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TRANSCRIPT
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HSEHealth & Safety
Executive
Evaluation of performancedeterioration in compacted
strand wire ropes
Prepared by the Health and Safety Laboratoryfor the Health and Safety Executive 2006
RESEARCH REPORT 487
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HSEHealth & Safety
Executive
Evaluation of performancedeterioration in compacted
strand wire ropes
Paul McCann BEng
Health andSafety LaboratoryHarpur Hill
BuxtonDerbyshire
SK17 9JN
Compacted strand ropes are used by the offshore industry in a number of applications. The compactedstrand construction offers improved performance compared with other stranded rope constructions, allowing its use for safety critical operations, including cranes and diving systems. However, previous work by Reading University, to evaluate the fatigue performance of galvanised compacted strand rope (HSE research project number 3422) and HSL, to evaluate the effects of variations in capping technique (HSE research projectnumber 3596) had identified a potential deterioration in the mechanical properties of the rope over time. This deterioration occurred regardless of whether the rope was in service or storage during this time period. Therope manufacturer, was made aware of this problem and production of this type of rope was temporarily suspended while the problem was evaluated and manufacturing techniques revised. Production has nowresumed using modified procedures. Field Engineering Section of the Health and Safety Laboratory (HSL) were requested by Mr J MacFarlane of Offshore Safety Division (OSD), HSE, to evaluate the extent of this problem. This investigation involved two sizes of rope, 25 mm and 32 mm diameter subjected to both dynamic and static loading.This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the author alone and do not necessarilyreflect HSE policy.
HSE BOOKS
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Crown copyright 2006First published 2006All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted inany form or by any means (electronic, mechanical, photocopying, recording or otherwise) without the priorwritten permission of the copyright owner.Applications for reproduction should be made in writing to: Licensing Division, Her Majesty's Stationery Office, St Clements House, 2-16 Colegate, Norwich NR3 1BQ or by e-mail to [email protected]
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CONTENTS1 INTRODUCTION......................................................................................... 12 COMPACTED STRAND ROPES................................................................ 23 TEST SET UP ............................................................................................. 43.1 Test Programme ...................................................................................... 43.2 The dynamic test facility .......................................................................... 43.3 The static test facility ............................................................................... 83.4 Sample preparation ................................................................................. 84 PREMATURE SOCKET FAILURES......................................................... 104.1 Examination of failed sockets ................................................................ 104.2 Hardness tests....................................................................................... 15
4.3 Charpy impact tests ............................................................................... 164.4 Tensile tests .......................................................................................... 164.5 Other premature failures........................................................................ 175 25 MM DIAMETER ROPE RESULTS....................................................... 185.1 25 mm static test results ........................................................................ 185.2 25mm dynamic test results .................................................................... 206 32 MM DIAMETER ROPE RESULTS....................................................... 246.1 32 mm static test results ........................................................................ 246.2 32 mm dynamic test results ................................................................... 26
6.3 32 mm samples with instrumented sockets ........................................... 297 COMPARISON BETWEEN 25 MM AND 32 MM ROPE ........................... 347.1 Static Testing......................................................................................... 347.2 Dynamic Testing .................................................................................... 348 COMMENTS ............................................................................................. 379 RECOMMENDATIONS............................................................................. 3910 REFERENCES ...................................................................................... 40
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EXECUTIVE SUMMARY
Objectives
Compacted strand ropes are used by the offshore industry in a number of applications. Thecompacted strand construction provides improved performance, allowing its use for safety
critical operations, including cranes and diving systems.
However, previous work by Reading University, to evaluate the fatigue performance of
galvanised compacted strand rope, identified a potential deterioration in the mechanical
properties of the rope over time. This deterioration occurred regardless of whether the rope was
in service or storage during this time period.
Field Engineering Section of the Health and Safety Laboratory (HSL) were requested by Mr. J.
MacFarlane of Offshore Safety Division (OSD), HSE, to evaluate the extent of this problem.
This investigation involved two sizes of rope, 25 mm and 32 mm diameter subjected to both
dynamic and static loading.
Main Findings
Static testing of both sizes of wire rope, identified a general reduction in mechanical properties
over time. The static breaking load is estimated to have dropped below the manufacturers
specified minimum breaking load after four months for 25 mm rope and 16 months for 32 mm
rope. In the worst case, the breaking load of the rope was 12% below the manufacturers
specified minimum breaking load. However while this is significant, it is unlikely to have
critical safety implications considering the safety factors of 5:1 quoted for these ropes.
During dynamic testing none of the 25 mm test-pieces and only three of the 32 mm test-pieces
met the manufacturers specified minimum breaking load. Dynamic testing identified variationsin rope properties during the programme but did not identify any reliable trends in rope
deterioration.
The dynamic testing programme, suffered from premature failure of the sockets used to
terminate the test-pieces, particularly on 32 mm rope. Failures remained ongoing throughout the
test programme and premature failure occurred during 40% of the 32 mm dynamic tests. This
was a dynamic effect that was not replicated during static testing. Strain measurement identified
very high strain levels in the socket during dynamic testing. Evidence suggests that premature
failure did not result from defective sample preparation.
Examination and testing of these sockets showed that they were compliant with RR-S-550D
Federal Specification Sockets Wire Ropes, the standard to which they were supplied.Metallographic examination identified that failure had occurred by a single stage overload
event. The initiation point showed localised ductile failure, followed by brittle cleavage.
Examination of sockets revealed several potential initiation points including, internal socket
grooves, manufacturers markings, manufacturing defects and a lack of tolerances on
dimensional variations. It is likely that improvements could be made to socket design and
quality control, but such changes would have cost implications.
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Recommendations
The rope manufacturer, was informed of the problem of strength deterioration with a
recommendation to undertake corrective actions. Production of this type of rope was
temporarily suspended while manufacturing techniques were revised. Production has now
resumed using modified procedures and strength deterioration is no longer a concern.
However, the premature failure of rope sockets at relatively low loads during high strain rate
loading may be more significant. In service these sockets are single line components used in
safety critical applications, and the effects of a premature failure could be potentially
catastrophic.
The sockets used during this project were open spelter sockets in both normal and heat treated
conditions. All sockets were supplied by the same manufacturer. However, this problem may
not be confined to one type and size of socket. While a full investigation of the problem was
beyond the scope of this project, further work investigating the reasons for premature failure inthese sockets and the potential for similar behaviour in other socket types is recommended.
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1 INTRODUCTION Compacted strand ropes are used by the offshore industry in a number of applications. The
compacted strand construction offers improved performance compared with other stranded rope
constructions, allowing its use for safety critical operations, including cranes and diving
systems.
However, previous work by Reading University, to evaluate the fatigue performance of
galvanised compacted strand rope (HSE research project number 3422) and HSL, to evaluate
the effects of variations in capping technique (HSE research project number 3596) had
identified a potential deterioration in the mechanical properties of the rope over time. This
deterioration occurred regardless of whether the rope was in service or storage during this time
period.
The rope manufacturer, was made aware of this problem and production of this type of rope was
temporarily suspended while the problem was evaluated and manufacturing techniques revised.Production has now resumed using modified procedures.
Field Engineering Section of the Health and Safety Laboratory (HSL) were requested by Mr. J.
MacFarlane of Offshore Safety Division (OSD), HSE, to evaluate the extent of this problem.
This investigation involved two sizes of rope, 25 mm and 32 mm diameter subjected to both
dynamic and static loading.
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2 COMPACTED STRAND ROPES Compacted strand refers to a method of producing a rope with compacted strands. The strand,
consisting of round wires, is drawn through a dye or rolls, plastically deforming these wires.
This process reduces strand diameter, improves contact conditions between the individual wires
in each strand and the separate strands and raises mechanical properties. Compacted strand
ropes are made by a number of manufacturers. A comparison between the breaking strength of
compacted strand ropes and their nearest non-compacted equivalent is shown in Figure 1, this
graph is based on published information.
