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Analysis of fatigue life of the steel cord used in tires in unidirectional and bidirectional bending
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Structural Integrity Procedia 00 (2018) 000–000 Available online www.sciencedirect.com Available online at at www.sciencedirect.com Structural Integrity Procedia 00 (2018) 000–000
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Procedia Structural Integrity 13 00 (2018) 1626–1631 Structural Integrity Procedia (2016) 000–000
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ECF22 - Loading and Environmental effects on Structural Integrity ECF22 - Loading and Environmental effects on Structural Integrity
Analysis of fatigue life of the steel cord used in tires in Analysis of fatigue life of the steel cord used in tires in unidirectional bending XV Portuguese Conference on Fracture,and PCFbidirectional 2016, 10-12 February 2016, Paço de Arcos, Portugal unidirectional and bidirectional bending a Robert Kruzelaa, of Malgorzata Ulewicz Thermo-mechanical modeling a high pressure Robert Kruzel , Malgorzata Ulewicza turbine blade of an Czestochowa University of Technology, Faculty of Civil Engineering, ul. Akademicka 3, 42-201 Częstochowa, POLAND airplane gas turbine engine Czestochowa University of Technology, Faculty of Civil Engineering, ul. Akademicka 3, 42-201 Częstochowa, POLAND a a
P. Brandãoa, V. Infanteb, A.M. Deusc*
Abstract AbstractaDepartment of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, The bresults of fatigue life tests of steel cord used in constructionPortugal machinery tires are presented in this paper. Steel wire used in tires, IDMEC,ofDepartment oftests Mechanical Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, The results life of steelEngineering, cord for usedtransferring inInstituto construction machinery are presented in this paper. Steel wire used tires, called the steelfatigue cord, is largely responsible largePortugal loads, but tires unfortunately it quickly undergoes fatigue. The in fatigue called cord, is largely responsible for transferring large loads, but unfortunately it quickly undergoes fatigue. The fatigue life cCeFEMA, of the thissteel steel cord is influenced by many factors related to its construction, workmanship quality, as well as the method of Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, life of this steel each cord is influenced by In many factors related toPortugal its construction, workmanship as well as the method of of bending during working cycle. laboratory tests, the fatigue life of steel cords wasquality, compared in the conditions bending during working bending, cycle. Inonlaboratory fatigue fatigue life of testing steel cords wasforcompared in Inthe conditions of unidirectional andeach bidirectional a speciallytests, built,the innovative machine steel cords. fatigue life tests, unidirectional bidirectional bending, on a specially built, innovative fatigueFor testing machine for steel cords.the Innumber fatigue life tests, a fundamental and difference is visible in the number of cycles leading to breaking. one-way bending process, of cycles Abstract difference a fundamental is visible the numberbending. of cyclesInstead leadingoftothe breaking. For one-way process, the number of cycles is significantly greater than for the in bidirectional wire fracture process,bending a steel cord elongation process takes is significantly greater for the bending. Instead of the wire fracture process,ofa failures steel cord takes place that increases thethan number of bidirectional fatigue cycles, which is reflected directly in the number in elongation the tires ofprocess construction During their operation, modern aircraftcycles, engine components aredirectly subjected to number increasingly demanding operating conditions, place that increases the number of fatigue which is reflected in the of failures in the tires of construction machines especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent machines one of Published which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict © degradation, 2018 The Authors. by Elsevier B.V. © 2018 The Authors. Published by Elsevier B.V. the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation © 2018 The under Authors. Published by B.V. Peer-review responsibility of Elsevier the ECF22 organizers. Peer-review of the ECF22 organizers. company,under wereresponsibility used to obtain and organizers. mechanical data for three different flight cycles. In order to create the 3D model Peer-review under responsibility ofthermal the ECF22 needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were Keywords: fatigue strength; steel cord; crack propagation; unidirectional bending; bidirectional bending obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D Keywords: fatigue strength; steel cord; crack propagation; unidirectional bending; bidirectional bending rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a 1. model Introduction can be useful in the goal of predicting turbine blade life, given a set of FDR data.
