Background information

Ship and Offshore structures are continuously exposed to environmental (e.g. wind, waves) and operational loads. Over a service life of typically 20 years such cyclic loadings with variable amplitudes will induce fatigue damage. For operators of ship and/or offshore structures it is of interest to know when the accumulated fatigue damage associated with nucleating or growing fatigue cracks impairs the structural integrity to an unacceptable level. 

Structural details such as stiffened panels, frames and trusses have a dominating uniaxial stress state and are characterized by their orthotropic stiffness. However, there are structural details present in ship and offshore structures where a more complex (multiaxial) stress state can be induced by loading, such as in bilge keels and bracket ends which are subjected to a complex loading, or by geometry, such as in welds of inclined attachments on a plate. Such multiaxial stress states decrease fatigue resistance significantly.

Currently, fatigue design of ship and offshore structures is based on results from uniaxial and constant amplitude fatigue tests. However, the fatigue life estimates that are obtained with such an approach can be non-conservative for structural details in a multiaxial stress state. Various multiaxial fatigue criteria have been developed to improve these estimates but they do not cover the gap concerning the fundamentals of multiaxial fatigue and experimental results or operational practice. Particularly for non-proportional variable amplitude loading cases the compatibility of these criteria is weak. Better estimates of where and when fatigue damage becomes critical will contribute to the optimization of maintenance and repair and will reduce costs, downtime and risks.


The 4D-Fatigue project will develop fundamental knowledge by investigation of the phenomenon of multiaxial fatigue in welded structural details. By modelling and experimental testing this fundamental knowledge will be related to real practice. this will enable to make better fatigue life estimates which will reduce costs and downtime for maintenance and repair. Also, it will reduce the risks involved with fatigue failure.


The 4D-Fatigue project has received substantial subsidy from STW which is realized by ten national partners. These partners vary from offshore operators, oil & gas companies, shipyards, class societies to governmental institutions. A Joint Industry Project (JIP) is currently under development in order to involve international companies as well.


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