DNV DNV CLASSIFICATION NOTES 30.7 PDF

Introduction 

Fatigue cracks and fatigue damages have been known to ship designers for several decades. Initially the obvious remedy was to improve detail design. With the introduction of higher tensile steels (HTS-steels) in hull structures, at first in deck and bottom to increase hull girder strength, and later on in local structures, the fatigue problem became more imminent. 

In the DNV Rules for Classification of Ships. the material factor f1o which give the ratio of increase in allowable stresses as a function of the material yield point was initially introduced in 1966. The factor is varying with the yield point at a lower than linear rate, this to give some (but insufficient) contribution to the general safety against fatigue fracture of higher tensile steels. However, during recent years a growing number of fatigue crack incidents in local tank structures in HTS steels have demonstrated that a more direct control of fatigue is needed. 

This Classification Note is intended to give a general background for the rule requirements for fatigue control of ship structures, and to provide detailed recommendations for such a control. The aim of the fatigue control is to ensure that all parts of the hull structure subjected to fatigue (dynamic) loading have an adequate fatigue life. Calculated fatigue lives, calibrated with the relevant fatigue damage data, may give the basis for the structural design (steel selection, scantlings and local details). Furthermore, they can form the basis for efficient inspection programs during fabrication and throughout the life of the structure. 

To ensure that the structure will fulfil its intended function, fatigue assessment, supported where appropriate by a detailed fatigue analysis, should be carried out for each individual type of structural detail which is subjected to extensive dynamic loading. It should be noted that every welded joint and attachment or other form of stress concentration is potentially a source of fatigue cracking and should be individually considered.