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Fretting wear behaviors of hoisting rope wires in acid medium
Materials and Design 55 (2014) 50–57
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Fretting wear behaviors of hoisting rope wires in acid medium Linmin Xu a,b, Dekun Zhang b,⇑, Yan Yin b, Songquan Wang a, Dagang Wang a a b
School of Mechatronic Engineering, China University of Mining & Technology, Xuzhou 221116, China School of Materials Science and Engineering, China University of Mining & Technology, Xuzhou 221116, China
a r t i c l e
i n f o
Article history: Received 11 July 2013 Accepted 18 September 2013 Available online 27 September 2013
a b s t r a c t The fretting wear behaviors of hoisting rope wires in acid medium were investigated in this paper. Fretting wear tests of steel wires were conducted on a self-made fretting wear rig, and their fretting running characteristics, coefficient of friction, dissipated energy and wear morphology were analyzed. The results show that the relative sliding between steel wires can be promoted in the acid medium. As the contact load increases, the fretting of steel wires changes from a slip regime to a mixed one, and the coefficient of friction decreases significantly. Moreover, the coefficient of friction changes from about 1.2 in the dry friction environment to about 0.5 in the acid medium. Energy loss presents the same variation trend. Wear scar depth is larger in the acid medium than in the dry friction environment. The primary wear mechanism in the dry friction environment is peeling as compared to peeling, particle attrition and corrosion in the acid medium. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Wire rope is an important bearing component in mine hoisting systems. Fretting wear and fretting corrosion usually occur between the contacting wires of the steel wire rope. It is subjected to repeated stretching and bending, which results in fretting wear of the steel wires under different displacements amplitude and loads [1,2]. Moreover, the humid and oxygen-rich working environment of the wires can cause corrosion. Therefore, the fretting wear behavior of steel wires will change somewhat in corrosive environments [3,4]. According to the published statistics [5,6], the failure proportion among scrapped wires caused by corrosion is about 70–80%. Therefore, to prolong the service life of the wire rope, it is necessary to study the fretting wear behavior of steel wires under corrosive conditions. Zhang [7–10] and others mainly studied the fretting wear mechanism of hoisting rope between steel wires and fatigue failure behavior under dry friction and alkaline corrosive environments. Li [11] studied the fretting corrosion characteristics of the Zr-4 alloy in Na2SO4 solution. Han et al. [12] studied the fretting behavior of self-piercing riveted aluminum alloy joints under different interfacial conditions. Ramesh and Gnanamoorthy [13] found that the fretting wear damage was frequently reported in the races of rolling element bearings and leaded to the increased noise and vibration in the total machinery. Zhou et al. [14,15] studied fretting wear and fretting fatigue performance of a single aluminum wire and found that the cable fatigue failure was mainly caused by plas⇑ Corresponding author. Tel.: +86 13952207958. E-mail address: [email protected] (D. Zhang). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.09.046
tic deformation, wear and cracking due to fretting. Perier et al. [16] studied fretting wear behavior of wire rope in sodium chloride solutions and pointed out that the lubrication and galvanized layer can effectively reduce corrosion and fretting fatigue caused by the sodium chloride solution. Ding and Dai [17] studied fretting wear characteristics of a titanium alloy in seawater and analyzed the impact of sea water on the coefficient of friction and wear. As mentioned above, most studies are concentrated on the fretting wear of materials under dry friction and marine environments, few of them have been focused on fretting wear behaviors of wire rope in acid medium. According to the literature [18], acid rain is a problem in South China and this may be a problem for steel wires in this area. The wire damage caused by the interaction between the acid medium and fretting wear is far greater than that caused by simple corrosion and fretting wear. Therefore, the objective of this paper is to explore the fretting properties and damage mechanisms of steel wires in acid medium. 2. Experimental details Fig. 1 is a self-made fretting corrosion test rig. This rig consists of a driving device, movement device, load-measuring device and lifting platform. The upper wire specimen was installed in the force-measuring device and the lower one was fixed on the specimen slider. The slide moves left/right due to the screw rotation that is driven by the step motor, and this achieves the relative horizontal position adjustment of both the upper and lower wire specimens. The loading device moves under the action of the step motor and which results in the movement of the horizontal position adjustment device and force-measuring device. When the con-
L. Xu et al. / Materials and Design 55 (2014) 50–57
tact load between the upper and lower wire specimens reach the present value, the step motor rotates and the eccentric derive device moves. The displacement amplitude was maintained at a value proportional to the eccentricity of the driving wheel. A typical steel wire based on the request of GB/T8918-2006 [19] was used in this paper. Fretting wear tests of perpendicular steel wires in corrosive environments were performed on the self-made fretting wear rig. Hoisting rope is made of high-quality carbon structural steel, of which the components (in wt.%) are 0.84% C, 94.62% Fe, 4.53% Zn and its allowances are S and P. The hardness and tensile strength are 365 HV0.1 and 1600 MPa, respectively. The main test parameters are as follows: Displacement amplitude of ±150 lm, contact force of 10–30 N, frequency of 1.2 Hz, fretting cycles ranging from 1 to 1 104 and room temperature. The coal mine water in South China was tested and the pH value of the acid medium was about 2.97. The friction forces were recorded during the experiment. According to the friction (Ft) – displacement (D) – cycle (N) curve, the fretting running regional characteristics of steel wire and the final average coefficient of friction could be obtained. In addition, the effect of acid medium on the fretting wear of steel wire was analyzed. The impact of the acidic medium on the fretting wear behavior of the steel wire was analyzed. After each test, the length and width of the wire abrasion gap were measured using an optical microscope with video imaging device, and the maximum wear depth of the gap was calculated according to the following formula [20]:
hmax
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi a 2 max ¼ R R2 2
ð1Þ
where R is the radius of the wire and amax is the maximum width of the fretting wear scar. An optical microscope and scanning electron microscope were used to observe the sample morphology of the wear scar. 3. Results and discussion 3.1. Analysis of wire fretting running characteristics Fig. 2 shows the fretting running curves of the steel wires under dry friction and acid medium. In the dry friction environment,
Fig. 1. Schematic diagram of fretting corrosion apparatus rig: (1) fixator; (2) step motor; (3) loading device; (4) horizontal slider; (5) step motor; (6) horizontal position adjustment device; (7) force measuring device; (8) fretting amplitude measuring device; (9) setting; (10) specimen slider; (11) scaling-down device; (12), eccentric drive device and (13) step motor.
