Fatigue might be the governing limit state in marine structures mainly induced by the waves and wind. Therefore, using a lap joint for the shell plating to replicate the appearance of a riveted joint for a retro yacht is critical due to the limited fatigue resistance. For this joint type cracks initiated at the root, which should always be avoided due to hard detectability, might appear. Arc- and laser-welded lap joints are researched to estimate fatigue resistance and reduce the risk of root failure.
The hot spot peak stress of weld toe and weld root notches is insufficient to reflect the fatigue strength, explaining why an effective notch stress parameter is used, able to deal with both weld toe and weld root notch induced fatigue. Semi-analytical weld notch stress formulations are established in order to avoid solid finite element modelling to capture the effective notch stress. Particular attention is paid to the lap joint characteristic extreme weld notch load carrying level. A second-order weld load carrying stress formulation is introduced. Since the (laser) weld is typically small in comparison to the plate thickness, a dedicated way of weld modelling is proposed, assuming a shell finite element type of formulation, allowing to capture the far field stress for both the weld toe and weld root notch accurately.
Fatigue tests are performed to obtain the laser-welded lap joint mid-cycle fatigue resistance information, including the material characteristic length as effective notch stress coefficient. Particular attention is paid to mean (residual) stress effects in comparison to the arc-welded joint equivalent to explain the fatigue performance.
Comparing the lap joint fatigue resistance in arc-welded and laser-welded configurations to the resistance of other joint types, the results turned out to be aligned. Investigating different material characteristic parameters, either an effective point or an effective line is used. A line based parameter shows a better performance. The main reason is that the actual notch stress gradient is explicitly considered. The mean (residual) stress seems to be larger for arc-welded joints. Furthermore, the mean (residual) stress correction seems at the same time to be one reason for toe induced- rather than expected root induced failure. For arc-welded joints, the change in failure location induced by stress level dependent secondary bending moments is another reason. The fatigue resistance parameter confidence for laser-weld weld joints is relatively low in comparison to the confidence for arc-welded ones, as a result of data size: respectively ≈190 and ≈3000.
When not considering the change in secondary bending moments in the fit of a Se-N curve, the overall fit parameters and quality do not change significantly for arc-welded lap joints. However, the influence on the effective stress range is substantial and changed the value on average by 4.7 % and up to 14.7 %.
Considering only the mid-cycle fatigue resistance, laser welding is superior to arc welding when exposed to low load and response ratios resulting from the lower residual stress level. The FAT class design curve of arc-welded joints for the hot spot structural stress concept with a load and response ratio of 0.5 can be used with a mean stress correction to estimate the fatigue resistance of laser welds.

The current fatigue assessment methods for naval ships are intentionally conservative. This is in part due to uncertainties in the fatigue capacity (i.e. fatigue resistance) of the ship: the nominal and hot spot structural stress incorporate implicit conservatism due to the inherent data-scatter and the Linear Damage Accumulation Model (LDAM) by Palmgren and Miner is not sufficiently accurate to assess random variable amplitude loading histories. The safe-life engineering solution to this is to employ a safety factor. This typically reduces the critical damage from 1.0 to 0.5, or even 0.2, based on engineering judgement. The main flaw of the Linear Damage Accumulation Model is that it does not properly account for the effects of the loading history. To address this flaw the non-linear probabilistic fatigue damage accumulation model by Leonetti is researched and thoroughly validated with variable amplitude fatigue data from the literature. The combination between the effective notch stress and the model from Leonetti poses significant improvements in the quality of the fatigue life predictions. The effective notch stress is used to formulate a generalised fatigue resistance curve (reducing scatter due to different geometries, hot spot types and mean stress ratios), after which the non-linear probabilistic fatigue damage accumulation model is used to work towards a generalised damage accumulation model. For the considered variable amplitude fatigue databases from literature, the Effective Notch Stress Concept (ENSC) - Non-Linear Damage Accumulation Model (NLDAM) combination proves to reduce the lifetime ratio scatter index (1:1.13, instead of 1:1.28) the most in relation to the DNV method (Hot Spot Structural Stress concept with the LDAM), allowing to reduce the implicit conservatism in the fatigue-life model.

Marine structures are mostly exposed to mild and moderate sea-states, where the structural response is elastic. Characteristics of environmental loading are highly stochastic in nature, resulting in stress-states that might be multiaxial. This thesis attempts to identify the source and effect of these multiaxial stress-states on fatigue damage in frigate type structures.
A stress invariant based method called the Projection-by-Projection approach was adopted to estimate damage in the presence of multiaxial stress-states. This method was tested for its applicability and modified, where necessary, to better cope with the encountered stress-states. The design rules by the International Institute of Welding (IIW) were also used to be able to distinguish physical phenomena from methodological properties.
The available data consisted of strain measurements from the United States Coast Guard (USCG) Cutters Bertholf and Stratton. The Bertholf was equipped with the WaMoS wave radar. Measured wave data was reinforced with simulated waves to make the research independent of encountered sea-states.
It was concluded that multiaxiality is most relevant at low damage estimates (i.e.: low wave). The governing influence factor is the incoming wave direction. It was also shown that multiaxial stress-states may significantly contribute to fatigue damage (up to a factor 2, depending on the structural detail and location in the vessel) at an estimated life time of 2 · 10^{6} cycles.