Page 49 - SSAB Welding Handbook Edition 2

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Welding handbook 8.0 Fatique in welded joints

Fig. 8.4 shows the difference between stresses from The fatigue strength is normally presented in a
the external load and the actual stresses acting on the SN-diagram, sometimes called a Wöhler diagram.
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welded joints. Compressive stress is not considered a In a SN-diagram, the logarithm of the stress range is
contributing factor to the fatigue process. plotted on the y-axis and the logarithm of the number
of cycles to failure is plotted on the x-axis. The data
comes from testing specimens at different stress
σ levels and registering the number of cycles to failure.
Below a certain stress level, the specimen will not
fail at a predetermined number of cycles and these
Yield stress
samples are called run outs. Fig. 8.6 shows an
example of a SN-diagram for Strenx 700 and a butt
Due to residuals
stresses weld in Strenx 700.
∆ σ

t 1000
Actual loading
Stress range/MPa 100 Strenx 700, Parent metal

Fig. 8.4: The effect of residual stress close to the welded joint.
Strenx 700, RO, Parent metal
Strenx 700, 2.3%, Parent metal
Strenx 700, Transverse butt weld
Force Strenx 700, 50%, Parent metal
Strenx 700, Transverse butt weld, 50%
10 Strenx 700, Transverse butt weld, 2.3%
1,00E+04 1,00E+05 1,00E+06 1,00E+07
Endurance/cycles

Fig. 8.6: Example of a SN-diagram of Strenx 700 and a fully
penetrated butt weld.

The data points in fig. 8.6 show a scatter in the
number of cycles to failure. Normally, the data points
are shown by a 50% probability of failure line and
the “2.3%” line, which is generally the characteristic
fatigue strength (2.3% probability of failure).

The sloped line in fig. 8.6 is described by formula 8.1.

∆ σ N = C Formula 8.1
m
Where m and C are constants, N is the number of
cycles to failure and Δσ is the stress range. m is the
inverse slope in a logarithmic-logarithmic diagram.
The constant C depends mostly on the geometry, for
Fig. 8.5: Examples of different fatigue cracks from different welded example, the geometry and the quality of the welded
joints. joint.

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