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when Fn = 10 N and 15 N, all Ft–D curves in the fretting process are irregular parallelograms, which apparently illustrates the relative slip between the friction pairs and the slip regime. As the contact load exceeds 15 N, the Ft–D curves become perfect parallelograms during the early period (about 100 cycles), which represents complete slipping of the contacting wires. With the increase of the fretting cycles, the Ft–D curve shifts to an elliptical shape, indicating that the contact interface experiences significant plastic deformation and a mixed regime. However, in the acid medium, the Ft–D curves are parallelograms for the entire testing process, regardless of the load. This illustrates the slip state and slip regime between contacting wires. The friction force increases with the increase of the number of fretting cycles according to all the figures. The fretting running regimes of steel wires under different contact loads in dry friction and acid medium are shown in Table 1. As the contact load increases, the running regime of wires changes from a slip regime to a mixed one, which reveals that shear stress of the friction pair surface increases with the increase of the load and that the plastic deformation occurs on the contact interface. However, in the dry friction environment, the wire fretting regime starts to shift into the mixed regime slip regime at Fn = 20 N, while the fretting operates in the slip regime in the acid medium. This shows that the acid medium significantly alters the fretting regime of steel wires and forces the fretting regime into a full slip state under smaller loads than in the dry friction environment; this illustrates that the lubrication of the acidic medium allows the relative sliding of contact steel wires to occur more easily. 3.2. Coefficient of friction Variation of the coefficient of friction vs. fretting cycles in dry friction environment and acid medium under different loads is shown in Fig. 3. Under dry friction conditions, the curve is very low and stable at the beginning of the friction test (a few to hundreds of cycles), namely, the running-in period of friction test. This is due to the oxide film and membrane fouling of the steel surface. The surface film was destroyed with the relative movement between the friction pair. The friction coefficient increased rapidly and reached to the peak caused by the production of the new friction contact surface. Subsequently, the friction coefficient decreased because of the lubricating action of the third body (wear debris). However, the experimental displacement amplitude was relatively high, so the debris was easy to discharge from the contact area, which made the peak value not easy to determine. At last, the formation and overflow of the wear debris at the contact interface reached a dynamic balance, so the curve became stable. In the acid medium, fretting of steel wires operates in a slip regime, and the curve of the coefficient of friction is typically characterized by five stages: running-in, rapid increase, peak, rapid reduction and stable. An increase of contact load induced a shortened runningin period due to the increase in surface shear stress. The acid medium had a certain influence on the fretting transition phase. Under small loads (10 N and 15 N), the acid medium obviously prolonged the running-in period, but the fretting transition period was shorter under higher loads (20 N, 25 N and 30 N). The coefficient of friction decreased as the load increased, which was mainly affected by the contact area, stress state and wear debris lubrication of the steel surface convex peak. Under small loads, the contacting material between the steel wires was in the elastic state, and the contact pressure and contact area of the surface convex peak were small. In the reciprocating fretting process of steel wires, the convex peak mutually crisscrossed, which caused the surface shear stress and coefficient of friction to become relatively higher. Under high loads, the micro-convex peak between the steel wires yielded under positive pressure, and two surfaces were in elastoplastic
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Fig. 2. Fretting running curve of steel wires in two environments.
Table 1 Fretting running regimes of steel wires in different loads in dry friction and acid medium environment. Load Dry friction Acid medium
10 N S S
15 N S S
20 N M S
25 N M S
Annotated: S represents Slip Regime; M represents Mixed fretting Regime
30 N M S
states. When the contact area of the micro-convex peak increased, the effect of friction became significant. However, wear debris was not easily discharged from the contact surface as the cycle number increased, and this played the role of lubrication in the tests. The friction force decreased and thus the coefficient of friction decreased. The coefficient of friction in the acid medium was significantly smaller relative to the dry friction environment. At a load of 10 N, coefficient of friction in the acid medium decreased from 1.2 (dry friction) to 0.5.
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1.4
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0.50
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At the beginning of the test, the properties of the surface film of steel wires were affected by the acid medium and the thickness of the surface film increased. At low loads, the contact area between steel wires was small and friction force was not large, thus leading to an extension of the running-in period. When loads increased, the friction force became large and the running-in period was shortened because of the corrosion from the acid medium. During the rising period, steel wires contacted each other directly when the surface film was damaged, and the adhesion effect was enhanced, which made it difficult for the acidic medium to enter the contact interface. When reaching the peak stage, the acid medium was able to enter the contact interface due to the loads and the regulatory role of wear debris as well as the isolation effect of wear debris, and this resulted in sustained lubrication as well as a corresponding decrease in the coefficient of friction. In the stable stage, the fluid lubrication film of the acid medium was formed on the contact surface of steel wires. 3.3. Friction dissipation energy Friction generates energy and most of it is released in the form of heat. Fouvry et al. [21–23] introduced the energy method to study fretting behaviors of metallic materials. Energy consumption is important in a tangential fretting process, and the differences of the friction force–displacement curves primarily manifested in dissipation energy differences. Through the calculation of the area surrounded by the Ft–D curves, the relationship of the dissipation energy with the number of cycles can be obtained and the material damage in the view of energy can be analyzed. The friction work of the friction dissipation energy in a single loop in the tangential fretting mode can be defined as:
E¼
D X F t DdD
ð2Þ
D
From the fretting map theory, the Ft–D curve has three basic forms: linear, elliptical and parallelogram. For the linear type of Ft–D curve, the change of displacement relies on the control of material elastic deformation, so friction dissipation energy is minimal and can be approximated as zero. For elliptic partial slip Ft–D curves and parallelogram type of full slip Ft–D curves with corresponding plastic deformation in the tests, the dissipation energy is equal to the area of the Ft–D curve. Fig. 4 shows the variation of dissipation energy with cycle numbers in both dry friction and acid medium under various loads.