1. Introduction ©With 2016 the Thegrowth Authors. by Elsevier B.V. an increase in demand for more advanced tires is observed, which would ofPublished the automotive industry, With thetransfer growth of the automotive an increaseof inand demand more advanced tires is observed, which would Peer-review under the responsibility of the industry, Scientific Committee PCF 2016.for effectively generated overloads to the pavement assure safe operation. Currently, for tire manufacturing effectively transfer the generated overloads to the pavement and assure safe operation. Currently, for tire manufacturing exists a complex combination of rubber mixes, steel wires and textile fibers, such as nylon or polyester, which are joined Keywords: High Pressure Turbine of Blade; Creep; Finitesteel Element Method; Model; Simulation. exists a complex combination rubber mixes, wires and 3D textile fibers, such as(1984). nylon or polyester, are joined together in the vulcanization process, Vedeneev (2012), VERT (2007), Massoubre The greatest which influence on the together in the vulcanization process, Vedeneev (2012), VERT (2007), Massoubre (1984). The greatest influence on the life and load capacity of tires, used in all types of vehicles is shown by the internal structure of the tire and the life and load capacity of tires, used in all types of vehicles is shown by the internal structure of the tire and the reinforcement used. For reinforcing tires, either steel wire or its products in the form of strands or ropes are used, called reinforcement used. For reinforcing tires, either steel wire or its products in the form of strands or ropes are used, called the steel cord, Tashiro and Tarui (2003), Bekaert catalogue (1982), Krmela (2017), Noor and Tanner (1985), which, the steel cord, Tashiro and Tarui (2003), to Bekaert (1982), more Krmela (2017), and Tanner (1985), which, embedded in an elastomer, are intended make catalogue the construction rigid, whileNoor imparting to it the appropriate embedded in an elastomer, are intended to make the construction more rigid, while imparting to it the appropriate 2452-3216 © 2018 The Authors. Published by Elsevier B.V. 2452-3216 © 2018 Authors. Published Elsevier B.V. Peer-review underThe responsibility of theby ECF22 organizers. * Corresponding Tel.: +351of218419991. Peer-review underauthor. responsibility the ECF22 organizers. E-mail address: [email protected] 2452-3216 © 2016 The Authors. Published by Elsevier B.V.
Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the ECF22 organizers. 10.1016/j.prostr.2018.12.342
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resistance to tensile forces. Single steel wires have a diameter ranging from 0.15 to 0.38 mm and are the most often manufactured as brass- or zinc-coated wires, Golis, Błażejowski and Pilarczyk (1998). The reliability of operation of steel cord used in the construction of tires or conveyor belts guarantees the safety of operation of the rubber product. The service properties of steel cord are significantly influenced by the chemical composition of steel used, as well as the wire drawing and cord winding technologies. Wires intended for the production of steel cord, made of unalloyed pearlitic steel, containing from 0.70 to 0.95% of carbon, belong to a group of unalloyed steels of a quality class designed for drawing or cold rolling, Ashby and Jones (2005), Czarski, Skowronek and Matusiewicz (2015), Grygier et al. (2016). The fatigue strength of those wires largely depends on the metallurgical purity of the material, and particularly on the contents of oxygen, silicon and Sulphur. The presence of non-metallic inclusions in steel strongly reduces its fatigue strength, because a high stress concentration occurs around impurities, leading, as a consequence, to material fracture, Golis, Błażejowski and Pilarczyk (1998), Ashby and Jones (2005), Berisha et al. (2015), Grygier (2016). The problem of the fracture of products made of pearlitic steel being subjected to plastic working or being in operation, has been arising interest among many researchers for a number of years now, since the fatigue strength of steel cord affects the life of tires and, as a consequence, the safety of their operation, Golis, Błażejowski and Pilarczyk (1998), Grygier (2016), Feng (2015), Kruzel and Suliga (2015), Rao, Daniel and Mc Farlane (2001), Grygier and Rutkowska-Gorczyca (2015), Kruzel, Suliga and Sosna, (2015), Grygier and Rutkowska-Gorczyca (2016), Lee et al. (1994).. Considering the fact that the fatigue properties of cord have a decisive effect on the cord behavior in different working conditions, the understanding and determination of these properties is crucial, especially as the cord constitutes up to 25% of the tire weight. Steel cords used for production of passenger car tires have splices of two to five fibers (e.g. 2 x 0.25 + 2 x 0.25; 2 x 0.25 + 1 x 0.30 - the first number means the number of middle wires and the second number of braided wires with adequate their diameter), while cords used for tires intended for trucks and special construction machines are of multiwire constructions (e.g. 2 x 0.30; 3 x 0.30 + 6 x 0.30). Currently, wishing to reduce their tire manufacturing cost, companies, instead of steel cords of large sizes (such as 4 x 0.28), increasingly often use cords with a smaller number of wires (e.g. 2 x 0.3). Such an approach is acceptable, provided that the cord reinforcement has the appropriate strength parameters. In connection with the above, the present study has made an attempt to evaluate the effect of the fatigue of steel cord on its strength properties, depending on its construction and the type of steel used. 2. Materials and methods The starting material for testing were steel cords of the following constructions: 3 x 0.25 + 6 x 0.35 (cord A); 2 x 0.25 +2 x 0.25 (cord B) and 2 x 0.30 (cord C) all made of steel D76, whose chemical composition is given in Table 1. Steel cords of the 2 x 0.30 construction (cords D, E, E, G), made of different steel grades, were also subjected to testing. All the cords tested were made in the Cord Production Plan (Železárny a drátovny Bohumín) in the Czech Republic. Table 1. Chemical composition of steel used for cords A-C. Material type D76
C
Mn
Si
K
0.76
0.61
0.19
0.007
Content of the chemical element, % S Cr Ni Cu 0.011
0.025
0.025
0.025
Al
Mo
N
0.002
0.005
0.025
The cords were subjected to testing for the force of breaking the cord in a whole (on a ZWICK Z/100 testing machine adapted specially for this purpose) in accordance with standard PN-EN ISO 6892-1:2010, after having previously been fatigued on a universal testing machine constructed at the Czestochowa University of Technology (Fig. 1). The above-mentioned machine enabled tests to be conducted in either unidirectional or bidirectional cord bending conditions, and its principle of operation is depicted schematically in Fig. 2 (R - bending roller radius, bending arc length, A, B - beginning and end of specimen contact with the bending roller, hm - test specimen length). The complete unidirectional bending on a testing machine is understood as bending the cord from its straightened state to a bent state and then back to the initial state, while the complete bidirectional bending is understood as transition from a bent state to a straightened state, and then to an oppositely bent state. The frequency of variations in fatigue machine loading is the number of cycles, N, per minute, i.e. the complete motions of the cord to and from.
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Table 2. Chemical composition of steel used for cords D-G. Material type (cord) C86D2 (cord D) C82D2 (cord E) C80K (cord F) C72DP (cord G)
Content of the chemical element, % C
Mn
Si
K
S
Cr
Ni
Cu
Al
Mo
N
0.87 0.81 0.79 0.73
0.66 0.62 0.67 0.59
0.21 0.18 0.22 0.28
0.006 0.007 0.006 0.005
0.015 0.021 0.019 0.016
0.052 0.064 0.056 0.071
0.043 0.039 0.056 0.049
0.041 0.054 0.087 0.077
0.004 0.006 0.009 0.005
0.007 0.009 0.006 0.008
0.015 0.016 0.018 0.026
Fig. 1. The fatigue testing machine of the authors' design.
(a)
(b) Fig. 2. Schematic diagram of the machine for cord fatigue testing: (a) unidirectional bending conditions; (b) bidirectional bending conditions.
An analysis of the microstructure of wires used for cords, made in compliance with the recommendations of standard PN-EN 10323:2005, was also made within the study using an Axiovert 25 optical microscope. The microstructures were observed on Nital-etched micro-sections. The microscopic examination did not show any significant differences in the structure of wire specimens tested. Figure 3 shows a sample microstructure image obtained for wire of steel D76. The microstructure depicted in the photograph, composed mainly of pearlite, shows cold work effects after performed plastic working. The material was not subjected to recrystallizing annealing, as evidenced by the oblong, heavily deformed structure. All of the wires tested were made of unalloyed pearlitic steel and the employed cold drawing process yielded a large degree of their plastic deformation. A similar deformation degree (in the range of 80–90%) has been observed for wires of eutectoid steel, Grygier and Rutkowska-Gorczyca (2016).
