Failure analysis of a cableway rope
Carlo Mapelli
*, Silvia Barella
Sezione Materiali per Applicazioni Meccaniche – Dipartimento di Meccanica, Politecnico di Milano, via La Masa 34, 20156 Milano, Italy
a r t i c l e
i n f o
Article history:
Received 20 November 2008 Accepted 6 December 2008 Available online 13 January 2009 Keywords: Wire rope Cableway Fatigue Relative sliding
a b s t r a c t
This study is focused on the failure analysis of a cable way rope composed by different wires in which the magnetoscopic analysis has pointed out a significant and abnormal number of failure indications after a relatively short time of service. Such a phenomenon has taken place in one of the longest worldwide cableway plant and it is interesting that the performed microstructural analysis has clearly indicated that the applied steel seems featured by a good soundness of the very fine perlite micro-structure that has not been interested in any way by the decarburation phenomena. Moreover, the realized micro-hardness tests indicate a reliable homogeneity of the strength properties featuring the steel along the wire section. On the other hand, the fractographic analysis clearly indicates that the source of the failure mechanism is the initiation of the cracks in the surface of the z-shaped wire of the outer wire layer in contact with the wires of the inner layer. The cou-pling of the different results seems to indicate that the most probable cause of the failure mechanism has to be found in the excessive pressure applied on the rope and to the con-sequent sliding between the external wire layer and the inner adjacent one that produces debris and the nucleation of the crack.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
A wire rope consists of many wires twisted together to make a complex structure combining axial strength and stiffness with flexibility under the bending action[1]. The producers of wire rope offer a wide range of rope types, in which the wires can be organized in according to different configuration for achieving achieve an acceptable performance in a wide range of safety critical applications. These elements are long lasting when properly used and maintained.
The rope is constructed by laying several strands around a core. The strands themselves have a central wire around which the single metallic wires are helically wrapped[2]. The larger fraction of load is carried by the strand. On the other hand, the wires are the basic elements of a rope and they are made by patented high strength steel, realized through the application of a carbon steel featured by a carbon content generally close to the eutectoid composition (0.77wt% C). The choice of such a steel grade is related to the aim of slowing or completely avoiding the growth process of the fatigue cracks through the obstacle role opposed by the alternate lamellas of ferrite and iron carbide that feature the perlite characterizing these eutec-toid steels.
The helical configuration of rope construction leads to shear stress components rising among the surfaces of wires in con-tact due to the action of the load directed along the main axis of the rope. Thus, the combination of the geometrical config-uration of the applied load is the source of shear stresses between wires in contact. These shears could represent the origin of a crack that leads to fatigue failure. On the other hand, the geometrical configuration is generally suitable to avoid the con-sequence of such a failure event. Actually, the high contact force due to friction among the adjacent wires tends to prevent
1350-6307/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2008.12.011
* Corresponding author. Tel.: +39 (0)2 2399 8272; fax: +39 (0)2 2399 8202. E-mail address:carlo.mapelli@polimi.it(C. Mapelli).
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Engineering Failure Analysis
failure.
In the great majority of applications the rope is subjected to repeated bending and variable tension, especially on the cableway plants, where the main source of load variability is related to wind action, application and releasing of load at the passage of the cable and shifting of the wire along its development during the maintaining procedure. This last operation implies a change in the curvature assumed by the rope (Fig. 1) that can stay in two different positions associated to two dif-ferent states of stress and strain (Fig. 2):
a convex curvature of 34 m assumed by the portion of rope staying on the sustaining shoes; a concave curvature of 2000 m assumed by the rope in between two successive columns.
There are two class of ropes applied in the cableway plant: the cable sustaining ones lied on the column shoes and the pulling rope – linked to each cable in order to perform the traction (Fig. 3) – that is coated by a zinc layer to protect the rope from corrosion processes. The possible twistering between the two ropes caused by the wind action is one of the more det-rimental event due to the heating and the wear processes that can be involved during the passage of the cables.
Moreover, the rope is also subjected to high contact stresses and longitudinal sliding at points of contact with sheaves and winch drums. As a result of its arduous service, rope can be worn out, compromising safety and structural properties, so spe-cific inspection procedures should be implemented to prevent tragic failure[3,4].
This cableway is one of the longest at worldwide level and it uses ropes 4500 m long. The costs related to a stop during the service and the safety aspects lead to avoid frequent changing of the rope and to identify the source of rapid degradation. In the analysed cases a three-strand multi-wire rope sustaining a cableway has an early loss of functionality and it became un-safe in 36 months due to the detection of an excessive number of revealed failure in a 0.8 m length. The inspected section of the rope is located on the sustaining sliding plate fixed over the column of the plant.
Fig. 1. Layout of the failed rope composed by different layers (the z-shaped ones in the outer layers and the round one in the inner layers) and an overall diameter of 67mm.
The dangerous situation has been detected after the indication of possible failures pointed out by magnetoscopic controls. In the analysed part of the inspected rope 27 wires were broken on the external layer (featured by z-shape section of the wire) and 56 wires have been recognized as cracked. After this measurement procedure the inspection has been immediately extended to other two plants featured by the same design and also in these other two cases the concentrated and dangerous failure events have been pointed out. The aim of this study is the identification of the mechanism causing the failure and the origin of such a process.
2. Experimental procedure
The sustaining damaged rope has been opened and disaggregated for sampling the different wires and during this oper-ation a significant amount of fine metallic debris fell down. The debris and traces of materials on the wire surfaces have been analysed by SEM–EDS to identify the source of such a material.
A metallographic examination realized to characterize the microstructure and a hardness profile were performed on ten different wires paying attention to the characteristics of the section normal to the wire axis. They have been sampled in two different positions of the wire rope not far from the broken end. Before metallographic sample preparation and SEM analysis, dirt and lubricant residues were removed by cleaning in an ultrasonic bath in acetone.
