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Wear 255 (2003) 12331237 Case study Research on the fatigue and fracture behavior due to the fretting wear of steel wire in hoisting rope D.K. Zhang, S.R. Ge, Y.H. Qiang College of Mechanoelectrical and Materials Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221008, PR China Abstract Hoisting steel rope is an important component of the winding equipment in coalmines. Fretting wear and its induced fatigue and fracture of wires have been the major failure modes of the hoisting ropes. In this paper, a series of experiments on the fretting friction and wear of steel wires were performed on an elastic beam oscillation test rig. The worn wires after fretting tests were put into fatigue test on the servo-fatigue test machine. The research results demonstrated that the fretting wear depth of the steel wires increased with the increasing fretting cycles and contact loads. The fatigue life of the steel wires with fretted damage was inverse proportional to the wear depth, and then to the fretting cycles and contact loads. The fretting wear and fatigue mechanisms were analyzed through SEM morphologies of fretting wear scars and fracture sections. It was found that the wear mechanism of fretting wire depended on the contact loads. For low contact loads, light scratch modes dominated the fretting of wires. When contact loads increased, third body abrasion appeared on the worn surfaces. 2003 Elsevier Science B.V. All rights reserved. Keywords: Fretting wear; Fatigue life; Fracture section; Steel wires 1. Introduction Hoisting rope is an important component of the winding equipment in coalmines, because its intensity and fatigue life have great effect on the reliability level of winder op- erations. Structurally, hoisting rope was twisted tightly be- tween strands and strands, wires and wires 1. Therefore, small relative sliding among the strands and wires occurred when the rope was subjected to the axial tension load and bending stretch load on the drum and guide wheel, which resulted in the fretting between steel wires. The fretting of steel wires caused fretting damage, crack initiation, propa- gation and fracture failure of the wires. Investigation revealed that fretting wear, fretting fatigue andfracturehavebeenidentifi edasthemajorcausesforwire failure of steel ropes under tensile load because it reduced the cross-section area of wires and provided initial sites for fatigue fracture 24. From this point of view, reducing the fretting damage of steel wires has been considered as an effective method to prolong the service life of hoisting rope. Waterhouse and coworkers 57 stated that the contact load infl uences the fretting wear. The fretting damage in the surface layer nearest to the outer surface of the rope was Corresponding author. Fax: +86-516-3888682. E-mail address: dkzhang98 (D.K. Zhang). caused by the contact stress. Hence, fretting wear, resid- ual stress and bending stress in ropes promoted the early initiation of fatigue cracks and caused fracture of single wires. Guo 8 investigated the wear resistance mechanism of polymer materials to prevent fretting damage and to re- duce the wear of the steel wires. In this paper, experiments were carried out on the fretting of steel wires. Fatigue tests were also performed. The relation between the fatigue life of fretted wires, fretting cycles and contact loads, was in- vestigated. The wear mechanism of the steel wire fretting was analyzed. 2. Experimental details In the reported researches, different test apparatuses to study the fretting wear of steel wires were established. The experience of these works indicated that generating an ac- curate oscillation of the wire specimen was the key tech- nique to obtain a good test result for the fretting wear of steel wires. In order to perform the test even closer to the actual conditions of the fretting of hoisting ropes, the cross contact of the two wires was designed to conduct the fret- ting test, which is schematically shown in Fig. 1 9. Two wire specimens were fi xed on the holders and aligned at 90 to each other. We designed a special test apparatus for the 0043-1648/03/$ see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0043-1648(03)00161-3 1234D.K. Zhang et al./Wear 255 (2003) 12331237 Fig. 1. Schematic diagram of the contact and test rig for steel wire fretting tests: 1, motor; 2, motor holder; 3, eccentric wheel; 4, elastic beam; 5, spring beam; 6, lower holder; 7, lower specimen; 8, upper specimen; 9, supporting pole; 10, loading arm; 11, upper holder; 12, dead weight. fretting test of wires. The fretting oscillation in this appa- ratus was obtained by an elastic bending beam. The lower oscillating specimen in the stroke direction was fi xed on an elastic frame