Measurement of abrasive wear on wire ropes using non-destructive electro-magnetic inspection

OIPEEC Technical Meeting Kraków September 1999 Chaplin, C.R., Ridge, I.M.L. & Tytko, A.A.* ODN 0679 The University of Reading * University of Mining and Metallurgy, Kraków Poland/The University of Reading Measurement of abrasive wear on wire ropes using non-destructive electro-magnetic inspection Summary This paper discusses the problems associated with the non-destructive inspection of wire ropes which have been subject to abrasive wear. Wear is a common form of degradation on wire ropes, which is difficult to quantify by visual inspection since, although it has the advantage that it is on the surface of the rope, very small changes in wear scar size can be associated with a relatively large volume of material lost. Electromagnetic inspection is accepted as the most effective technology for locating rope degradation but debate continues as to the interpretation of quantitative measurements. For damage such as abrasive wear the assessments are made by reference to the loss of metallic area signal (LMA), however measured. Possible variation between sets of equipment raises the questions of calibration and interpretation of the signals from the apparatus. The authors report upon experiments to assess the accuracy of three proprietary magnetic non-destructive test heads on a variety of rope constructions with artificial (and hence quantifiable) abrasive wear. The results obtained show a surprising consistency for distributed wear, and confirm that LMA measurements give a snap-shot of cross sectional area, not the critical effective area loss. 1. Introduction Abrasive wear is a common form of degradation on wire ropes, which is difficult to quantify by visual inspection since, although it has the advantage that it is on the surface of the rope, small changes in the width of the wear scar on the wires can be associated with a relatively large volume of material lost. Electromagnetic inspection has long been regarded as the most effective technology for locating rope degradation but debate continues as to the interpretation of quantitative measurements. For damage such as abrasive wear the assessment will be made by reference to the loss of metallic area signal (LMA), however measured. Complications can arise in the interpretation of LMA because the threshold length of degradation over which LMA is recorded accurately, is affected significantly by the sensor technology in use. However this is not a problem here provided wear is uniform over a sufficient length. But there are additional problems associated with aggregating area loss to infer the effective load bearing area of the rope and so ascertain the quantitative influence on strength, and by implication, on service endurance (Chaplin, 1992). 13

The Non-destructive Testing of Rope The use of different sensors employing different technologies in different sets of equipment raises questions about calibration and interpretation of the signals from the apparatus. The primary issue here is how the indicated loss of magnetic area, relates to the actual loss of steel area, observing that the area loss in a real wire rope will apply to different wires at different cross sections. Figure 1: Measured nominal LMA (left) and effective LMA (right) for a uniformly abraded rope. If a rope had been exposed to uniform abrasive wear, the apparent effect would be as illustrated in the left of Figure 1, in which it is evident that only a few outer wires in each strand have been affected. On this snap shot cross section the area loss is of the order of 3 or 4%. However the real effect on the rope is that over quite a short distance of rope each wire in turn comes to the crown of the strand so that every wire is worn to the same degree and the effect is much as illustrated on the right of Figure1, where it can be seen that the effective area loss is very much higher, say 15%. The loss on load bearing capacity depends on the critical take up of load away from the worn part of a wire. This apparent multiplier on LMA will be even greater when wear is one-sided, and only one or two strands are affected at one location. This paper is concerned with experiments to assess the accuracy of three proprietary magnetic non-destructive test heads on a variety of rope constructions with artificial (and hence quantifiable) abrasive wear, and in particular to identify whether magnetic measurements of LMA relate to the nominal or effective area loss. The problem is illustrated in a highly schematic manner in Figure 2 in which a nominal rope consists of four very thick wires. Each wire has experienced local wear to the extent that half the cross section has been lost. Within a short length of the rope there will be no load transfer and rope strength will reflect the aggregate area loss and fall by 50%. However the magnetic measurement of LMA will depend on flux transfer between wires: so with good contact between wires, flux will transfer very easily and the apparent loss of area will be that evident at a defined section, namely 50% of one of the four wires, i.e. 12.5% of the total. With well spaced wires, poorly 14

