Settings

Now showing1 - 9 of 9
  • Some of the metrics are blocked by your 
    consent settings
    Item type:Publication, Access status: with files ,
    Development of a floe-ice breaking simulation for the generation of propeller–ice load spectra
    (2023-08-16)
    Sapp, Lina
    Within the scope of this work, an icebreaking simulation for floe ice is developed and implemented. The existing failure descriptions of local failure by bending and global failure by cracking as well as failure by direct rotation are extended by a newly described local failure by cracking. This combines the failure initiation of cracking with the fracture pattern of failure by bending. The simulation is validated for both the fracture forces at the bow and the generated breaking pattern by comparison with observations. Furthermore, the impact of different simulative as well as physical parameters on the model is investigated by a parameter study. The simulation can be applied to arbitrary ice floe fields in combination, in conjunction with the bow contour of different icebreaking vessels, to calculate the breaking pattern including the ice fragments that migrate under the bow. In this work, the PSRV S.A. Agulhas is primarily used, with additional comparative simulations for the RV Polarstern. Additionally, an algorithm is developed and applied to generate load and torque spectra based on the computed ice cusps from the icebreaking simulation. Finally, the generated torque spectrum is compared with field measurement data.
    Publicationtype: Master Thesis
      89  144
  • Some of the metrics are blocked by your 
    consent settings
    Item type:Publication, Access status: with files ,
    Entwicklung von Methoden zum KI-gestützten Propellerentwurf im frühen Entwurfsstadium
    (2022-08-10)
    Strecker, Maike
    ;
    Abdel-Maksoud, Moustafa
    The aim of the work is to develop an AI-based design method for the creation of a propeller geometry that serves as a starting point for the design process. It should be possible to consider boundary conditions such as the maximum diameter, the required thrust or the available power. Based on a literature review regarding possible AI approaches or methods, these are evaluated in terms of their suitability for supporting propeller design. They range from simple regressions to artificial neural networks. Subsequently, the AI approaches suitable for the task are to be selected and corresponding models are to be implemented from them under consideration of the mentioned boundary conditions. The models are to be trained on the basis of available data from propeller series. In order to assess their quality, the calculated and available free-running characteristics are to be compared for an example propeller. Based on the comparison, neural networks are best suited for this application.
    Publicationtype: Master Thesis
      170  570
  • Some of the metrics are blocked by your 
    consent settings
    Item type:Publication, Access status: with files ,
    Fracture mechanics simulation of welded joints at sub-zero temperatures considering the weld shape
    (2022-01-27)
    Sanhen, Nils-Erik
    Welded joints are the most common method for connecting steel components, but they are also points of weakness, as they contain, among other things, initial cracks or are unwelded gaps. Most of the fatigue life of a welded joints takes place during the phase of macro crack growth, fracture mechanics models are therefore widely used method to predict the fatigue life. In this thesis, a procedure is developed how fracture mechanics simulations can be conducted automatically for a large number of welded joints with heterogeneous geometries. Therefore, an integrated procedure is developed in which parametric simulation models are generated, fracture mechanically analysed and the results together with their metadata are saved in a selectable master file. Then, the fatigue lives are calculated from the numerical results via the Paris-Erdogan equation and compared with the experimentally determined data. It is shown that by adapting the Paris-Erdogan parameters, high accuracy can be achieved in predicting fatigue lives under sub-zero temperatures. Moreover, a method is described in which a low effort verification of the Paris-Erdogan parameters C and m, separately from each other, by comparing numerical and experimental fatigue lives in a logarithmic space.
    Publicationtype: Master Thesis
      61  111
  • Some of the metrics are blocked by your 
    consent settings
    Item type:Publication, Access status: with files ,
    Untersuchung eines LNG-Kompensators im kryogenen Bereich
    (2020-08)
    Englert-Dudek, Olivia
    ;
    Ehlers, Sören
    In shipping, engines powered by heavy fuel cause high level of climate effecting emmissions. LNG is therefore considered as an alternative to heavy fuel, as combustion of LNG will result in a significant reduction in emissions. This thesis examines the potential use and its feasibility for use of a LNG expansion joint in ships. For this purpose, an expansion joint is tested in a climate chamber to determine its behaviour. In this experiment, the expansion joint is cooled down with liquid nitrogen down to -160 °C in order to measure the force and strain on the expansion joint. The experiment is then simulated with a FE-analysis, whereby a stress analysis is also carried out using FEM. The last step of the verification of the usability of expansion joints in the LNG sector is the calculation of the fatigue life. For this step the maximum stress at the nodes is extracted from the simulation. With this information, using the fatigue class FAT 225, the operational strength is determined in the form of a maximum number of life cycles that an expansion joint can sustain in operation before failure occurs. Based on the knowledge gained from the experiment and simulation, a number of 2220 life cycles can be determined. This corresponds, for a refuelling interval of 14 days, a life expectancy of about 85 years and thus significantly exceeds the average operating time of ships, both in tourism and in freight transport. The results of this thesis provide a basis for research in this field. In the future it is considered useful to conduct further experiments for validation. In addition, further experiments may consider a separation of bellows and pipe of the expansion joint. This could ensure a more reliable evaluation of the material behaviour in the bellows and pipe.