Optical analysis samples were prepared by grinding and polishing and they are etched in 2% Nital (2%H2NO3solution in
demineralised water applied for 10 s).
Micro-hardness tests were performed by a 100 g load and applying an indentation time of 15 s. The hardness values are a mean of three tests take at the same distance from the surface.
The fractured surfaces were observed by Scanning Electron Microscope coupled by Energy Dispersion Spectrometer (SEM–EDS) in order to define the morphology of the cracked surface, which provides useful information about the failing process.
3. Results and discussion
The analysis of the residuals present on the wire surface points out that the presence of zinc in very limited and is often associated with some traces of iron oxide (Figs. 4 and 5,Tables 1 and 2).
This observation seems to exclude a significant role played by the friction produced during twisting of the ropes, because a high heating would have certainly produced a lager zinc deposit due to the low melting point of such a chemical element. Visual examination of the rope shows (Fig. 6) fractures without traces of visible necking of the wires that indicate that the failure process has taken places without plastic deformation. The lateral surfaces of the wire samples were covered by lu-bricant and corrosion products and wear damages were not visible at a macroscopic level. On the other hand, a lot of wires are clearly cracked and some were even completely broken.
The microstructural analysis is aimed at determining the role played by the possible defects revealed at a microstructure level for developing the failure. The metallographic analysis performed on the sample taken from the wires show the typical very fine perlitic microstructure (Figs. 7 and 8).
The structure is very fine and this is the typical and the expected microstructure after a patenting process. The SEM obser-vation better points out this very fine perlite structure, but in any case there is not evidence of a decarburization phenomena that can be the cause of an alloy softening which can allow a surface damage. Thus, this source of the damaging phenomena has to be excluded (Figs. 9 and 10).
The absence of a detrimental heterogeneity of the microstructure features is confirmed by the detected Vickers micro-hardness profiles that do not show anomalous values and the harness is constant on all sample sections (Figs. 11 and 12).
Fig. 4. Example of a zone near the damaged region with the presence of the residual debris found on the wire surface.
Fig. 5. Example of a zone near the damaged region with the presence of the residual debris found on the wire surface.
Table 1
Average chemical composition of the residual identified on the selected area inFig. 4.
wt% O P Ca Fe Zn 1 21.57 78.43 2 19.72 80.28 3 30.13 2.23 67.64 4 14.67 3.60 0.94 73.22 7.56 Table 2
Average chemical composition of the residual identified on the selected area inFig. 5.
wt% O Si Cl Ca Fe
1 30.85 0.79 68.36
2 8.17 91.83
However, the observations performed by SEM about the fracture morphology allow to identify clearly a fatigue failure mechanism on the cracked surfaces. The developed cracks can be divided into two main different zones, in which a toe shape zone is clearly present and this indicates the origin of the fracture process, so the fracture starts on the surface and the fa-tigue cracks grow within the wire resistant section. The failure mechanism is the same for all the examined wires (Figs. 13 and 14). Moreover, sample observations show the cracks origin: in the two observed samples the initiation is always in the wire region in contact with the more internal wires. The plausible failure explanation is a fatigue mechanism initiated be-tween the surface of the external wire and the internal ones. This contact causes many shears which produced coloured bands visible on the wire surface (Fig. 15).
Fig. 6. Piece of wire rope with indication of crack wires. The cracks produced during the service are the only marked ones.
Fig. 7. Optical metallography of the cross-section of sample 1 featured by a very fine perlitic microstructure.
The damaging phenomenon can be due to an excessive state of stress induced between the wire surfaces which leads to a relative sliding between those surfaces that produces the decries and the crack initiation[5–7]. Provided the good micro-structural state of the wire microsructure, the fracture nucleation is probably related to the limited size of the sustaining shoe that produces a significant stress concentration due to the too small area on which the rope load is distributed. 4. Conclusions
The performed analysis have pointed out that the failure mechanism interesting the cracked perlitic wires composing the rope is due to:
a fatigue mechanism begun with the nucleation of cracks always localized on the surface of the z-shaped wires of the outer layer constituting the rope;
the surface interested by the crack nucleation is clearly recognized by the assumed toe shape of the nucleated cracks and it is always the surface in contact with the adjacent wires;
Fig. 9. Example of the microstructure near the wire surface of a damaged region.
Fig. 10. Example of microstructure near the wire surface of a damaged region.
the absence of microstructure alteration of the perlite and of surface decarburation exclude that the initiation mechanism can be related with the detrimental modification of the perlitic steel;
the presence of a significant amount of steel debris after the separation of wires composing the rope seems to indicate that the source of the cracks is due to a relative sliding between the outset wire layer and the adjacent inner one;
Fig. 12. Example of a direction of the z-shape wire along which the micro-hardness profile has been measured.
Fig. 13. Sample 1 fracture surface at low magnification (left) and at high magnification (right).
this sliding has been probably promoted by an excessive stress produced by an underestimation of the size featuring the sustaining shoe fixed on the supporting column in which a strong stress concentration takes place.
References
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[4] Torkar M, Arzenšek B. Failure of crane wire rope. Eng Fail Anal 2002;9:227–33.
[5] Hobbs RE, Ghavami K. The fatigue of structural wire strand. Int J Fatigue 1982;4(2):69–72.
[6] Beretta S, Boniardi M. Fatigue strength and surface quality of eutectoidic steel wires. Int J Fatigue 1999;21:329–35. [7] Schrems K, Maclaren D. Failure analysis of a mine hoist rope. Eng Fail Anal 1997;4(1):25–38.
