Common Structural Rules - Common Structural Rules for Bulk Carriers and Oil Tankers, January 2019 - Part 1 General Hull Requirements - Chapter 9 Fatigue - Section 1 General Considerations - 4 Methodology
4 Methodology
4.1 Principles
4.1.1 General
Appropriate fatigue strength of structural details is ensured by use of:
- Detail design standards given in Ch 9, Sec 6, providing specific design requirements.
- Fatigue strength assessment by fatigue life calculation, based on three different methods for hot spot stress calculation: simplified stress analysis, very fine mesh finite element stress analysis and fatigue screening assessment.
4.2 Simplified stress analysis
4.2.1 Procedure based on simplified stress analysis, required in Ch 9, Sec 4, is used to determine the hot spot stress at weld toe of longitudinal stiffener end connections, given in Ch 9, Sec 2, [1.1].
Nominal stresses are calculated by using analytical method based on beam theory according to Ch 9, Sec 4, [3] and Ch 9, Sec 4, [4]. Hot spot stresses are obtained by multiplying nominal stresses by stress concentration factors (SCF) of the considered detail according to Ch 9, Sec 4, [5.2].
4.3 Finite element stress analysis
4.3.1 Procedure based on finite element stress analysis, required in Ch 9, Sec 5, is used to determine hot spot stress at weld toe of specified structural details, from very fine mesh models.
The hot spot stress is generally highly dependent on the finite element model used for representing the structure.
General procedure for the calculation of hot spot stress at weld toe for any welded details except for web stiffened cruciform joints is given in Ch 9, Sec 5, [3.1]. Procedure for the calculation of hot spot stress at the flange connections for web stiffened cruciform joints is given in Ch 9, Sec 5, [4]. Calculation of local stress for non-welded area is provided in Ch 9, Sec 5, [3.2].
A list of details for which the fatigue assessment is to be made through a compulsory very fine mesh finite element analysis or through the compliance with the design standard given in Ch 9, Sec 6 if a very fine mesh finite element analysis is omitted, is given respectively in Ch 9, Sec 2, Table 1 and Ch 9, Sec 2, Table 3.
4.4 Fatigue screening assessment
4.4.1 A fatigue screening procedure is used to assess the fatigue strength of specified structural details, given in Ch 9, Sec 2, [2.1.3]. The screening procedure is based on screening hot spot stress at weld toe of specified structural details determined by multiplying the stresses obtained from a local fine mesh finite element model, required in Ch 7, Sec 3, by stress magnification factor η of the considered detail, given in Ch 9, Sec 5, Table 2.
4.5 Fatigue design standards
4.5.1 Detail design standards given in Ch 9, Sec 6 are provided to ensure improved fatigue performance of critical structural details. Alternative detail design configurations may be accepted subject to demonstration of satisfactory fatigue performance.
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