Wind Energy

320 J. Bergers et al.
60.2 Fatigue Tests
It is generally assumed that fatigue strength decreases with an increasing wall
thickness. To estimate the influence of wall thickness more exactly, however,
there are different approaches depending on the national standards and the
type of construction, showing a wide difference in the reduction factor (see
Fig. 60.1).
When using high strength steel, smaller wall thicknesses usually suffice.
Structures with many welds become more economical by thus also decreasing
weld volume and post weld treatment for extending the remaining fatigue life.
According to current standards, the choice of high strength steels with yield
strengths up to 460 MPa is considered as mild steels and therefore no influence
on the fatigue resistance can be taken into account. Fatigue resistance of higher
steel grades is not considered in the standards.
To estimate the influence of yield strength, wall thickness and post weld
treatment on the fatigue strength, various fatigue tests are carried out. It
is known from former investigations at the University of Karlsruhe [7], that
high strength steels up to a yield strength of 1,100 MPa do not show any disadvantage
compared to mild steels regarding the fatigue strength. In several
cases even better results for higher steel grades were achieved. For structures
with low notch effects, high strength steel particularly exhibits better
fatigue strength. One possibility to reduce the influence of the notch form is an
effective post weld treatment, which is also investigated in the fatigue tests at
present.
In Fig. 60.2 results from former investigations on crane specific details
in Karlsruhe, investigations made in Italy [8] and the first results of the
ks-factor
1
0.9
0.8
0.7
0.6
0.5
16
25 71
0.4
0 20 40 60 80 100 120 140 160
*Tubular joints
**Plates
wall thickness t [mm]
Schumacher [5]* 16
Eurocode [4]/GL [3]** 25
Gurney [2]*/**
CIDECT [6]*
Fig. 60.1. Influence of wall thickness according to different standards
( )
t
( )
0.14
( )
t
22 0.25
t
( )
16 0.38
t
0.2