Minimumb
reakin
gforce(kN)
1500
1000
500
0
Non-compacted strand
Compacted strand
Figure 2 Rope Construction10 15 20 25 30 35 40
Rope diameter (mm)Figure 1 Comparison of minimum breaking strengthof compacted strand and non-compacted equivalent
The rope used during this project was a 34 LR, 2160 grade galvanised WSC. This is a multi-
strand, low rotational (LR), compacted strand rope. The rope consists of 34 shaped strands
around a central wire strand core (WSC), each consisting of seven wires. The designation 2160
identifies to the original tensile grade of the wire used. The construction of this rope is shown in
Figure 2. The ropes used in this investigation were right hand lay, meaning that the strands twist
in the same direction as a right hand thread while the wires in the strand twist in the opposite
direction.
This project used two sizes of rope, 25 mm and 32 mm diameter. The manufacturers specified
mechanical properties for these ropes are shown in Table 1.
Table 1 Breaking strength of compacted strand 34 LR 2160 gradegalvanisedRope dia. (mm) Mass (kg/100m) Minimum breaking strength
(kN) (tonne)
25 312 572 58.3
32 505 927 94.5 At the time of testing, the relevant British Standard for stranded wire ropes was BS 302
Stranded steel wire ropes- Parts 1 to 8. This has subsequently been partially superseded by BS
EN 12385 Steel wire ropes Safety Parts 1 to 5.
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BS 302 Part 1 Specification for general requirements (1987) includes a formula for
calculating minimum breaking force and specifies values for the minimum breaking force factor
(K) for different rope constructions.
Minimum Breaking Force (kN)= Breaking force factor (K) x Rope diameter2
x wire tensile grade
1000
BS EN 12385 Part 4 Stranded ropes for general lifting applications (2002) quotes minimum
breaking forces for this basic rope construction. A comparison of minimum breaking forces is
given in Table 2. The figures quoted are for the basic rope construction and do not consider the
effects of the compacted strand production method.
Table 2 Minimum breaking strengths for 34 x 7 stranded wire rope (2160 grade)Rope dia.
(mm)BS 302-1 (1987) (calculated) BS EN 123854 (2002)
(specified/ calculated)Mass Minimum breaking Mass Minimum breaking
(kg/100m strength (kg/100m) strength(kN) (tonne) (kN) (tonne)
25 - 429.3 43.8 284 472.5 48.2
32 - 703.4 71.7 465 774 78.9
As there had been a temporary suspension of production of compacted strand rope, 800 m of
each rope diameter was manufactured as part of a special order for HSL to use during this
investigation. This rope was supplied as Order number D9201488/92, full details of the ropes as
supplied are shown in Table 3.
Table 3 Details of rope supplied from test certificatesRope Test certificate no. Rope no. Product Code Date Actual Actual
Dia made Dia. (mm) Breaking(mm) Load (t)
25* Z50404A/W9201708/98 Z50404A 25.034074S5B02 18/11/98 25.72 60.46
Z50404B/W9201708/98 Z50404B 25.72 60.46
32 Z50405A/W9201709/98 Z50405A 32.034074S5B01 22/10/98 33.14 96.00
* The 25 mm rope was supplied in two separate 400 m lengths
The test certificate specifies a coefficient of utilisation (or safety factor) of 5:1 for these ropes.
Safe working loads for these ropes would be 11.66 tonnes and 18.9 tonnes for 25 mm and 32
mm ropes respectively.
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3 TEST SET UP
3.1 TEST PROGRAMME
Samples from each rope diameter were tested at measured time intervals. The initial
programme, proposed that each batch of tests should consist of the following:
Three static tests to destruction.
One dynamic test to destruction.
Three dynamic tests without destruction.
Three static tests taken from one end of the damaged dynamic samples above.
Prior to commencing the main test programme, commissioning tests were carried out toevaluate the viability of the test procedure and facility.
The unexpected premature failure of sockets caused a necessary deviation from the original
timescales and test programme. Changes to test programme are discussed in the Results sections
(Section 5, for 25 mm rope and Section 6, for 32 mm rope).
3.2 THE DYNAMIC TEST FACILITY
The Field Engineering impact track is a large scale dynamic test facility at HSL, Buxton. The
facility uses the natural contours of the surrounding valley to accelerate free rolling impact
trucks, running on a double set of rails. This can be used to generate impact energies up to 1 MJdepending on the test configuration and the combination of trucks used. The facility is shown in
Figure 3. This facility has previously been used for dynamic tests on a range of components
including wire ropes used in the mining industry and metallic and non-metallic lifting slings.
The track consists of four separate sections, on the southern side of the valley there is a 76 m
long section with gradient of 1 in 4, leading into a 87 m long section with a gradient of 1 in 7.
On the northern side of the valley there is a 87 m section with a gradient of 1 in 7. Between the
two opposing slopes there is a short level section 18 m long, which is generally selected as the
position of impact. This level section is covered and has overhead mountings for
instrumentation or cameras. The adjacent building houses track control and data acquisition
equipment.
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A schematic illustration of the impact test technique is shown in Figure 3. A number of impact
(or hammer) trucks are available, with different masses. The dynamic force introduced into a
test-piece was controlled by varying the position on the gradient of the Southern slope fromwhich the impact truck was released. The track has been calibrated in terms of impact speed and
energy generated for each impact truck released from any given height. Losses due to friction,
bearing inefficiency, etc have been taken into account.
Throughout the test programme, drop heights were varied in an attempt to control the energy
applied to the test sample and limit the potential for premature failure of the rope sockets.
This programme used a single impact truck, weighing 5500 kg. This truck consists of two
substantial steel plates connected by transverse members. The trucks open structure, allows it
to pass over obstructions on the track. On the inner surfaces of these plates were mounted two
catcher plates. The impact surfaces of these catcher plates mount thin walled crushable tubes
minimising damage and ringing between the catcher plates and T piece on impact. This
truck is shown in Figure 5.
The test-piece was fixed on the level section of the track. One end of the test rope sample was
connected to a fixed mounting point between the inner rails of the track. This mounting point
contained an integral Strainstall type 5316, 1500 kN shear pin type loadcell serial number
64908. The loadcell mount is shown in Figure 6. Throughout the test programme, this loadcell
was calibrated annually by the manufacturer, in accordance with HSL calibration procedure
FE/CP/20 Loadcells General. There was no significant variation in this calibration during
the test period. Loadcell output was recorded using a Nicolet high speed data logger, operating
at 50 kHz.
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(a) Before release
Hammer
Disposable
Rope
Truck
Support
Test
(b) After release
Fi
Slope
xed load cellmounting
Southern NorthernSlope
(c) Overhead view of hammer truck and T-piece
Figure 4 Schematic illustration of Loading mechanism during dynamic testing
The free end of the test sample was connected to a flying T piece, which is shown in Figure
5. Each rope diameter had a specific T piece. Prior to the test, the T piece was held in place
on a disposable wooden support and a nominal rope tension was introduced to hold the sample
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in the correct position and reduce rope sag. As the impact truck passed over the T piece, it
was collected by the catcher plates, introducing a dynamic load.
In the event of a rope failure, the impact truck was halted by a 10.6 tonne brake truck positioneda short distance up the Northern slope. This truck bridged both rails and sat on skids covered by
a high friction material.
The displacement of the T piece, on impact was measured using a line scanning camera (for
tests carried out before the 15th
August 2000) or high speed video system operating at 4,500
frames per second (for tests after the 15th
August 2000). Both systems were mounted on a
moveable trolley, directly above the T piece. Line scan camera data was analysed using
software developed in house by Control and Instrumentation Section (CIS). High speed video
footage was recorded by Visual Presentation Section (VPS) and analysed using OPTIMAS
software.
