A high and reliably reproducible preload level of bolts is essential for the fatigue behaviour of ring flange connections in wind energy. For the yield-point controlled tightening method as a potential alternative, the missing normative foundations for large bolt diameters must be created in steel construction.

In accordance with this objective, the authors identify the stress state in the bolt during installation through measurements. In addition, plastic deformations of the HV-bolts are quantified by 3D scans. Further experimental investigations evaluate the structural integrity of the connection and the effect of high preloads in terms of the load bearing behaviour. First results indicate no reduction of resistance within the connection and HV-bolts have a sufficient deformation capacity against high preloads. Based on that experimental scope of a joint national research project, this contribution discusses the applicability of yield-point controlled tightening for HV-bolts.


Bolted ring flange connections are still the predominant technology to join sections of wind-turbine steel-towers. The fatigue loads of bolts during operational time are significantly influenced by the pressure body between the compressed flanges, which in turn depends on two main scattering parameters - geometric flange imperfections and the preload level. With standardised torque controlled tightening methods, the achievable preload level is strongly dependent on the friction conditions of the paired threads (DASt 024, 2018; DIN EN 1993-1-8, 2010). Instead of a direct measurable control variable, only the friction conditions ensure a correlation between the applied torque and the incorporated preload. Recent investigations have shown that the friction conditions of bolts vary widely due to weather conditions during transport or on construction site and thus also the resulting preload (Rutkowski, 2011). This impact considerably reduces the reliability of such methods, which is also reflected in the tightening factor αA (VDI 2230-1, 2015).

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