Background 

A wire rope broke while lifting a load of reinforcing steel estimated to weigh 2.5 - 3 tonnes. The precise sequence of events leading to the failure were not known, but the load did not drop because the rope jammed in the gap between sheave and support bracket. The rope was 2 years old and was stated to be maintained by regular lubrication. Bounce of the load (Activity 1) was possible during lifting. The rope sheave diameter was 520 mm, and it was known to have been in service longer than the rope. 

The insurance loss adjustor (whose job is to advise the insurance company on the quantum of loss and whether the existing policies covered the causes and consequences of the failure) commissioned a failure investigation to ascertain whether the rope was overloaded at the time of failure, or whether poor maintenance had been responsible for premature breakage.  The resolution of these issues would determine whether or not insurance cover existed for the various consequences of the accident, and would also indicate if an increase in premium was necessary to offset a possible higher level of risk. 

Three pieces of rope were supplied to assist in this investigation - these comprised the two broken ends together with a section of rope taken well away from the failed ends. The purpose of this latter piece was to check the load capacity of the rope, via tensile testing, at the time of failure. 

Visual Observations 

The rope was a general engineering 18 strand non-spin type designed as 12x7(6/1)/6x7(6/1) - see the illustration below. 

This designation is a shorthand form which summarises the information contained in the image.  Thus this rope had an inner layer of 6 strands of wires, with each strand comprising 7 wires wrapped as 6 outer wires around 1 inner wire, while the outer layer is formed by 12 strands of wires wrapped in the same way.  The inner core of the wire is fibre. Resistance to rope spin is provided by opposing twist directions of the inner layer (anticlockwise) and outer layer (clockwise), whereby the load-induced torque tends to cancel out.  Further information on wire rope design, damage and the effect of sheave size can be found by following this hyperlink and looking at

thisweb page.

As received, rope lubrication was deficient to dry, with slight corrosion evident on the outside of the rope.Figure 1 shows the 2 broken ends of the rope. Close inspection of the rope near to the fracture plane showed that a number of wires were cracked in outer and inner strands (Figures 2 & 3). Cracks on both inner and inner layers were associated with flattened regions on the wires.Cracking was also observed on wires well away

from this region (Figure 4).

Figure 1 Figure 2

Figure 3 Figure 4

Proceed to second part of case study.

Tensile Testing 

Before performing tensile testing of the rope, it was necessary to establish the original grade and size of the wire rope.  This would indicate what degradation of properties had taken place over the service life, and provide an indicator of the severity of service and quality of maintenance.  The only information that the operator could supply, was that the rope was a 1770 MPa grade.  Thus it was necessary to measure the diameter of wires near to the break (average approximately 1.5 mm) and the rope diameter (approximately 21.5 mm). 

Use the information contained in the wire rope manufacturer's table of properties (Activity 2) to find the most likely original rope diameter and breaking force. 

Tensile testing was performed on a 1.5 m length sample using a wire rope testing machine, giving a measured failure load of 232 kN. It should be noted that this load did not represent complete failure of the rope, but rather fracture of 11 strands (66 wires) out of a total of 18 strands (108 wires).  Two inner and five outer strands remained unbroken.Examination of the 66 wires that broke indicated that only 10 of them had a flat fracture surface, which would be indicative of the presence of an initial fatigue crack, with the rest showing ductile failure modes. 

Referring back to the information on original rope diameter and breaking force, use Activity 3 to compare this load with the observed value for breaking load in the tensile test. What conclusions can be drawn regarding the likely influence of pre-existing fatigue cracks? 

Fractography 

A number of individual broken wires were cut off the fractured ends and examined at low magnification using stereo binoculars, and at high magnification in a scanning electron microscope (SEM).  The total number of wires in all strands was 108, and 20 wires were selected from the outer strands and 11 from the inner strands. The wires were de-rusted and ultrasonically cleaned in a de-greasing agent. 

Typical SEM observations of the fracture surfaces are given below at both a low and a high magnification, together with information on the number of wires in the sample which were similar. 

Type 1: Tensile cup-and-cone fracture - 3 occurrences - 1 wire in outer strands 2 wires in inner strands.

 Low magnification fractograph of cup-and-cone.