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On a daily basis, prior to commencing testing, the data-logger and amplifiers were calibrated
using a Time 2003S dc voltage source, in accordance with HSL calibration procedure,
FE/CP/38 Digital data-loggers. The line scan and high speed cameras were calibrated by
movement of a target against a fixed scale.
Test data was analysed using Sigmaplot 2000 for Windows, version 6.10, the load data and
displacement data from the linescan camera (or high speed value) were used to create load
displacement curves. An in-house Sigmaplot transform was used to calculate peak load, peak
displacement and energy absorbed by the rope.
3.3 THE STATIC TEST FACILITY
Static tests were carried out using an RDP / Avery 74N8, 4 MN, long bed universal test machine
with digital control, situated at HSL, Sheffield. This machine was calibrated on an annual basis
to BS EN 10002 Part 2 and NAMAS specifications. There was no significant variation in this
calibration during the test period.
Loading rates of 133.3 and 200 kN per minute were used for static tests on 25 mm and 32 mm
samples respectively.
Rope extension under load was measured using a Wallace extensometer, a non contact optical
device which could track the relative movement of two marked positions on the surface of the
rope. Rope extension was measured over a gauge length of 1500 mm , across the centre of the
sample. This extensometer was calibrated before every use.
Load and displacement data was recorded using a notebook PC with a logger card running HSL
DAQ studio software developed in-house by Control and Instrumentation Section, HSL.
Test data was analysed using Sigmaplot 2000 for Windows, version 6.10, the load data and
displacement data from the Wallace extensometer were used to create load displacement
curves. An in-house Sigmaplot transform was used to calculate peak load, peak displacement
and energy absorbed by the rope.
3.4 SAMPLE PREPARATION
The test rope was stored on the reel, under cover, in a heated building. Samples were removed
and prepared in batches as required.
The test samples were prepared by experienced technicians using best practice rope cappingtechniques as described in the Ropemans Handbook and current mining industry guidance.
Sample preparation complied with BS 7035 Code of practice for Socketing of stranded wire
ropes.
The test samples were terminated at each end, with a galvanised open spelter type socket.
However, due to premature failure of these sockets (Section 4), these were replaced with higher
strength, heat-treated, un-galvanised sockets). For 25 mm test-pieces, 1 sockets were used,
suitable for 24 26 mm rope. For 32 mm test-pieces, 11/4 1
3/8 sockets suitable for 32 35
mm rope were used. The principal dimensions of the socket, as supplied by the manufacturer,
are shown in Table 4. These dimensions are compliant with RR-S-550D Federal Specification
Wire Ropes (1980) socket type A.
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As a requirement of RR-S-550D, the inner cone (or basket) of each socket had two
circumferential grooves. These are intended to help secure the resin cone within the socket.
RR-S-550D specifies the number and depth of these grooves but not their position.
Table 4 Principal dimensions of the open spelter socket (as supplied by themanufacturer)
(
mouth)
A B C D E F G H I
1 2 Deep
25mm 21 85
32mm 25
AB
C
D
E F
G
IH
Position of
groove from
267 102 123 51.0 28.7 45.2 91.0 51.0 95.5 2.4
333 117 138 63.5 38.1 58.0 110 63.5 121 105 2.4
All dimensions in mm
RR-S-550D specifies that sockets should be manufactured from steel with a minimum tensile
strength of 70,000 lbs per square inch (482.6 N/mm2) and a minimum elongation of 15%. The
sockets used during this programme were purchased in several batches, minimum breaking
loads for the 32 mm sockets were specified as 102.0 to 136.0 tonnes.
Sockets were secured with resin cappings (or cones). These were prepared using two part
Wirelock resin capping kits. All spelter sockets were discarded after a single use. Spelter
sockets were not recovered and re-used between tests.
Dynamic test samples measured nominally 15 m in length. End terminations were orientated at
90o
to each other on the axis of the rope to allow connection between the load cell mounting and
T piece. Static test samples measured nominally 2.67 m in length.
Test samples were conditioned prior to test. Conditioning is required to fully bed the resin
cones into the socket, prior to the first service loading. Early tests used a 10 tonne hanging
mass for conditioning. The procedure was subsequently amended and later tests (after February
1999) used an RDP / Avery 4 MN test machine. The conditioning process involved loading
samples to 20% of the minimum breaking load of the rope and holding that load for a minimum
period of 5 minutes. The conditioning loads were 115 kN and 185 kN for 25 mm and 32 mm
rope respectively.
Where static samples were prepared from damaged dynamic samples, they were taken from one
end of the original test-piece and retained one of the original terminations. Re-capping
techniques were identical to those used for preparation of the original wire rope samples. A
record was kept of any broken wires found during the re-capping process.
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Macroscopic features on the fracture surface indicated that failure initiated near the outer wall of
the socket at a position 23 mm from the socket mouth. This corresponds with the positions of
three potential initiators; the position of minimum wall thickness, a circumferential internal
socket groove and the external manufacturers stamp. The position of initiation is identified in
Figure 9.
Scanning Electron Microscopy revealed that the majority of the fracture occurred by brittle
cleavage with a small area of ductile microvoid coalescence near the initiation site. The socket
had failed from a single stage overload. Metallography revealed a normalised structure of ferrite
with bands of slightly spherodised lamellar pearlite. There were elongated inclusions and
general banding indicative of the hot working associated with a forged component.
Drillings were taken for chemical analysis and these results are shown in Table 6. The federal
specification RR-S-550D states that sockets should be manufactured from steel meeting
material standard FED-STD-66 class 1035 or 1038, with a typical carbon content of 0.32% to0.42% and a manganese content of 0.6% to 0.9%.
These sockets would have met the specifications for both grades. However, this socket would
not have met the British standard BS 463 Part 2 Specification for sockets for wire ropes
(1970), which specifies a maximum carbon content of 0.30%.
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Table 6 Chemical Analysis ofsocket 320002
% +/- % error
Carbon 0.37 0.01Silicon 0.20 0.02
Manganese 0.72 0.02
Phosphorous 0.025 0.002
Sulphur 0.014 0.002
Chromium 0.18 0.02
Molybdenum
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4.2
4.1.5 Socket vss25-2This was the only 25 mm socket failure, this socket is shown in Figure 12. Vss25-2, failed in a
similar manner to the 32 mm sockets, ejecting a fragment approximately 59 mm long. Failure
initiated at a point 17 mm from the socket mouth, wall thickness at this point was 9.7 mm. Wall
thickness at the socket mouth varied between 6.5 mm and 8.8 mm (the manufacturer specifies a
nominal wall thickness of 8.25 mm). Failure occurred by brittle cleavage with a small ductile
area at the position of initiation. The socket had failed from a single stage overload.
Metallography revealed structures indicative of a forged product.
HARDNESS TESTS
Hardness tests were carried out using a 10 kg load, in accordance with BS 427 Method for
Vickers hardness test and for verification of Vickers hardness test machines (1990). Results are
shown in Table 7. These results have a specified accuracy of+/- 3%
Table 7 Hardness testing of failed socketsHardness Average Equivalent tensile
Range Hardness strength (N/mm2)
320002 198-213 203 -Vss32-1 183-195 191 652
Vss32-2 215-224 219 751
Vss32-3 210-213 211 706
Vss25-2 228-238 233 771
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4.3
4.4
Hardness values were consistent with observed microstructures. Equivalent tensile strengths
calculated from these results exceeded the minimum requirement of 483 N/mm2
specified in
RR-S-550D.
CHARPY IMPACT TESTS
Charpy impact tests were carried out on test-pieces removed from socket vss32-1 and 320002,
in accordance with BS EN 10045 Part 1 Charpy impact tests on metallic materials; test
method (V and U notches) (1990). Tests were carried out at ambient temperature, -5oC, -10
oC, and -20
oC.
