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Failure Analysis of a Crane Rope
Submitted by:Muhammad Arfan (LS1101201)
Syed Imran Jawaid (LS1105202)
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PROBLEM STATEMENT
A wire rope broke while lifting a load of
reinforcing steel estimated to weigh 2.5 - 3
tones. The precise sequence of events leading to
the failure were not known, but the load did not
drop because the rope jammed in the gap between
sheave and support bracket.
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ANALYSIS STEPS
Case has been analyzed in following three steps;
First Step
Background
Rope Bounce Analysis
Visual Observations
Second Step
Tensile Testing
Rope Damage Analysis
Fractography
Third Step Summary
Conclusion
Recommendations
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FIRST STEP
Background
Rope Bounce Analysis
Visual Observations
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ROPE AND SHEAVE DETAILS
The rope was a general engineering 18 strandnon-spin type designed as 12x7(6/1)/6x7(6/1)
The rope had an inner layer of 6 strands of wires,
with each strand comprising 7 wires wrapped as 6outer wires around 1 inner wire, while the outer
layer is formed by 12 strands of wires wrapped in
the same way.
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ROPE AND SHEAVE DETAILS
The inner core of the wire is fibre. Resistance to rope spin is provided by opposing
twist directions of the inner layer (anticlockwise)
and outer layer (clockwise), whereby the load-
induced torque tends to cancel out.
The rope sheave diameter was 520 mm, and it was
known to have been in service longer than the rope.
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SAMPLING
Three pieces of rope were supplied to assist in thisinvestigation;
These comprised the two broken ends together
with a section of rope taken well away from the
failed ends. The purpose of this latter piece was to
check the load capacity of the rope, via tensile
testing, at the time of failure.
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ROPE BOUNCE ANALYSIS
It is worth considering what the possible effects of ropebounce could be, and how they might contribute to thisfailure. There are different possible effects of rope bounce,
depending on;
whether the rope stays in the groove, or whether it jumps out of the groove
Four possibilities are discussed;
Rope Stays in Groove-No effect
Rope Stays in Groove-Extra Load Induced
Rope Jumps out of Groove-Transient Impulsive Load Induced
Rope Jumps out of Groove-James between Sheave and Boom
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CASE-I (ROPE STAYS IN THE GROVE-NO
EFFECT)
This is incorrect, rope bounce will lead to some levelof transient impulsive load on the rope, which will
add to the dead weight being lifted.
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CASE-II(ROPE STAYS IN THE GROVE-EXTRA LOADINDUCED)
Yes-any bounce of the load would add atransient impulsive force. In extreme cases, this
may double the load being lifted, giving a possible
upper limit to the apparent load of about 5-6 kN.
If the measured breaking load for a piece of wire
rope is around 50 - 60 kN, rope bounce may have
caused the failure to occur. So, validity of this
reason is subject to the tensile testing results. And ifit stands valid, the cause of such a low failure load
would still require investigation.
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CASE-III(ROPE JUMPS OUT OF GROOVE-
TRANSIENT IMPULSIVE LOAD INDUCED)
This might be the result if the rope jumped out ofthe groove and ran freely on the sheave shaft.
However the clearance between sheave wheels and
the carrier is usually quite small. Information on the
measured breaking load in a tensile test will assist
in indicating whether the failure reflected a
transient impulsive load or some more serious
source of loading.
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CASE-IV (ROPE JUMPS OUT OF GROOVE-
JAMES BETWEEN SHEAVE AND BOOM)
This is possible and, if it occurred, would causehigh loads on the rope if the crane operator did not
detect it and there was no trip mechanism installed
to prevent such an event. Such a severe effect of
rope bounce would imply poor lifting practice.
If the measured breaking load in a tensile test is
significantly higher than the stated load being lifted,
and there is no obvious cause of a different ropestate at the fractured region, then this possibility
must be considered.
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Conclusion of Rope Bounce Analysis:
Final endorsement with regard to this activity is
referred to tensile testing results.
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VISUAL INSPECTION
Visual inspection of the rope was performed forfollowing;
Rope specifications
Lubrication and CorrosionAny visible cracks near fracture plane
Location of cracks (if, any)
Any link of cracks with different areas (flattened,round, elliptical etc)
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VISUAL INSPECTION
Rope specifications was same as quoted (RopeSpecification)
As received, rope lubrication was deficient to dry, with
slight corrosion evident on the outside of the rope.