 High magnification fractograph from the central region of the cup-and-cone fracture.

Type 2: Flat twisted failure -2 instances in outer strands.

Type 3: Flat semi-elliptic regions present - 26 cases; 17 in outer strands, 9 in inner strands

 Example 1

 High magnification fractograph from flat semi-elliptic region shown with arrow.

 Example 2 Using the fractographic information from Activity 4, determine the mechanism of failure indicated by types 1-3.

Summary and Conclusions 

The information gleaned so far from this case study is summarised below:1. Overall, the strength of the rope was reduced by the presence of fatigue cracks - this is evidenced by the observed tensile strength of

232 kN compared with the manufacturers stated breaking load of 332 kN.2. The observed breaking load is still very much higher than the stated load being lifted at the time of failure (some 25 - 30 kN). Thus the

rope must still have failed through application of an overload relative to its current strength level. The failure was not solely due to the presence of fatigue cracks (whose existence is fairly normal in wire ropes and explains the requirement for regular maintenance and high factors of safety).

3. The cause of this overload is not clear, but bouncing of the load might have allowed the rope to jump from its groove and jam between sheave and boom during winding. If this state of affairs could exist for a short time undetected, and the rope winding was continued, a very significant overload could be applied to the rope.

4. The cause of the fatigue cracks needs clarification. They can initiate as a result of bending stresses induced by too small a sheave diameter. The recommended diameter is 18x rope diameter which equals 432 mm for the resent rope. The actual sheave diameter was 520 mm, which should have been sufficient. As the sheave was older than the rope, however, it is possible that wear of the sheave groove has had an influence. Fatigue cracks can result from deformed surface regions where ductility thus becomes exhausted, particularly if surface damage from abrasion occurs. This would be exacerbated by any decrease in sheave groove diameter, which could occur by wear during service, and by poor lubrication practice (which was apparently the case).

The conclusion to be drawn from this investigation is that the presence of fatigue cracks has lowered the breaking load of the rope by some 30%. However, the breaking load is still 232 kN, very much higher than the stated lifting load of 25 - 30 kN. The fractographic work has indicated ductile fracture in all wires, demonstrating that the rope metallurgy is up to specification. The most likely cause of the fracture seems to be rope jamming between sheave and groove, probably due to bounce during lifting. The cause of the bouncing is unknown. 

In insurance terms, poor maintenance is not a prime cause of the failure, which would have been "sudden and unexpected" when it occurred. Cover should exist for such circumstances.Recommendations 

Any good failure investigation leads to recommendations aimed at avoiding the problem in the future or, at least, reducing its likelihood of occurring. Recommendations in the present case are:

1. Ensure adequate lubrication is maintained in the rope.

2. Re-groove the sheave at regular intervals and, particularly, when the rope is replaced by a new one.

3. Control lifting to avoid bouncing and install detectors which are activated by rope coming off the sheave.

4. Monitor condition of rope by surface inspection and tensile testing.

References - to fatigue of wire ropes.1. J Llorca and V Sanchez-Galvez (1989) Fatigue and Fracture of Engineering Materials and Structures Vol. 12 No. 1 pp31-452. RE Hobbs and K Ghavami (1982) International Journal of Fatigue April 1982 pp69-72

3. NF Casey and WK Lee (1989) International Journal of Fatigue Vol. 11 No. 2 pp78-84.

4. M Alani and M Raoof (1997) Effect of mean axial load on axial fatigue life of spiral strands, International Journal of Fatigue Vol. 19 No. 1 pp1-11

5. K Coultate (1997) Magnetic attraction of wire rope testing, Materials World Vol. 5 September 1997 pp519-520.

6. K Schrems and D Maclaren (1997) Failure analysis of a mine hoist rope, Engineering Failure Analysis, Vol. 4 No. 1 pp25-38.

7. MD Kuruppu, A Tytko and TS Golosinski (2000) Loss of metallic area in winder ropes subject to external wear, Engineering Failure Analysis, Vol. 7 No. 3 pp199-207.

8. J-I Suh and SP Chang (2000) Experimental study on fatigue behaviour of wire ropes, International Journal of Fatigue Vol. 22 pp339-347.