    Publicationtype: Master Thesis
      182  1459
  • Some of the metrics are blocked by your 
    consent settings
    Item type:Publication, Access status: with files ,
      81  537
  • Some of the metrics are blocked by your 
    consent settings
    Item type:Publication, Access status: with files ,
    The measurement accuracy of instrumented ship structures under local loads using strain gauges
    (2019)
    Böhm, Angelo
    ;
    Ehlers, Sören
    The State of the Art measurement set up for ice loads measurements is as follows. On four to five frames there are strain gauges attached on each end for measurements of shearstrain differences. With the help of a finite element analysis (FEA) the load strain relation of the instrumented section can be determined. This load strain relation is displayed in the so-called influence coefficient matrix which is obtained by the unit load principle. Nevertheless after the many past measurements the following questions arise. How accurate is the mapping of the influence coefficient matrix? Which structures are suitable for measurements of shear-strain differences? How can the existing method be improved and what measurement uncertainties are inherent in the overall system? In order to achieve the goal of this work, the theory of shear-strain measurement was presented. In addition to shear-strain measurement, the theory of shear-strain measurement on ships for the determination of ice loads was explained. Besides a grillage model which consists of five frames and one plate was modeled and point loads and pressures were investigated by finite element analysis. Under the given conditions symmetrical structures with a good load alignment are suitable for measurements of shear-strain differences. Besides, the author shows how the influence coefficient matrix maps the response of the structure. The best mapping of the influence coefficient matrix could be achieved with a double instrumentation of the frames. The location of the load, the load length and the pressure distribution within the contact area were one of the biggest uncertainties. The author shows how the already existing measurement system can determine loads in the middle of the frames. In addition an approach to estimate the load length is presented as well. At the end there is a recommendation for further investigations of this kind.
    Publicationtype: Master Thesis
      168  309
  • Some of the metrics are blocked by your 
    consent settings
    Item type:Publication, Access status: with files ,
    The measurement of the Young's modulus of ice with ultrasonic waves
    (2018)
    Randhawa, Kulbir Singh
    ;
    Ehlers, Sören
    Many techniques have been used to measure the Young’s modulus of ice. The purpose of this bachelor thesis is to develop a more effective and precise technique. The impulse-echo method has been used to measure the longitudinal ultrasonic velocities of three different types of specimens. From this, the Young’s modulus has been calculated. The specimens included homogeneous, inhomogeneous and a cracked polycrystalline ice Ih sample. The homogenous samples studied were found to be elastic isotropic, whereas the inhomogeneous and cracked sample were found to have an anisotropic elastic behaviour. For the cracked specimen, different decreases of the Young’s moduli were measured in different directions. The results showed an overall scatter and gives therefore only temporary and rough values for the longitudinal sound velocity in a homogenous polycrystalline ice specimen. Further, the applied method is applicable to detect different types of ice with different composites and manufacturing, and to identify a change of elastic properties after crack.
    Publicationtype: Bachelor Thesis
      554  3086
  • Some of the metrics are blocked by your 
    consent settings
    Item type:Publication, Access status: with files ,
    Investigation of constraint effect on fatigue crack growth rate measurements
    (2018)
    Hochfellner, Veronika
    ;
    Ehlers, Sören
    For the purpose of reliable life prediction the fatigue crack growth behaviour of a material has to be well-known. Therefore standardized fatigue crack growth tests are made with test specimens of the material. The obtained fatigue crack growth data is taken as material specific data and is given without any information of the test conditions and used specimen geometry. However, in several studies a geometry dependency of the test specimen on the fatigue crack growth data is found. Hence in this investigation experiments on fatigue crack growth behaviour are made with two different specimen geometries, the middle tension specimen M(T) and the compact (tension) specimen C(T). The direct current potential drop (DCPD) method is used to determine the crack lengths which is calibrated with the aid of beach marks and crack detection gauges. Two different configurations of the DCPD method are implemented to gain acceptable results. With the obtained data fatigue crack growth curves are established which are analysed regarding the geometry influence of the used test specimens. Discrepancies in the fatigue crack growth curves between the tested specimens are found. With these findings an approach is tested to create a material curve which is independent of the specimen geometry. However, this approach is not leading to satisfactory results. The implementation of the DCPD method in its main configuration on the used experimental setup proves to be applicable for fatigue crack growth tests on different geometries and will be used in further studies at low temperatures.
    Publicationtype: Master Thesis
      332  1343
  • Some of the metrics are blocked by your 
    consent settings
    Item type:Publication, Access status: with files ,
    Strain-gauge-based measurements of ice loads on ships
    (2018)
    Zorn, Till
    ;
    Ehlers, Sören
    Design loads are a central element of ship construction, because they influence plate and frame thickness. For the construction of ships that are destined to navigate in cold areas, it is extremely important to take into account the loads of ice impacts and their location on the hull. But ice is a particularly inconsistent material. To measure ice-induced loads, a lot of measurement has been undertaken, many of them used strain gauges. However, long-term strain gauge measurements are rare. In this bachelor’s thesis the state-of-the-art strain-gauge measurements are to be reviewed and problems with strain-gauge measurements will be discussed. Suggestions for improvement on problems like long cable complexity, big measurement setup, complicated installation, external power supply and long-term maintenance by humans while measuring will also be submitted. Therefore, different systems and measurement possibilities should be compared in the context of long-term strain gauge measurements. A feasible solution for the mentioned problems, but also to identify the location and force of ice impact, will be designed.
    Publicationtype: Bachelor Thesis
      374  1569