All samples were taken longitudinally from the body of the socket baskets. Test samples from
320002 were of the standard geometry, measuring 10 mm by 10 mm by 55 mm. Test samples
from vss32-1 were of a reduced geometry, measuring 7.5 mm by 10 mm by 55 mm and results
were scaled up to a 10 mm by 10 mm equivalent. Results from Charpy impact tests are shown in
Table 8.
Table 8 Charpy impact testing of failed socketsTemp Energy Average 10 x 10
(oC) Range (J) Energy (J) equivalent (J)
320002 ambient 35 - 39 37 37
Vss32-1 -5 23 - 26 24 32
-10 9 - 18 14 18
-20 5 - 9 7 9
Examination of fractured surfaces identified a mixture of brittle cleavage and ductile microvoid
coalescence fracture modes. There was a significant reduction in Charpy impact energies as the
test temperature was reduced. Tests at 5oC showed a reduction in impact energy of 35%. Tests
at 10o
C showed a reduction in impact energy of 62%. It is possible that temperatures this lowcould be experienced in an offshore environment.
TENSILE TESTS
Tensile test-pieces were removed longitudinally from the body of the socket basket of socket
320002. Tensile tests were carried out in accordance with BS EN 10002 Part 1 Tensile testing
of metallic materials; method of test at ambient temperatures (1990). Tensile results are shown
in Table 9.
Table 9 Tensile testing of failed sockets
Proof stress UTS Elongation(N/mm2) (N/mm2) (%)
320002 369 - 400 601 - 617 20.0 22.1 These samples exceeded the tensile strength and elongation requirements (minimum 483 N/mm
2
UTS and 15% elongation specified by RR-S-550D. The results were lower than the equivalent
tensile strengths calculated from hardness testing.
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4.5 OTHER PREMATURE FAILURES
Following the replacement of the basic galvanised sockets with higher strength equivalents and
the modifications to the conditioning procedure, four further premature failures occurred. All of
these failures occurred during tests on 32 mm rope. These sockets were identified as 32 mm
tests 6, 7, 10 and 16.
A visual examination of the failed sockets identified similar features to those previously
encountered and a thorough metallurgical investigation was not considered to be necessary.
Failed socket 32mm-6 (320021) is shown in Figure 13.
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5 25 MM DIAMETER ROPE RESULTS
5.1 25 MM STATIC TEST RESULTS
Static tests on 25 mm rope were carried out over an extended period of 40 months. Results for
these static tests are shown in Table 10.
Table 10 Results for 25 mm Static Tests
Date no. Peak Max Max Energy NotesLoad Disp* Disp (kJ)(kN) (mm) (%)
16/12/98 1 583 83 5.5 28 Unused Rope
2 583 80 5.3 29 Unused rope
3 551 70 4.7 22 Unused rope. First run aborted at 470kN (problem
experienced with loading rate).
08/12/99 4 535 35 2.3 11.2 Unused Rope
5 536 33 2.2 10.8 Unused Rope
6 537 31 2.1 9.9 Unused Rope
17/01/00 7 494 19 1.3 0.8 Sample taken from dynamic test-piece no. 7.
3 broken wires found on recapping.
8 500 40 2.7 15.0 Sample taken from dynamic test-piece no. 8
9 500 22 1.5 5.4 Sample taken from dynamic test-piece no. 9
15/01/02 10 505 25 1.7 8.4 Unused Rope
11 505 28 1.9 6.6 Unused Rope
12 497 27 1.8 7.2 Unused Rope
21/03/02 13 482 25 1.7 6.4 Sample taken from dynamic test-piece.
14 494 24 1.6 5.9 Sample taken from dynamic test-piece
15 490 26 1.7 6.1 Sample taken from dynamic test-piece
* Displacement over a gauge length of 1500 mm
All test samples failed within the body of the rope. There was some distortion of the socket in
the vicinity of the pin-holes but this was not considered to be significant.
One of the three samples in the first batch, tested on 16th
December 1998 was below the
manufacturers specified minimum breaking load of 572 kN. However, problems were
experienced with the test machine and this result is not conclusive. Load against displacement
traces for these three tests are shown in Figure 14. All of the test samples, including damaged
test-pieces, were above the minimum values for this basic construction (without dye forming) as
calculated from BS302 Part 1 (429.3 kN) and BS EN 12385 Part 4 (472.5 kN).
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Load(kN)
600
500
400
300
200
100
0
Test 1
Test 2
Test 3
0 10 20 30 40 50 60 70 80
Displacement (mm)
Figure 14 Load vs displacement for static tests carried out on 16/12/98
There was a clear decrease in breaking loads and energy to failure for the unused 25 mm rope
over the test period. All tests carried out on subsequent dates failed to meet the manufacturers
minimum breaking loads. This is illustrated in Table 11.
Table 11 Deterioration of static breaking load for unused 25 mm rope.
Date Month Mean Load % of Min Mean Max Energy to
tested (kN) Breaking Load Disp (%) failure (kJ)
11/98 0 593.2 104 - -12/98 +1 583* 102 5.2 26.3 12/99 +13 536 94 2.2 10.6 01/02 +38 502 88 1.8 7.4
* Excludes test 3, where problems were encountered
From static test results, it was estimated that after four months of use or storage, the 25 mm rope
would have a breaking strength less than the minimum value of 572 kN specified by the
manufacturer. Deterioration in elongation to failure is also significant, percentage elongation
dropped from 5.2 in the first batch of tests to 1.8 in the last batch.
No broken wires were discovered when recapping the two batches of 25 mm ropes which had
previously experienced dynamic loading. A comparison of the performance of unused rope and
rope damaged by dynamic testing is shown in Table 12.
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5.2
Table 12 Comparison of mean static test results for 25 mm rope in the unused
and damaged conditions
Unused Damaged by dynamic test
Date Peak Max Energy Date Peak Max Energy
Load Disp (kJ) Load Disp (kJ)(kN) (%) (kN) (%)
08/12/99 536 2.2 10.6 17/01/00 498 1.8 7.1 15/01/02 502 1.8 7.4 21/03/02 489 1.7 6.1
As expected, samples taken from damaged ropes which had been previously loaded using the
Field Engineering Impact Track gave a consistently lower breaking load than unused samples
from the same batch. Damaged ropes showed a clear reduction in breaking strength and energy
to failure over time in a similar manner to undamaged ropes.
25MM DYNAMIC TEST RESULTS
The available impact energies for releases from each drop height used during these tests were
calculated and are shown in Table 13. These figures include potential losses due to friction, etc.
Release heights were varied during the test program, in an attempt to control the likelihood of
premature failure of sockets.
Table 13 Impact speeds and energies for the 5500
kg truck
Drop Track Impact Total
Height Distance Speed Energy
(m) (m) (m/s) (kJ)
3.0 21 6.83 128.2
3.5 25 7.45 152.6
4.9 35 8.81 213.6
5.6 40 9.42 244.1
Dynamic tests on 25 mm rope were carried out over an extended period of 38 months. Results
for these tests are shown in Table 14.
Generally, the 25 mm dynamic test event had a duration of 50 70 milliseconds from the onset
of loading until failure occurred. Where there was no failure, the time to reach the peak loadvalue is quoted. In two cases, this event had a greater duration (up to 101.7 ms), however it
should be noted that the impact events were complex in nature, with localized variations in the
load / time trace, therefore quoted durations are approximate and variations are not necessarily
significant.
The first and second batches of samples (tested on 2nd
February 1999 and 28th
April 1999) were
conditioned by suspending a load of 10 tonne, lower than the conditioning load used for
subsequent tests. Faulty conditioning was considered as a potential contributing factor in
premature socket failure and the conditioning procedure was amended. However, socket failures
continued to occur, principally on 32 mm rope.