When loads of 10 N and 15 N are applied in the dry friction environment, the fretting runs in the slip regime, and the dissipation energy has three-stage characteristics (see Fig. 4a). During the initial fretting, the dissipation energy is relatively stable. As the cycle number increases, the dissipation energy increases rapidly and reaches a maximum. After that, as the cycle number further increases, the dissipation energy first declines, then increases slightly and finally becomes stable. Fretting runs in the mixed regime under loads of 20 N, 25 N and 30 N, so the dissipation energy curve is obviously different from that in the slip regime. During the initial stage, because there is relative slip at the contact interface, the Ft–D curve has a parallelogram shape, which induces high dissipation energy. As the number of cycles increases, the tangential slip severity on the contact interface decreases, and the material continuously hardens. The Ft–D curve gradually becomes a straight line and thus the friction dissipation energy decreases rapidly. Then the curve opens in the shape of an ellipse, during which the dissipation energy increases slightly, but the energy is still less than that in the early fretting stage. At this stage, the dissipation energy exhibits some instability, and this is mainly because the Ft–D curve has multiple conversions between the linear and elliptical shapes. During the stable stage, with the further increase of the cycle number, the friction dissipation energy tends to be stable and the Ft–D curve maintains the elliptic shape. Fig. 4b shows the trends of the dissipation energy with numbers of cycles under different loads in the acid medium. At a displacement amplitude of 150 lm, the fretting modes are in the sliding state regardless of the load, and the dissipation energy trend was similar to that of the steel tested in the dry friction environment in the sliding state. This is because the acid medium promoted friction and made relative slip between friction pairs easier. Comparing Fig. 4a with b, during initial fretting in the acid medium, the dissipation energy curves are lower than those in the dry friction environment. However, as the cycle number increases, the dissipation energy became higher than that in the dry friction case, which indicates that the change of fretting wear in the acid medium is less sudden during the initial stage, and the acid medium changes the thickness of the surface film. Later, corrosion from the acid medium accelerated fretting wear. Meanwhile, the cleaning effect of the medium promoted relative sliding between contacting surfaces. After the operation, the dissipation energy in the acid medium was higher than that in the dry friction environment. Fig. 4 also shows that the dissipation energy increases as the contact load increases, which indicates that the tangential force increases as the contact load increases.
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30N
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(b) acid medium Fig. 4. Variation trend of dissipation energy in two environments under different loads with cycle change.
3.4. Interaction between fretting wear and corrosion In 1949, Zelder first proposed that the interaction between corrosion and abrasion often appeared to accelerate each other [24]. Our test results agree with this, i.e. fretting wear loss in the acid medium is larger than that in the dry friction environment (Fig. 5), and both are influenced by lubrication and corrosion of the acid medium. From the above discussion, the acid medium changes the wire running area, which transitions from the mixed regime to the slip regime. Moreover, the acid medium promotes the relative sliding of the steel wires. However, the pH value of the acid medium is lower, causing pitting corrosion of the matrix material. After corrosion, the wire surface is easily scraped by abrasive particles or washed off by liquid flow particles. The alternating
function of corrosion and wear increased the wear of the steel wires, which was in consistent with the result of Ren [20]. As can be seen from Fig. 5, the wear loss of the steel wires also affects the contact load. The wear depth of the steel wires increases as the load increased in both environments. Under low loads, the contact stress between the upper and lower samples is small, and contact interface produces elastic deformation. In this case, smaller wear debris easily overflow from the fretting zone, and the specimen surface becomes slightly scratched. As the contact loads increase, the contact stress also increases, and plastic deformation occurs on the two contacting surfaces, which causes enhanced wear loss. Fig. 6 shows the optical morphologies of grinding cracks in the steel wires. As can be seen from the images obtained in the dry
L. Xu et al. / Materials and Design 55 (2014) 50–57
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Acid Specimen Dry Specimen
Wear depth/µm
50 40 30 20 10 0 10N
15N
20N
25N
30N
Different Loads Fig. 5. The depth of wear scar of the fretted steel wires in two environments.
friction environment, the wear scar surface is covered by reddish brown debris (Fig. 6a and b), which indicates that the friction pairs resulted in an oxidation reaction, while that does not occur in the acid medium. This is because the friction surface of the metallic material forms a certain thickness oxidation film for the election of the oxygen. The periodically regenerated oxidation film is easy to break off and then causes wear. Moreover, both the wear scar size increases with the increase of the contact load, which is consistent with the results of wear scar depth. Fig. 6c and d shows the optical microscopy of wear topography of wires in an acid medium under different contact load conditions. As can be seen, under the condition of the acid medium, the more obvious the effect of the ‘‘cleaning’’ of fluid medium is, the smoother the surface of the wear scar is. There is a small amount of wear debris adhering to the surface of scar center, which causes the
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formation of the furrows. Some pits with small diameters can be observed on the worn surface for the corrosive effect of the acid medium. Scar size is in contact with the increase of the contact load. The generated debris is continuously dissolved by the H+ ions in acid medium, while the corrosion pits form on the new surface of matrix material at the same time. The wear loss of the wire is increased due to the positive synergism of corrosion and wear in acid medium. The action of H+ ions causes the excretion of abrasive and grinding debris. A new round of fretting wear occurs between the contact interfaces, which results in a slow increase of the friction coefficient. Fig. 7 exhibits the SEM morphologies of wear scar in different contact load conditions under the fretting amplitude of 150 lm. As can be seen from the figure, the wear of the contact interfaces becomes more serious, for fretting basically on the mixed regime and the slip regime (Fig. 7a–e). In certain load and fretting amplitude, the contact point of the surface plastically flows along the sliding direction with a strong plastic deformation. As a result, the scar shows elliptical shape in the direction of motion. Wear surface produces the uneven morphology caused by the strong plastic deformation (Fig. 7a–e) and there are many pits and particles or flake debris on the contact interface (Fig. 7c and d). Owing to a cyclic process of adhesion, shear, and then adhesion, shear occurring on the surface of the material, furrows begin to arise after the formation of third substance which makes micro-cutting on the metal surfaces. The area and depth of wear scar increase as the contact load increases. Meanwhile, the contact fatigue of surface becomes serious with the increase of the contact stress between wire contact surfaces. The marking off of wear debris and fatigue micro-cracks can be found on the wear scar (Fig. 7d and e). The cyclic alternating stress and the friction force lead to plastic deformation and surface hardening on the friction surface microbulge, which result in the generation of fatigue cracks on the defective surface. Meanwhile, the flake and particle debris fall off due to the further expansion of the fatigue cracks. It can be found that the
Fig. 6. Optical morphologies of the fretted steel wire in two environments. (a) Fn = 20 N, dry friction, (b) Fn = 25 N, dry friction, (c) Fn = 20 N, acid medium and (d) Fn = 25 N, acid medium.
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Fig. 7. SEM morphologies of wear scars in dry friction environment. (a) Fn = 10 N, (b) Fn = 15 N, (c) Fn = 20 N, (d) Fn = 25 N and (e) Fn = 30 N.