Figure 1 shows the 2 broken ends of the rope.
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VISUAL INSPECTION
Close inspection of the rope near to the fractureplane showed that a number of wires were cracked
in outer and inner strands (Figures 2 & 3)
Fig.2 Fig.3
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VISUAL INSPECTION
Cracks on both inner and inner layers wereassociated with flattened regions on the
wires. Cracking was also observed on wires well
away from this region (Figure 4).
Fig.4
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SECOND STEP
Tensile Testing
Rope Damage Analysis
Fractography
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TENSILE TESTING
Before performing tensile testing of the rope, it was
necessary to establish the original grade and size of thewire rope. This would indicate what degradation ofproperties had taken place over the service life, andprovide an indicator of the severity of service and
quality of maintenance. The only information that the operator could supply,
was that the rope was a 1770 MPa grade.
Thus it was necessary to measure the diameter of wires
near to the break (average approximately 1.5 mm) andthe rope diameter (approximately 21.5 mm).
Information contained in the wire rope manufacturer'stable of properties has been used to find the most likely
original rope diameter and breaking force.
ROPE DIAMETER AND BREAKING LOAD
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ROPE DIAMETER AND BREAKING LOAD
ESTIMATION
Manufacturer's data for a range of generalengineering ropes which bracket the measured size
information is given in Table 1.
ROPE DIAMETER AND BREAKING LOAD
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ROPE DIAMETER AND BREAKING LOAD
ESTIMATION1st Approximation: 26mm diameter
It might initially be thought that the diameter
of both rope and individual wires would have
reduced in service. However, although the rope
diameter would decrease due to bedding in of
the strands, It is unlikely that wire diameter
would change in the absence of significant
plastic deformation
ROPE DIAMETER AND BREAKING LOAD
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ROPE DIAMETER AND BREAKING LOAD
ESTIMATION2nd Approximation: 24mm diameter
This appears to be the correct specification for the
original load, and the error in measuring wire
diameter is at least 0.02 mm.
Original rope diameter 24 mm, wire diameter 1.5
mm and as manufactured rope breaking force = 332
kN.
ROPE DIAMETER AND BREAKING LOAD
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ROPE DIAMETER AND BREAKING LOAD
ESTIMATION3rd Approximation: 22mm diameter
Although the rope diameter corresponds to the
value measured, the wire diameter is too small. Inservice, rope stretch and 'bedding-in' would be
expected to reduce the apparent rope diameter, but
the wire diameter should remain unchanged unless
plastic deformation has occurred.
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TENSILE TESTING
The applied load (232 kN) did not represent
complete failure of the rope, but rather fracture of
11 strands (66 wires) out of a total of 18 strands
(108 wires).
Other results are presented in Table below;
The measured breaking force for the section cut
from the rope was 232 kN (approximately 23.2
tonnes). This is some 30% lower than would be
expected.
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ROPE DAMAGE ANALYSIS
Typical damage to the rope is shown in Figure 4.
This damage is analyzed to determine the likely cause
of this damage and to investigate the probable
inference of this for the present failure.
Fig.4
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ROPE DAMAGE ANALYSIS
High Strands
The damage is not due to a high strand although
at first glance the surface of wires in Figure 4 may
seem worn due to the presence of the flat spots.
Close inspection reveals cracks associated with the
flat regions. Little evidence of any severe abrasion is
evident.
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ROPE DAMAGE ANALYSIS
Fatigue- Small Sheave
Although cracks are visible in the wires in Figure
4, which is likely to indicate operation of a fatigue
mechanism, no wires have been bent out of place.
Repeated bending over too small a sheave would
lead to such movement of individual wires.
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ROPE DAMAGE ANALYSIS
Fatigue- Normal Sheave
The observed damage appears to correlate withfatigue damage occurring through bending undernormal loads over a sheave of the correct size. The
flat regions are surface deformation, possiblyoccurring as a result of groove wear during service.
Plastic deformation to regions of the surface ofsome wires has occurred during service. Fatiguecracks have been induced at some of thesehardened regions during bending over the sheave.
