PSI - Issue 19

J. Rudolph et al. / Procedia Structural Integrity 19 (2019) 575–584 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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The application of COMPONENT fatigue curves (for welded parts) and strain-controlled test data of STANDARD specimens (for unwelded parts) is the basic differentiator between the methodological approaches for welds and unwelded parts in EN 13445-3 [4]. This differentiation is equally crucial for the understanding of differences between different design codes concerning the fatigue assessment of welds, particularly in the high cycle fatigue regime (HCF). Assessment results based on (load-controlled) component fatigue curves of welds will not be comparable to assessment results based on (strain-controlled) material fatigue curves of unnotched, unwelded and polished standard specimens in combination with the app lication of some “weld factor” (Stress Concentration Factor - SCF, Fatigue Strength Reduction Factor - FSRF). The nominal stress approach (e.g. design S-N curves of Eurocode 3 [8] and IIW Guidelines [1]-[3]) constituted the basis of the EN 13445 S-N curves for weld details. The design S-N curves are described by the equivalent stress range Δσ, and the fatigue life in cycles N. C is a constant dependent on the fatigue class a nd a slope of m = 3 was imposed when deriving design curves up to 5·106 cycles (m = 5 up to 1·108 cycles for variable amplitude loading). The design curves refer to particular weld details (see Figure 2), because the stress concentration effect of the weld detail is already included in the S-N curve. The assignment of the welding details to the design curves (fatigue classes) is based on (the equally revised) table 18-4 in [4].

10000

1000

100 eq. stress range Δσ [MPa]

32 45 56 71 90

40 50 63 80 100

10

1E+02 1E+03 1E+04 1E+05 1E+06 1E+07 1E+08 1E+09

al lowablecycles N [-]

Fig. 2. Fatigue design curves for welded components according to [4]

As already noted in [9], the original design curves were approximately two standard deviations below the mean curves, representing 97.7% probability of survival. When deriving the higher survival probability S-N curves required for EN 13445 it was noted that the Eurocode 3 or IIW design S-N curves were approximately one standard deviation of log N apart. Thus, unless there was new evidence to change it, the fatigue class chosen for a given detail and potential failure site in EN 13445 was one fatigue class below the Eurocode 3/IIW fatigue class. In the IIW designer’s guide to the structural hot -spot stress approach [10] the design S-N curves fatigue class 90 and 100 are recommended for different weld types and quality in the framework of the structural stress approach. The determination of stresses for fatigue assessment of welded components and zones is detailed in section 18.6 of EN13445-3 [4] and was discussed in detail in [5]. Detailed in Annex NA of EN 13445-3 [4] the determination of the structural hot-spot stress can be based on (see Figure 3): • Linear extrapolation of the stress on the surface (usually at distances l1=0.4e and l1+l2=1.0e) in shell type or brick type finite element models; • Quadratic extrapolation of the stress on the surface (usually at distances l1=0.5e and l1+l2=1.5e and l1+l2+l3=2.5e) in shell type or brick type finite element models;