OIPEEC Technical Meeting Kraków September 1999 coupled magnetically the aggregate flux carried by the rope under conditions of saturation would be the true 50% of that carried by the undamaged saturated rope. Figure 2: Parallel flux paths. So the question is does the rope NDT instrument indicate nominal or effective, and are different instruments similar in this respect? It is essential that the NDT operator understands these issues, and the geometrical implication of the LMA signal of the instrument he is using in order that an accurate diagnosis can be made. In this regard, as with the threshold length, it is to be expected that instruments will differ, and give different indications for different rope constructions. 2. Preparation of rope samples Abrasive wear is concerned with the removal of material from the surface of the rope, and as such is usually encountered in running rope applications, caused when the rope contacts another hard object. Wear can occur on any rope construction that might be used in a running application. In this series of experiments, three rope constructions were used: 34LR die-formed (or compacted) multi-strand bright rope 1960 N/mm 2 grade 6 x 25 (12/6 + 6F/1) + IWRC RHL bright rope 1960 N/mm 2 grade 6 x 36 (14/7 + 7/7/1) + FC RHO bright rope 1960 N/mm 2 grade These constructions are illustrated in Table 1 which also presents data for the metallic cross sectional area of each of the ropes used, and lists the various forms of abrasive wear induced on each rope. Abrasive wear will occur either uniformly distributed about a rope cross section or will be predominantly one-sided. It is important to be able to distinguish whether the loss is uniform or not, as non-uniform wear will have a proportionately greater effect on the residual endurance of the rope. In order to investigate whether the NDT apparatus could distinguish between similar losses of cross-sectional area either evenly or unevenly distributed on the rope section, samples were made which included both distributions of wear. 15

The Non-destructive Testing of Rope Thus patches of uniform wear were created 0.75m long (greater than the threshold length of any of the NDT test heads used) at light, medium and severe levels. In the case of the medium and severe abrasion, regions of one sided wear were also created. Wire Cross Rope diameter/ Pattern of wear induced rope section Wire CSA Low (around the rope) 34 LR, 28 mm Medium (around and on one side of the rope) die-formed 335.8 mm 2 Heavy (around and on one side of the rope) Low (around the rope) 6x25F IWRC 28 mm Medium (around and on one side of the rope) RHL 379.3 mm 2 Heavy (around and on one side of the rope) Low (around the rope) 6x36 FC 28 mm Medium (around and on one side of the rope) RHO 313.3 mm 2 Heavy (around and on one side of the rope) Table 1: Data for the wire ropes used in the investigation. After preliminary trials, the method finally adopted for preparation of samples involved using a drum sander driven by an electric drill, with the severity and uniformity controlled visually. Figure 3 illustrates the appearance of the multi-strand and sixstrand ropes for the three different grades of wear. Figure 3: Appearance of the abraded rope samples. 16

OIPEEC Technical Meeting Kraków September 1999 Each rope sample was made from a length of rope about 10 m long. The samples had aluminium caps at each end which were secured by casting with resin. A hole in each of these caps allowed attachment via a D shackle to a line from a winch which was used to pull the samples through the test head at a constant speed. zoom wire rope cross section c LMA wire c θ δ LMA wire = ( δ/2) 2 (θ/2-0.5 sin θ) [mm 2 ] θ = 2arcsin(c/δ) where δ: wire diameter c: abraded wire cord width [rad] [mm] [mm] Figure 4: Measurement of LMA for a six strand rope. 3. Measurement of induced abrasive wear In order to be able to assess the accuracy of the various NDT instruments, it was necessary to measure the actual loss of metallic area (LMA) which had been created on each of the rope samples. The LMA has been assessed from a measurement of the wear scars on the abraded rope surface, and is defined explicitly as proportional area lost over a cross sectional slice of the rope in terms of aggregated wire areas. The wear scars were measured using a technique similar to that proposed by 17

The Non-destructive Testing of Rope Nishioka (1966) which created a print of the rope's surface, from an enlargement of which measurements could be made. Although not invented for this purpose, this technique has been previously employed successfully by the third author (Tytko, 1998) to measure LMA. Figure 4 illustrates the procedure employed in this series of experiments: the surface of the abraded rope is cleaned; the wear scars are marked with layout blue ; when dry, double sided adhesive tape is used to lift the print; the print is enlarged using a photocopier (x4); the characteristic value c is measured from which to calculate the LMA. The method for determining the LMA for the die-formed multi-strand rope is essentially the same, but each strand is treated as a wire and the value of c measured accordingly (Figure 5). moderate LMA strand c zoom c θ δ LMA strand = ( δ/2) 2 (θ/2-0.5 sin θ) [mm 2 ] severe imprint c θ = 2arcsin(c/δ) where δ: wire diameter c: abraded wire cord width [rad] [mm] [mm] Figure 5: Measurement of LMA for a die-formed multi-strand rope. 4. Electro-magnetic Non-destructive testing 4.1 NDT equipment For this series of electromagnetic experiments, three sets of NDT equipment were tested: LMA125 produced by Non-destructive Technology Inc (USA); MD120 produced by Meraster (Poland); Ropescan (UK). 18

OIPEEC Technical Meeting Kraków September 1999 LMA125 Magnetic head and sensors Recorder and data acquisition system MD120 Magnets: rare earth permanent LF/LMA sensor: DC main flux sensor coil Analogue 2 channel recorder with external data acquisition system: 400Hz 9 bit PCMCIA memory card, PC software Ropescan Magnets: ferrite solid magnets LF: differential coil LMA: Hall effect sensors Digital 4 channel recorder with internal data acquisition system: 400Hz 9 bit PCMCIA memory card, PC software Magnets: ferrite permanent LF/LMA sensor: DC main flux sensor coil with ferrite concentrator External 2 channel analogue Gould recorder Table 2: Basic parameters of the electro-magnetic test equipment used in tests. 19

