Fatigue lifetime has proven to be a leading design parameter for naval vessels. In the field of fatigue analysis and design of welded joints, two overall methods are distinguished: the temporal and spectral approach. Using empirical models, the spectral approach is typically preferred due to its efficiency, even though damage estimates are conservative. With the temporal approach, nonlinear effects can be captured, providing higher accuracy at the cost of additional computation time.

Within this framework, an improvement of the spectral approach was pursued, focusing on incorporating the mean stress effect. Past efforts to include mean stress corrections were continued by focusing on applying the Walker correction.
Nonlinear effects on the mean stress response were investigated and were found to be quite small for the case of a pontoon vessel. This enabled the use of the still water bending moment as a single input to the spectral mean stress model. By using Walker in the spectral mean stress correction, an overall reduction of conservatism of 21% was achieved.

Future research should focus on testing new wave cases, applying additional fatigue models and investigating the effects for hull shapes of naval vessels.

Fatigue is typically a governing limit state for maritime structures, and weld joints are the most critical locations. Various fatigue damage criteria have been developed involving either an intact or cracked geometry parameter, incorporating local or global information. In this thesis, the effective notch stress based total life fatigue damage criterion has been used in order to improve the accuracy of fatigue lifetime prediction and reduce the workload of engineers in the early design stage. First, the Battelle structural stress calculation procedure is clarified and examined with three models. A hot spot type A, B and C mesh convergence study are conducted. In order to quantify the influence of mesh quality, several ugly mesh models are tested with their good mesh counterparts. For validation purposes of the total life model, a large scale specimen fatigue test result has been used and the mesh quality investigations are included. The DNVGL based life time estimate is provided for the sake of comparison. Finally, several conclusions and recommendations are provided for future applications.

Fatigue strength is typically a governing limit state for marine structures. Predicting an accurate fatigue lifetime is important for structures (e.g. wind turbines) that are supposed to be out at sea for decades. It is valuable for all type of structures that encounter stochastic sea loadings to optimise the design so that fatigue failure is predicted for, or material is saved in preventing over conservative estimates. This research contributes to the investigation of multiaxial fatigue concepts by getting insight in the mode-III loading & response conditions for the effective notch stress concept. The mode-III loading & response will in this research be represented by a tubular welded joint loaded under torsion. A semi-analytical formulation is developed to describe the weld notch shear stress distribution analytically. This formulation can be used as input for the effective notch stress concept. The formulation is based on three components, the notch stress, weld load carrying stress and far field stress component. Furthermore, the static force and moment equilibrium is satisfied. The weld load carrying stress component is based on a weld load carrying stress estimate. This coefficient is used because the weld geometry causes a local change in stiffness, a shift in neutral axis, meaning the weld becomes load carrying up to some extent. The distribution must be corrected for this phenomenon which is done by this estimated term. The geometry is a tubular welded joint. This specimen can be loaded with torsion which in the case of the tubular joint cause mode-III shear in the through thickness direction at the weld notch level. The weld load carrying stress estimate is based on the geometry ratios of the assessed specimens. It is individually fitted for all 6,300 geometries by comparing the results to stress distribution obtained with 2D finite element analysis. The results of the individual fitted load carrying stress estimates are used as input of a multi variable polynomial regression analysis. The regression analysis led to a polynomial function that describe the load carrying stress estimate which uses geometry ratios as input. The far field information and geometry dimensions are the input for the semi-analytical formulation to obtain the weld notch shear stress distributions analytically. Accurate results are obtained and therefore the formulation can be used as input for the effective notch stress concept. The effective notch stress concept is also based on a material characteristic length, ρ*, over which the stress distribution can be integrated to obtain the effective notch stress. This parameter is obtained using a log likelihood regression analysis and led to a value of ~1 [mm]. Although this is a good result, the confidence regarding the value is low due to low variety in the experimental data set and little data in general. The findings of this study will be used in further research regarding the 4D fatigue project and the effective notch stress concept in general at the Delft University of Technology.