Exactly, what is ”characteristic fatigue strength” for a welded joint?
When performing a fatigue assessment of a welded joint, the joint is characterized by a “joint class” or a FAT-value, which is based on the “characteristic fatigue strength” of the joint. Most standards and design codes define characteristic fatigue strength according to one of the following definitions:
1) Mean fatigue strength minus 2 standard deviations measured in log(N), which is equivalent to a failure probability of 2.3%. Reference .
2) 75% confidence level of 95% probability of survival for log(N). Reference 
3) 95% survival probability referred to a two-sided confidence level of the mean of 75%. Reference .
Please read the attached .pdf-file to see the difference and the investigations made in Reference .
In fact, the fatigue strength differs rather much if the number of tested specimens is small, depending on what definition the "characteristic fatigue strength" is based upon. However, from an engineering point of view, they may be considered as equal if the number of tested specimens is larger than 20 and the scatter of the test result is moderate (s < 0.20 referring to log(N)).
Avoid Formula 6.4 (= Table 6.6) in Reference . It leads to an increased safety margin and imposes additional structural weight and costs.
Take as an example ten specimens that are tested in a fatigue test rig and the standard deviation of the test series is 0.20 referring to log(N). Then, if the same FAT-value is to be determined, the fatigue strength of the joint needs to be 10% higher if the assessment is based on Table 6.6 in Reference , compared to if the assessment were instead based on 75% confidence level of 95% probability of survival for log(N), as advised in Reference .
Reference . See below.
Reference . EN 1990:2002 Eurocode - Basis of structural design.
Reference . EN 13001-3-1:2013 Cranes - General design - Part 3-1: Limit states and proof competence of steel structure safety.
Reference . Hobbacher, A. Recommendations for Fatigue Design of Welded Joints and Components. Second edition, Springer International Publishing, Switzerland 2016. ISBN 978-3-319-23756-5.
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