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Load against displacement traces for the first batch of tests are shown in Figure 15. There are
clear differences between Figures 14 and 15, which illustrate the complex nature of the dynamic
event. Of particular interest are the differences between the traces for no failure (test 1), socket
failure (test 2) and rope failure (test 3). In test 2, the socket failed at a much lower load and
displacement than the rope failure in test 3.
The third batch of samples (tested 30th
November 1999) were capped using specially supplied
heat treated sockets and were proof loaded to 20% of minimum breaking load (115 kN or 11.7
t). Only one of these samples was tested to destruction. Static samples were taken from the other
three dynamic ropes and tested on 17th
January 2000.
Table 14 Results for 25 mm Dynamic Tests
Date no. Track Time to Peak Max Max Energy Notes
distance
(m)
failure
(ms)
Load
(kN)Disp*
(mm)
Disp
(%)
(kJ)
02/02/99 1 25 - 435.2 386 2.6 72.6 No failure**2 40 55.6 416 298 2.0 88.4 Socket failure T (moving) end
3 35 79.8 550.4 316 2.1 126.2 Rope failure
28/04/99 4 40 61.9 542.1 297 2.0 125.1 Rope failure
5 40 62.4 537 212 1.4 93.4 Rope failure
30/11/99 6 40 54.0 500.4 241 1.6 93.9 Sample tested to destruction. Rope failure.
7 25 51.2 471.6 234 1.6 49.8 No failure**
8 25 50.6 451.9 254 1.7 44.6 No failure**
9 25 88.8 483.7 234 1.6 50.3 No failure**
22/01/02 10 40 56.3 569.2 387 2.6 132.0 Sample tested to destruction. Rope failure.
11 21 101.7 498.6 310 2.1 - No failure**. Partial displacement.
12 21 67.5 525.5 306 2.0 19.0 No failure**. Partial displacement.
13 21 63.6 527.5 302 2,0 23.0 No failure**. Partial displacement.
14 40 59.1 559.5 373 2.5 134.9 Sample tested to destruction. Rope failure
in two positions
* Displacement at point of failure or maximum extension of sample under load.** Where no failure is recorded, then the test was not to destruction. In this case, time to peak load is quoted
instead of time to failure.
Displacement lost at peak load, absorbed energy quoted as indicator only.
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600500400300200100
0
-100
0 100 200 300 400 500 600 700
Displacement (mm)
Fi ure 15Load vs displacement for dynamic tests carried out on 02/02/99
)
Load(kN
Test 1
Test 2
Test 3
The fourth batch of samples tested on 22nd
January 2002, peak load to failure showed a
substantial improvement on previous batches including the original test batch, almost meeting
the manufacturers specified minimum breaking load. For the samples that were not tested to
destruction, the displacement trace was lost at the approximate position of peak load. It was not
possible to calculate accurate values of energy absorbed and the values in Table 14 are quoted
as indicator only. During the test on sample 14, failure occurred at two positions within the
body of the rope.
The deterioration of dynamic breaking load over time is summarised in Table 15. These figures
only represent dynamic tests to failure, where the rope failed rather than the socket.
Table 15 Deterioration of dynamic breaking load for 25 mm rope.
Date Month Load (kN) % of Min Max disp. Energy to
tested Breaking Load (%) failure (kJ)
02/99 +3 550.4 96 2.1 126.2 04/99 +5 539.6 94 1.7 109.2 11/99 +12 500.4 87 1.6 49.8 01/02 +38 564.4 99 2.6 133.4
All samples failed to meet the manufacturers specified minimum breaking load requirements of
572 kN although, dynamic samples 10 and 14, failed at loads only slightly below this minima.All of the rope failures were above the minimum values for this basic construction (without dye
forming) as calculated from BS302 Part 1 and BS EN 12385 Part 4. There were no clear trends
in terms of deterioration in breaking load or energy to failure over time.
In the single instance where a socket failed prematurely (2nd
February 1999), the socket failed at
416 kN, approximately 73% of the minimum breaking load. The load against displacement trace
for this test is shown in Graph 2. This is substantially below the load measured for failure to
occur in the body of the rope (550.4 kN) and the load measured where failure did not occur
(435.2 kN). Further details of this socket and its mode of failure are given in Section 4.1.5.
Absorbed energy to failure was also lower than for failures occurring in the body of the rope.
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A comparison of static and dynamic loads to failure is shown in Table 16. Initially, dynamic
tests produced a peak load to failure well below the peak loads measured during static testing.
However, this trend was reversed for the final batch of tests, carried out on 22nd
January 2002.
Table 16 Comparison of static and dynamic breaking load for 25 mmrope.
Date Month Static Dynamic
tested Mean Load Mean Max Load (kN) Max. disp.(kN) disp.(%) (%)
11/98 0 593.2 - -12/98 +1 583 5.2 - -02/99 +3 - - 550.4 2.1 04/99 +5 - - 539.6 1.7 11/99 +12 - - 500.4 1.6 12/99 +13 536 2.2 - -01/02 +38 502 1.8 564.4 2.6
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6 32 MM DIAMETER ROPE RESULTS
6.1 32 MM STATIC TEST RESULTS
Static tests on 32 mm rope were carried out over an extended period of 48 months. Results for
these static tests are shown in Table 17.
All test samples failed within the body of the rope. There was minimal distortion of the socket
in the vicinity of the pin-holes, however this was not considered to be significant.
Table 17 Results for 32 mm Static Tests
Date no. Peak Max Max Energy Notes
Load
(kN)Disp
(mm)
Disp
(%)
(kJ)
15/12/98 1 1,000 92 6.1 59 Unused Rope. First run aborted at 580kN due toproblem with machine loading rate.
2 980 92 6.1 56 Unused rope
3 980 91 6.1 56 Unused rope.
08/08/00 4 914 30 2.0 14.9 Unused Rope.
5 888 35 2.3 19.4 Unused rope
6 907 33 2.2 17.7 Unused rope.
14/09/00 7 917 - ** - ** - Unused rope. Strain-gauged socket
29/05/02 8 870 91 6.1 36.0 Unused rope.
9 876 39 2.6 18.6 Unused rope.
10 891 42 2.8 13.6 Unused rope. Failure commenced at 580 kN
31/10/02 11 923 30 2.0 14.8 Sample taken from dynamic test-piece no. 14.
2 broken wires found on recapping.
12 889 30 2.0 10.8 Sample taken from dynamic test-piece no. 15.
1 broken wire found on recapping.
13 889 28 1.9 12.0 Sample taken from dynamic test-piece no. 18.2 broken wires found on recapping.
* Displacement over a gauge length of 1500 mm
** No displacement was measured for this test. The failure of sample 8 was a two-stage process. Peak load occurred at a displacement of 37mm and an
absorbed energy of 9.9 kJ.
The first batch of ropes, tested on 15th
December 1998, failed at loads well above both the
manufacturers specified minimum expected breaking load of 927 kN and the certificated test
load.
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25
The second batch of rope samples were not tested until 8th
August 2000, this delay resulting
from the problems encountered with premature failure of the 32 mm sockets during dynamic
tests. This batch showed a significant drop off in terms of peak loads and displacements and
failed at a load below the manufacturers specified minimum breaking strength, as did all
subsequent batches.
The third batch of samples tested on the 29th
May 2002 are shown in Figure 16. The failure of
sample 8 occurred as a two-stage process. Partial failure and peak load occurred at a
displacement of 37 mm and an absorbed energy of 9.9 kJ, however, final failure occurred at a
lower load but much greater extension (91 mm). Under certain lighting conditions the Wallace
extensometer experienced problems with contrast, resulting in difficulties in distinguishing
between the optical target and its background and a noisy data signal.
The fourth batch of tests (tested 31st
October 2002) were comparable with the third batch of
tests (tested 29th
May 2002). However, it should also be noted that this was damaged rope from
dynamic test-pieces and that samples were known to contain broken wires.
