Wind energy turbines

G - 12 Design of a resonance-based testing facility as a frame system

In order to further advance future research on tower constructions of wind turbines, knowledge about the fatigue behaviour is of great importance. The cyclic loads from wind and wave loading lead to high numbers of load cycles throughout their life time. In order to reproduce the fatigue stress of wind turbines well experimentally, large-scale cyclic tests on structural elements are necessary. For this, high numbers of load cycles and high input forces must be realised. The resonance-based testing method is an efficient way to apply the required cyclic load. Two contrarotating imbalanced motors are used to generate the input force. The excitation frequency of the motors is set close to the natural frequency of the system. This leads to resonance and consequently to a dynamic magnification of the forces. In order to preserve the existing energy in the system as much as possible without adding more and more energy, damping and friction mechanisms must be reduced. To minimise the energy dissipation, a closed frame as a mechanical system could be a solution for a resonance-based testing facility.

Within the scope of the work, a design for a oscillating frame system as a resonance-based testing facility is to be developed. First of all, the resonance-based testing method and oscillating frame systems are studied. Then two or three pre-designs are worked out. Then, one design should be selected, which is designed in more detail under specified boundary conditions. For this purpose, a simple numerical model is created to analyse the vibration behaviour and to design the components. The topic can be worked on as projectwork, as bachelor thesis or as a master thesis. Depending on interest and level of knowledge, the task is formulated jointly.

Contact person:
Dipl.-Ing. Clara Schramm
Phone: +49 351 463-41117
E-Mail:

G - 11 Large-scale tower tests on the load-bearing behaviour of segmented concrete towers 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 towards 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 usually not glued or mortared together, but stacked dry on top of each other and connected by external tendons.

Far more than half of all newly installed wind turbines are manufactured using this construction method. In this way, it is possible to erect turbines with total heights of 240 m and more. Despite the great practical relevance, the complex load-bearing behaviour of such concrete towers with dry joints is neither fully understood nor can it be reliably modelled. Therefore, large-scale experiments with model towers on a scale of approximately 1:10 are being carried out at the Institute of Concrete Structures. The aim of the tests is to record the real load-bearing behaviour of such towers and to develop calculation models based on this. Within the scope of the thesis, a detailed examination of such tower structures and of the tower tests is to be carried out. In particular, the following points are to be dealt with:

• Literature research on tower structures of wind turbines and their load-bearing behaviour
• Documentation and support in the execution of the tower tests
• Evaluation of selected test results
• Comparison of the test results with existing calculation models

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

G - 10 3D scan of concrete segments 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 towards 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 usually not glued or mortared together, but stacked dry on top of each other and connected by external tendons. Due to the dry joints between the segments, unevenness on the segment surfaces can significantly influence the load-bearing behaviour of the tower structure. In order to avoid major unevenness, the segments are therefore ground during production using CNC-controlled grinding machines. However, despite the grinding process, an 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 high-precision 3D scan. Based on this, an investigation of the effects of imperfections can be carried out. Within the scope of the work, the following points are to be dealt with in particular:

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

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

G - 9 Friction tests on concrete segments with dry joints

Concrete segmental construction has been an established construction method for several decades. They are mainly used in segmental bridge construction and for wind turbine towers. The prefabrication of the segments in precast plants results in numerous advantages, such as shorter erection times, higher component quality and the feasibility of more complex formwork geometries. The joints that inevitably occur between the individual segments are usually bonded or dry. The formation of dry joints with flat ground joint surfaces, i.e. without shear keys, has become increasingly important in recent years. As a result, a simplified deconstruction as well as a reuse of the segments is possible. This contributes to resource-efficient and sustainable construction.

The use of dry concrete segment joints is already common, but especially the load-bearing behaviour of such joints under fatigue loads is neither fully understood nor can it be modelled reliably so far. Therefore, static and dynamic friction tests are carried out on concrete shear segments with dry ground joints. Within the scope of the work, these tests should be considered in more detail. Especially the following points are to be solved:

• Literature research on concrete segment construction with dry joints
• Support/documentation of the static friction tests
• Evaluation of the test results
• Numerical comparative investigations (no previous knowledge of numerical simulations required).

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

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 - 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 - 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: