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Structural failures of rope-based systems
Engineering Failure Analysis 16 (2009) 1929–1939
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Structural failures of rope-based systems G. Piskoty *, M. Zgraggen, B. Weisse, Ch. Affolter, G. Terrasi Empa, Swiss Federal Laboratories for Materials Testing and Research, Ueberlandstrasse 129, CH-8600 Duebendorf, Switzerland
a r t i c l e
i n f o
Article history: Received 21 September 2008 Accepted 16 October 2008 Available online 26 October 2008 Keywords: Structural failures Damage tolerance Rope failures Stress corrosion cracking
a b s t r a c t The aim of this paper was to emphasise the importance of making a broad collection of hypotheses at the beginning of failure investigations as a precondition for efficiency and correct conclusions. For this purpose, four case studies of structural failures are presented, where the failed load carrying structure was based on wire ropes (steel cables) and the incidents occurred despite safety precautions. The investigations are not dealt with in depth – they are merely summaries intended to highlight the failure hypotheses and the procedure in finding the failure causes. Some generalised conclusions are formulated at the end of each case study. In this way, the knowledge and experience derived from a failure analysis can be archived in memory and called upon during future investigations – regardless of whether or not a wire rope is present in the failed system. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction When starting a failure analysis it is common practice to compile a catalogue of failure hypotheses and to plan the investigations based on it. The precondition for the success of this approach is that the catalogue of failure hypotheses includes the real failure cause. Otherwise, there is a large likelihood of misinterpretation of the results and delivering incorrect conclusions. Therefore, the goal of the present paper is to emphasise the importance for a broad spectrum of hypotheses and to derive some general ideas for completing hypothesis-catalogues, based on four case studies of structural failures. In all cases the failed load-bearing structure was based on wire ropes and that the incident occurred despite safety precautions. A brief introduction to rope-based structures is appropriate before the case studies are presented. Ropes are used especially in such structures where high load capacity combined with flexibility of the load carrying element and an inherent redundancy (damage tolerance) are required, such as in ropeways. The reason for these special characteristics is that ropes are made of many single woven wires. In rope-based systems, the structure and the functional part of the system are often combined. For example, the hauling rope of a chairlift represents the main part of the load carrying structure and also acts as the load transmission element for moving the hangers. 2. Rupture of a rope due to local fatigue 2.1. Introduction There grips of monocable ropeways [1] can be detachable or fixed. In the first case, the hangers (e.g. chairs, cabins or T-bars) are separated from the rope during the station transit. This allows the passengers to board comfortably at reduced * Corresponding author. Tel.: +41 44 823 40 58; fax: +41 44 823 40 11. E-mail address: [email protected] (G. Piskoty). 1350-6307/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2008.10.004
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speed of the hangers. As a positive side-effect, the fatigue of the rope is homogenous, because the location of grips on the rope changes continuously and randomly during operation. The following accident occurred on a ropeway with fixed grips. In this kind of ropeway, the carriers remain permanently attached to the rope during operation. Therefore, always the same rope segments experience the additional bending and pressure stresses at the grips. In order to avoid an accelerated localised fatigue of the rope, all of the grips have to be relocated by a given distance (approximately 0.5 m) at regular intervals; typically twice a year. 2.2. Accident During regular operation of the ropeway the hauling rope ruptured close to a grip. 2.3. Investigations The visual investigation of the rope revealed typical characteristics of fatigue fracture. Many broken wires were found also in the region of almost all the other grips. It seemed obvious that the prescribed relocations of the grips had been neglected. However, the maintenance records showed that the grips were relocated at the specified time intervals and distances in accordance to the manual. The fatigue rupture of the rope was a mystery. In order to collect some clues, the employees who had relocated the grips were interviewed. It turned out that the grips were relocated by two alternating teams: one team had relocated the grips in the direction of travel, while the other team relocated them in the opposite direction. As a result, the grips alternated between the same two positions (Fig. 1). Obviously, the manufacturer did not see the necessity to prescribe a direction for the grip-relocation in the manual, as both possible directions were equivalent. 2.4. Generalized conclusion The combination of inherently correct processes can neutralize their desired effects and cause a failure. The findings should be evaluated not only individually but also in all possible combinations.
3. Rope-detachment from an anchor drum 3.1. Introduction Anchor drums represent a well-established method for anchoring wire ropes. By this kind of anchorage, the final sequence of the rope is wrapped around a fixed drum with a soft surface, e.g. wood (Fig. 2). The main part of the service load is transmitted between the rope and the drum by friction. Behind the drum, only the small residual force has to be absorbed, using a clamp. The residual force is given by the following equation:
F2 ¼
F1 F1 ¼ eal e2pNl
ð1Þ
a: Enlacement angle of the rope around the drum, l: friction coefficient between the rope and the drum’s surface, N: number of windings.
rope
clamp
Fig. 1. Possible relocation directions of the grips (the lower region of the hangers is not depicted).
G. Piskoty et al. / Engineering Failure Analysis 16 (2009) 1929–1939
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k
clamp F2
support beam
F1
Fig. 2. Construction of the anchor drum.
For example, if the friction coefficient is 0.2 and the rope is wrapped around the drum four times, the residual force amounts to only 1% of the service load (F 2 ffi F 1 =100). In general, the clamp has to be designed just for a small fraction of the service force in the rope, allowing a smooth clamping, without excessive local pressure on the rope’s surface. This advantage becomes especially important for applications where the rope has to be shifted periodically as in the final case study in this paper. 3.2. Accident During regular operation of a ropeway, the end of a hauling rope holding the cabin in position slipped off the clamp of the anchor drum. The rope detached from the cabin and the cabin became free. Fortunately, a safety equipment (track rope break [1]) prevented the crash of the cabin. 3.3. Investigations The visual inspection of the anchor revealed no apparent anomalies. As an excessive overload during operation could be excluded, an insufficient load capacity of the anchorage was assumed. This obvious hypothesis had to be proved with a load test. Since the rate of change of the service load in the rope was low, a quasi-static test set-up was built. During the test, the force in the anchored rope was increased gradually up to the triple of the highest service load. However, the clamp did not fail. The hypothesis of insufficient load capacity of the anchor had to be rejected. At a loss to explain the failure and test result, the disassembly of the test set-up was started. At first, the load in the rope was removed. In doing so, a small amount of slippage was observed between the clamp and the rope. By repeating the loading and unloading cycle, the rope slipped incrementally through the clamp, even when the fluctuating tensile force in the rope remained considerably below the regular operational force. Strangely enough, the slippage took place not during the increase but during the reduction of the load. The unexpected system behaviour could be explained after a detailed analysis of the process. The service load fluctuated slowly in accordance with the load status, position or acceleration of the cabin etc. within a regular range. If the tensile load increased, the elastic deflection of the support beam became larger. As a consequence, the distance between the drum and the rope clamp decreased ðF 1 ") k #, Fig. 3). Due to the reduced distance, the residual force collapsed ðk #) F 2 Þ and therefore the relationship (1) was no longer satisfied. In order to restore the required equilibrium between the forces on the both side of the drum, the rope crept around the drum in a clockwise direction until the residual force increased to the required value again.
k↓
F2↓
slippage
F1 Fig. 3. Disturbed force equilibrium in the rope on both sides of the drum during increased service load (the elastic deformation of the support beam is exaggerated).
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When the operational load decreased, the support beam started to flex back into its original position. Therefore, the distance between the drum and the clamp got larger ðF 1 #) k "Þ. Because of the missing rope length between the drum and the clamp, the load started to increase in that rope section. There was no possibility for pulling back the rope over the drum, as it would have required a force several times higher than the current service load (interchanged indices ‘‘1” and ‘‘2” in Eq. (1)). Obviously, the yielding took place at the clamp which was designed just for a small fraction of the service load. During the operation of the ropeway this asymmetric procedure was repeated periodically, until the rope had slid out of the clamp. 3.4. Generalized conclusions Structures should be analysed also under consideration of fluctuating service loads, even if classical dynamic failure causes – such as vibrations, fatigue or wear – can be excluded. A failure can be caused by hidden coupling effects in a system. A simple elastic structure can react to a variable load with asymmetric behaviour. This can lead to the accumulation of small negative effects and finally cause failure.
4. Collapse of a hoisting platform 4.1. Introduction The accident involves the crash of a hoisting platform in a two-storey shop. The platform was guided along a rail fixed to the wall and was moved up and down with the help of an electric hoist (Fig. 4). The load capacity of the hoist was doubled with the use of a moving pulley. As a safety precaution, a load limiter was built into the hoist’s drive, controlling the torque on the drum. It was adjusted to switch off the hoist at a 20% overload. 4.2. Accident The worker intended to transport a heavy box from the lower to the upper floor of the shop. While raising the platform, the hoist suddenly stopped and the platform came to a halt between the upper and lower floor. The worker assumed that the platform had jammed in the guide thus activating the load limiter. In order to leave the platform, he pressed the ‘‘down” button on the hoist’s control unit. By doing so, the rope failed suddenly and the platform fell to the lower floor. 4.3. Investigations The visual investigation revealed that the rope had derailed from the moving pulley because the groove of its plastic wheel was partially damaged. However, the causality between that defect and the accident was unclear, since the load limiter had worked properly and the rope had withstood the load until the platform was lowered. The accident was reconstructed in the laboratory. For the test, the original equipment was used with a new rope. By raising a metal block, representing the moved load, the rope derailed at the pulley and jammed between the metal fork and the
hoist
upper floor pulley control unit box lower floor
platform
Fig. 4. Situation before the accident.
wall
fixed end of the rope
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Fig. 5. Derailed steel cable jammed between the fork and the plastic wheel of the moving pulley.
plastic wheel (Fig. 5). The hoist was correctly stopped by the load limiter. Obviously, the real reason why the load limiter came into action on the occasion of the accident was not the jamming of the guide wheels of the platform, as assumed by the worker. After the stop of the hoist the rope could still carry the load, despite its noticeable local damage in the jammed zone. The safety concept seemed to be effective. However, after switching the hoist to ‘‘down” direction, the rope fractured immediately, in an analogous manner to the accident. After analysis of the process, the reason for the rope rupture became clear. Once the rope had jammed at the pulley, the equilibrium between the two sections of the rope was no longer established. During the lifting, the external load concentrated gradually in the rope section connected to the hoist. As soon as the load in that rope section exceeded the allowed overload of 20%, the hoist was stopped by the load limiter. However, during the subsequent lowering of the platform, the service load concentrated in the rope section between the pulley and the fixed end of the rope. The hoist got unloaded and the tension in the rope sequence with the fixed end doubled. This excessive load was higher than the remaining load capacity of the rope in the jammed zone. The load limiter could not prevent this procedure as it did not register the tension in that rope section after the malfunction of the pulley. 4.4. Generalized conclusions A failure can restrict the region which is being monitored by safety equipment. A structural member can be overloaded not only during energy input, but also in the passive phase of the system (e.g. during lowering a load).
5. Track rope damage 5.1. Incident The incident occurred on a forty years old reversible aerial ropeway with classic design: The carriage of the cabin rolls on two track ropes and is moved by a hauling rope [1]. During regular operation of the ropeway, an employee in the upper terminal heard a loud bang. He did not waste any time with exploring the source of the unusual sound, and immediately shut down the ropeway. With the help of binoculars, the he spotted a serious track rope damage with protruding wires in a distance of around 60 m from the terminal. The damaged rope section was observed also from a helicopter. This risk evaluation revealed that all 21 Z-formed wires in the outer layer of the track rope were fractured, reducing the rope’s metallic cross-section by more than 50% (Figs. 6 and 7). As there was fear of ropeway collapse, the 53 passengers were evacuated by helicopter.
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Fig. 6. Cross-section of the track rope. Outer layer with 21 Z-wires (all of them broke during the incident) and inside layers containing 61 round wires.
Fig. 7. Lower half of the damaged track rope section with protruding broken Z-wires 80 m above ground level (t: distance between the adjacent track ropes of the same track).
5.2. Background information Such a serious track rope damage was considered in the state of the art safety regulations of ropeways to be practically impossible [2]. This until then well-proved philosophy was based primarily on the fact that even a large number of wire fractures is tolerable without risk of a rope rupture. The reason for this failure tolerance is that single wire fractures due to normal deterioration of a rope are randomly distributed in both the axial and circumferential directions of the rope. This fact has two positive consequences: The reduction of the rope’s metallic cross-section is everywhere negligible, as the probability of many wire fractures in the same cross-section is very low. A broken Z-wire cannot protrude out of the rope’s compound as the adjacent wires constrain it (Fig. 6). As an additional positive effect, the tensile force in each broken wire builds up gradually again, due to friction forces between the adjacent wires. Therefore, the minor weakening of the rope by a broken wire is further limited to its short adjacent section on both sides of the flaw. The growth of the number of single wire fractures is monitored periodically by means of magneto-inductive tests. If the number of wire fractures within a reference length approaches the prescribed tolerable limit, the rope is replaced. In the case under consideration, the last magneto-inductive test had been carried out shortly before the incident. The number of detected wire fractures was well below the tolerable limit. Obviously, the incident had to be initiated by effects which were not covered by this state of the art safety concept.
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5.3. Investigations The investigation started on site with documentation of the damage and measuring of several parameters such as the geometry of the rope, the length of lay and – based on the propagation velocity of a transversal wave in the rope – the tensile force. The preliminary investigations provided evidence only for the exclusion of some hypotheses, but none for the failure mechanism. Therefore, the rope was unloaded and its damaged section was cut out and transported to the laboratory for further investigations. Right at the beginning of the examinations in the laboratory, metallic deposits were detected on the rope’s surface, distributed along a line of about 6 m in length (Fig. 8, left). It appeared that the deposits must have arisen from a galling process under unfavourable tribological conditions. As the same kind of deposits was also detected adjacent to all of the wire fractures (Fig. 8, right), a link between the incident and the deposits seemed obvious. However, (a) the source of the deposits and (b) the triggered failure mechanism were still unclear. Accordingly, the investigation was focused on these two issues. 5.3.1. Localization of the deposit’s origin The chemical analysis (EDX and WDX) revealed that the counterpart of the rope during the galling process was composed of structural steel with a carbon content of approximately 0.2% (Z-wire: approximately 0.6%). During operation, only the passenger cabin’s carriage could have had such a tribological contact with the damaged rope section, namely in the case of an imaginable derailment. However, no such galling traces were found on the carriage. The search for the galling partner had to be continued by reconstructing the location of the rope in the past. According to the standard regulation, the track ropes were shifted every 12–14 years. The aim was to avoid excessive fatigue in the most highly stressed sections of the rope, i.e. at the ends of the tower saddles and in the terminal. During each rope shift, the clamp behind the anchor drum was carefully loosened (Fig. 9), so that the rope slipped over the drum and moved towards the lower terminal. After a shift of approximately 25 m, the clamp was tightened again until the next shift.
Fig. 8. Metallic deposits on the rope’s surface (left) and next to breaking point of a Z-wire (right).
hauling rope track rope
saddle
carriage of the cabin
anchor drum
clamp
Fig. 9. Sketch of the upper terminal.
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Fig. 10. Traces of galling on the saddle’s surface.
Fig. 11. Cracks on the surface of a Z-wire, detected by magnetic particle method.
Before the incident, the track ropes were shifted three times. At the first relocation, the damaged segment of the rope lay exactly on the deflection saddle, which is made of structural steel. Obviously, this part was considered as the most likely counterpart to the rope during the galling process. In order to prove this hypothesis, the groove of the saddle was inspected visually after dismantling the rope. A cast was made of the groove and its surface was compared to the surface of the rope under an optical microscope. As the topology of the surfaces qualitatively matched, there was no doubt that the real galling partner had been found (compare Fig. 8 with Fig. 10, right). 5.3.2. Determination of the triggered failure mechanism by the deposits At the beginning of this part of the investigation, the original location of the single wire fractures on the rope’s surface was determined. The outer layer of Z-wires was cut at a reference cross-section of the rope where the compound was still intact. Afterwards, the individual wire sections were weighted and their lengths were calculated based on the specific weight of the profile wire. With this rather uncommon measurement method secondary damages on the wire’s surface could be avoided. This investigation showed that the fractures were distributed over a distance of round 6 m and there were some groups containing up to four adjacent wires with roughly equal lengths. In other words, those wires had fractured in the same crosssection of the rope, forming fracture clusters. The surface of some Z-wires was tested for cracks by the magnetic particle method. This method revealed numerous cracks oriented perpendicular to the main stress direction (Fig. 11), indicative for corrosion fatigue. However, the corrosion chemistry analysis of the deposit’s aqueous extract detected only slight traces of corrosion-promoting substances and a slightly acidic environment (pH 5.4–5.7). The next step was the fractographic and metalographic investigation of the wires. Most of the fractured surfaces showed a similar macroscopic and microscopic appearance. They exhibited only little macroscopic deformation and were hardly corroded (Fig. 12, left). The last point indicated that almost all wires fractured within a short period of time and not long before the incident. The main part of the broken surfaces showed the characteristic of micro-ductile overload under the scanning electron microscope. However, adjacent to the fracture origin there was a typically 0.7 mm deep intergrannular zone with boundary gaps in places (Fig. 12, right). In the longitudinal cross-section of the Z-wires small cavities isolated from the ambient air were found between the deposits and the wire’s surface (Fig. 13). These findings provided a clue for hydrogen-induced stress corrosion cracking (H-SCC) as the primary damage mechanism. Below the deposits all the preconditions for H-SCC were given [3]:
G. Piskoty et al. / Engineering Failure Analysis 16 (2009) 1929–1939
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Fig. 12. Fracture surface of a Z-wire (left) and detailed view of the intergranular zone with numerous boundary gaps (right).
Fig. 13. Etched longitudinal micro-section of a Z-wire in the area of metallic deposits.
a susceptible material such as high strength steel (Rm > 1000 MPa [4]) ) the strength of the broken Z-wires was at 1500 MPa (based on the measured hardness of 450 HV10) and probably even higher in the region of the detected friction induced martensitic layer, tensile stress ) due to normal service loads, catalysts, such as sulphur ) present in nearly all steels in the form of manganese sulphide, cavities with an absence of oxygen ) regions below the deposits. In fact, the increased hydrogen concentration at the crack tips could be proved by microelectrochemical measurements, using glass microcapillary. The extent of the embrittlement due to H-SCC was semi-quantified with the help of a regular wire twisting test. For this purpose, two samples were taken from the same Z-wire but from different sections: one with, the other without deposits. The samples were twisted until fracture. The large embrittlement of the wire in the region of the deposits was apparent (Fig. 14). The question, whether the crack could became instable during regular operation of the ropeway, was assessed by means of linear elastic fracture mechanics. The estimated tensile stress in a Z-wire was in the range of r ffi 400–500 MPa, not including the additional bending stress induced temporary during passing of the cabin. Considering a crack length equal to the size of the typical intergranular area (affi 0.7 mm) the stress intensity factor amounts to
K ffi 1:12
pffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffi p a r ¼ 21—26 Mpa m
ð2Þ
The fracture toughness of a medium carbon steel like the material of the Z-wires lies at room temperature in the range of pffiffiffiffiffi about K IC ¼ 40—50 Mpa m [5]. As the incident happened at an air temperature far below the freezing point, the instability of the deepest intergranular cracks is plausible.
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Fig. 14. Two sections of the same Z-wire after the twisting test in the region of the produced breakage.
5.4. Summary of the sequence of events Based on the findings of the investigation, the chronological sequence of the incident could be reconstructed as follows. 5.4.1. 1979 Galling process At the first relocation of the rope a galling process between the rope and the deflection saddle took place as the saddle had a rather small deflection radius and an oversized grove radius. The corresponding high compressive stress peaks between the track rope and the groove, combined with poor lubrication, led to damage in the surface of both components. In fact, the personnel could still remember the jerky sliding of the rope during the shift, but at that time nobody was concerned with possible long-term consequences. 5.4.2. 1979–2004 Stress corrosion cracking After the first relocation of the track rope, a slow deterioration process started below the deposits by H-SCC. The cracks were not randomly distributed, but they were concentrated at the particular location of the deposits. Therefore, the likelihood of fracture-clusters of many adjacent wires was high. During this time period, the rope section with the deposits was moved towards the lower station in increments of around 25 m on the occasion of every rope shift. 5.4.3. 2004. 04. Last magneto-inductive tests At the time of the last magneto-inductive tests, most of the Z-wires were not fractured, just cracked. Since the applied magneto-inductive method could detect only fully fractured wires, the test did not reveal the real risk. 5.4.4. 2004. 04. - 12 Fast fracturing of several wires The deepest wire-cracks approached the critical length for unstable crack propagation. 5.4.5. 2004. 12. Dynamic chain reaction (incident) As soon as three adjacent wires fractured in the same cross-section, they protruded from the rope compound, leading to torsional unbalance between the remaining rope layers and therefore to abrupt twisting of the rope. The heavy rotation triggered a chain reaction, leading to rupture of the entire outer layer of the rope. The dynamics of the process could be backtracked based on the shape of some broken wires, standing perpendicular to the axis of the rope with a hook at the end (Figs. 7 and 15). The distance between the hooks’ centres and the axis of the rope was approximately as large as the distance to the adjacent track rope (t = 550 mm). Obviously, the wires must have been twisted at high speed and must have collided with the adjacent rope. The end section of the wire was fractured at one of the cracks due to the force of inertia at impact. In fact, some of the broken end sections were found on the ground, flung up to 70 m from the vertical plane of the ropeway. After the investigation several bicable ropeways, which were susceptible to the same failure mechanism, were controlled. In many cases metallic deposits on the track ropes and even cracks were detected. The reason why in the past no similar incident had occurred could be that the regular deterioration of the rope (fatigue, fretting) progresses usually faster than the failure mechanism due to H-SCC. Hence, the ropes were replaced due to detected fatigue fractures before the intergranular cracks became instable. 5.5. Generalized conclusions The primary cause of a failure can lie far back in history of the system. A redundant system can fail if the preconditions for redundancy are violated.
G. Piskoty et al. / Engineering Failure Analysis 16 (2009) 1929–1939
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Fig. 15. Broken Z-wire protruding perpendicular to the axis of the rope with a hook at the end.
‘‘Inspected and approved” does not necessarily equal ‘‘flawless” (each measuring method has its detection limits in size and type of the flaws). Systems can be affected by different failure mechanism simultaneously. A slow failure mechanism can remain undetected as long as the service time is limited by a quicker one.
6. Summary The generalised conclusions given in this paper represent only some possible examples of how the knowledge of a particular failure analysis can be generalised and collected in order to support future investigations. In this way, the experience made by a failure analysis can be strengthened in one’s memory. Furthermore, it becomes more recallable and applicable for different kinds of failed systems. As a result, the efficiency of the failure analysis procedure increases and the likelihood of incorrect conclusions decreases. Acknowledgements The second failure was originally investigated under the leadership of Prof. Dr. G. Oplatka, retired head of the Institute of Ropeway Technology at the Swiss Federal Institute of Technology Zurich (ETH-Z). The authors thank him for providing the background information for this paper. References [1] [2] [3] [4] [5]
Wallis-Tayler AJ. Aerial or wire rope-ways: their construction and management. Kessinger Publishing, LLC; 2007. Eidgenössisches Verkehrs, Energiewirtschaftsdepartement. Pendelbanhnverordnung. Bern: Eidgenössische Drucksachen – und Materialzentrale; 1988. Kuron D. Wasserstoff und Korrosion. Kuron Verlag; 2000. Lange G. Systematische Beurteilung technischer Schadensfälle. DGM-Verlag; 1993. p. 235 ff. Ashby MF, Jones DRH. Eng Mater 1. 3rd ed. Elsevier; 2005.
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Structural failures of rope-based systems G. Piskoty *, M. Zgraggen, B. Weisse, Ch. Affolter, G. Terrasi Empa, Swiss Federal Laboratories for Materials Testing and Research, Ueberlandstrasse 129, CH-8600 Duebendorf, Switzerland
a r t i c l e
i n f o
Article history: Received 21 September 2008 Accepted 16 October 2008 Available online 26 October 2008 Keywords: Structural failures Damage tolerance Rope failures Stress corrosion cracking
a b s t r a c t The aim of this paper was to emphasise the importance of making a broad collection of hypotheses at the beginning of failure investigations as a precondition for efficiency and correct conclusions. For this purpose, four case studies of structural failures are presented, where the failed load carrying structure was based on wire ropes (steel cables) and the incidents occurred despite safety precautions. The investigations are not dealt with in depth – they are merely summaries intended to highlight the failure hypotheses and the procedure in finding the failure causes. Some generalised conclusions are formulated at the end of each case study. In this way, the knowledge and experience derived from a failure analysis can be archived in memory and called upon during future investigations – regardless of whether or not a wire rope is present in the failed system. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction When starting a failure analysis it is common practice to compile a catalogue of failure hypotheses and to plan the investigations based on it. The precondition for the success of this approach is that the catalogue of failure hypotheses includes the real failure cause. Otherwise, there is a large likelihood of misinterpretation of the results and delivering incorrect conclusions. Therefore, the goal of the present paper is to emphasise the importance for a broad spectrum of hypotheses and to derive some general ideas for completing hypothesis-catalogues, based on four case studies of structural failures. In all cases the failed load-bearing structure was based on wire ropes and that the incident occurred despite safety precautions. A brief introduction to rope-based structures is appropriate before the case studies are presented. Ropes are used especially in such structures where high load capacity combined with flexibility of the load carrying element and an inherent redundancy (damage tolerance) are required, such as in ropeways. The reason for these special characteristics is that ropes are made of many single woven wires. In rope-based systems, the structure and the functional part of the system are often combined. For example, the hauling rope of a chairlift represents the main part of the load carrying structure and also acts as the load transmission element for moving the hangers. 2. Rupture of a rope due to local fatigue 2.1. Introduction There grips of monocable ropeways [1] can be detachable or fixed. In the first case, the hangers (e.g. chairs, cabins or T-bars) are separated from the rope during the station transit. This allows the passengers to board comfortably at reduced * Corresponding author. Tel.: +41 44 823 40 58; fax: +41 44 823 40 11. E-mail address: [email protected] (G. Piskoty). 1350-6307/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2008.10.004
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speed of the hangers. As a positive side-effect, the fatigue of the rope is homogenous, because the location of grips on the rope changes continuously and randomly during operation. The following accident occurred on a ropeway with fixed grips. In this kind of ropeway, the carriers remain permanently attached to the rope during operation. Therefore, always the same rope segments experience the additional bending and pressure stresses at the grips. In order to avoid an accelerated localised fatigue of the rope, all of the grips have to be relocated by a given distance (approximately 0.5 m) at regular intervals; typically twice a year. 2.2. Accident During regular operation of the ropeway the hauling rope ruptured close to a grip. 2.3. Investigations The visual investigation of the rope revealed typical characteristics of fatigue fracture. Many broken wires were found also in the region of almost all the other grips. It seemed obvious that the prescribed relocations of the grips had been neglected. However, the maintenance records showed that the grips were relocated at the specified time intervals and distances in accordance to the manual. The fatigue rupture of the rope was a mystery. In order to collect some clues, the employees who had relocated the grips were interviewed. It turned out that the grips were relocated by two alternating teams: one team had relocated the grips in the direction of travel, while the other team relocated them in the opposite direction. As a result, the grips alternated between the same two positions (Fig. 1). Obviously, the manufacturer did not see the necessity to prescribe a direction for the grip-relocation in the manual, as both possible directions were equivalent. 2.4. Generalized conclusion The combination of inherently correct processes can neutralize their desired effects and cause a failure. The findings should be evaluated not only individually but also in all possible combinations.
3. Rope-detachment from an anchor drum 3.1. Introduction Anchor drums represent a well-established method for anchoring wire ropes. By this kind of anchorage, the final sequence of the rope is wrapped around a fixed drum with a soft surface, e.g. wood (Fig. 2). The main part of the service load is transmitted between the rope and the drum by friction. Behind the drum, only the small residual force has to be absorbed, using a clamp. The residual force is given by the following equation:
F2 ¼
F1 F1 ¼ eal e2pNl
ð1Þ
a: Enlacement angle of the rope around the drum, l: friction coefficient between the rope and the drum’s surface, N: number of windings.
rope
clamp
Fig. 1. Possible relocation directions of the grips (the lower region of the hangers is not depicted).
G. Piskoty et al. / Engineering Failure Analysis 16 (2009) 1929–1939
1931
k
clamp F2
support beam
F1
Fig. 2. Construction of the anchor drum.
