Wind energy turbines

G - 5 Investigation of the propagation conditions of the acoustic signals resulting from prestressing wire breaks in the acoustic emission analysis (SEA).

Acoustic emission analysis (SEA) is a non-destructive testing method that can draw conclusions about a change in the condition of the structure based on the analysis of the acoustic waves emitted by the structure. An application of SEA can be found in structural monitoring, detection and localization of prestressing wire breaks, and materials research, among others. At the IMB of the TUD, research is being conducted to detect and localize possible wire breaks of tendons in wind turbines using SEA.

In order to form real boundary conditions of wind turbines even in laboratory tests, a prestressing frame with dimensions of 12 m × 4 m was set up, where a total of four tendons were prestressed, each with a length of about 10 m. The tendons were separated by Dremel. The installed tendons were artificially cut with the help of Dremel. The acoustic signals generated as a result were measured by the acoustic emission sensors, which were placed in a distributed manner in the tensioning frame. The attached picture shows, for example, the cutting process of a tendon in the tensioning frame and the measurement technique used.

The measured signals show different characteristics of the propagation path from the fracture location of the tendon to the measurement sensor and are to be analyzed with respect to the propagation conditions. In the analysis of the signals, a distinction is made between qualitative and quantitative evaluation. In the former, some acoustic emission features (SE-features) such as peak amplitude, signal energy are extracted and statistically evaluated. In the latter, the whole transient waveforms of the signals are considered and evaluated in both time and frequency domains.

Depending on the students' interests and the type of student work (project or diploma thesis), the scope of work can be customized. For a diploma thesis, knowledge of programming in Python or Matlab is required. A detailed assignment will be worked out accordingly.

Contact person:
Dipl.-Ing. Ronghua Xu
Phone: 0351-463-33776
E-mail:

G - 4 3D scan of concrete segment rings of wind turbines

Wind turbines already account for the largest share of electricity generation from renewable energies in Germany. To further increase the performance of wind turbines, the trend is toward ever taller towers and larger rotor diameters. Especially for large hub heights, so-called hybrid towers have become successfully established. The upper part of such towers is composed of several steel sections, whereas the lower part consists of prestressed precast concrete rings. The individual concrete segments are stacked dry on top of each other and braced together by means of external tendons. Due to the dry joint formation between the segments, unevenness on the segment surfaces can significantly influence the load-bearing behavior of the tower structure. To avoid major unevenness, the segments are therefore machined with a CNC-controlled grinding machine during manufacture. However, despite the grinding process, ideal evenness of the segment surfaces cannot be guaranteed. For this reason, the actual surface geometry of model segments of wind turbines (scale 1:10) is to be recorded by means of a 3D scan and, based on this, an investigation of the effects of imperfections is to be carried out. Within the scope of the work, the following points are to be dealt with in particular:

• Support for 3D scanning of the segments
• Evaluation of the 3D scan data with respect to existing imperfection patterns
• Investigation of possibilities to implement imperfections in FE models
• Numerical investigations of the influence of imperfections on the (joint) load-bearing behavior (no previous knowledge of numerical modeling required)

Contact person:
Dipl.-Ing. Florian Fürll
Phone: +49 351 463-32317
E-Mail:

G - 3 Development of a test and measurement concept for large-scale tower tests on model segments of wind turbines

In order to achieve the expansion targets in the field of renewable energies, the further cost reduction of wind power generation in manufacturing and operation is inevitable. At the current tower heights, about 45% of the total construction costs are attributable to the tower structure. Therefore, further optimization of the support structure is imperative for more economical power generation. Especially for large hub heights, so-called hybrid towers have established themselves as the most economical option in recent years. With the new modular tower generation, additional vertical joints break down the concrete segments into smaller components, so that half, third or quarter shells make a significant contribution to reducing transport costs and assembly effort compared with other concepts. However, the decomposition of the tower structure into individual precast concrete segments leads to a complex load-bearing behavior that is neither fully understood nor can be reliably modeled so far. Within the scope of this work, a test and measurement concept is to be developed for investigating the influence of vertical joints on the load-bearing behavior of segmented tower structures. The aim is to be able to record the actual load-bearing behavior of the tower structure under different loading situations with the experiments.

Contact person:
Dipl.-Ing. Florian Fürll
Phone: +49 351 463-32317
E-Mail:

G - 2 Design and layout of a resonance test rig for fatigue tests on concrete beams

Wind turbines play a central role in the expansion of renewable energies. According to the current design status, however, they have so far only been designed for a service life of 25 years. For safe use of the turbines beyond this, the design principles need to be adapted, which in turn requires a more precise understanding of the changes that occur in the material during the service life.

The tower structures of wind turbines are subjected to bending stresses due to wind and wave loads. This results in linearly varying strain gradients in the tower cross-section. This non-uniform loading leads to material degradation in the more heavily stressed cross-sectional areas and consequently to stress redistribution. This effect is to be investigated in more detail in tests on fatigue-stressed concrete beams. For these tests, a test rig is to be developed in which the large stresses required are achieved under very high loading frequencies by exploiting the resonance principle with small excitation forces. Since this excitation produces strong vibrations that are conducted into the supports, an appropriate damping element should also be provided in the trainer.

Within the scope of this work, the following subtasks are to be processed for this purpose:

• Literature research on the dynamics of bending beams
• Compile the necessary boundary conditions for fatigue tests on concrete beams.
• Design of a test facility for these investigations
• Dimensioning of the main elements of the test facility
• (Drawing of the dimensioned test facility - master thesis only)

Contact person:
Dennis Birkner M.Sc. 
Phone: 0351 463-39677 
E-Mail:

G - 1 Effects of unevenness on joint surfaces of segmented tower structures for wind turbines.

In order to achieve the expansion targets in the field of renewable energies, the further cost reduction of wind power generation in manufacturing and operation is inevitable. More powerful turbine classes and the development of new locations in low-wind regions or forest areas require the construction of higher tower constructions. In recent years, so-called hybrid towers have become established, especially for large hub heights. The upper part of such towers is composed of several steel sections, whereas the lower part consists of prestressed precast concrete rings. The individual precast concrete rings are connected by external tendons that run inside the tower and are anchored in the foundation and in the intermediate adapter. Transmission of shear forces between the individual segments is provided by frictional resistance in the horizontal joints. Current design models assume ideally flat flange surfaces at the top and bottom of the segments and thus assume a constant normal stress distribution at the compression connection.

Experimental investigations show, however, that a highly non-uniform normal stress distribution occurs in the joints as a result of a pure prestressing load. The reason for this lies in imperfections occurring on the joint surfaces. Investigations into the accuracy of the joint surfaces on real segments for wind turbines make it clear that the surfaces are not ideally flat despite grinding processes, but exhibit unevenness. Numerical simulations show that especially the occurring waviness on the joint surfaces can significantly influence the joint bearing behavior and the joint bearing capacity.

Within the scope of the work, the effects of the waviness occurring on the joint surfaces on the joint bearing behavior are to be investigated by means of analytical and numerical calculations. For this purpose, parameter studies are to be carried out, among other things, in order to find out the most unfavorable manifestations of the waviness with respect to the joint bearing capacity.

Contact person:
Dipl.-Ing. Florian Fürll
Phone: +49 351 463-32317
E-mail: