Wire Ropes, Elements and Definitions - Wire Ropes: Tension, Endurance, Reliability - page 11

fatigue tensile strength of the wire ends is increased slightly over that of the wire in
the free length. This hardening of the wire ends together with the lamella clamp
provides a high probability that the wire breaks in the free length.
In the fluctuating tensile tests the stress is constant over the whole cross-section
as shown in Fig. 1.5 . The total stress is composed of a constant middle stress r m
and a stress amplitude r a ,

r ¼ r m r a :

Because of the risk of buckling, the stress amplitude r a normally should be smaller
than the middle stress r m . Tensile fatigue tests with a compressive section can only
be done with very short wires.
Simple bending test, one sheave. In this method the wire moves over one sheave
forwards and backwards, Woernle ( 1929 ), Donandt ( 1950 ) Müller ( 1961 ) etc. The
wire is loaded by a constant tensile force S and a fluctuating bending. For this test,

the middle stress is r m ¼ r t ; m þ r b = 2
and the stress amplitude is r a ¼ r b = 2 :

The fluctuating bending stress r b exists only in one small segment of the wire
cross-section in the outside wire bow, as shown in Fig. 1.5 . In the other small wire
segment lying on the sheave, the bending stress is compressive. There is no wire
breakage to be expected from this stress, especially if this stress is reduced—as is
normal—by a tensile stress r t,m .
Reverse bending test, two sheaves. In this method the wire first moves over one
sheave and then over a second sheave with reverse bending forwards and back-
wards, Schmidt ( 1964 ). In another method the wire is bent and reverse bent over
sheave segments, Unterberg ( 1967 ). For both methods, the wire is loaded by a
constant tensile force S and a fluctuating bending. For the wire reverse bending test

the middle stress is r m ¼ r t ; m

and the stress amplitude is r a ¼ r b :

The fluctuating bending stress r b exists only in two small segments of the wire
cross-section, as shown in Fig. 1.5 .
The pressure between the wire and the single sheave or two sheaves is small
and can be neglected. The advantage of this bending test method is that the tensile
stress r m , and the bending stress can be chosen quite freely. There should be only
a tensile force chosen that is large enough to ensure that the wire lies securely on
the sheave in contact with the bow.
Rotary bending test. In a rotary bending machine, the wire is bent in a free bow
around its own axis. By turning the wire, the stress in an outer fibre of the bent wire
changes from compressive to tensile stress and back again. In one turn of the wire
around the wire axis, each of the outside fibres of the wire is stressed by a complete
cycle of longitudinal stress. The stress amplitude ±r a decreases linearly from the