All of the test samples, including damaged test-pieces, were above the minimum values for this
basic construction (without dye forming) as calculated from BS302 Part 1 (703.4 kN) and BS
EN 12385 Part 4 (774 kN).
There was a general decrease in breaking load of the unused 32 mm rope over time. This is
shown in Table 18.
Table 18 Deterioration of static breaking load forunused 32 mm rope.Date
tested
Month Mean Load
(kN)
% of Min
Breaking Load
Mean Max.
disp. (%)
Energy to
failure (kJ)
10/98 0 942 102 - -
12/98 +2 986.7 106 6.1 57
08/00 +22 903 97 2.2 17.3
05/02 +43 879 95 2.7* 16.1*
* Excludes sample 8
Figure 16 Load vs displacement for static tests carried out on 29/05/02
Displacement (mm)
0 10 20 30 40 50 60 70 80
Load(kN)
0
100
200
300
400
500
600
700
800
900
1000
Test 8
Test 9
Test 10
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6.2
From static testing it was estimated that after sixteen months of use or storage, the 32 mm rope
would have a breaking strength less than the minimum value of 927 kN specified by the
manufacturer.
A comparison of the performance of unused rope and rope damaged by dynamic testing is
shown in Table 19. Only one set of static test data was available for damaged rope and this
indicated that while load to failure had increased, extension and energy to failure were
significantly decreased.
Table 19 Comparison of mean static test results for 32 mm rope in the unused
and damaged conditions
Unused Damaged by dynamic test
Date Peak Max Energy Date Peak Max Energy
Load Disp (kJ) Load Disp (kJ)(kN)
(%)(kN)
(%)29/05/02 873 4.1* 37.2* 31/10/02 894 1.9 12.5
* Includes sample 8
32 MM DYNAMIC TEST RESULTS
The available impact energies for releases from each drop height used during these tests were
calculated and are shown in Table 20. These figures include potential losses due to friction, etc.
Release heights were varied during the test program, in an attempt to reduce the likelihood of
premature failure of sockets.
Table 20 Impact speeds and energies for the 5500
kg truck
Drop Track Impact Total
Height Distance Speed Energy
(m) (m) (m/s) (kJ)
4.9 35 8.81 213.6 5.6 40 9.42 244.1
11.3 80 13.32 488.2 13.0 90 14.30 562.4
Dynamic tests on 32 mm rope were carried out over an extended period of 48 months. Results
for these tests are shown in Table 21.
Generally, the 32 mm dynamic test event had a duration of 30 70 milliseconds from the onset
of loading until failure occurred. Where there was no failure, the time to reach the peak load
value is quoted. Where premature failure of the socket occurred the duration of the test tended
to be reduced.
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While testing the initial batches of samples, serious problems were encountered with premature
failure of the rope sockets used. These sockets failed at loads considerably below the minimum
breaking load, for example; test sample 4 failed at a load of 576.6 kN, approximately 62% of
the minimum breaking load. These unexpected circumstances made it necessary to deviate from
the project plan, evaluate the ramifications of this problem and verify the suitability of the
socket type being used.
Table 21 Results for 32 mm Dynamic Tests
Date no. Track Time to Peak Max Max Energy Notes
Distance
(m)
failure
(ms)
Load
(kN)Disp*
(mm)
Disp
(%)
(kJ)
26/11/98 1 90 30.4 775 169 1.1 121.4 Rope failure at fixed end.
2 90 52.8 668 - - -Socket failure at fixed end. No
displacement.
01/02/99 3 90 30.8 756.5 195 1.3 102.8 Socket failure T (moving) end.
4 90 - - - - - Socket failure T (moving) end. No data.
5 40 47.4 576.6 118 0.8 27.5Socket failure T (moving) end.
Partial displacement.
25/06/99 6 80 29.5 780 253 1.7 127.2 Socket failure T (moving) end
30/11/99 7 80 23.0 752 - - -Socket failure T (moving) end.
No displacement.
8 80 - 417
338
2.2
- Rope failure. Partial data.
15/08/00 9 90 33.4 873 361 2.4 164.0 Rope failure
13/09/00 10 90 31.1 776 - - - Socket failure (T) end. Straingauged socket
26/07/01 11 40 - - - - - Rope failure at socket. No data.
12 35 62.8 824 289 1.9 146.4 Partial rope failure 300 mm from fixed end.
13 35 65.5 830 293 2.0 147.4 Partial rope failure.
14/10/02 14 40 65.7 885.0 392 2.6 94.2 No failure **.
15 40 65.5 927.0 725 4.8 165.0 No failure **.
16 40 46.9 985.5 589 3.9 158.0 Socket failure at fixed end.
17 40 63.2 901.0 558 3.7 220.0 Rope Failure.
18 40 58.9 1066.0 453 3.0 86.0 No failure **.
* Displacement at point of failure or maximum extension of sample under load.
** Where no failure is recorded, then the test was not to destruction. In this case, time to peak load is quoted
instead of time to failure.
Partial results only.
No displacement measured for this test.
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The failure of the 32 mm dynamic sockets and the unpredictable nature of these tests led to
further complications, especially in terms of survivability of the normally robust impact track
instrumentation and the reliability of data gathering.
The first three batches of samples (tests 1 to 7) were conditioned by suspending a load of 10
tonne, substantially lower than the load used for subsequent tests. Faulty conditioning was
considered as a potential contributing factor in premature socket failure and the conditioning
procedure was subsequently amended.
Socket failures continued after the replacement of the galvanized sockets with heat treated
equivalents and changes to the proof loading technique (although in reduced numbers). This
suggests that these were not the only causes of premature failure.
Particular problems were encountered during test batches 2 (1st
February 1999) when a general
failure of the loadcell occurred. Without load data, displacement data could not be correctly
interpreted. Problems were again encountered during test batches 4 (30th
November 1999) and 6
(26th
July 2001), when the instrumentation wiring was severed during the test.
From the fifth batch onwards (15th
August 2000), samples were prepared using specially
supplied heat treated sockets and were proof loaded to 20% of their minimum breaking load
(185 kN or 18.8 t). However, socket failures continued, although to a lesser extent. One sample
in the fourth batch (26th
July 2001) and one sample in the fifth batch (14th
October 2002) failed
prematurely in the socket. In both cases, failure occurred in heat treated sockets with improved
mechanical properties.
The seventh batch of samples tested on 14th
October 2002, appeared to show an improvement on
earlier test results. Load against displacement for these tests is shown in Figure 17. However,
the high number of socket failures which occurred during the early stages of the 32 mm rope
dynamic testing have made it difficult to draw any clear conclusions in terms of ropedeterioration.
Load
kN
1000
800
600
400
200
0
test 14Test 15Test 16Test 17Test 18
0 100 200 300 400 500 600 700
Displacement (mm)
Figure 17 Load vs dis lacement for dynamic tests carried out on 14/10/02
Where the sockets failed prematurely, the socket failed well below the loads required for failure
in the body of the rope. In the worst case a socket failed at 576.6 kN, 62% of the manufacturers
specified minimum breaking strength for this rope.
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6.3
The change in dynamic breaking load over time is summarised in Table 22. These figures only
represent dynamic tests to failure, where the rope failed rather than the socket. All samples
failed to meet the manufacturers specified minimum breaking load requirements of 927 kN. All
of the rope failures were above the minimum values for this basic construction (without dye
forming) as calculated from BS302 Part 1 and BS EN 12385 Part 4. Although test 1 was only
barely above the minimum breaking load of 774 kN specified by BS EN 12385.
These results appear to indicate that breaking load of the rope increased over time, however due
to problems with premature failure, the number of available results is limited and this trend may
be misleading.
Table 22 Deterioration of dynamic breaking load for 32 mm rope.
Date Month Load % of Min Max. disp. Energy to
tested (kN) Breaking Load (%) failure (kJ)
11/98 +1 775 84 1.1 121.4 08/00 +22 873 94 2.4 164.0 10/02 +48 901 97 3.7 220.0
A comparison of static and dynamic loads to failure is shown in Table 23. Initially, dynamic
tests produced a peak load to failure well below the peak loads measured during static testing.
