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Fatigue Design Based on S-N Data

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Fatigue Design Based on S-N Data

Fatigue analysis of welded components is based on the long term distribution of stresses from either a known load history or from design rules.

The capacity with respect to the fatigue strength is characterized by S-N curves, which give the relationship between the stress ranges applied to a given detail and the number of constant amplitude load cycles to failure.

Depending on the kind of stresses used in the calculation, the fatigue assessment may be categorised by the so-called ‘nominal stress approach’, ‘hot spot stress approach’ and ‘notch stress approach’. The three stresses are defined as follows:

Nominal stress A general stress in a structural component calculated by beam theory based on the applied loads and the sectional properties of the component. The sectional properties are determined at the section considered (i.e. the hot spot location) by taking into account the gross geometric changes of the detail (e.g. cut-outs, tapers, haunches, brackets, changes of scantlings, misalignments, etc.). The nominal stress can also be calculated using a coarse mesh FE analysis or an analytical approach. Hot spot stress A local stress at the hot spot (point with highest stress) where cracks may be initiated. The hot-spot stress takes into account the influence of structural discontinuities due to the geometry of the connection, but excludes the effects of the weld itself. The hot spot stress is also referred to as structural stress or geometric stress.

Notch stress A peak stress at the root/toe of a weld or notch taking into account stress concentrations due to the effects of structural geometry as well as the presence of the weld.

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General 2
Nominal stress 2
Structural stress 6
Notch stress 7
Hot spot stress 8
Stress range 10
Fatigue resistance according to DNV-RP-C203 12
Introduction 12
General (2.4.1) 12
Failure criterion inherent the S-N curves (2.4.2) 12
S-N curves and joint classification (2.4.3) 13
S-N curves in air (2.4.4) 14
S-N curves in seawater with cathodic protection (2.4.5) 15
S-N curves for tubular joints (2.4.6) 16
S-N curves for free corrosion (2.4.9) 17
Effect of fabrication tolerances (2.6) 18
Stress Concentration Factors (3) 18
Stress concentration factors for tubular butt weld connections (3.3.7) 19
DNV Appendix A 23
References 44

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