The Non-destructive Testing of Rope Table 2 presents the basic parameters of these three sets of equipment, which are essentially the same in that they operate with permanent magnets and have both local fault (LF) and loss of metallic area sensors. All three pieces of equipment have been used in service inspections. 4.2 NDT test track In order to ensure that each of the rope samples was tested by each NDT head under similar conditions, a test track was designed and built. The track, which is illustrated schematically in Figure 6, has four main features: the winch ensures that ropes are tested at the same constant speed; the track is fabricated from aluminium so as to not interfere with the magnetic testing (this also means it is not too heavy for handling); the track keeps the rope samples clean and helps to prevent the picking up of iron debris which may be lying around the test area; it can be dismantled to allow easy transport for on-site assessment of any NDT head. 1 - wire rope, 2 - magnetic head, 3 - area (LMA) sensor, 4 - distance sensor, 5 - track segment, 6- guide pulley, 7 - fixing bolts, 8 - winch, 9 - gear box and motor, 10-12V DC battery, 11 - haulage rope Figure 6: Rope NDT test track. 4.3 Calibration of the electromagnetic testing heads The LMA sensor for each of the testing heads used was calibrated for each rope before testing. Calibration was carried out as described by the ASTM standard (ASTM, 1993). This 'static' method of calibration (in which the rope is static in the test head) operates by comparing the LMA output when a rod of known cross section is removed from alongside the rope in the head (Figure 7). 20

OIPEEC Technical Meeting Kraków September 1999 The value of LMA in the worn rope is given by (1): 100 WCS RSM LMA = [%] (1) RCS WSM where LMA [%] : percentage loss of metallic rope area WCS [mm 2 ] : metal rod/wire cross sectional area RCS [mm 2 ] : test rope aggregate wire cross sectional area WSM [mv] : LMA signal amplitude with calibration rod inserted RSM [mv] : LMA signal amplitude with worn rope. Figure 7: Calibration of the LMA sensor. 5. Results Figures 8, 9 and 10 present the results obtained for each of the rope constructions used in this series of experiments: the 34LR multi-strand, six-strand ordinary lay with FC and six-strand Lang's lay with IWRC. Each figure presents traces allowing the comparison of the three sets of apparatus, along with an ideal schematic diagram of the measured wear. The traces for apparatus A and B have been recorded digitally using a PCMCIA card and computer software to process the results, while for apparatus C this option was not available, and the figures show a copy of the paper plot obtained. Figures 11-13 plot for each of the three ropes the relationship between the actual value of wear (x axis) and that indicated by each of the three sets of NDT apparatus (y axis). Each of these sets of data has a linear relationship fitted which is presented on the appropriate graph. 21

The Non-destructive Testing of Rope Figure 8: LMA traces from each of the three sets of NDT apparatus and an ideal trace (bottom) for various levels of abrasive wear on a 34LR multi-strand rope. Figure 9: LMA traces from each of the three sets of NDT apparatus and an ideal trace (bottom) for various levels of abrasive wear on a 6x25F RHL + IWRC rope. 22

OIPEEC Technical Meeting Kraków September 1999 Figure 10: LMA traces from each of the three sets of NDT apparatus and an ideal trace (bottom) for various levels of abrasive wear on a 6x36 RHO + FC rope. Figure 11: Relationship between the geometrical value of rope LMA and measured LMA (by NDT head) for each of the sets of NDT equipment for the 34LR multi-strand rope. 23

The Non-destructive Testing of Rope Figure 12: Relationship between the geometrical value of rope LMA and measured LMA (by NDT head) for each of the sets of NDT equipment for the 6x25F RHL + IWRC rope. Figure 13: Relationship between the geometrical value of rope LMA and measured LMA (by NDT head) for each of the sets of NDT equipment for the 6x36 RHO + FC rope. 24

