Core protrusion can be caused by shock loading as well as improper installation.
Rotation-Resistant wire ropes on hoists must be installed with extra care and proper handling to prevent rope damage during installation. The lay length of a rotation-resistant rope must not be disturbed during the various stages of installation.
By introducing twist or torque into the rope, core slippage may occur—the outer strands become shorter in length, the core slips and protrudes from the rope. In this condition, the outer strands become overloaded because the core is no longer taking its designed share of the load. Conversely, when torque is removed from a rotation-resistant rope core slippage can also occur. The outer strands become longer, and the inner layers or core become overloaded, reducing service life and causing rope failure.
Corrosion Breaks (Poor Lubrication)
Wire ropes that fail due to corrosion are usually indicative of improper lubrication. Corrosion is easily identified by the pitted surface on individual wires of the rope. Broken wires do not usually show evidence of tension, abrasion, and fatigue. The extent of the damage to the interior of the rope is extremely difficult to determine; consequently, corrosion is one of the most dangerous causes of rope deterioration.
Cut or Shear (Externally Induced Damage)
In this type of wire rope damage, the wire will be pinched down and cut at broken ends or will show evidence of a shear-like cut. This condition is evidence of mechanical damage cause from an external source or wear or damage on equipment components such as a broken flange on a sheave or sharp drum grooves.
Fatigue Breaks (Repetitive Bending Wear)
Hoist wire ropes are subject to a lot of repetitive bending over sheaves, which causes the wire to develop cracks in its individual wires. These broken wires often develop in the sections that repetitively move over sheaves.
The smaller the sheave is in relationship to the diameter of the wire rope, the higher the bending fatigue. This process will become escalated as a rope travels on and off of a grooved single layer drum, which also causes a bending cycle.
Fatigue breaks often develop in segments as stated before these segments are usually the part of the rope surface that comes into direct contact with a sheave or drum. Because this is caused by external elements rubbing, often these breakages are external and visible for the eye to see.
Once broken wires start to appear, it creates a domino effect and quickly much more will appear. Square ends of wires are common for fatigue breaks. These breaks are considered a long term condition and are to be considered part of the normal to the operating process.
Tension Failure (Overload / Shock Loading)
Tension failure is caused by overloading of the wire rope. The overloading can be based upon the original strength of a new wire rope or for the remaining strength of a used wire rope.
When a wire rope has failed due to excessive tension, the broken rope will show one end of broken wire coned and the other cupped. Necking down of the broken ends is typical of this type of break. Tension breaks are often caused due to shock loading a slack rope that induces excessive impact stress on the rope.
CRANE 1 provides experienced wire rope analysis and recommendations:
We have included a troubleshooting checklist of the possible causes for early wire rope failure below. Call your nearest CRANE 1 office for expert analysis, lubrication and replacement ropes that are installed by trained professionals.
ABRASION DAMAGE
CORE PROTRUSION/SLIPPAGE
CORROSION DAMAGE
CRUSHING
DIAMETER REDUCTION
FATIGUE
HIGH STRANDING
JUMPING THE SHEAVE
KINKING
LAY LENGTHENING/TIGHTENING
LOOPED WIRES
UNBALANCED ROPE
A failure analysis of a broken multi strand 71mm steel wire rope used in the main towing winch was carried out. The wire rope was failed during a bollard pull test. The wire rope was a new one and had failed during the first use. The wire rope was in IWRC/ RHO 6X41 constructions. Fig.1 shows the typical cross section of the wire rope. The failure investigation is performed by chemical and metallurgical examinations.
Fig.1 Cross section of the wire rope
OBJECTIVE
This investigation focuses on determining the
(i) chemistry and grade of steel employed,
(ii) the uniformity and cleanliness of the microstructure of the rope steel and the effect of microstructure on crack initiation and propagation, and
(iii) the nature and extent of failure mechanisms as revealed by the test results and fractography .
EXPERIMENTAL APPROACH
Samples were cut from wire ropes for chemical analysis, tensile testing, and hardness testing and metallography.
1) Chemical Analysis
Chemical analyses were carried out as per Wet Chemistry Method
2) Mechanical Testing
Hardness measurements were taken by Vickers hardness-testing machine.
For tensile testing of wires, samples 150 mm long with75 mm between grips were used. Tests were conducted at room temperature.
3) Metallography
For microstructural analysis, specimens were mounted, polished, and etched and microscopically examined as per ASTM E-407
To study fracture morphology, fractured surfaces were inspected under the scanning electron microscope (SEM)
RESULT AND DISCUSSION
1) Chemical analysis of steel wire rope is presented in Table 1. The analysis showed that it is made of high carbon steel corresponding to AISI 1074 grade, and galvanized with zinc to resist corrosion.
