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    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
      59  111
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    Statistical assessment of post-weld treated welded joints by burr grinding
    (2020-05)
    Wang, Xiru
    ;
    Ehlers, Sören
    Burr grinding is one of the post-weld improvement methods that are widely used in various industries to improve the fatigue strength of welds: bridges, offshore structures, ships and land-based structures. In this paper, main influencing parameters and effect of burr grinding technique on the fatigue strength for butt-welded, longitudinal welded, doubling plates, load-carrying transverse, non-load carrying transverse welded and T-joints as well as I-section with cope hole and out-of-plane longitudinal welds has been evaluated, based on the extracted 395 fatigue data points from 34 existing publications. The specimens used in these studies with yield strength 175 MPa < σYS <1100 MPa, which were tested with stress ratio -1 < R < 0.5, are analyzed and assessed with statistical methods. The assumed value of S-N slope m = 4 and proposed FAT classes seem to be applicable for the extracted data. More experimental testing is needed to draw the conclusion more exactly.
    Publicationtype: Bachelor Thesis
      161  451
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    Item type:Publication, Access status: with files ,
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    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
      331  1315
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    Fatigue crack growth rate measurements in GL D36 grade welded structural steel
    (2018)
    Göpfrich, Josef
    ;
    Ehlers, Sören
    In this master thesis, the differences in the crack growth behavior of welded and base material is investigated. The direct current potential drop method is used to document the crack propagation. The material are tested using middle tension samples. The test is carried out with cyclic loading, a force ratio of 0.1 and a frequenze of approximately 33 Hz. Based on the test result a calibration curve is created using the Johnson formular. With the help of crack and strain gauges the calibriation curve is validated. It is shown that the Johnson formular generates an accurated description of the crack propagation. The calibration curve gives an electric current independent comparision of crack propagation and voltage drop. The conducted work in this thesis has shown that the welded material has a higher crack resistance, then the base material. The difference is especially apparent for low stress intensity factor. The findings in this thesis can be used to investigated welded and base material at low temperatures.
    Publicationtype: Master Thesis
      684  883