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Fig. 2.38 Woehler diagram,
wire rope C, Warr. 8 9 19-
IWRC-zZ, resin socket
500
fictitious life
fatigue strength
finite life
fatigue strength
400
N
mm 2
300
S min /d 2 = 176 N/mm 2
200
S min /d 2 = 0 und
S min /d 2 = 352 N/mm 2
100
10 4
10 6
10 8
10 5
10 7
number of load cycles N
The mean number of load cycles N is calculated with ( 2.102 ) and the constants.
The number of load cycles where with a certainty of 95 % the highest quantile c
% (for example, 10 or 1 %) of the wire ropes are broken can be calculated with
lg N c ¼ lg N k Tc lg s :
ð 2 : 103 Þ
The standard deviation lg s is determined with the regression calculation. The
constant k Tc has to be calculated as a mean value for the region of the wire rope
forces being considered, Stange ( 1971 ).
In contrast to the rope bending fatigue tests, all the known tension fatigue tests
have been carried out up to wire rope breakage mostly without detecting any
outside wire breaks or other discard criteria. Magnetic inspection to detect inner
wire breaks during the fatigue tests has not been used up to now, Feyrer and
Wehking ( 2006 ). For practical purposes in connection with safety requirements,
for the time being it seems reasonable to evaluate the number of load cycles N 1 as
that which—with a certainty of 95 %—not more than 1 % of the wire ropes under
consideration are broken. It can be expected that up to this limit possible rope
defects will be detected and show that the rope has to be discarded. For wire ropes
without safety requirements, the number of load cycles N 10 may be used as hav-
ing—with a certainty of 95 %—not more than 10 % of the ropes broken.
2.6.2.3 Woehler Diagram
With the help of ( 2.102 ) a Woehler diagram can be drawn for the sphere of finite
life strength. The test results let us see that the sphere of finite life fatigue strength
ends for not much more than N = 1,000,000. There are only a small number of test
results available above this number of load cycles and from these results it is not
possible to derive the relation between the acting forces and the number of load
cycles. Supposing a more or less constant fatigue strength does not exist, a ficti-
tious continuation of the fatigue strength line according to Haibach ( 1989 ) can be
drawn as a conservative form, Sonsino ( 2005 ). To be on the safe side the fictitious
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