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ROPE DAMAGE ANALYSIS
Inference- secondary Effect
This seems to be a correct inference from the tensile testdata. Some fatigue cracking of wires is present in the rope, andthe breaking load has been reduced by some 30%. Nonetheless,this load of 232 kN (23.2 tonnes) is much higher than the load
reportedly being lifted at the time of failure (2.5 - 3 tonnes). Thefatigue cracking is unlikely to have been a direct contributor tothe failure, unless one rope section was more defective thanindicated by this single tensile test.
A 30% reduction in breaking load due to the presence offatigue cracks implies that the mechanism is unlikely to havebeen a primary cause of the failure, unless the section of ropewhich failed was more defective than the test piece.
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ROPE DAMAGE ANALYSIS
Inference -Primary Effect
It is unlikely that fatigue cracking in the wires
was a primary contributory factor in this failure. The
observed breaking load of 232 kN (23.2 tonnes) is
much greater than the load reportedly being lifted. It
is worth keeping in mind, for possible future review,
the possibilities that either the loadwas much greater
than reported or that a particular rope section wasmore defective than implied by the tensile test.
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FRACTOGRAPHY
A number of individual broken wires were cut off the fractured ends
and examined at low magnification using stereo binoculars, and athigh magnification in a scanning electron microscope (SEM).
The total number of wires in all strands was 108, and 20 wires were
selected from the outer strands and 11 from the inner strands.
The wires were de-rusted and ultrasonically cleaned in a de-greasingagent.
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FRACTOGRAPHY
Summary of fracture types observed with number of occurrence is
presented in Table below;
TotalWires
Tensile Cup and cone fracture Flat Twisted FailureFlat semi-elliptical Regions
present
Inner
Strands
Outer
StrandsTotal
Inner
Strands
Outer
StrandsTotal
Inner
Strands
Outer
StrandsTotal
31 1 2 3 0 2 2 9 17 26
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FRACTOGRAPHY
Typical SEM observations of the fracture surfaces are given below at
both a low and a high magnification.
Type 1: Tensile Cup and cone fracture
Low magnification fractograph of cup-and-cone. High magnification fractograph from the central
region of the cup-and-cone fracture.
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FRACTOGRAPHY
Type 2: Flat Twisted Failure
Low magnification image High magnification image
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FRACTOGRAPHY
Type 3: Flat semi-elliptical Regions present
Low magnification image High magnification fractograph from flat semi-
elliptic region shown with arrow.
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FRACTOGRAPHY ANALYSIS
Based on the available information deduced up till this stage, fast
fracture and fatigue are the likely contenders for identification ofsubject failure.
Different fracture modes are considered to determine the
mechanism of failure indicated by these types.(1~3).
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FRACTOGRAPHY ANALYSIS
Type-1 Cleavage
This is not cleavage, which is the micro-
mechanism of brittle fracture. The macrograph
shows a tensile cup-and-cone fracture which is
indicative of ductile overload failure. The outerregion is expected to show shear and the inner
region to be ductile.
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FRACTOGRAPHY ANALYSIS
Type 1-Microvoid Coalescence
Micro-void coalescence (MVC) is the micro-
mechanism of ductile tensile fracture. In fine scale
microstructures, the voids are quite small and the
initiating inclusions or second phase particles may
be sub-micron in size. The white network in the
picture represents the tear ridges between voids.
Only 3 out of the 31 wires have failed purely by
tensile overload (~ 10%).
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FRACTOGRAPHY ANALYSIS
Type 2-Cleavage
Torsional overload failure has occurred right
across the section, except for the small white point
which is the final region of shear. The failure will be
ductile, not brittle.
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FRACTOGRAPHY ANALYSIS
Type 2-Microvoid Coalescence
MVC is the correct micro-mechanism of fracture,
and it has occurred under a shear stress, thus the
voids are slanted in the direction of applied stress. 2
wires in the outer strands failed by torsionaloverload from the rope experiencing twisting during
failure.
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FRACTOGRAPHY ANALYSIS
Type 3-Microvoid Coalescence
Small flat semi-elliptic regions are usually a sign of
the presence of a pre-existing defect and would not
generally be expected to have occurred by MVC.
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FRACTOGRAPHY ANALYSIS
Type 3-FatigueThis is the most likely explanation for the presence of the small
semi-elliptic regions seen on many of the selected wires. They are
associated with the flattened regions, but fatigue striations cannot be
resolved in this fine scale quenched and tempered microstructure.