connected to the bending beam at the posi- tion near the fi xed end. An eccentric wheel oscillated the other side. The upper stationary wire specimen was fi xed to the load arm holding the dead weights. In this designed rig, the fretting amplitude was maintained at the value pro- portional to the eccentricity of the driving wheel. Our ex- periments confi rmed that this test rig was very good at keeping constant fretting amplitude and it was easier to ad- just the contact load and oscillation amplitude in the test process. The steel wires from 619 point contact ropes were taken as specimens in the research. The diameter of the steel wire was 1mm. The steel wire was carbon steel with the con- position given in Table 1. The hardness of steel wire was 365HV. The fretting tests were carried out in ambient labo- ratory and dry friction condition after the specimen surfaces had been cleaned with alcohol. The experiments were performed at the 75?m amplitude and 3.3Hz oscillation frequency. The contact load changed from14to40N.Acomputerrecordedthefrictioncoeffi cient in the fretting process. The length and width of wear scars weremeasuredafterfrettingtests.Themaximumworndepth was applied to characterize the fretting wear. The relation betweenthemaximumweardepthandmaximumwearwidth Table 1 Chemical composition of wire specimens (wt.%) CompositionPercentage Fe98.71 Mn0.39 Si0.02 Ni0.01 C0.87 S0.001 P0.001 was expressed as the following equation: hmax= R ? R2 ?amax 2 ?2 where R is the radius of wire, and amaxthe maximum width of the fretting wear scar. To decrease or eliminate the randomicity of wire fracture and to accelerate the fatigue tests, fretting wear depth of wire specimen needed to enlarge. So the fretted wires ob- tained from the fretting tests with 150?m amplitude and 20h fretting time were then put into fatigue tests. The fa- tigue tests of the fretted wires were performed on the MTS 810 servo-fatigue test rig in a pullpull loading mode. The tension in fatigue tests of fretted wires was fl uctuated be- tween 200 and 1000N. The frequency in the fatigue test was 9Hz. The two ends of the specimens were clamped with special clamps, which had a testing length of 50mm. The test cycles for the specimens broken at the worn notch were recorded as the fatigue life of fretted wires. 3. Results and discussions 3.1. Fretting wear The friction coeffi cient curve in Fig. 2 was the test result of 19N contact load after 109min fretting wear. It indicated that the friction force had positive and negative variation because of the reciprocating sliding of fretting wear. The mean friction coeffi cient was taken to evaluate the fretting wear of steel wires. Fig. 3 shows the friction coeffi cient changes to fretting wear time under different contact loads. The plotted curves in this fi gure indicate that the friction co- effi cients in the loads over 29N increased at the beginning of fretting wear and then they stabilized at the values about 0.22. For contact loads of 14, 19 and 24N, the friction co- effi cients were higher at the beginning of fretting wear and then decreased to the steady value. The relation of the steady D.K. Zhang et al./Wear 255 (2003) 123312371235 Fig. 2. Recording curve of friction coeffi cients in fretting of steel wires. Fig. 3. Variation of friction coeffi cients with contact loads. friction coeffi cients to the contact loads is shown in Fig. 4. In our opinion, such friction coeffi cient behavior in fretting of steel wires depended on the real contact area, contact state and lubricant role of debris. When the contact load was light, the elastic contacts between the fretting surfaces made the asperities interlock each other and the friction coeffi cients resultedinhighvalues.Incontrast,theheavyloadsdeformed the asperities on the surfaces and the surface contact came Fig. 4. Varying trend of friction coeffi cient with contact load at steady fretting period. Fig. 5. Variation of wire wear depth with fretting cycles at 29N contact load. into elasticplastic states. Therefore, the friction coeffi cients of fretting of steel wires decreased with heavy contact loads. There were oxide fi lms formed on the fretting surface. With proceeding of fretting on the wire surface, the ox- ide fi lm gradually broke and the contact area increased. Thus, the friction coeffi cient increased during the initial period. As fretting cycles increased, a large amount of oxi- dized debris was produced, which played a lubrication role like ball bearings between the two surfaces to decrease the friction coeffi cients. When the two surfaces attained an adaptive roughness and the debris output and removal was in a balanced state, the friction coeffi cient reached a steady value. On the other hand, the real contact area increased in the fretting process, which