Wind Energy

∆σN [N/mm²]
100
10
54 Benefits of Fatigue Assessment with Local Concepts 295
1
1.E+05 1.E+06 1.E+07 1.E+08
Number of cycles N [-]
mod. IIW
FAT225
BM, R = −1,
res. stresses
HAZ, R = −1
HAZ, R = 0.4
Fig. 54.2. Predicted crack initiation life for the top tripod tubular joint
N (NSA)
N (NSA)
1.E+13
1.E+17
brace crown
1.E+12
1.E+15
brace saddle
brace heel
mean
1.E+11
100%
1.E+10
1.E+13
–100%
chord crown
1.E+09
brace crown
chord saddle
1.E+11
brace saddle
1.E+08
mean
100%
1.E+09
1.E+07
–100%
1.E+07
1.E+07 1.E+09 1.E+11 1.E+13 1.E+07 1.E+10 1.E+13 1.E+16 1.E+19
Number of cycles N (SSA)
Number of cycles N (SSA)
Fig. 54.3. Comparison of structural stress approach to notch stress approach
of dimensions and different crack locations. The compliance of the structural
stress approach (SSA) with the NSA is excellent leading to a best-fit hot spot
S–N curve FAT98 (compared to 100 N mm −2 as the reference value) and an
average plate thickness exponent k of 0.245. The heel points are an exception
because the underlying volume weld model with the gross section according
to AWS results in too steep flank angles.
The benefits of the local approach are used for an application to the case
of single-side welded joints for which insufficient test data is available. Here
the comparison with a reduced hot-spot S–N curve as recommended by offshore
standards shows an overestimation of lifetime by the SSA, which cannot
be explained by the slight overestimation of strength reduction for short gaps
mentioned in [1]. For the analysed tubular joints, local bending effects due to
the weld shape stay uncovered by the hot-spot method, especially for the
brace saddle locations.