In fatigue loaded components such as tower structures of wind turbines, linearly varying strain gradients occur in the cross-section due to cyclic wind and wave loads. As a result, the outer edge of the cross-section is subjected to larger stresses than the inner part of the cross-section. This causes stiffness degradation of the concrete at the outer edge of the cross-section and, consequently, stresses are gradually redistributed to the inner part of the component. This effect decreases the stress at the outer edge and thus extends the service life of fatigue loaded components.
To investigate the effect of stiffness degradation more thoroughly, large concrete beams were experimentally subjected to fatigue loading. This was conducted in a resonance-based testing facility, which made it possible to carry out investigations up to 6.5⋅107 load cycles at a loading frequency of 18 Hz, and thus reaching the very high cycle fatigue range. Beam specimens with two different concrete strengths were tested at two distinct stress levels each. In this contribution, the particular design of the experimental setup in this unique testing facility will be presented, as well as the experimental results. It became evident that this testing facility is suited to purposefully cause stiffness degradation in the beam specimens. These ranged from individual cracks to complete destruction of the specimens in the most stressed zone. Due to the stiffness change in the edge zone, stresses redistributed into the inner part of the cross-section.
In summary, it became clear that this effect enabled the beam specimens to endure a significantly larger number of load cycles than estimated, for example, according to Model Code 2010 for the respective stresses. Since the fatigue assessment is often decisive in the design of fatigue loaded components, this can save a great amount of material if taken into account accordingly.