Fig. 8. SEM morphologies of wear scars in acid medium. Fn = 10 N, (b) Fn = 15 N, (c) Fn = 20 N, (d) Fn = 25 N and (e) Fn = 30 N.
injurious mechanism contains mainly abrasive wear, surface fatigue and oxidation when the wire acts on the mixed regime and the slip regime. In the acid medium, there is no adhesion phenomenon on the wear scar surface (Fig. 8) and there is more severe plastic deformation, which indicates that there was no adhesive wear during fretting because the solution acted as a barrier. Due to the ‘‘washing’’ of the fluid medium, the wear scar surface is smooth, and there is only a small amount of debris that remain adhered (Fig. 8a–d). Moreover, the typical abrasive wear characteristics of the furrow can be seen from Fig. 8e. Because of the corrosive action of the acid medium, there are small-diameter pits in the grinding trace surface (Fig. 8a and e).
Above all, as the contact load increases in the dry friction environment (Fig. 7e) and in the acid medium (Fig. 8c and e), the contact stress in the steel wire between the contact surfaces also increases, which also makes the wear scar surface develop microcracks and traces of debris fatigue shedding. Therefore, in the case of dry friction, the wear mechanisms are abrasive wear, adhesive wear, fatigue wear and oxidation as compared to abrasive wear and fatigue wear which are dominant in the acid environment. Acid medium plays the dual role of lubrication and corrosion on the steel wire. The wear depth of the steel wire increased because the wear debris was constantly dissolved by acid medium and pitting corrosion appeared on the surface of the matrix material, which showed that the corrosive effect was more pronounced.
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4. Conclusions The acid medium change the steel wire running regime and increase the relative slip between the steel wires, which made the fretting mode transition from a mixed regime to a slip regime. When the load increased, the running regime shifted to a mixed regime. The fretting corrosion process of the wire rope was affected by the medium and contact loads. In the acid medium, the coefficient of friction decreased significantly from about 1.2 in the dry friction environment to about 0.5. Meanwhile, the coefficient of friction decreased as the load increased. In the acid medium, the dissipation energy trend was similar to that of the slip regime in the dry friction environment, and energy loss increased as the load increased. Due to the interaction between corrosion and wear whereby they accelerate each other, the grinding trace surface in the acid medium was smoother, and wear loss was greater than in the dry friction environment. An increase of contact load resulted in a smoother surface and more wear loss. Wire wear in the dry friction environment was affected by the delamination and oxidation wear mechanisms, but in the acid medium, the wear mechanisms were mainly abrasive wear and corrosion wear. According to this paper, the synergism between corrosion and wear is positive in acid medium, which increases the wear loss of the steel wire. Acknowledgments Financial supports from the National Natural Science Foundation of China (Grant No. 50875252), the Youth Fund of China University of Mining and Technology (No. 2010QNB06), the University Postgraduate Research and Innovation Projects of Jiangsu province in 2012, is gratefully acknowledged. References [1] Li YF. Study of the measurement of the effect of coal resource development on resource environment in mining area. J Univ Sci Technol 2009;38(4):607–8. [2] Liang B, Bai GL, Wang Y, Jiang LG. Environmental effect and risk forecast of surface mining on groundwater. J Univ Sci Technol 2007;36(3):315–9.
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[3] Zhu RP, Pan SC, Gao DP. A numerical study of stress state parameters and fretting wear parameters in fretting fatigue. Eng Mech 1998;11(4):116–22. [4] Wang CH, Yao G, Yin WQ. Analyses of axial fatigue and fracture characteristics of small diameter wire rope. Steel Wire Products 2001;4(2):35–8. [5] Zhang DK, Ge SR, Zhu ZC. Friction and wear performance on fretting wear of steel wires in hoisting ropes. J China Univ Mining Technol 2002;31(5):367–70. [6] Ge SR, Qu RL. Reliability technology of mine hoist. Xuzhou: China University of Mining and Technology Press; 1994. [7] Zhang DK, Ge SR, Qiang YH. Research on the fatigue and fracture behavior due to the fretting wear of steel wire in hoisting rope. Wear 2003;255:1233–5. [8] Zhang DK, Shen Y, Xu LM, Ge SR. Fretting wear behaviors of steel wires in coal mine under different corrosive mediums. Wear 2011;271:866–74. [9] Zhao WJ, Zhang DK, Zhang ZF, Wang SQ, Wang SB. Effect of contact load on the fretting of steel wire under alkaline corrosive environment. Tribology 2012;32(3):306–12. [10] Shen Y, Zhang DK, Wang DG, Xu LM. Effect of contact load on the fretting wear behavior of steel wire. Tribology 2010;4:404–8. [11] Li JW. Fretting corrosion property of Zr-4 alloy in Na2SO4 solution. Tribology 2007;9(5):406–9. [12] Han L, Chrysanthou A, O’Sullivan JM. Fretting behaviour of self-piercing riveted aluminium alloy joints under different interfacial conditions. Mater Des 2006;27(3):200–8. [13] Ramesh R, Gnanamoorthy R. Development of a fretting wear test rig and preliminary studies for understanding the fretting wear properties of steels. Mater Des 2006;27(2):141–6. [14] Zhou ZR, Goudreau S, Fiset M, Cardou A. Single wire fretting fatigue tests for electrical conductor bending fatigue evaluation. Wear 1995;181:537–43. [15] Zhou ZR, Fiset M, Cardou A, Cloutier L, Goudreau S. Effect of lubricant in electrical conductor fretting fatigue. Wear 1995;189:51–7. [16] Périer V, Dieng L, Gaillet L, Tessier C, Fouvry S. Fretting-fatigue behavior of bridge engineering cables in a solution of sodium chloride. Wear 2009;267:308–14. [17] Ding HY, Dai ZD. Fretting wear characteristics of titanium alloy in seawater. Rare Metal Mater Eng 2007;5(5):778–80. [18] Zhou ZR, Vincent L. Mixed fretting regime. Wear 1995;181–183:531–6. [19] GB/T8918-2006. Steel wire rope for important purposes (ISO 3154: 1988, MOD). Standardization Administration of the People’s Republic of China; 2006. p. 1–7. [20] Ren PD. Study on the fretting corrosion behavior of steels. Chengdu: Southwest Jiaotong University; 2005. [21] Fouvry S, Kapsa Ph, Vincent L. Quantification of fretting damage. Wear 1996;200:186–205. [22] Fouvry S, Kapsa PH, Vincent L. Wear analysis in fretting of hard coatings through a dissipated energy concept. Wear 1997;203–204:393–403. [23] Mohrhacher H, Blanpain B, Cells JP, et al. Oxidational wear of N coating on tool steel and nitrided tool steel in unlubricated fretting. Wear 1995;188:130–7. [24] Jiang XX, Li SZ, Li S. Corrosion wear of metal. Beijing: Chemical Industry Press; 2003. p. 282.
