Project Description

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. Such multiaxial stress states are induced by complex loading and/or geometry and can 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 lifetime estimates that are obtained with such an approach can be non-conservative for structural details in a multiaxial stress state. Various multiaxial fatigue methods have been developed aiming to improve these estimates but they do not succeed in covering the gap between the fundamentals of multiaxial fatigue and experimental results or operational practice. Particularly for multiaxial fatigue which is induced by non-proportional variable amplitude loading there exists no consensus on which method provides the most accurate fatigue lifetime estimates.

Objectives

The objective of this research is to identify a unified method for the assessment of multiaxial fatigue in welded joints under non-proportional, variable amplitude loading. Therefore, the 4D-Fatigue project will develop fundamental knowledge by investigation of the phenomenon of multiaxial fatigue and relating this to real practice by modelling and experimental testing. Eventually, the aim is to improve fatige lifetime estimates which enables to further optimize maintenance and repair and to reduce costs, downtime and risks involved with fatigue failure.