OIPEEC Technical Meeting Kraków September 1999 6. Discussion and conclusions The first observation which may be made concerning these results is how similar the shapes of the traces from different sets of NDT equipment are. The graphs shown in Figures 11, 12 and 13 show that for this form of wear, the LMA sensor accuracy for all three chosen sets of equipment is very high, the coefficients in the linear relationships varying between 0.83 and 0.98 (where 1.00 with zero intercept would indicate direct 1:1 correspondence). Not only is the shape of the trace consistent, but the measure of the LMA is as well, which is particularly surprising given the very different nature of the three systems. Table 3 summarises these results along with other measures of the equipment: NDT set Accuracy, resolution & sensitivity of apparatus Wire rope construction 34LR multistrand 6x 25F +IWRC RHL 6x36 + FC RHO LMA125 error (1) ± 0.12 ± 0.19 ± 0.12 accuracy (2) 0.91 0.94 0.97 noise ration (3) 0.18 0.19 0.21 resolution (4) ~ 0.50 ~ 0.35 ~ 0.40 MD120 error (1) ± 0.23 ± 0.23 ± 0.25 accuracy (2) 0.83 0.92 0.84 noise ration (3) 0.13 0.26 0.30 resolution (4) ~ 0.50 ~ 0.40 ~ 0.50 Ropescan error (1) ± 0.22 ± 0.14 no data accuracy (2) 0.98 0.95 no data noise ration (3) 0.32 0.40 no data resolution (4) ~ 0.50 ~ 0.50 no data (1) error - this gives the 90% confidence bound (as a % of wire rope cross sectional area) (2) the slope of the linear model i.e. the coefficient for the LMA in the linear relationship (as defined above) (3) the noise ratio - this is the ratio of the amplitude of the noise to the lowest detectable signal (as a % of wire rope cross sectional area) (4) the lowest detected value of the LMA signal (as a % of wire rope cross sectional area) Table 3: Resolution of the equipment use in the investigation made on the basis of the results of the electromagnetic inspection of wire ropes with induced abrasive wear. From the measurements made in these tests, it is not possible to comment on the threshold or averaging length the minimum length of a uniform anomaly for which the sensor provides an accurate measurement of a rope s LMA (Weischedel, 1999), since these rope samples were made with wear over a sufficient length to avoid that issue. However, what can be seen is that for abrasion or LMA spread consistently along a rope, the LMA equipment measures the loss of area on a snap-shot single section of the construction. However, it is noticeable that the true LMA is consistently under-estimated. An error of this sign cannot be explained in terms of static calibration as noted by Golosinski & Tytko (1998). 25

The Non-destructive Testing of Rope With correct calibration and knowledge of the physical distribution, this information will allow prediction of the residual strength (directly) and fatigue endurance (by careful interpretation). Chaplin (1992) and others proposed that if a rope is subject to uniform abrasion (or loss of material by any means) along its outer wires on the outer strands, then effectively the rope behaves as if every outer wire in the cross section were abraded (as shown in Figure 1), as each strand outer wire comes to the surface in turn. The effect of this is that a LMA NDT signal for uniformly distributed wear, even if a precise measure of cross-section, significantly under-estimates strength and fatigue loss. The under-estimate is even more inaccurate if the wear is along one side of the rope affecting only one or two strands. Ridge & Chaplin (1998) and Chaplin, Ridge & Zheng (1999) have used the technique of effective area for predicting the influence on the fatigue endurance of a range of forms of rope degradation (including abrasive wear) with some success. By combining the measure of the LMA provided by the rope inspection apparatus with visual assessment of the uniformity of the wear, it is possible to predict the residual fatigue endurance and hence remaining service life, of the abraded rope. However, such calculations are dependant on being able to relate the LMA NDT signal to actual geometrical changes. This requires calibration using realistic rope defects in combination with more simplistic static rod calibrations. 7. References ASTM (1993) ASTM E 1571-93 Practice for electromagnetic examination of ferromagnetic steel wire rope ASTM Standard, 1993. Chaplin, C.R. (1992) The inspection and discard of mooring lines Reading Rope Research pub. by Noble Denton & Associates, London 1992. Chaplin, C.R., Ridge, I.M.L. & Zheng, J. (1999) Rope degradation and damage HSE Final Project Report Contract No. MaTSU/8865/3564. Golosinski, T.S. & Tytko, A.A. (1998) Magnetic examinations of wire ropes: loss of metallic area (LMA) measurement with Hall effect sensors OIPEEC Bulletin 75(1998) 27-36 ISSN:1018-8819. Nishioka, T. (1966) Surface condition and fatigue of wire rope Wire World International 8(1966)3 May/June 67-73. Ridge, I.M.L. & Chaplin, C.R. (1998) Final Report to Research Sponsors Reading Rope Research, The University of Reading, Department of Engineering 1998. Confidential report to sponsors - to be published. Tytko, A.A. (1995) Modelowanie zuzycia zmeczeniowego i diagnostyka lin stalowych (Modelling of fatigue wear and diagnostic of steel wire ropes) Rozprawy Monografie 65, Wydawnictwa AGH ISSN: 0867-6631 (in Polish). Weischedel, H.R. (1999) Electromagnetic wire rope inspection: signal generation, filtering and computer aided rope evaluation OIPEEC Technical Meeting Kraków September 1999. 8. Acknowledgements The authors gratefully acknowledge support of the UK Health and Safety Executive. 26