9. M Torkar and B Arzensek (2002) Failure of crane wire rope, Engineering Failure Analysis, Vol. 9 No. 2 pp227-233.

Failure Analysis of Wire Rope

Brett A. Miller, Stork Technimet Inc.

From: B.A. Miller, Failure Analysis of Wire Rope, Advanced Materials and Processes, Vol 157 (No. 5), May 2000, p 43–46

Abstract: Mechanical properties of wire ropes, their chemical composition, and the failure analysis process for them are described. The wires are manufactured from high-carbon, plain carbon steel, with high-strength ropes most often manufactured from AISI Grade 1074. During visual failure examination, the rope, strand, and wire diameters should all be measured. Examination should also address the presence or absence of lubricant, corrosion evidence, and gross mechanical damage. Failed wires can exhibit classic cup-and-cone ductile features, flat fatigue features, and various appearances in-between. However, wires are often mechanically damaged after failure. Most nondestructive evaluation (NDE) techniques are not applicable to wire rope failures. Electron microscope fractography of fracture surfaces is essential in failure analysis. Fatigue is the most important fracture mode in wire ropes. Metallographic features of wire ropes that failed because of ductile overload and fatigue are described.

Keywords: Fractography; Metallography; Wire rope

Material: 1074 (Nonresulfurized carbon steel), UNS G10740

Failure types: Ductile fracture; Fatigue fracture

Introduction

Among load-bearing steel constructs, wire ropes and cables are possibly the most widely used and most highly stressed. Wire ropes are assemblages of intertwined steel wires and wire strands for pulling or lifting. They serve in critical applications and in severe environments, and failures are common. One particularly damaged rope that failed via fatigue is shown in Fig. 1. From a metallurgical failure analysis perspective, wire rope failures can appear deceptively mundane. However, investigation of these failures involves many complicated and unique considerations that are worthy of review.

Fig. 1  This photograph of a severely cracked wire rope exhibits wire fractures that result from an improper service environment.

This article includes a description of the mechanical properties of wire ropes, their chemical composition, and a detailed discussion of the failure analysis process.

Mechanical properties

The mechanical properties of a wire rope result from the individual wires and their arrangement or construction. Unlike most other metallic components, rope wires are stressed alternately in tension, compression, torsion, and shear. The wires and strands are designed to slide in relation to each other, distributing the complicated applied stresses more effectively. For this reason, wire rope is often called a “machine.” As machines, ropes require prudent inspection, maintenance, and periodic replacement. The inherent friction coefficient between bare steel strands mandates presence of adequate lubrication to allow the requisite wire movement.Wire rope constructions are identified by the number of strands, the number of wires per strand, the type of core, the “lay” (length) direction, left or right handedness, and many other attributes. Rope grades are rated in tons of breaking strength. The individual wires provide strength, whereas the construction dictates service characteristics. It is generally accepted that many smaller wires provide better fatigue resistance, while fewer, larger wires provide better abrasion resistance. Independent wire rope cores (IWRC) provide better crushing resistance than fiber cores.Because of the complex geometry of the assembled wires, the ultimate tensile strength is not equivalent to a large wire of equal cross-sectional area. A certain percentage of the load on individual wires produces shear stresses, rather than axial tensile stresses, and steels exhibit lower (approximately 30%) ultimate

strength in shear. Nevertheless, a synergy of mechanical properties is produced, providing excellent elastic bending properties.

Chemical composition

Wire rope specifications are somewhat anachronistic compared with those of most steel components. Although the engineering community has become increasingly more specification-driven, no standards require steel wire ropes to be manufactured from a particular composition, with identified elemental or impurity limits. The wires are manufactured from high-carbon, plain carbon steel, with high-strength ropes most often manufactured from AISI Grade 1074. Wire rope conformance is typically dependent upon mechanical properties alone, with composition dictated by the ability of the finished product to satisfy the requisite strength level after wire drawing and rope manufacture.

Wire grades

Labeled grades of wires are of historical significance, but are somewhat vague. The following grades have been specified: iron, traction steel, cast steel, extra strong cast steel (a.k.a. mild plow steel), plow steel, improved plow steel, extra-improved plow steel, and extra-extra-improved plow steel. Lower-strength grades have been almost completely discontinued. Improved plow steel, extra-improved plow steel, and extra-extra-improved plow steel are primarily used in engineering applications, with approximate comparative strength ratings of 1:1.15:1.26, respectively.

