Development and optimization of resonance-based test methods | ResoWind
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Project data
| Titel | Title TP ABT.Reso.TUD: Entwicklung und Optimierung resonanzbasierter Prüfmethoden für axial- und biegebeanspruchte Tragstrukturelemente im Verbundvorhaben ResoWind: Resonanzbasierte Prüfmethoden für kosten- und zeitoptimierte Lebensdaueruntersuchungen an Tragstrukturelementen von Windenergieanlagen | SP ABT.Reso.TUD: Development and optimization of resonance-based test methods for structural elements under axial and bending loads within the joint research project ResoWind: Resonance-based testing methods for cost- and time-optimized lifetime studies on support structure elements of wind turbines Förderer | Funding Bundesministerium für Wirtschaft und Klimaschutz (BMWK) Zeitraum | Period 12/2019 – 11/2022 Teilprojektleiter | Subproject manager Prof. Dr.-Ing. Steffen Marx Bearbeiterin | Contributor Dipl.-Ing. Clara Schramm Projektpartner | Project partners Fraunhofer-Institut für Windenergiesysteme (IWES), Bremerhaven | Testzentrum Tragstrukturen Hannover (TTH), Leibniz Universität Hannover |Bauunternehmung Gebr. Echterhoff GmbH & CO. KG, Westerkappeln |Vallourec Deutschland GmbH, Düsseldorf |
Report in the annual report 2021
Resonance cleverly used
Two imbalanced rotors to generate vertical forces to Demonstrator II
Experimental testing of big structural elements, such as those used in wind energy turbines, is extremely time-consuming and expensive. In particular, the study of fatigue behaviour, which is of great importance as a result of high cyclic stresses due to wind and shaft loading, can become very inefficient with conventional servo-hydraulic actuators. The idea: resonance-based testing. Therefore, the Institute of Concrete Construction of the University of Hannover has already developed a resonance-based testing rig. To imply the excitation to the system simple imbalanced rotors were used to generate cyclic fatigue loads with minimal energy input and simultaneously high excitation frequency.
Within this project, the mission is to continue the research on the resonant testing method and to make it more practicable for different types of applications. Upon completion it will be possible to do experimental testing of large structural elements at high frequencies (20–50 Hz) and with a very large number of cycles (N > 107) in an energy and time-saving way.
To achieve the project goals, two demonstrators will be developed. Demonstrator I, for testing specimens under axial tensile or compressive loading and Demonstrator II for testing structural elements subjected to bending. By using two imbalanced rotors, which rotate in opposite directions a vertical oscillation will be generated. The excitation frequency of the rotors is close to the first natural bending frequency of the system. Due to the resonance effect, a relatively small excitation force will result in strong reaction forces. The Demonstrator I functions as a cantilever system with an articulated bearing. Vertically prestressed tension springs are used to apply a specific stress to the specimen around which the system oscillates. The specimens are placed either below (compression test) or above (tension test) the cantilever beam. After numerical considerations and the analysis of the testing rig, the realisation in the laboratory will follow. The most challenging aspect of the design of the Demonstrator II is to find its nodes of deflection. The aim here is to identify the vibration nodes of the specimen (a steel tube) so that a critical vibration transmission to the environment can be reduced to a minimum.
Report in the annual report 2020
Testing by resonance
Schematic illustration of a beam support in its oscillation nodes in a resonance-based test facility
During their 20 years-service life, wind turbines are subjected to high dynamic loads. Especially offshore turbines, subjected to both wave and wind loads, are exposed to load cycles as high as 107. Therefore the research on the fatigue behaviour of structural elements of offshore wind turbines is of great importance.
Until now high and ultra-high cycle fatigue tests for service life studies have been performed solely on small-scale specimens. Due to size effects, only large-scaled testing provides relevant measurement results that can be transferred to real structural scales. Fatigue tests of structural elements with high numbers of load cycles are commonly carried out with servo-hydraulic actuators. Because of the high energy demand and the limited loading frequencies of the actuators, this method is no longer efficient for tests on large structural elements. That’s why a resonance test rig was developed at the Institute of Concrete Construction of the Leibniz University Hanover. With this test method, based on synchronized imbalance rotors, it is possible to generate cyclic fatigue loading with minimum energy consumption and a high excitation frequency at the same time. However, the strong oscillations being transmitted to the test hall are limiting factors for resonance tests on large structural elements such as those used in the wind energy sector.
Within the framework of the joint project ResoWind in cooperation with the Fraunhofer Institute for Wind Energy Systems (IWES) and the Test Centre for Support Structures Hanover (TTH), the resonant testing method is being optimized to enable tests on large-scale structures. Demonstrators are being designed for fatigue-stressed support structures of offshore wind turbines. They enable tests at both axially (tensile and compressive) loaded and bending stressed structural elements. The focus of designing the resonant bending fatigue test is on the development of an oscillation node support. Consequently, the critical transmission of oscillations to the environment is being minimized. The demonstrators for axial compressive and tensile loads combine the advantage of low energy costs compared to conventional hydraulic testing technology with the flexibility to use test specimens with almost any geometry.