Page 48 - SSAB Welding Handbook Edition 2

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8.0 Fatique in welded joints Welding handbook

8.3 Fatigue strength in welded joints
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The main topic of this section is the fatigue strength
of welded joints. The differences between what
affects the fatigue strength of the base material and
that of the welded joints are summarized in table 8.1. Force

Base material Welded joints
Fig. 8.2: Examples of discontinuities around the weld toe.
Surface condition is very Joint geometry and weld quality
important determines the fatigue strength The area around the welded joint has stress
Strong influence of No influence of mean stress, concentrations due to the welded joint itself.
mean stress use stress range
Fig. 8.3 gives an idea of the difference between
Strong influence of No influence of material strength nominal stress and the stress concentration close
material strength
to the weld. The picture shows the stress over the
Table 8.1: Differences between fatigue strength of base material and thickness of a plate with and without a weld. The
welded joints. stress concentration depends on the joint geometry
and the geometry of the transition between the weld
metal and the parent material. A smoother transition
Table 8.1 shows that the fatigue strength of welded gives less stress concentration and higher fatigue
joints is independent from the strength of the parent strength consequently.
material. The fatigue strength of welded joints is
also much lower than that of the parent material. Welded joint
The next section will explain why this is.
Load
Usually, a fatigue failure can be divided into three
stages: initiation, propagation and final failure. Nominal
stress
This means that the total number of cycles can be
divided into cycles for initiation (Ni) and cycles for
propagation (Np, Ntot=Ni+Np). A welded joint Stress
always has discontinuities such as cold laps, under concentration
cuts, and slag particles around the weld toe, see
fig. 8.2. These discontinuities act like a fatigue
Welded joint
crack that has already been initiated. For welded
joints, the number of cycles to initiation is 10-30%
of the total numbers of cycles, but for a polished
parent material, the number of cycles to initiation
is about 90%. Fatigue in the welded joint is said Fig. 8.3: Stress concentration close to weld over the thickness of the
to be governed by propagation and in the parent plate.
material by initiation. The propagation rate is
independent of the strength of the material and this
is the reason why welded joints in high strength The last effect comes from the residual tensile
steel have the same fatigue strength as those in low stresses around the joint caused by the welding
strength steel. process, see chapter 4.16. These stresses can add to
the stress from the external load. The welded joint
is then subjected to tensile stress even if the external
load is compressive.

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