For example, if the friction coefficient is 0.2 and the rope is wrapped around the drum four times, the residual force amounts to only 1% of the service load (F 2 ffi F 1 =100). In general, the clamp has to be designed just for a small fraction of the service force in the rope, allowing a smooth clamping, without excessive local pressure on the rope’s surface. This advantage becomes especially important for applications where the rope has to be shifted periodically as in the final case study in this paper. 3.2. Accident During regular operation of a ropeway, the end of a hauling rope holding the cabin in position slipped off the clamp of the anchor drum. The rope detached from the cabin and the cabin became free. Fortunately, a safety equipment (track rope break [1]) prevented the crash of the cabin. 3.3. Investigations The visual inspection of the anchor revealed no apparent anomalies. As an excessive overload during operation could be excluded, an insufficient load capacity of the anchorage was assumed. This obvious hypothesis had to be proved with a load test. Since the rate of change of the service load in the rope was low, a quasi-static test set-up was built. During the test, the force in the anchored rope was increased gradually up to the triple of the highest service load. However, the clamp did not fail. The hypothesis of insufficient load capacity of the anchor had to be rejected. At a loss to explain the failure and test result, the disassembly of the test set-up was started. At first, the load in the rope was removed. In doing so, a small amount of slippage was observed between the clamp and the rope. By repeating the loading and unloading cycle, the rope slipped incrementally through the clamp, even when the fluctuating tensile force in the rope remained considerably below the regular operational force. Strangely enough, the slippage took place not during the increase but during the reduction of the load. The unexpected system behaviour could be explained after a detailed analysis of the process. The service load fluctuated slowly in accordance with the load status, position or acceleration of the cabin etc. within a regular range. If the tensile load increased, the elastic deflection of the support beam became larger. As a consequence, the distance between the drum and the rope clamp decreased ðF 1 ") k #, Fig. 3). Due to the reduced distance, the residual force collapsed ðk #) F 2 Þ and therefore the relationship (1) was no longer satisfied. In order to restore the required equilibrium between the forces on the both side of the drum, the rope crept around the drum in a clockwise direction until the residual force increased to the required value again.
k↓
F2↓
slippage
F1 Fig. 3. Disturbed force equilibrium in the rope on both sides of the drum during increased service load (the elastic deformation of the support beam is exaggerated).
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When the operational load decreased, the support beam started to flex back into its original position. Therefore, the distance between the drum and the clamp got larger ðF 1 #) k "Þ. Because of the missing rope length between the drum and the clamp, the load started to increase in that rope section. There was no possibility for pulling back the rope over the drum, as it would have required a force several times higher than the current service load (interchanged indices ‘‘1” and ‘‘2” in Eq. (1)). Obviously, the yielding took place at the clamp which was designed just for a small fraction of the service load. During the operation of the ropeway this asymmetric procedure was repeated periodically, until the rope had slid out of the clamp. 3.4. Generalized conclusions Structures should be analysed also under consideration of fluctuating service loads, even if classical dynamic failure causes – such as vibrations, fatigue or wear – can be excluded. A failure can be caused by hidden coupling effects in a system. A simple elastic structure can react to a variable load with asymmetric behaviour. This can lead to the accumulation of small negative effects and finally cause failure.
4. Collapse of a hoisting platform 4.1. Introduction The accident involves the crash of a hoisting platform in a two-storey shop. The platform was guided along a rail fixed to the wall and was moved up and down with the help of an electric hoist (Fig. 4). The load capacity of the hoist was doubled with the use of a moving pulley. As a safety precaution, a load limiter was built into the hoist’s drive, controlling the torque on the drum. It was adjusted to switch off the hoist at a 20% overload. 4.2. Accident The worker intended to transport a heavy box from the lower to the upper floor of the shop. While raising the platform, the hoist suddenly stopped and the platform came to a halt between the upper and lower floor. The worker assumed that the platform had jammed in the guide thus activating the load limiter. In order to leave the platform, he pressed the ‘‘down” button on the hoist’s control unit. By doing so, the rope failed suddenly and the platform fell to the lower floor. 4.3. Investigations The visual investigation revealed that the rope had derailed from the moving pulley because the groove of its plastic wheel was partially damaged. However, the causality between that defect and the accident was unclear, since the load limiter had worked properly and the rope had withstood the load until the platform was lowered. The accident was reconstructed in the laboratory. For the test, the original equipment was used with a new rope. By raising a metal block, representing the moved load, the rope derailed at the pulley and jammed between the metal fork and the
hoist
upper floor pulley control unit box lower floor
platform
Fig. 4. Situation before the accident.
wall
fixed end of the rope
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Fig. 5. Derailed steel cable jammed between the fork and the plastic wheel of the moving pulley.
plastic wheel (Fig. 5). The hoist was correctly stopped by the load limiter. Obviously, the real reason why the load limiter came into action on the occasion of the accident was not the jamming of the guide wheels of the platform, as assumed by the worker. After the stop of the hoist the rope could still carry the load, despite its noticeable local damage in the jammed zone. The safety concept seemed to be effective. However, after switching the hoist to ‘‘down” direction, the rope fractured immediately, in an analogous manner to the accident. After analysis of the process, the reason for the rope rupture became clear. Once the rope had jammed at the pulley, the equilibrium between the two sections of the rope was no longer established. During the lifting, the external load concentrated gradually in the rope section connected to the hoist. As soon as the load in that rope section exceeded the allowed overload of 20%, the hoist was stopped by the load limiter. However, during the subsequent lowering of the platform, the service load concentrated in the rope section between the pulley and the fixed end of the rope. The hoist got unloaded and the tension in the rope sequence with the fixed end doubled. This excessive load was higher than the remaining load capacity of the rope in the jammed zone. The load limiter could not prevent this procedure as it did not register the tension in that rope section after the malfunction of the pulley. 4.4. Generalized conclusions A failure can restrict the region which is being monitored by safety equipment. A structural member can be overloaded not only during energy input, but also in the passive phase of the system (e.g. during lowering a load).
5. Track rope damage 5.1. Incident The incident occurred on a forty years old reversible aerial ropeway with classic design: The carriage of the cabin rolls on two track ropes and is moved by a hauling rope [1]. During regular operation of the ropeway, an employee in the upper terminal heard a loud bang. He did not waste any time with exploring the source of the unusual sound, and immediately shut down the ropeway. With the help of binoculars, the he spotted a serious track rope damage with protruding wires in a distance of around 60 m from the terminal. The damaged rope section was observed also from a helicopter. This risk evaluation revealed that all 21 Z-formed wires in the outer layer of the track rope were fractured, reducing the rope’s metallic cross-section by more than 50% (Figs. 6 and 7). As there was fear of ropeway collapse, the 53 passengers were evacuated by helicopter.
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Fig. 6. Cross-section of the track rope. Outer layer with 21 Z-wires (all of them broke during the incident) and inside layers containing 61 round wires.