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Technical Report
Fretting wear behaviors of hoisting rope wires in acid medium Linmin Xu a,b, Dekun Zhang b,⇑, Yan Yin b, Songquan Wang a, Dagang Wang a a b
School of Mechatronic Engineering, China University of Mining & Technology, Xuzhou 221116, China School of Materials Science and Engineering, China University of Mining & Technology, Xuzhou 221116, China
a r t i c l e
i n f o
Article history: Received 11 July 2013 Accepted 18 September 2013 Available online 27 September 2013
a b s t r a c t The fretting wear behaviors of hoisting rope wires in acid medium were investigated in this paper. Fretting wear tests of steel wires were conducted on a self-made fretting wear rig, and their fretting running characteristics, coefficient of friction, dissipated energy and wear morphology were analyzed. The results show that the relative sliding between steel wires can be promoted in the acid medium. As the contact load increases, the fretting of steel wires changes from a slip regime to a mixed one, and the coefficient of friction decreases significantly. Moreover, the coefficient of friction changes from about 1.2 in the dry friction environment to about 0.5 in the acid medium. Energy loss presents the same variation trend. Wear scar depth is larger in the acid medium than in the dry friction environment. The primary wear mechanism in the dry friction environment is peeling as compared to peeling, particle attrition and corrosion in the acid medium. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Wire rope is an important bearing component in mine hoisting systems. Fretting wear and fretting corrosion usually occur between the contacting wires of the steel wire rope. It is subjected to repeated stretching and bending, which results in fretting wear of the steel wires under different displacements amplitude and loads [1,2]. Moreover, the humid and oxygen-rich working environment of the wires can cause corrosion. Therefore, the fretting wear behavior of steel wires will change somewhat in corrosive environments [3,4]. According to the published statistics [5,6], the failure proportion among scrapped wires caused by corrosion is about 70–80%. Therefore, to prolong the service life of the wire rope, it is necessary to study the fretting wear behavior of steel wires under corrosive conditions. Zhang [7–10] and others mainly studied the fretting wear mechanism of hoisting rope between steel wires and fatigue failure behavior under dry friction and alkaline corrosive environments. Li [11] studied the fretting corrosion characteristics of the Zr-4 alloy in Na2SO4 solution. Han et al. [12] studied the fretting behavior of self-piercing riveted aluminum alloy joints under different interfacial conditions. Ramesh and Gnanamoorthy [13] found that the fretting wear damage was frequently reported in the races of rolling element bearings and leaded to the increased noise and vibration in the total machinery. Zhou et al. [14,15] studied fretting wear and fretting fatigue performance of a single aluminum wire and found that the cable fatigue failure was mainly caused by plas⇑ Corresponding author. Tel.: +86 13952207958. E-mail address: [email protected] (D. Zhang). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.09.046
tic deformation, wear and cracking due to fretting. Perier et al. [16] studied fretting wear behavior of wire rope in sodium chloride solutions and pointed out that the lubrication and galvanized layer can effectively reduce corrosion and fretting fatigue caused by the sodium chloride solution. Ding and Dai [17] studied fretting wear characteristics of a titanium alloy in seawater and analyzed the impact of sea water on the coefficient of friction and wear. As mentioned above, most studies are concentrated on the fretting wear of materials under dry friction and marine environments, few of them have been focused on fretting wear behaviors of wire rope in acid medium. According to the literature [18], acid rain is a problem in South China and this may be a problem for steel wires in this area. The wire damage caused by the interaction between the acid medium and fretting wear is far greater than that caused by simple corrosion and fretting wear. Therefore, the objective of this paper is to explore the fretting properties and damage mechanisms of steel wires in acid medium. 2. Experimental details Fig. 1 is a self-made fretting corrosion test rig. This rig consists of a driving device, movement device, load-measuring device and lifting platform. The upper wire specimen was installed in the force-measuring device and the lower one was fixed on the specimen slider. The slide moves left/right due to the screw rotation that is driven by the step motor, and this achieves the relative horizontal position adjustment of both the upper and lower wire specimens. The loading device moves under the action of the step motor and which results in the movement of the horizontal position adjustment device and force-measuring device. When the con-
L. Xu et al. / Materials and Design 55 (2014) 50–57
tact load between the upper and lower wire specimens reach the present value, the step motor rotates and the eccentric derive device moves. The displacement amplitude was maintained at a value proportional to the eccentricity of the driving wheel. A typical steel wire based on the request of GB/T8918-2006 [19] was used in this paper. Fretting wear tests of perpendicular steel wires in corrosive environments were performed on the self-made fretting wear rig. Hoisting rope is made of high-quality carbon structural steel, of which the components (in wt.%) are 0.84% C, 94.62% Fe, 4.53% Zn and its allowances are S and P. The hardness and tensile strength are 365 HV0.1 and 1600 MPa, respectively. The main test parameters are as follows: Displacement amplitude of ±150 lm, contact force of 10–30 N, frequency of 1.2 Hz, fretting cycles ranging from 1 to 1 104 and room temperature. The coal mine water in South China was tested and the pH value of the acid medium was about 2.97. The friction forces were recorded during the experiment. According to the friction (Ft) – displacement (D) – cycle (N) curve, the fretting running regional characteristics of steel wire and the final average coefficient of friction could be obtained. In addition, the effect of acid medium on the fretting wear of steel wire was analyzed. The impact of the acidic medium on the fretting wear behavior of the steel wire was analyzed. After each test, the length and width of the wire abrasion gap were measured using an optical microscope with video imaging device, and the maximum wear depth of the gap was calculated according to the following formula [20]:
hmax
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi a 2 max ¼ R R2 2
ð1Þ
where R is the radius of the wire and amax is the maximum width of the fretting wear scar. An optical microscope and scanning electron microscope were used to observe the sample morphology of the wear scar. 3. Results and discussion 3.1. Analysis of wire fretting running characteristics Fig. 2 shows the fretting running curves of the steel wires under dry friction and acid medium. In the dry friction environment,
Fig. 1. Schematic diagram of fretting corrosion apparatus rig: (1) fixator; (2) step motor; (3) loading device; (4) horizontal slider; (5) step motor; (6) horizontal position adjustment device; (7) force measuring device; (8) fretting amplitude measuring device; (9) setting; (10) specimen slider; (11) scaling-down device; (12), eccentric drive device and (13) step motor.
