Wire Ropes, Elements and Definitions - Wire Ropes: Tension, Endurance, Reliability - page 8

Because of early yielding in parts of the cross-section of the non-straightened
wires and even if the wires are straightened in the normal way, the elasticity
module can only evaluated precisely enough if the wire has yielded before over the
whole cross-section. However, that does not mean that the two parts of a broken
wire resulting from a tensile test can be used for the evaluation of the elasticity
module. These wire parts cannot be used because they may have new inherent
stresses due to buckling from the wire breaking impact, Unterberg ( 1967 ).
For straightened wires from wire ropes, Wolf ( 1987 ) evaluated a mean elasticity
module E = 199,000 N/mm 2 . For new wires, Häberle ( 1995 ) found the mean
elasticity module E = 195,000 N/mm 2 . Together with other measurements—after
loading the wires close to the breaking point—a mean elasticity module has been
evaluated for the stress field of practical usage. This mean elasticity module of
rope wires made of carbon steel in the following is E = 196,000 N/mm 2 .
The elasticity module decreases a little with larger upper stresses. For drawn
corrosion resistant wires with the steel number 1.4310 and 1.4401, Schmidt and
Dietrich ( 1982 ) evaluated the elasticity module E = 160,000 N/mm 2 , respec-
tively, E = 150,000 N/mm 2 .
1.1.6 Wire Endurance and Fatigue Strength
1.1.6.1 Test Methods, Definitions
The wires in wire ropes are stressed by fluctuating tension, bending, pressure and
torsion. For a long time wires have been tested in different testing machines under
one or a combination of these fluctuating stresses. The tests with combined
stresses, especially bending and pressure, have been done with the aim to imitate
the stresses in a wire rope. Such tests have been done by Pfister ( 1964 ), Lutz
( 1972 ), Pantucek ( 1977 ) and Haid ( 1983 ). However the test results do not come up
to expectations, or only imperfectly. The wire endurance for example has even
been increased when the wires—loaded by fluctuating bending—are loaded in
addition by fluctuating pressure. This effect can probably be attributed to a strain
hardening of the wire surface. An overview of the test methods with single or
combined stresses has been described by Wolf ( 1987 ).
Nowadays, wire fatigue tests are normally tests with only one fluctuating
stress—mostly a longitudinal stress. The test methods with fluctuating longitudinal
stresses are:
• Tensile fatigue test (wire under fluctuating tensile force)
• Simple bending test (fluctuated bending of the wire over one sheave)
• Reverse bending test (fluctuating bending of the wire over two sheaves or
sheave segments)
• Rotary bending test (wire bending by rotating the bent wire).