above concepts.