However, this trend was reversed for the final batches of tests, carried out in May and October
2002.
Table 23 Comparison of static and dynamic breaking load for 32 mmrope.
Date Month Static Dynamic
tested Mean Load Mean Max Load (kN) Max. disp.
(kN) disp.(%) (%)
10/98 0 942 - - -11/98 +1 - - 775 1.1 12/98 +2 987 6.1 - -08/00 +22 903 2.2 873 2.4 05/02 +43 879 2.7 - -10/02 +48 - - 901 3.7
32 MM SAMPLES WITH INSTRUMENTED SOCKETS
In order to better understand the mechanism of premature socket failure, two 32 mm rope
samples (one dynamic test-piece and one static test-piece) were prepared, each with one
instrumented socket. The objective of this work was to initiate premature socket failure in the
dynamic test-piece and compare strain levels between dynamic and static loading.
Samples were prepared as per other test-pieces, each sample using one galvanised and one
higher strength socket. Basic galvanised sockets were used for the instrumented socket rather
than the higher strength alternatives to help initiate failure. Prior to application of the strain
gauges, dimensions and concentricity of the sockets were measured. These dimensions are
shown in Table 24.
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The two sockets were both slightly non-concentric. The wall thickness measured at all locations
was greater than the nominal figure of 10 mm quoted by the manufacturer.
Table 24 Dimensions of instrumented socketsSocket Strain- Wall thickness Concentricity
gauge (mm)
Static A 12.66 Top 11.98(SS) B 16.00
C 19.76 D 22.64 E 16.23
11.9612.36
10.19
Dynamic A 12.54 Top 11.92
(SD) B 16.05C 19.12
D 22.56
E 16.1011.75
11.89
10.44
Ten strain-gauges, consisting of five 90o
rosettes, were attached to the socket, by trained
technicians. This allowed measurement of both longitudinal and hoop strains. The position and
orientation of these gauges are shown in Figure 18. Clearly gauges had to be placed on flat
prepared surfaces and could not be placed on or near the potential initiation sites identified bymetallurgical examination of the failed sockets (See Section 4). Strain gauges were calibrated by
the application of a range of precision resistors, up to 10,000 microstrain.
During dynamic testing, instrumentation cables made it necessary to position the instrumented
socket at the fixed end of the rope.
Load was measured as in previous tests. Displacement measurements were not taken. Load and
strain data was recorded using a Nicolet high speed data logger, operating at 50 kHz. The data-
logger and amplifiers were calibrated using a Time 2003S dc voltage source, in accordance with
HSL calibration procedure, FE/CP/38 Digital data-loggers.
The dynamic test was carried out on 13th September 2000. The static test was carried out on 14th
September 2000. The static test-piece failed at a load of 917 kN, failure occurring in the body of
the rope. The dynamic test-piece failed at a load of 776 kN, failure occurring in the moving
socket.
For the static test, the variation of both load and microstrain against time are shown in Figure
19. For the dynamic test, the variation of both load and microstrain against time is shown in
Figure 20. A summary of peak loads and maximum strain values is given in Table 25.
In both static and dynamic tests, the measured hoop strain was substantially greater than the
longitudinal strain. Measured microstrain in the dynamic socket was generally higher than in the
static socket.30
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Table 25 Summary of peak loads and microstrain in the test socket during static anddynamic testing
Load Hoop Strain Longitudinal Strain
(kN) A B C D E A B C D E
Static 918 2325 5818 3671 -27 9724 645 27 253 1781 176 Max
473 281 117 -306 397 -336 -1045 -773 0 -568 Min
Dynamic 776 8654 11822 16517 10276 12339 2080 540 252 241 26 Max
-1377 -495 -53 0 -146 -887 -1620 -1290 -2583 -2132 Min
TOP FACE
A B C D
E
25
105
140
28
56
84
56
E
112
BOTTOM FACE
Figure 18 Strain gauge positions for tests on instrumented sockets
During the static test, hoop strain reached levels where localised plastic deformation would beexpected to occur at positions A, B, C and E. The highest levels of hoop strain were recorded at
positions B and E, diametrically opposite each other, 56 mm from the socket mouth. The lowest
hoop strain was recorded at position D, adjacent to the wide part of the cone. The greatest
longitudinal strain was recorded at position D, however, from strain levels at this position, it is
unlikely that localised yielding was taking place.
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32
Strain traces for the dynamic test were complex in nature. Hoop strains reached levels where
localised deformation and subsequent failure would be expected to occur. Hoop strain was high
at all gauge positions, particularly in the mid-sections of the cone. The highest longitudinal
strains were compressive in nature, which could be expected given the nature of the event. The
highest longitudinal compressive strain was, once again, measured at position D.
It should be noted that at higher levels of strain, (greater than 10,000 microstrain) these strain
gauges would be outside their operating envelope. Measurements of higher levels of strain are
likely to be non-linear and would not produce an accurate measurement of strain.
Force/Time
Force(kN)
0
250
500
750
1000
Longitudinal Strain
Microstrain
-2000
0
2000
4000
6000
Hoop Strain
Time (s)
0 50 100 150 200 250 300 350 400 450 500
Microstrain
0
5000
10000
15000
A
B
C
DE
Figure 19 Load and strain variations measured during static testing
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0
250
500
750
1000
-2000
0
2000
4000
6000A
B
C
D
E
Hoop Strain
0
5000
10000
15000
Force/Time
Force(
kN)
Longitudinal Strain
Microstrain
Microstrain
0.00 0.01 0.02 0.03 0.04 0.05 0.06
Time (s)
Figure 20 Load and strain variations measured during dynamic testing
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7.2
7 COMPARISON BETWEEN 25 MM AND 32 MM ROPE
7.1 STATIC TESTING
A comparison of load, displacement and energy for failure during static testing on both sizes of
rope is shown in Table 26. For comparison purposes, the age of the rope under test has been
broken down into 12 month periods. As expected, the larger rope generally gave higher loads
and energies absorbed to failure. Percentage elongation to failure was slightly greater for 32 mm
than 25 mm rope.
Deterioration over time was more significant for 25 mm rope than for 32 mm rope. The 25 mm
rope suffered a 16% loss in strength (based on manufacturers specified minimum breaking
load) over a period of 38 months, while the 32 mm rope suffered a 7% loss over 43 months.
Without detailed knowledge of production variables, it is not possible to propose an explanation
for the differences in severity of deterioration.
Table 26 Comparison of load, displacement and energy for failure duringstatic tests on 25 mm and 32 mm unused rope
25 mm rope 32 mm rope
Age Mean % Min. Mean Mean Mean % Min. Mean Mean
(Month) Load Break disp. Energy Load Break disp. Energy(kN) Load (%) (kJ) (kN) Load (%) (kJ)
0 593.2 104 - - 942 102 - -1 to 12 583* 102 5.2 26.3 986.7 106 6.1 57
13 to 24 536 94 2.2 10.6 903 97 2.2 17.3
37 to 48 502 88 1.8 7.4 879 95 2.7** 16.1
* Excludes 25 mm test 3 where technical problems were encountered
** Excludes 32mm test 8 which failed in two stages
DYNAMIC TESTING
A comparison of load, displacement and energy for both rope and socket failure during dynamic
testing on both sizes of rope is shown in Table 27. For comparison purposes, the age of the rope
under test has been broken down into 12 month periods.
As expected, the larger rope generally gave higher loads and energies absorbed to failure.Breaking loads expressed as a percentage of manufacturers minimum specified breaking load
where comparable between the two rope sizes. There were no clear indications of any trends in
these results.It should be noted that many ropes were not tested to failure and that some of these
figures represent single results.
