DNV DNV CLASSIFICATION NOTES 30.7 PDF

Introduction 

Fatigue cracks and fatigue damages have been known to ship designers for several decades. Initially che 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. 

ln the DNV Rules for Classification of Ships, the material fat:torf1, which gives the ratio ofincrcase 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 in order to give som~ (hut 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 made from 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 rnle requirements for fatigue control of ship structures, and to provide detailed recommendations for such control. The aim of the fatigue control is to ensure that all parts of the hull structure subjected to fatigue (dynamic) loading have adequate fatigue life. Calculated fatigue lives, calibrated with the relevant fatigue damage data, may give the hasis 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 servicelife of the structure. 

To ensure that the structme will fulfil its intended function, fatigue assessment, suppo1ted where appropriate by a detailed fatigue analysis, should he carTied out for each individual type of structural detail subjected lo extensive dynamic loading. lt 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.