Fig. 7. Lower half of the damaged track rope section with protruding broken Z-wires 80 m above ground level (t: distance between the adjacent track ropes of the same track).
5.2. Background information Such a serious track rope damage was considered in the state of the art safety regulations of ropeways to be practically impossible [2]. This until then well-proved philosophy was based primarily on the fact that even a large number of wire fractures is tolerable without risk of a rope rupture. The reason for this failure tolerance is that single wire fractures due to normal deterioration of a rope are randomly distributed in both the axial and circumferential directions of the rope. This fact has two positive consequences: The reduction of the rope’s metallic cross-section is everywhere negligible, as the probability of many wire fractures in the same cross-section is very low. A broken Z-wire cannot protrude out of the rope’s compound as the adjacent wires constrain it (Fig. 6). As an additional positive effect, the tensile force in each broken wire builds up gradually again, due to friction forces between the adjacent wires. Therefore, the minor weakening of the rope by a broken wire is further limited to its short adjacent section on both sides of the flaw. The growth of the number of single wire fractures is monitored periodically by means of magneto-inductive tests. If the number of wire fractures within a reference length approaches the prescribed tolerable limit, the rope is replaced. In the case under consideration, the last magneto-inductive test had been carried out shortly before the incident. The number of detected wire fractures was well below the tolerable limit. Obviously, the incident had to be initiated by effects which were not covered by this state of the art safety concept.
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5.3. Investigations The investigation started on site with documentation of the damage and measuring of several parameters such as the geometry of the rope, the length of lay and – based on the propagation velocity of a transversal wave in the rope – the tensile force. The preliminary investigations provided evidence only for the exclusion of some hypotheses, but none for the failure mechanism. Therefore, the rope was unloaded and its damaged section was cut out and transported to the laboratory for further investigations. Right at the beginning of the examinations in the laboratory, metallic deposits were detected on the rope’s surface, distributed along a line of about 6 m in length (Fig. 8, left). It appeared that the deposits must have arisen from a galling process under unfavourable tribological conditions. As the same kind of deposits was also detected adjacent to all of the wire fractures (Fig. 8, right), a link between the incident and the deposits seemed obvious. However, (a) the source of the deposits and (b) the triggered failure mechanism were still unclear. Accordingly, the investigation was focused on these two issues. 5.3.1. Localization of the deposit’s origin The chemical analysis (EDX and WDX) revealed that the counterpart of the rope during the galling process was composed of structural steel with a carbon content of approximately 0.2% (Z-wire: approximately 0.6%). During operation, only the passenger cabin’s carriage could have had such a tribological contact with the damaged rope section, namely in the case of an imaginable derailment. However, no such galling traces were found on the carriage. The search for the galling partner had to be continued by reconstructing the location of the rope in the past. According to the standard regulation, the track ropes were shifted every 12–14 years. The aim was to avoid excessive fatigue in the most highly stressed sections of the rope, i.e. at the ends of the tower saddles and in the terminal. During each rope shift, the clamp behind the anchor drum was carefully loosened (Fig. 9), so that the rope slipped over the drum and moved towards the lower terminal. After a shift of approximately 25 m, the clamp was tightened again until the next shift.
Fig. 8. Metallic deposits on the rope’s surface (left) and next to breaking point of a Z-wire (right).
hauling rope track rope
saddle
carriage of the cabin
anchor drum
clamp
Fig. 9. Sketch of the upper terminal.
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Fig. 10. Traces of galling on the saddle’s surface.
Fig. 11. Cracks on the surface of a Z-wire, detected by magnetic particle method.
Before the incident, the track ropes were shifted three times. At the first relocation, the damaged segment of the rope lay exactly on the deflection saddle, which is made of structural steel. Obviously, this part was considered as the most likely counterpart to the rope during the galling process. In order to prove this hypothesis, the groove of the saddle was inspected visually after dismantling the rope. A cast was made of the groove and its surface was compared to the surface of the rope under an optical microscope. As the topology of the surfaces qualitatively matched, there was no doubt that the real galling partner had been found (compare Fig. 8 with Fig. 10, right). 5.3.2. Determination of the triggered failure mechanism by the deposits At the beginning of this part of the investigation, the original location of the single wire fractures on the rope’s surface was determined. The outer layer of Z-wires was cut at a reference cross-section of the rope where the compound was still intact. Afterwards, the individual wire sections were weighted and their lengths were calculated based on the specific weight of the profile wire. With this rather uncommon measurement method secondary damages on the wire’s surface could be avoided. This investigation showed that the fractures were distributed over a distance of round 6 m and there were some groups containing up to four adjacent wires with roughly equal lengths. In other words, those wires had fractured in the same crosssection of the rope, forming fracture clusters. The surface of some Z-wires was tested for cracks by the magnetic particle method. This method revealed numerous cracks oriented perpendicular to the main stress direction (Fig. 11), indicative for corrosion fatigue. However, the corrosion chemistry analysis of the deposit’s aqueous extract detected only slight traces of corrosion-promoting substances and a slightly acidic environment (pH 5.4–5.7). The next step was the fractographic and metalographic investigation of the wires. Most of the fractured surfaces showed a similar macroscopic and microscopic appearance. They exhibited only little macroscopic deformation and were hardly corroded (Fig. 12, left). The last point indicated that almost all wires fractured within a short period of time and not long before the incident. The main part of the broken surfaces showed the characteristic of micro-ductile overload under the scanning electron microscope. However, adjacent to the fracture origin there was a typically 0.7 mm deep intergrannular zone with boundary gaps in places (Fig. 12, right). In the longitudinal cross-section of the Z-wires small cavities isolated from the ambient air were found between the deposits and the wire’s surface (Fig. 13). These findings provided a clue for hydrogen-induced stress corrosion cracking (H-SCC) as the primary damage mechanism. Below the deposits all the preconditions for H-SCC were given [3]:
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Fig. 12. Fracture surface of a Z-wire (left) and detailed view of the intergranular zone with numerous boundary gaps (right).