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when Fn = 10 N and 15 N, all Ft–D curves in the fretting process are irregular parallelograms, which apparently illustrates the relative slip between the friction pairs and the slip regime. As the contact load exceeds 15 N, the Ft–D curves become perfect parallelograms during the early period (about 100 cycles), which represents complete slipping of the contacting wires. With the increase of the fretting cycles, the Ft–D curve shifts to an elliptical shape, indicating that the contact interface experiences significant plastic deformation and a mixed regime. However, in the acid medium, the Ft–D curves are parallelograms for the entire testing process, regardless of the load. This illustrates the slip state and slip regime between contacting wires. The friction force increases with the increase of the number of fretting cycles according to all the figures. The fretting running regimes of steel wires under different contact loads in dry friction and acid medium are shown in Table 1. As the contact load increases, the running regime of wires changes from a slip regime to a mixed one, which reveals that shear stress of the friction pair surface increases with the increase of the load and that the plastic deformation occurs on the contact interface. However, in the dry friction environment, the wire fretting regime starts to shift into the mixed regime slip regime at Fn = 20 N, while the fretting operates in the slip regime in the acid medium. This shows that the acid medium significantly alters the fretting regime of steel wires and forces the fretting regime into a full slip state under smaller loads than in the dry friction environment; this illustrates that the lubrication of the acidic medium allows the relative sliding of contact steel wires to occur more easily. 3.2. Coefficient of friction Variation of the coefficient of friction vs. fretting cycles in dry friction environment and acid medium under different loads is shown in Fig. 3. Under dry friction conditions, the curve is very low and stable at the beginning of the friction test (a few to hundreds of cycles), namely, the running-in period of friction test. This is due to the oxide film and membrane fouling of the steel surface. The surface film was destroyed with the relative movement between the friction pair. The friction coefficient increased rapidly and reached to the peak caused by the production of the new friction contact surface. Subsequently, the friction coefficient decreased because of the lubricating action of the third body (wear debris). However, the experimental displacement amplitude was relatively high, so the debris was easy to discharge from the contact area, which made the peak value not easy to determine. At last, the formation and overflow of the wear debris at the contact interface reached a dynamic balance, so the curve became stable. In the acid medium, fretting of steel wires operates in a slip regime, and the curve of the coefficient of friction is typically characterized by five stages: running-in, rapid increase, peak, rapid reduction and stable. An increase of contact load induced a shortened runningin period due to the increase in surface shear stress. The acid medium had a certain influence on the fretting transition phase. Under small loads (10 N and 15 N), the acid medium obviously prolonged the running-in period, but the fretting transition period was shorter under higher loads (20 N, 25 N and 30 N). The coefficient of friction decreased as the load increased, which was mainly affected by the contact area, stress state and wear debris lubrication of the steel surface convex peak. Under small loads, the contacting material between the steel wires was in the elastic state, and the contact pressure and contact area of the surface convex peak were small. In the reciprocating fretting process of steel wires, the convex peak mutually crisscrossed, which caused the surface shear stress and coefficient of friction to become relatively higher. Under high loads, the micro-convex peak between the steel wires yielded under positive pressure, and two surfaces were in elastoplastic
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Fig. 2. Fretting running curve of steel wires in two environments.
Table 1 Fretting running regimes of steel wires in different loads in dry friction and acid medium environment. Load Dry friction Acid medium
10 N S S
15 N S S
20 N M S
25 N M S
Annotated: S represents Slip Regime; M represents Mixed fretting Regime
30 N M S
states. When the contact area of the micro-convex peak increased, the effect of friction became significant. However, wear debris was not easily discharged from the contact surface as the cycle number increased, and this played the role of lubrication in the tests. The friction force decreased and thus the coefficient of friction decreased. The coefficient of friction in the acid medium was significantly smaller relative to the dry friction environment. At a load of 10 N, coefficient of friction in the acid medium decreased from 1.2 (dry friction) to 0.5.
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At the beginning of the test, the properties of the surface film of steel wires were affected by the acid medium and the thickness of the surface film increased. At low loads, the contact area between steel wires was small and friction force was not large, thus leading to an extension of the running-in period. When loads increased, the friction force became large and the running-in period was shortened because of the corrosion from the acid medium. During the rising period, steel wires contacted each other directly when the surface film was damaged, and the adhesion effect was enhanced, which made it difficult for the acidic medium to enter the contact interface. When reaching the peak stage, the acid medium was able to enter the contact interface due to the loads and the regulatory role of wear debris as well as the isolation effect of wear debris, and this resulted in sustained lubrication as well as a corresponding decrease in the coefficient of friction. In the stable stage, the fluid lubrication film of the acid medium was formed on the contact surface of steel wires. 3.3. Friction dissipation energy Friction generates energy and most of it is released in the form of heat. Fouvry et al. [21–23] introduced the energy method to study fretting behaviors of metallic materials. Energy consumption is important in a tangential fretting process, and the differences of the friction force–displacement curves primarily manifested in dissipation energy differences. Through the calculation of the area surrounded by the Ft–D curves, the relationship of the dissipation energy with the number of cycles can be obtained and the material damage in the view of energy can be analyzed. The friction work of the friction dissipation energy in a single loop in the tangential fretting mode can be defined as:
E¼
D X F t DdD
ð2Þ
D
From the fretting map theory, the Ft–D curve has three basic forms: linear, elliptical and parallelogram. For the linear type of Ft–D curve, the change of displacement relies on the control of material elastic deformation, so friction dissipation energy is minimal and can be approximated as zero. For elliptic partial slip Ft–D curves and parallelogram type of full slip Ft–D curves with corresponding plastic deformation in the tests, the dissipation energy is equal to the area of the Ft–D curve. Fig. 4 shows the variation of dissipation energy with cycle numbers in both dry friction and acid medium under various loads.