Lubricants

The lubricant applied to wire ropes also provides a measure of corrosion resistance in relatively benign service environments. For enhanced corrosion resistance, wire rope is available in a galvanized form, but with a 10% reduction in mechanical strength from identically sized “bright” (non-plated) carbon steel rope, unless it is drawn after galvanizing. Austenitic stainless steel grades are available for more severe environments, but can also be substantially lower in strength than plain carbon steel ropes, depending on wire size and rope construction.

Microstructure

As with all engineering materials, the physical and mechanical properties of steel rope wires are a function of the microstructure, which is a function of processing. Rope wires are very heavily drawn, with severely cold-worked microstructures. The typical longitudinal and transverse microstructures of a high strength wire rope are shown in Fig. 2. The structure consists of pearlite and ferrite grains that have been drawn down so far that the grain boundaries are not easily resolved. Relatively low levels of nonmetallic inclusions are required in high quality rope wires, to provide better fatigue resistance and more uniform mechanical properties.

Fig. 2  Typical longitudinal (left) and transverse (right) microstructure of high strength steel wire rope. 2% Nital etch (500×)

Failure analysis process

Preliminary Investigation

The preliminary portion of an investigation is of great importance in wire rope analyses, similar to all materials failure investigations. All possible information should be gathered, including the purchased rope specification, service history, service environment, estimated loads, and maintenance history. However, it is not unusual to receive very little reliable information concerning a rope failure. This is often due to a general industrial misunderstanding of the complexity of wire rope constructions and service characteristics. After the available information has been collected, the failure analysis procedure should be planned, including identification of analytical tests and the location of test samples.

Visual examination

Since a prudent investigator will progress systematically from least to most destructive analytical methods, a thorough visual evaluation is typically undertaken first. The visual examination of wire rope and cable failures is analogous to other metallurgical failures, except for the fact that hundreds of individual fracture surfaces may be present. Inspection generally includes photographic documentation of all pertinent observations. The rope, strand, and wire diameters should all be measured. It is especially important to measure all different wire sizes that may be present. This is necessary to identify the rope construction. Examination should also address the presence or absence of lubricant, corrosion evidence, and gross mechanical damage. The identification of cracks, kinks, doglegs, and abrasion on the rope is also important.

Failure features

Failed wires exhibit classic cup-and-cone ductile features, flat fatigue features, and various appearances in-between. However, wires are often mechanically damaged after failure, as the separated wires are often dragged through sheaves or abraded against other components. In addition, the extreme energy dissipation upon failure often results in considerable post-fracture damage that can be misleading to an investigator. Relatively rapid general corrosion can also accompany wire rope failures as the protective lubricant is removed and the natural passive oxide layer on the wires is disrupted.

Nondestructive examination

Most nondestructive evaluation (NDE) techniques are not applicable to wire rope failures. The size of the individual wires confounds most inspection methods, requiring the rope to be addressed as a whole construction. The most prevalent NDE methods are electromagnetic, magnetic flux, and magnetic flux leakage for steel ropes. These methods can detect loss of cross-sectional area caused by corrosion or degradation, and some types of flaws. NDE of wire ropes is used more often in determining when ropes in service need to be replaced rather than during metallurgical failure analysis.Wire ropes have a finite service life, and visual examination is the most prevalent method for determining when ropes require replacement. Applicable specifications, such as ASME and ANSI, typically consider the number of visibly broken wires in a prescribed length, or lay, to be an acceptance criterion. These criteria differ with regard to the rope design and service criticality, and whether the rope is in running or standing service.