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Fig. 3. Microstructure of wire (steel D76) used for cord A-C; the longitudinal section.
3. Results and discussion At the first stage of the investigation, steel cords A, B and C were cut into 800 mm-long sections and appropriate loads were selected to them (Table 3). For the cord A, a load of 0.40 kN was applied; for the cord B, a load of 0.30 kN; while for the cord C a load of 0.20 kN. After mounting a respective load, steel cord was put in a universal fatigue life testing machine that enabled a unidirectional and a bidirectional bending processes to be conducted. As shown by preliminary tests, the basic difference between the unidirectional and bidirectional bending processes is visible in the number of cycles leading to a cord rupture. For the unidirectional bending process, the number of cycles is significantly greater than for bidirectional bending. In the preliminary tests, when conducting the unidirectional bending process for 24 hrs, the cord wire fatigue level necessary for a specimen rupture was not attained. In view of the fact that for unidirectional bending tests the number of cycles would be too large and fatigue tests for those cord types would take a long time, it was decided to conduct further tests under bidirectional bending conditions, where a specimen undergoes very large plastic deformations in two directions, alternately, in a single cycle. The fatigue effect is then obtained in a shorter time and the comparative results are correct and equally good. So, each steel cord was put to a fatigue test that lasted, respectively, 15, 30, 60 and 120 minutes. During the course of tests, the number of cycles was determined, which is to be understood as the number of complete bidirectional bends. During the tests, the number of bends was counted several times in relation to an assumed time unit, namely 60 s, to obtain a result of 45 complete bends/minute. The cord was bent bi-directionally with cyclically varying tensile loads and the rope rotating around its own axis. The obtained test results are provided in Table 4. During the bending, variable (tensile, bending and compressive) cyclic stresses occurred in the cord, which caused a cord fatigue that manifested itself in wire cracking with a characteristic fatigue fracture surface and growing specimen elongation. As shown by the data in Table 4, for test cords made of high-carbon steel D76, regardless of the cord construction, a reduction in cord tensile strength was observed with increasing number of bends. The decrease of steel cord strength under the effect of variable bending stress is not a linear relationship. After 675 fatigue cycles, regardless of the examined cord construction, a slight, statistically insignificant decrease in breaking force by approx. 0.22% was observed, compared to the control specimen. After 5400 fatigue cycles, the greatest drop in cord strength (by 12.95%) relative to the control specimen was observed for the cord C (of construction 2 x 0.30) in spite of applying the smallest load, while the least, for the cord A (a decrease by 5.60%). The obtained results confirm that the cord winding method, as well as the number of wires and their diameter, have a major effect on the fatigue resistance of cord. The fatigue life of steel cord is also influenced by splice making technology (either rotor or rotorless), workmanship and the material grade used. The process of steel cord elongation proceeds differently from the wire crack build-up process. For cords of a construction having a larger number of wires, a slight elongation of specimens subjected to fatigue testing was observed. Specimens of the A type cord with an initial length of 800 mm and construction 3 + 6 after the bidirectional bending process had a length of about 802 mm, which constitutes an increment in length by 0.2%, relative to specimens with a smaller number of wires in their constructions. For the C type cords of construction 2 x 0.30, no increment in total specimen length was observed, which remained at 800 mm. This was probably caused by the wires arranging relative to one another in multi-wire constructions, stretching of complete cords and a slight elongation of splices in the cords, or their straightening.