In order to control the potential for both rope and socket failure, it was necessary to vary the
drop height (and, therefore, the available energy and rate of loading) throughout the test
programme. This was less significant on 25 mm rope, where all tests to failure took place from a
release position of 35 m to 40 m, than for 32 mm rope where tests to failure took place from a
release height between 35 m and 90 m.
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Table 27 Comparison of load, displacement and energy for rope and socket failuresduring dynamic tests on 25 mm and 32 mm rope
Rope failures (excluding partial failures)
25 mm rope 32 mm rope
Age Time to Mean % Min. Mean Mean Time to Mean % Min. Mean Mean
(Month) failure Load Break disp. Energy failure Load Break disp. Energy(ms) (kN) Load (%) (kJ) (ms) (kN) Load (%) (kJ)
1 to 12 68.0 543.2 95 1.8 108.3 30.4 775.0 84 1.1 121.4
13 to 24 54.0 500.4 87 1.6 93.9 33.4 873.0 94 2.4 164.0
37 to 48 57.7 564.4 99 2.6 133.4 63.2 901.0 97 3.7 220.0
Socket failures
25 mm rope 32 mm rope
Age Time to Mean % Min. Mean Mean Time to Mean % Min. Mean Mean
(Month)failure Load Break disp.
Energyfailure Load Break disp.
Energy(ms) (kN) Load (%) (kJ) (ms) (kN) Load (%) (kJ)
1 to 12 55.6 416.0 73 2.0 88.4 40.1 695.3 75 1.2 112.1
13 to 24 - - - - - 27.0 764.0 82 - -
37 to 48 - - - - - 46.9 985.5 106 3.9 158.0
The earlier tests on 32 mm rope carried out during the first 24 months, showed a much shorter
time to failure. During these tests, the release point was 90 m compared with 40 m used in later
tests and the rate of loading was correspondingly higher.
As a result of variations in release height, the dynamic tests were not generally comparable.
However in the later stages of the programme, tests were carried out on both sizes of rope usinga release height of 40 m. A comparison of the results for these tests is shown in Table 28.
As expected, 32mm rope gave greater loads, extensions and energies to failure, the single socket
failure occurred at a load above the manufacturers specified minimum breaking load. No other
significant trends were identified
Table 28 Comparison of dynamic tests with a release height of 40 m (energy available244.1 kJ)
25 mm rope 32 mm rope
Age Load % Min. Disp. Mean Type of Load % Min. Disp. Energy Type of(Month (kN) Break (%) Energy failure (kN) Break (%) (kJ) failure
Load (kJ) Load
37-48 498.6 87 2.1 132.0 None 885.0 92 2.6 94.2 None
525.5 92 2.0 - None 927 100 4.8 165.0 None
527.5 92 2.0 - None 1066 115 3.9 158.0 None
569.2 100 2.6 132.0 Rope 901 97 3.0 158.0 Rope
559.5 98 2.5 134.9 Rope 985.5 106 3.90 220.0 Socket
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Socket failures predominated on 32 mm rope sockets with only a single instance of failure on 25
mm sockets. While it seems likely that this was a genuine size effect, there was insufficient
information to explain the causes.
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8 COMMENTS
8.1 Static testing of both sizes of wire rope, identified a general reduction in breaking load,extension to failure and energy absorbed to failure over time.
8.2 During this test programme the static breaking load for both rope sizes dropped belowthe manufacturers specified minimum breaking load. This is estimated to have
occurred after four months for 25 mm rope and 16 months for 32 mm rope.
8.3 The worst deterioration encountered resulted in a static breaking load which was 12%below the manufacturers specified minimum breaking load. While significant, this is
unlikely to have critical safety implications considering the coefficients of utilisation (or
safety factors) of 5:1 quoted for these ropes.
8.4 During dynamic testing none of the 25 mm test-pieces met the manufacturers specifiedminimum breaking load.
8.5 Dynamic testing of 25 mm rope identified variations in rope properties during theprogramme but did not identify any consistent trends in rope deterioration.
8.6 During dynamic testing of 32 mm ropes, only three samples met the manufacturersspecified minimum breaking load. All three of these samples were in the final batch.
8.7 Dynamic testing of 32 mm rope appeared to identify an improvement in rope propertiesover time. However, these tests were beset with technical problems resulting from
premature failure of sockets and the identified trends in rope deterioration may not be
reliable.
8.8 All of the test samples, including damaged test-pieces, were above the minimum valuesfor the basic non dye formed version of this rope construction, as calculated from BS
302-1 (1987) and BS EN 12385-4 (2002).
8.9 The dynamic testing of 32 mm rope, suffered from premature failure of the sockets usedto terminate the test-pieces. Premature failure occurred during 40% of dynamic tests.
There was a single premature failure of a 25 mm socket. This was a dynamic effect that
was not replicated during static testing. Strain measurement identified very high strain
levels in the socket during dynamic testing.
8.10 Modifications to the procedures used to make samples did not prevent prematurefailures occurring. Evidence suggests that premature failure did not result fromdefective sample preparation.
8.11 Examination and testing of these sockets showed that they were compliant with RR-S-550D Federal Specification Sockets Wire Ropes, the standard to which they were
supplied.
8.12 Metallographic examination of several failed sockets identified that failure had occurredby a single stage overload event. The initiation point showed localised ductile failure,
followed by brittle cleavage.
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8.13 Examination of sockets revealed several potential initiation points including, internalsocket grooves, manufacturers markings, manufacturing defects and a lack of
tolerances on dimensional variations. It is likely that improvements could be made to
socket design and quality control, but such changes would have cost implications.
8.14 This work did not fully identify the cause or implications of these premature failures. Itis possible that other types of sockets may suffer similar problems when subjected to
dynamic loading
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9 RECOMMENDATIONS Since this work was initiated, the manufacturer has altered their manufacturing processes and
strength loss over time is no longer believed to be a concern.
Premature failures in the sockets used to terminate wire rope samples were ongoing throughout
the test programme. The problem of premature failure was not solved by changes to preparation
procedures or the use of higher strength sockets. Failures occurred primarily on the 32 mm
socket size.
The sockets failed unexpectedly at relatively low loads during high strain rate loading. In
service these sockets are single line components and the effects of a premature failure could be
potentially catastrophic.
The sockets used during this project were open spelter sockets in both normal and heat treated
conditions. All sockets were supplied by the same manufacturer. However, this problem maynot be confined to one type and size of socket. While a full investigation of the problem was
beyond the scope of this project, further work investigating this phenomenon would be
advisable.
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10 REFERENCES
BS 302 - 1 Stranded steel wire ropes- Part 1: Specification for general requirements (1987) BS EN 12385-1 Steel wire ropes Safety Part 1: General requirements (2002) BS EN 12385-2 Steel wire ropes Safety Part 2: Definitions, designation and classification. (2002) BS EN 12385-4 Steel wire ropes Safety - Part 4: Stranded ropes for general lifting applications(2002) BS 7035 Code of practice for Socketing of stranded wire ropes (1989) Ropemans Handbook (1980) NCB Notes for guidance on the resin capping of wire ropes (1994) British Coal Corporation RR-S-550D Federal Specification Sockets, Wire Ropes (1980 Amended 1986) HSL Letter Report Failure of a Wire Rope Socket H. Pitts
(1999)
HSL Letter ReportExamination of Wire Rope Sockets
H.Pitts
(1999)
BS 463 Part 2
Specification for sockets for wire ropes
(1970)
BS 427
Method for Vickers hardness test and for verification of Vickers hardness test machines
(1990)
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BS EN 10045 Part 1 Charpy impact tests on metallic materials; test method (V and U notches) (1990) BS EN 10002 Part 1 Tensile testing of metallic materials; method of test at ambient temperatures (1990)
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Printed and published by the Health and Safety ExecutiveC30 1/98
Published by the Health and Safety Executive10/06
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