Wire Ropes Under Tensile Load - Wire Ropes: Tension, Endurance, Reliability - page 140

2.6 Wire Ropes Under Fluctuating Tension
2.6.1 Conditions of Tension-Tension Tests
A wire rope can only be loaded in connection with rope terminations. The wire
rope and the terminations form one unit and the results from tension-tension tests
refer only to this unit. In order to determine the actual characteristics of the wire
ropes themselves fairly accurately, it is therefore necessary to use terminations
which exert only minimal influence. In any case, it is difficult to avoid the effect of
the terminations completely. Even a wire rope breakage occurring in the free
length of the rope is no certain indication that there is no influence from the
termination.
Of all known terminations, resin sockets exert the least influence on the
endurance of wire ropes. Just how small this influence is can be seen by the small
deviation in the numbers of load cycles reached in repeated tension-tension tests
with specimens of the same rope, see the following Figs. 2.39 and 2.40 . Results
gained using metal sockets have much greater deviation with smaller numbers of
load cycles.
The tension-tension tests for determining rope characteristics as described here,
in particular those used to determine rope endurance, are conducted using resin
sockets. Normally the ropes are lubricated. The rope ends, which were degreased
before fitting the resin sockets, were lubricated again on the outside of the sockets
after fitting.
The temperature needs to be kept low during tension-tension fatigue tests to
ensure that the lubrication remains fairly effective. A top limit can be set at about
50 C or for a lubricant with very high viscosity at the most 60 C. The temper-
ature increases greatly with the diameter of the rope. A certain limit for the
frequency of testing in relation to the rope diameter cannot be given because the
maximal possible frequency also depends on the extension hysteresis occurring
during the load cycles. As Fig. 2.14 (Sect. 2.2.3 ) shows, the hysteresis effect
increases the greater the stress range is and the smaller the lower stress. Ventilation
can help to reduce the temperature.
The tension-tension fatigue tests normally end with rope breakage. Strand
breakage or rope deformations count as rope breakage too if they result in the tests
being discontinued. The results of rare tests where rope breakage occurs near the
terminations (about two times rope diameter) are to be disregarded as untrue for
the rope itself. Of 49 tests, three resulted in breakage occurring at a distance up to
two rope diameters and two others at a distance up to 2.5 rope diameters. Four of
these five reached a number of load cycles higher than the mean number.
Tension-tension fatigue tests with wire ropes are much rarer than bending tests.
Where the results of tension-tension fatigue tests have been published, it is often
not possible to evaluate them in common with other tests. As Chaplin and Potts
( 1991 ) pointed out, one problem is that there are no precise specifications laid
down for the wire ropes to be tested or for the test conditions. One other problem at