Civil Engineering Reference
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the wire terminations. Later on, it was shown in a great number of tests that in
practically all cases where the ratio is l/C = 1.58 the wires break in the free wire
length. Since 1990, therefore, all the tests with the Stuttgart rotary bending
machine have been done with the ratio l/C = 1.58. Because of the very small
stress reduction of only 4 % at the wire terminations, the maximum bending stress
amplitude can be considered as stress over the whole bending length.
The middle stress is practically r
m
& 0. As an example for the ratio l/C = 1.58
and
stress r
b
= 600 N/mm
2
,
the
bending
the
compressive
stress
is
only
r
m
= 965
(d/C)
2
= 0.0089 N/mm
2
.
1.1.6.3 Wöhler Diagram
Wolf (
1987
) did a great number of fatigue tests with the simple Stuttgart rotary
bending machine using wires taken from wire ropes. He straightened all the wires
in the same way with a special device before conducting the fatigue tests.
Figure
1.10
shows the numbers of bending cycles N resulting from a series of tests
with wires of 1 mm diameter taken from Seale ropes for varying amplitude of the
rotary bending stress (alternate bending stress on the whole circumference)
r
rot
& r
b,alt
. For the logarithm normal distribution of the bending cycles N, the
standard deviation increases in the usual way with decreasing bending stress
amplitude r
rot
.
99
1
2
3
4
5
%
95
90
80
70
60
50
40
30
20
rotary bending
stress
1 1200 N/mm
2
2 1100 N/mm
2
3 1000 N/mm
2
4
10
5
3
1
10
3
900 N/mm
2
800 N/mm
2
5
10
4
10
5
10
6
number of rotary bending cycles N
Fig. 1.10 Number of bending cycles for straightened wires, diameter d = 1 mm from Seale
ropes, Wolf (
1987
)
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