2) The microstructure observed under optical microscope and is shown in Figs. 2. It was typical of a drawn ferrite–pearlitic steel wire with heavily cold worked micro structure. Further examination of microstructure of the failed wires did not indicate any sign of metallurgical problems such as de- carburized layer, nonmetallic inclusions, or martensite formation. In addition, the wires were free from any sort of corrosion and pitting. Therefore, corrosion had no role in the failure of wires.
Fig-2 Longitudinal microstructures of EEIPS wire; an optical microscope image. Structure is of pearlite and ferrite that are severely deformed
3) Table 2 represents the hardness values of the wire rope at and nearby the broken ends. The results indicate relatively high values.
4) Table 3 represents the tensile values of the wire. The result indicates relatively less value comparing the metallographic results and the mill test certificate supplied by the Client. Figs. 3 showing Stress- Strain during tensile testing of the wire
The high hardness values, chemical composition, and the pearlitic structure of wires indicating that this is a type of extra extra improved plow steel (EEIPS) grade wire ropes. These types of wires have typically higher load-bearing capacity as compared with other grades. They are considered as heavy-duty wire ropes. The minimum tensile strength of EEIPS is 2160 N/mm2. (Ref. API Spec 9A)
Fig-3 Graph showing Stress- Strain during tensile testing of the wire
5) The fractured ends of group of wires were visually inspected. Majority of wires failed in shear, and the remaining had cup-and-cone fracture, some of which are shown in Fig. 4.
Fig.4 Typical pictures of the broken ends of wires
Fractographs of broken wires in the form of cup and cone and shear are shown in Fig. 5 and Fig.6. Tensile overload fracture occurs when the axial load exceeds the breaking strength of the wires. This type of fracture usually appears in ductile manner, either in the form of cup and cone or in shear mode. In the former case, there is a reduction at the fracture which is called necking, whereas in the case of the latter, fracture surface is inclined at 45degree to the wire axis. In both cases, ductile dimple formations are clearly observed and confirm the tensile overloading of wires.
Every wire rope failure will be accompanied by a certain number of tensile over load breaks. The fact that tensile overload wire breaks can be found therefore necessarily mean that the rope failed because of an overload. The rope might have been weakened by fatigue breaks. The remaining wires were then no longer able to support the load, leading to tensile overload failures of these remaining wires.
Only if the metallic area of the tensile overload breaks and shear breaks combined is much higher than 50% of the wire rope’s metallic cross section is it likely that the rope failed because of an overload.
Fig.5 Revealing the indications of ductile dimples on the fracture surface indicating the presence of ductile fracture mechanism
Fig.6 Revealing the indications of ductile dimples on the fracture surface indicating the presence of ductile fracture mechanism with shear lip and slant fracture
Majority of the metallic area suffers shear breaks and the remaining area suffers tensile over load breaks and hence it is evident that the shear breaks was predominant. Fractograph is clearly indicating the presence of ductile fracture mechanism with shear lip and slant fracture.
Shear breaks are caused by axial loads combined with perpendicular compression of the wire. Their break surface is inclined at about 45degree to the wire axis. The wire will fail in shear at a lower axial load than the pure tensile over load.
If a steel wire rope breaks as a consequence of jumping a layer or being wedged in, a majority of wires will exhibit the typical 45degree break surface.
In the instant case the wire rope was failed at 100 Ton or even less. As the breaking load of the wire rope is 353 Tons, there is no reason for a tensile over load breaks in an axial direction and that too considering the fact that the wire rope was failed during a bollard pull test. Fig. 7 shows the maximum stress generations in the wire rope at 100 Ton under normal bollard pull test. More over the metallurgical investigation is also not suggesting for any factors that fostering an axial overload failure.
Fig.7 Stress analysis at 100 Tons under normal bollard pull test
CONCLUSION
The failure of the wire rope was studied in detail. In order to investigate the problem metallurgical and mechanical post failure analyses were performed. The wire rope was made of AISI 1074 grade steel, and it was a type of EEIPS. The microstructure was composed of severely deformed and elongated ferrite–pearlite, and no other phase formation or nonmetallic inclusions could be detected. The morphologies of fractured surfaces indicated that the wires were mainly failed in shear mode and few in tensile mode. Owing to galvanized coating, the wires were free from corrosion.
The tensile strength of the wire material is less than the required value. The required tensile strength of EEIPS is 2160 N/mm2 and the obtained value is 2059 N/mm2. But this factor is not a reason for the current failure of the wire rope. The said point is substantiated by the following:
· Wire rope was failed at 100 Ton or even less whereas the breaking load of the wire rope is 353 Tons.
· The metallurgical investigation didn’t reveal any factors that fostering a premature axial tensile failure.
It is concluded that the wire rope was failed due to shear breaks. Shear breaks were caused by high axial loads combined with perpendicular compression of the wire. It is worthwhile to note that the rope was failed in its first usage. The shear break is linked to the lapses during the installation/ spooling of the wire rope.
The potential reasons for such a shear failure may attribute to any one or more of the following:
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