Fig. 13. Etched longitudinal micro-section of a Z-wire in the area of metallic deposits.
a susceptible material such as high strength steel (Rm > 1000 MPa [4]) ) the strength of the broken Z-wires was at 1500 MPa (based on the measured hardness of 450 HV10) and probably even higher in the region of the detected friction induced martensitic layer, tensile stress ) due to normal service loads, catalysts, such as sulphur ) present in nearly all steels in the form of manganese sulphide, cavities with an absence of oxygen ) regions below the deposits. In fact, the increased hydrogen concentration at the crack tips could be proved by microelectrochemical measurements, using glass microcapillary. The extent of the embrittlement due to H-SCC was semi-quantified with the help of a regular wire twisting test. For this purpose, two samples were taken from the same Z-wire but from different sections: one with, the other without deposits. The samples were twisted until fracture. The large embrittlement of the wire in the region of the deposits was apparent (Fig. 14). The question, whether the crack could became instable during regular operation of the ropeway, was assessed by means of linear elastic fracture mechanics. The estimated tensile stress in a Z-wire was in the range of r ffi 400–500 MPa, not including the additional bending stress induced temporary during passing of the cabin. Considering a crack length equal to the size of the typical intergranular area (affi 0.7 mm) the stress intensity factor amounts to
K ffi 1:12
pffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffi p a r ¼ 21—26 Mpa m
ð2Þ
The fracture toughness of a medium carbon steel like the material of the Z-wires lies at room temperature in the range of pffiffiffiffiffi about K IC ¼ 40—50 Mpa m [5]. As the incident happened at an air temperature far below the freezing point, the instability of the deepest intergranular cracks is plausible.
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Fig. 14. Two sections of the same Z-wire after the twisting test in the region of the produced breakage.
5.4. Summary of the sequence of events Based on the findings of the investigation, the chronological sequence of the incident could be reconstructed as follows. 5.4.1. 1979 Galling process At the first relocation of the rope a galling process between the rope and the deflection saddle took place as the saddle had a rather small deflection radius and an oversized grove radius. The corresponding high compressive stress peaks between the track rope and the groove, combined with poor lubrication, led to damage in the surface of both components. In fact, the personnel could still remember the jerky sliding of the rope during the shift, but at that time nobody was concerned with possible long-term consequences. 5.4.2. 1979–2004 Stress corrosion cracking After the first relocation of the track rope, a slow deterioration process started below the deposits by H-SCC. The cracks were not randomly distributed, but they were concentrated at the particular location of the deposits. Therefore, the likelihood of fracture-clusters of many adjacent wires was high. During this time period, the rope section with the deposits was moved towards the lower station in increments of around 25 m on the occasion of every rope shift. 5.4.3. 2004. 04. Last magneto-inductive tests At the time of the last magneto-inductive tests, most of the Z-wires were not fractured, just cracked. Since the applied magneto-inductive method could detect only fully fractured wires, the test did not reveal the real risk. 5.4.4. 2004. 04. - 12 Fast fracturing of several wires The deepest wire-cracks approached the critical length for unstable crack propagation. 5.4.5. 2004. 12. Dynamic chain reaction (incident) As soon as three adjacent wires fractured in the same cross-section, they protruded from the rope compound, leading to torsional unbalance between the remaining rope layers and therefore to abrupt twisting of the rope. The heavy rotation triggered a chain reaction, leading to rupture of the entire outer layer of the rope. The dynamics of the process could be backtracked based on the shape of some broken wires, standing perpendicular to the axis of the rope with a hook at the end (Figs. 7 and 15). The distance between the hooks’ centres and the axis of the rope was approximately as large as the distance to the adjacent track rope (t = 550 mm). Obviously, the wires must have been twisted at high speed and must have collided with the adjacent rope. The end section of the wire was fractured at one of the cracks due to the force of inertia at impact. In fact, some of the broken end sections were found on the ground, flung up to 70 m from the vertical plane of the ropeway. After the investigation several bicable ropeways, which were susceptible to the same failure mechanism, were controlled. In many cases metallic deposits on the track ropes and even cracks were detected. The reason why in the past no similar incident had occurred could be that the regular deterioration of the rope (fatigue, fretting) progresses usually faster than the failure mechanism due to H-SCC. Hence, the ropes were replaced due to detected fatigue fractures before the intergranular cracks became instable. 5.5. Generalized conclusions The primary cause of a failure can lie far back in history of the system. A redundant system can fail if the preconditions for redundancy are violated.
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Fig. 15. Broken Z-wire protruding perpendicular to the axis of the rope with a hook at the end.
‘‘Inspected and approved” does not necessarily equal ‘‘flawless” (each measuring method has its detection limits in size and type of the flaws). Systems can be affected by different failure mechanism simultaneously. A slow failure mechanism can remain undetected as long as the service time is limited by a quicker one.
6. Summary The generalised conclusions given in this paper represent only some possible examples of how the knowledge of a particular failure analysis can be generalised and collected in order to support future investigations. In this way, the experience made by a failure analysis can be strengthened in one’s memory. Furthermore, it becomes more recallable and applicable for different kinds of failed systems. As a result, the efficiency of the failure analysis procedure increases and the likelihood of incorrect conclusions decreases. Acknowledgements The second failure was originally investigated under the leadership of Prof. Dr. G. Oplatka, retired head of the Institute of Ropeway Technology at the Swiss Federal Institute of Technology Zurich (ETH-Z). The authors thank him for providing the background information for this paper. References [1] [2] [3] [4] [5]
Wallis-Tayler AJ. Aerial or wire rope-ways: their construction and management. Kessinger Publishing, LLC; 2007. Eidgenössisches Verkehrs, Energiewirtschaftsdepartement. Pendelbanhnverordnung. Bern: Eidgenössische Drucksachen – und Materialzentrale; 1988. Kuron D. Wasserstoff und Korrosion. Kuron Verlag; 2000. Lange G. Systematische Beurteilung technischer Schadensfälle. DGM-Verlag; 1993. p. 235 ff. Ashby MF, Jones DRH. Eng Mater 1. 3rd ed. Elsevier; 2005.