When loads of 10 N and 15 N are applied in the dry friction environment, the fretting runs in the slip regime, and the dissipation energy has three-stage characteristics (see Fig. 4a). During the initial fretting, the dissipation energy is relatively stable. As the cycle number increases, the dissipation energy increases rapidly and reaches a maximum. After that, as the cycle number further increases, the dissipation energy first declines, then increases slightly and finally becomes stable. Fretting runs in the mixed regime under loads of 20 N, 25 N and 30 N, so the dissipation energy curve is obviously different from that in the slip regime. During the initial stage, because there is relative slip at the contact interface, the Ft–D curve has a parallelogram shape, which induces high dissipation energy. As the number of cycles increases, the tangential slip severity on the contact interface decreases, and the material continuously hardens. The Ft–D curve gradually becomes a straight line and thus the friction dissipation energy decreases rapidly. Then the curve opens in the shape of an ellipse, during which the dissipation energy increases slightly, but the energy is still less than that in the early fretting stage. At this stage, the dissipation energy exhibits some instability, and this is mainly because the Ft–D curve has multiple conversions between the linear and elliptical shapes. During the stable stage, with the further increase of the cycle number, the friction dissipation energy tends to be stable and the Ft–D curve maintains the elliptic shape. Fig. 4b shows the trends of the dissipation energy with numbers of cycles under different loads in the acid medium. At a displacement amplitude of 150 lm, the fretting modes are in the sliding state regardless of the load, and the dissipation energy trend was similar to that of the steel tested in the dry friction environment in the sliding state. This is because the acid medium promoted friction and made relative slip between friction pairs easier. Comparing Fig. 4a with b, during initial fretting in the acid medium, the dissipation energy curves are lower than those in the dry friction environment. However, as the cycle number increases, the dissipation energy became higher than that in the dry friction case, which indicates that the change of fretting wear in the acid medium is less sudden during the initial stage, and the acid medium changes the thickness of the surface film. Later, corrosion from the acid medium accelerated fretting wear. Meanwhile, the cleaning effect of the medium promoted relative sliding between contacting surfaces. After the operation, the dissipation energy in the acid medium was higher than that in the dry friction environment. Fig. 4 also shows that the dissipation energy increases as the contact load increases, which indicates that the tangential force increases as the contact load increases.
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3.4. Interaction between fretting wear and corrosion In 1949, Zelder first proposed that the interaction between corrosion and abrasion often appeared to accelerate each other [24]. Our test results agree with this, i.e. fretting wear loss in the acid medium is larger than that in the dry friction environment (Fig. 5), and both are influenced by lubrication and corrosion of the acid medium. From the above discussion, the acid medium changes the wire running area, which transitions from the mixed regime to the slip regime. Moreover, the acid medium promotes the relative sliding of the steel wires. However, the pH value of the acid medium is lower, causing pitting corrosion of the matrix material. After corrosion, the wire surface is easily scraped by abrasive particles or washed off by liquid flow particles. The alternating
function of corrosion and wear increased the wear of the steel wires, which was in consistent with the result of Ren [20]. As can be seen from Fig. 5, the wear loss of the steel wires also affects the contact load. The wear depth of the steel wires increases as the load increased in both environments. Under low loads, the contact stress between the upper and lower samples is small, and contact interface produces elastic deformation. In this case, smaller wear debris easily overflow from the fretting zone, and the specimen surface becomes slightly scratched. As the contact loads increase, the contact stress also increases, and plastic deformation occurs on the two contacting surfaces, which causes enhanced wear loss. Fig. 6 shows the optical morphologies of grinding cracks in the steel wires. As can be seen from the images obtained in the dry
L. Xu et al. / Materials and Design 55 (2014) 50–57
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Acid Specimen Dry Specimen
Wear depth/µm
50 40 30 20 10 0 10N
15N
20N
25N
30N
Different Loads Fig. 5. The depth of wear scar of the fretted steel wires in two environments.
friction environment, the wear scar surface is covered by reddish brown debris (Fig. 6a and b), which indicates that the friction pairs resulted in an oxidation reaction, while that does not occur in the acid medium. This is because the friction surface of the metallic material forms a certain thickness oxidation film for the election of the oxygen. The periodically regenerated oxidation film is easy to break off and then causes wear. Moreover, both the wear scar size increases with the increase of the contact load, which is consistent with the results of wear scar depth. Fig. 6c and d shows the optical microscopy of wear topography of wires in an acid medium under different contact load conditions. As can be seen, under the condition of the acid medium, the more obvious the effect of the ‘‘cleaning’’ of fluid medium is, the smoother the surface of the wear scar is. There is a small amount of wear debris adhering to the surface of scar center, which causes the
55
formation of the furrows. Some pits with small diameters can be observed on the worn surface for the corrosive effect of the acid medium. Scar size is in contact with the increase of the contact load. The generated debris is continuously dissolved by the H+ ions in acid medium, while the corrosion pits form on the new surface of matrix material at the same time. The wear loss of the wire is increased due to the positive synergism of corrosion and wear in acid medium. The action of H+ ions causes the excretion of abrasive and grinding debris. A new round of fretting wear occurs between the contact interfaces, which results in a slow increase of the friction coefficient. Fig. 7 exhibits the SEM morphologies of wear scar in different contact load conditions under the fretting amplitude of 150 lm. As can be seen from the figure, the wear of the contact interfaces becomes more serious, for fretting basically on the mixed regime and the slip regime (Fig. 7a–e). In certain load and fretting amplitude, the contact point of the surface plastically flows along the sliding direction with a strong plastic deformation. As a result, the scar shows elliptical shape in the direction of motion. Wear surface produces the uneven morphology caused by the strong plastic deformation (Fig. 7a–e) and there are many pits and particles or flake debris on the contact interface (Fig. 7c and d). Owing to a cyclic process of adhesion, shear, and then adhesion, shear occurring on the surface of the material, furrows begin to arise after the formation of third substance which makes micro-cutting on the metal surfaces. The area and depth of wear scar increase as the contact load increases. Meanwhile, the contact fatigue of surface becomes serious with the increase of the contact stress between wire contact surfaces. The marking off of wear debris and fatigue micro-cracks can be found on the wear scar (Fig. 7d and e). The cyclic alternating stress and the friction force lead to plastic deformation and surface hardening on the friction surface microbulge, which result in the generation of fatigue cracks on the defective surface. Meanwhile, the flake and particle debris fall off due to the further expansion of the fatigue cracks. It can be found that the
Fig. 6. Optical morphologies of the fretted steel wire in two environments. (a) Fn = 20 N, dry friction, (b) Fn = 25 N, dry friction, (c) Fn = 20 N, acid medium and (d) Fn = 25 N, acid medium.
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Fig. 7. SEM morphologies of wear scars in dry friction environment. (a) Fn = 10 N, (b) Fn = 15 N, (c) Fn = 20 N, (d) Fn = 25 N and (e) Fn = 30 N.