Mechanical property evaluation

The mechanical strength of a failed wire rope cannot always be verified. Very often, additional regions of the rope have been damaged or have yielded, providing unrealistic results for subsequent tests. Tensile testing is the only strictly applicable mechanical test, as the breaking strength in tons is the sole guarantee provided by the supplier to the purchaser.Tensile strength of undamaged wire ropes can be verified in accordance with the requirements of ASTM A 931. The preferred tension test attachment method is socketing, which requires a measure of familiarity with wire rope handling. Direct clamping of wire ropes in V-shaped universal tensile machine jaws is not recommended, because of poor load distribution and crushing of the wires and strands.Sometimes the wire rope has been so distressed prior to and after a failure, that only microhardness testing on cross sections can evaluate the strength of a rope. Although the approximate tensile strength of the wires can be interpolated, and the wire rope strength can be estimated, these are unavoidably imprecise appraisals, and cannot accurately discriminate between rope grades or be relied upon as an acceptance/rejection criteria.On occasion, ropes have been intentionally misrepresented or even counterfeited, but such incidents are exceedingly rare. Ongoing industry efforts to better regulate and specify wire ropes are of great importance; however, no simple nondestructive method has been developed to confirm mechanical strength of a failed rope.

Chemical analysis

The chemical composition of a wire rope should be determined during the course of a failure investigation, but will not likely provide much insight. In rare cases of excessive, deleterious impurity content or possible counterfeit/substitution, the composition may be very important.Chemical analysis of any surface residue or corrosion product can also be helpful, especially in cases of corrosion fatigue or severe pitting. Other useful analytical techniques include energy dispersive X-ray spectrometry, X-ray diffraction, or a comparable surface science methodology. In questions concerning the presence or absence of lubricant, or if the identification of a wire rope lubricant is needed, infrared

spectrometry may provide answers. The fiber core could also be analyzed for evidence of overheating, grease degradation, and organic contamination.

Fractographic analysis

Electron microscope fractography of wire rope fracture surfaces is essential in failure analysis. Visual examination alone is not sufficient, as several hundred fracture surfaces are present and may not all be analogous fractures. In overload, steel wire ropes typically exhibit classic cup-and-cone ductile overload features.Fatigue is the most important fracture mode in wire ropes because it generally appears at a fraction of the rated strength, below the yield strength, and often without warning. The fractures are flat and do not exhibit any necking. Many secondary cracks often accompany the fractures, particularly in low stress/high cycle fatigue and corrosion fatigue. Well-defined fatigue striations are not always evident during fractographic examination, and this is likely due to the complicated loading experienced by running ropes and the cold-worked microstructure.It should also be noted that shear failures can exhibit flat or angular profiles that may be incorrectly identified as fatigue if the failure is not analyzed by fractography. Ductile failures are relatively straight-forward, exhibiting classic ductile features. It is not unusual to observe several fracture modes on the wires from a single failed rope.

Metallographic examination

Examination of metallographic cross sections can clarify many failure characteristics. Longitudinal sections through representative failed wires can confirm the type of fracture mode indicated during visual and fractographic examination. A typical ductile overload failed wire is shown in Fig. 3. Even if the fracture surface has been damaged or corroded, the telltale grain flow and necking may be apparent.

Fig. 3  Metallographic cross section through a wire that failed via ductile overload. 2% Nital etch (80×)

The longitudinal metallographic profile of a fatigue fracture is shown in Fig. 4. Sometimes a whitish surface layer of abrasion-induced untempered martensite is observed at the origin of cracks in severely abraded ropes. These profiles may also show grain flow evidence, suggesting cutting or pinching in cases

of flat fractures without fatigue cracking. The longitudinal microstructure near the fracture is shown in Fig. 5. Severe disruption of the linear structure is sometimes evident and may be very important if the fracture surfaces have been obliterated by corrosion.

Fig. 4  Cross section through a wire that failed by fatigue. Secondary cracks are evident. 2% Nital etch (32×)

Fig. 5  Some longitudinal microstructure disruption near the fracture surface in Fig. 4 is apparent. 2% Nital etch (500×)

Transverse cross sections near the fracture surfaces are often useful, particularly when evaluating the potential contribution of corrosion and mechanical cracking to a rope failure. Experience has shown that a steel hose clamp affixed around the rope prior to metallographic mounting and polishing can avoid inadvertent destruction of the wire strand and wire configuration. This allows for easy wire counting,

although wire dimensions will be misleading because the cross sections are oblong due to the helical preforming.

Data summation

The final phase of the failure analysis is a logical summation of all laboratory data, service history information, and technical knowledge. The data should indicate the failure mode and identify the contributing factors. It may be helpful to delineate the factors that did not contribute to failure, as this information can also