Discussion of the Fatigue Crack Model Used as Decision Basis for Probabilistic Inspection Planning

Abstract

Marine structures are often highly utilized structures and are operating in an environment dominated by dynamic loading. The fatigue limit state is of main concern and is governing the structural dimensions and the amount of resources utilized in inspection of several members and joints. There is a considerable uncertainty related to the parameters determining the fatigue life, and probabilistic analyses have therefore been used extensively in fatigue analysis. The models used both on describing the fatigue driving mechanisms and the deterioration mechanism, however, are always compromises between ability to describe nature and computational applicapability. In a state-of-the-art probabilistic fatigue analysis, major assumptions are made when establishing the crack growth model, such as: 1) the relation between membrane and bending stresses is assumed to be constant for all stress ranges, and one common stress component for the total stress is used to calculate fatigue crack growth. 2) The crack growth is assumed to follow a pattern with a constant relation between crack depth and crack length. This paper investigates the foregoing fundamental assumptions for the fatigue driving mechanism and shows the effect on time to the first inspection based on state-of-the-art probabilistic crack growth analysis for the detail of interest, for a simple but realistic jacket structure.