Progress
It is not possible to say after the initial round of testing
whether the swage fitting or the compression fitting is capable of
obtaining a higher failure load from the cable / termination
system. Although the 3mm swage fittings out-performed the
compression fittings it is believed that some simple improvements
could be made to the designs to improve the fittings in this
respect. Furthermore, if the slip element was removed from the load
/ extension curves for the swage fittings then it would look much
like that for the 6mm compression fittings. The designs have now
been modified and more testing shall be done to verify the
improvements.
Overlayed load extension
curves from 3mm swage stud tests
A six millimetre compression
fitting
Once the swaged system reached the same load at which the 3mm
compression system was failing it started to act in an
unpredictable manner. The cable started to pull out of the fitting
with a corresponding drop in load, if cold welding had not occurred
then the cable would have pulled out completely. The point at which
cold welding occurred was found to be unpredictable, if this is to
be the mechanism by which the fitting works such unpredictability
is completely unacceptable. This means that the useful working load
for the swage fitting is not higher than that of the swage fitting.
The 6mm compression fittings gave much more repeatable behaviour
than their 3mm equivalents. The test certificate was obtained for
the 6mm cable used, which had the results of the breaking load
tests performed by the cable manufacturer on it. The figure stated
on the certificate was 28.65kN. The average breaking load obtained
in these experiments was 30.26kN with a standard deviation of
183.5N, this means that the figure on the test certificate was
nearly 9 standard deviations lower than the mean value obtained by
these experiments.
The cable manufacturer was contacted with this information and
they retested another sample from the same reel of cable. Their
second result came back at 29.69kN. The pictures they sent over
from their test facility show the cable to be held at each end by a
set of wedge grips in a vertical twin lead-screw testing machine,
the wedges were coated in sandpaper to increase grip. The pictures
of the failed cable show an outer wire had broken at the point of
contact with a wedge. The implication is that the compression
fitting does no more damage to the cable than the test method used
by this cable manufacturer.
Overlayed load extension
curves from 6mm compression fitting tests