There are subtle differences in the appearance of the white ridgesbetween the MVC and the fatigue fractographs.26 wires show
evidence of the existence of small fatigue cracks. Such defects would
reduce the breaking load considerably, but note that the remainder
of the fracture surface shows evidence of ductile fracture, indicatingthat the toughness of the individual wires is high.
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FRACTOGRAPHY ANALYSIS
Type 3-Cleavage
The white tear ridges demonstrate the
operation of a ductile mechanism of failure.
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Third STEP
Summary
Conclusion
Recommendations
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SUMMARY
Overall, the strength of the rope was reduced by the presence of fatigue cracks -
this is evidenced by the observed tensile strength of 232 kN compared with themanufacturers stated breaking load of 332 kN.
The observed breaking load is still very much higher than the stated load being
lifted at the time of failure (some 25 - 30 kN). Thus the rope must still have failed
through application of an overload relative to its current strength level. The failure
was not solely due to the presence of fatigue cracks (whose existence is fairlynormal in wire ropes and explains the requirement for regular maintenance and
high factors of safety).
The cause of this overload is not clear, but bouncing of the load might have
allowed the rope to jump from its groove and jam between sheave and boom
during winding. If this state of affairs could exist for a short time undetected, andthe rope winding was continued, a very significant overload could be applied to the
rope.
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SUMMARY
The cause of the fatigue cracks needs clarification. They can initiate as a result of
bending stresses induced by too small a sheave diameter. The recommendeddiameter is 18x rope diameter which equals 432 mm for the resent rope. The
actual sheave diameter was 520 mm, which should have been sufficient. As the
sheave was older than the rope, however, it is possible that wear of the sheave
groove has had an influence. Fatigue cracks can result from deformed surface
regions where ductility thus becomes exhausted, particularly if surface damagefrom abrasion occurs. This would be exacerbated by any decrease in sheave groove
diameter, which could occur by wear during service, and by poor lubrication
practice (which was apparently the case).
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CONCLUSION
The conclusion to be drawn from this investigation is that the
presence of fatigue cracks has lowered the breaking load of the ropeby some 30%. However, the breaking load is still 232 kN, very much
higher than the stated lifting load of 25 - 30 kN. The fractographic
work has indicated ductile fracture in all wires, demonstrating that
the rope metallurgy is up to specification. The most likely cause ofthe fracture seems to be rope jamming between sheave and groove,
probably due to bounce during lifting. The cause of the bouncing is
unknown.
In insurance terms, poor maintenance is not a prime cause of the
failure, which would have been "sudden and unexpected" when it
occurred. Cover should exist for such circumstances.
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RECOMMENDATIONS
Recommendations in the present case are:
Ensure adequate lubrication is maintained in the rope.
Re-groove the sheave at regular intervals and, particularly, when the
rope is replaced by a new one.
Control lifting to avoid bouncing and install detectors which are
activated by rope coming off the sheave.
Monitor condition of rope by surface inspection and tensile testing.
S
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REFERENCES
1. J Llorca and V Sanchez-Galvez (1989) Fatigue and Fracture of Engineering Materials and Structures
Vol. 12 No. 1 pp31-45
2. RE Hobbs and K Ghavami (1982) International Journal of Fatigue April 1982 pp69-72
3. NF Casey and WK Lee (1989) International Journal of Fatigue Vol. 11 No. 2 pp78-84.
4. M Alani and M Raoof (1997) Effect of mean axial load on axial fatigue life of spiral strands,
International Journal of Fatigue Vol. 19 No. 1 pp1-11
5. K Coultate (1997) Magnetic attraction of wire rope testing, Materials World Vol. 5 September 1997
pp519-520.6. K Schrems and D Maclaren (1997) Failure analysis of a mine hoist rope, Engineering Failure Analysis,
Vol. 4 No. 1 pp25-38.
7. MD Kuruppu, A Tytko and TS Golosinski (2000) Loss of metallic area in winder ropes subject to
external wear, Engineering Failure Analysis, Vol. 7 No. 3 pp199-207.
8. J-I Suh and SP Chang (2000) Experimental study on fatigue behaviour of wire ropes, International
Journal of Fatigue Vol. 22 pp339-347.9. M Torkar and B Arzensek (2002) Failure of crane wire rope, Engineering Failure Analysis, Vol. 9 No. 2
pp227-233.