made the friction coeffi cient less dependent on the contact loads. So the friction coeffi cient maintained similar values in high load fretting tests. How- ever, lower contact loads wore out the contact surfaces slower and therefore the contact area had less infl uence on the friction coeffi cient. Fig. 5 shows wear depth of wire specimens in relation to fretting cycles at 75?m amplitude and 29N contact load. This indicates that the wear depth increases with the increas- ing cycles. However, it increased faster before 1.25 105 cycles and slower after this number of cycles. The variation curves of wear depth of fretting wires with different contact loads at 75?m amplitude and 6.98 105 cycles are shown in Fig. 6. The wear depth increases when the contact loads are increased. The wear mechanism of wire fretting was seen to be de- pendent on the fretting cycles and contact stress. For the low contact loads, the elastic contact occurred on the surface asperities and fi ne debris escaped easily out of the fretting wear zone. In this case, the light scratch mechanism dom- inated the fretting wear surfaces, seen in Fig. 7(a). When contact stress and fretting cycles increased, the variation of contact stress little affected the fretting wear. Much debris accumulated between the fretting zones, which formed the third body on contact surfaces, seen in Fig. 7(b). 1236D.K. Zhang et al./Wear 255 (2003) 12331237 Fig. 6. Variation of wire wear depth with contact load at 6.98 104 fretting cycles. 3.2. Fatigue and fracture The lateral notch of the SEM morphology in Fig. 8 in- dicates that the deepest fretting wear point occurred at the center of notch and caused the largest cross-section area loss at this point. The curves in Fig. 9 indicate that the wear depth and the percent of cross-section area loss at the deep- est point of worn wire, were proportional to the fretting time. The nominal stress at the deepest point of specimens was its highest value, which caused high stress concentration at Fig. 7. SEM morphologies of worn wires surface at different contact loads, 75?m amplitude and 1.25105fretting cycles: (a) 9N; (b) 34N. Fig. 8. SEM morphology of lateral notch of worn wires at 19N contact load, 150?m amplitude and 18h fretting time. Fig. 9. Variation of wear depth and cross-section area loss with the fretting time, load 19N, amplitude 150?m. this point, and then caused crack initiation, propagation and fracture of fretted wires. The test results of the wear depth and corresponding fa- tigue life are drawn in Fig. 10. The curves in this fi gure state that the fatigue life is in an inverse proportional relation to Fig. 10. Variation of wear depth and fatigue life with fretting time, load 19 N, amplitude 150?m. D.K. Zhang et al./Wear 255 (2003) 123312371237 Fig. 11. SEM morphologies of fracture section after 12h fretting time (34,174 cycles fatigue life). Fig. 12. SEM morphology of fracture section after 20h fretting time (29,251 cycles fatigue life). the fretting wear depth. For the upper specimens, the fatigue life was 36,035 cycles when the wear depth was 107.3?m. Its fatigue life decreased to 29,251 cycles when the wear depth increased to 158.8?m. Therefore, it was concluded that the fatigue life reduced with the increasing fretting time of the steel wires because the wear depth of the fretting wires increased. The fracture morphologies of wire specimens obtained from test of 19N contact load and 150?m amplitude are shown in Figs. 11 and 12. Two fracture morphologies were observed by SEM, respectively, for 34,174 and 29,251 cy- cles of fatigue life. The fracture sections were divided into four zones A, B, C and D to correspond to different fatigue phases. Zone A was the fretting worn notch, which might be the crack initiation source because of stress concentra- tion. The crack propagated along the outer areas of fracture section and exhibited multi-fatigue sources and fatigue steps in zone B. The morphologies in Figs. 11 and 12 indicate that the high fatigue life of the fretted wires had more fa- tigue steps, and the fracture surface topographies appeared fl atter and smoother because the fracture surfaces had an openclose contact in the crack growth process. When the crack propagation caused the overstress in wire specimens, they broke suddenly at the center zone C. After that the frac- ture crack inclined to the edge of wire specimens at a certain angle as shown in zone D. 4. Conclusions Thefrictioncoeffi cientsbetweenthefrettingofsteelwires decreasedwithincreasingcontactloads,butwhichstabilized when the contact loads attained a certain value. The wear depth of fretted wires depended strongly on the fretting time and contact stress, in which there was a direct proportional relation between them. The fretting wear notch on wi