Fig. 8. SEM morphologies of wear scars in acid medium. Fn = 10 N, (b) Fn = 15 N, (c) Fn = 20 N, (d) Fn = 25 N and (e) Fn = 30 N.
injurious mechanism contains mainly abrasive wear, surface fatigue and oxidation when the wire acts on the mixed regime and the slip regime. In the acid medium, there is no adhesion phenomenon on the wear scar surface (Fig. 8) and there is more severe plastic deformation, which indicates that there was no adhesive wear during fretting because the solution acted as a barrier. Due to the ‘‘washing’’ of the fluid medium, the wear scar surface is smooth, and there is only a small amount of debris that remain adhered (Fig. 8a–d). Moreover, the typical abrasive wear characteristics of the furrow can be seen from Fig. 8e. Because of the corrosive action of the acid medium, there are small-diameter pits in the grinding trace surface (Fig. 8a and e).
Above all, as the contact load increases in the dry friction environment (Fig. 7e) and in the acid medium (Fig. 8c and e), the contact stress in the steel wire between the contact surfaces also increases, which also makes the wear scar surface develop microcracks and traces of debris fatigue shedding. Therefore, in the case of dry friction, the wear mechanisms are abrasive wear, adhesive wear, fatigue wear and oxidation as compared to abrasive wear and fatigue wear which are dominant in the acid environment. Acid medium plays the dual role of lubrication and corrosion on the steel wire. The wear depth of the steel wire increased because the wear debris was constantly dissolved by acid medium and pitting corrosion appeared on the surface of the matrix material, which showed that the corrosive effect was more pronounced.
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4. Conclusions The acid medium change the steel wire running regime and increase the relative slip between the steel wires, which made the fretting mode transition from a mixed regime to a slip regime. When the load increased, the running regime shifted to a mixed regime. The fretting corrosion process of the wire rope was affected by the medium and contact loads. In the acid medium, the coefficient of friction decreased significantly from about 1.2 in the dry friction environment to about 0.5. Meanwhile, the coefficient of friction decreased as the load increased. In the acid medium, the dissipation energy trend was similar to that of the slip regime in the dry friction environment, and energy loss increased as the load increased. Due to the interaction between corrosion and wear whereby they accelerate each other, the grinding trace surface in the acid medium was smoother, and wear loss was greater than in the dry friction environment. An increase of contact load resulted in a smoother surface and more wear loss. Wire wear in the dry friction environment was affected by the delamination and oxidation wear mechanisms, but in the acid medium, the wear mechanisms were mainly abrasive wear and corrosion wear. According to this paper, the synergism between corrosion and wear is positive in acid medium, which increases the wear loss of the steel wire. Acknowledgments Financial supports from the National Natural Science Foundation of China (Grant No. 50875252), the Youth Fund of China University of Mining and Technology (No. 2010QNB06), the University Postgraduate Research and Innovation Projects of Jiangsu province in 2012, is gratefully acknowledged. References [1] Li YF. Study of the measurement of the effect of coal resource development on resource environment in mining area. J Univ Sci Technol 2009;38(4):607–8. [2] Liang B, Bai GL, Wang Y, Jiang LG. Environmental effect and risk forecast of surface mining on groundwater. J Univ Sci Technol 2007;36(3):315–9.
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[3] Zhu RP, Pan SC, Gao DP. A numerical study of stress state parameters and fretting wear parameters in fretting fatigue. Eng Mech 1998;11(4):116–22. [4] Wang CH, Yao G, Yin WQ. Analyses of axial fatigue and fracture characteristics of small diameter wire rope. Steel Wire Products 2001;4(2):35–8. [5] Zhang DK, Ge SR, Zhu ZC. Friction and wear performance on fretting wear of steel wires in hoisting ropes. J China Univ Mining Technol 2002;31(5):367–70. [6] Ge SR, Qu RL. Reliability technology of mine hoist. Xuzhou: China University of Mining and Technology Press; 1994. [7] Zhang DK, Ge SR, Qiang YH. Research on the fatigue and fracture behavior due to the fretting wear of steel wire in hoisting rope. Wear 2003;255:1233–5. [8] Zhang DK, Shen Y, Xu LM, Ge SR. Fretting wear behaviors of steel wires in coal mine under different corrosive mediums. Wear 2011;271:866–74. [9] Zhao WJ, Zhang DK, Zhang ZF, Wang SQ, Wang SB. Effect of contact load on the fretting of steel wire under alkaline corrosive environment. Tribology 2012;32(3):306–12. [10] Shen Y, Zhang DK, Wang DG, Xu LM. Effect of contact load on the fretting wear behavior of steel wire. Tribology 2010;4:404–8. [11] Li JW. Fretting corrosion property of Zr-4 alloy in Na2SO4 solution. Tribology 2007;9(5):406–9. [12] Han L, Chrysanthou A, O’Sullivan JM. Fretting behaviour of self-piercing riveted aluminium alloy joints under different interfacial conditions. Mater Des 2006;27(3):200–8. [13] Ramesh R, Gnanamoorthy R. Development of a fretting wear test rig and preliminary studies for understanding the fretting wear properties of steels. Mater Des 2006;27(2):141–6. [14] Zhou ZR, Goudreau S, Fiset M, Cardou A. Single wire fretting fatigue tests for electrical conductor bending fatigue evaluation. Wear 1995;181:537–43. [15] Zhou ZR, Fiset M, Cardou A, Cloutier L, Goudreau S. Effect of lubricant in electrical conductor fretting fatigue. Wear 1995;189:51–7. [16] Périer V, Dieng L, Gaillet L, Tessier C, Fouvry S. Fretting-fatigue behavior of bridge engineering cables in a solution of sodium chloride. Wear 2009;267:308–14. [17] Ding HY, Dai ZD. Fretting wear characteristics of titanium alloy in seawater. Rare Metal Mater Eng 2007;5(5):778–80. [18] Zhou ZR, Vincent L. Mixed fretting regime. Wear 1995;181–183:531–6. [19] GB/T8918-2006. Steel wire rope for important purposes (ISO 3154: 1988, MOD). Standardization Administration of the People’s Republic of China; 2006. p. 1–7. [20] Ren PD. Study on the fretting corrosion behavior of steels. Chengdu: Southwest Jiaotong University; 2005. [21] Fouvry S, Kapsa Ph, Vincent L. Quantification of fretting damage. Wear 1996;200:186–205. [22] Fouvry S, Kapsa PH, Vincent L. Wear analysis in fretting of hard coatings through a dissipated energy concept. Wear 1997;203–204:393–403. [23] Mohrhacher H, Blanpain B, Cells JP, et al. Oxidational wear of N coating on tool steel and nitrided tool steel in unlubricated fretting. Wear 1995;188:130–7. [24] Jiang XX, Li SZ, Li S. Corrosion wear of metal. Beijing: Chemical Industry Press; 2003. p. 282.