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Table 3. Fatigue testing parameters for cords A, B and C Cord
Cord construction
A
3+6
B
2+2 x 0.25
C
2x0.30
Number of cycles 675 1350 2700 5400 675 1350 2700 5400 675 1350 2700 5400
Length of sample, mm 800 800 800 800 800 800 800 800 800 800 800 800
Load, kN 0.40 0.40 0.40 0.40 0.30 0.30 0.30 0.30 0.20 0.20 0.20 0.20
Table 4. Results of measurements of tensile strength, Rm, with indicated number of fatigue cycles, N of steel used for cords D-G. Cord A B C
0* 1665.1 938.9 443.1
Number of cycles, N 675 1350 2700 1661.4 1655.8 1632.1 936.8 928.1 902.1 442.1 432.5 402.8
5400 1571.9 870.7 385.7
The fatigue strength of steel wire depends largely on the metallurgical purity of the material, and especially on its contents of non-metallic inclusions (silicon, sulphur and nitrogen), because it is around those impurities that high stress concentrations form, which result in wire cracking. Therefore, in the subsequent part of the investigation, the strength parameters were determined for cords made of different steel grades (cords D-G) of construction 2 x 0.30, while considering the fact that for this type of construction (cord C), the greatest difference in strength decrease after the bending process was observed. Subjected to testing was wire of a diameter of 1.60 mm, from which, after the drawing process, 0.30 mm - diameter wires were used for splicing the cord under consideration are obtained. The mechanical properties of the wire are given in Table 5. The bidirectional bending test was performed in accordance with the standard PN-ISO 7801: 1996 applicable to determining the resistance of wire to plastic deformation; whereas, the fatigue properties of cords made from the above wires are shown in Table 6. Table 5. The mechanical properties of respective 1.60 mm-diameter wires after heat treatment Strength [MPa]
Cord D E F G
Rm-average 1335 1285 1255 1175
Number of twists [100×d] Min.-Max. 1300-1340 1280-1300 1240-1260 1150-1190
Average 33 30 40 31
Min.-Max. 26-39 24-36 32-44 28-36
Number of bends 5 [mm] Average 10 10 10 10
Min.-Max. 8-12 8-12 9-11 9-11
Table 6. The mechanical and fatigue properties of finished 2 x 0.30 construction cords Cord D E F G
Breaking force, [N] Average 410 400 410 390
Min.-Max. 405-415 395-405 400-420 385-400
Time [min.]
Number of cycles
Length of sample, [mm]
Load, F [kN]
290 280 270 250
13824 13200 12076 11904
1270 1300 1320 1330
0.20 0.20 0.20 0.20
As demonstrated by the data in Table 7, cords D-G of construction 2 x 0.30 with the use of load of a magnitude of 0.20 kN showed a gradual decrease in strength after subjecting them to a specified fatigue. After 675 fatigue cycles, a slight (< 0.3%) decrease in breaking force was observed for all of the examined cords (D-G). By contrast, after 5400 fatigue cycles, the highest percentage drop (12.73 %) in tensile strength was observed for cord D, and the lowest (5.43
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%), for cord G. Cord D contained higher contents of non-metallic inclusions (0.87 % C, 0.021 % Si and 0.015 % S), compared to cord G ( 0.73% C, 0.28 % Si and 0.016 % S). The presence of such a large amount of non-metallic inclusions, especially in the form of oxides, may cause a reduction in material ductility, thereby making technological processes difficult and, in extreme cases, even leading to wires rupturing during the operation of the product. Table 7. The decrease in the strength of 2 x 0.30 construction steel cords D – G, depending on the number of fatigue cycles applied. Cord D E F
% /675 0.28 0.22 0.19
G
0.14
Percentage decrease in the strength/number of cycles % /1350 % / 2700 2.35 8.86 1.88 4.76 1.22 3.81 0.51
1.93
% /5400 12.73 10.21 7.29 5.43
4. Conclusions From the investigation carried out it can be concluded that the main signs of the fatigue wear of steel cords made of high-carbon steel is a reduction in their strength, cracking of wires and an elongation of the test specimen section. The distribution of fatigue cracks depends on the number of bends and the method of bending the rope section tested. Unidirectional bending on a testing machine causes lesser material fatigue, compared to bidirectional bending. The fatigue of steel cord caused by bidirectional bending results in a decrease in mechanical properties of the steel cord, regardless of the cord construction. The greatest drop in cord strength relative to the number of fatigue cycles occurred in the case of cord of construction 2 x 0.30 and a lower carbon concentration in the steel (0.73 %). By contrast, the least effect on the cord strength was shown by a test performed on the 3+6 construction cord, which confirms that the cord winding method, as well as the number of wires and their diameter have a key effect on the tension of steel cord. References Ashby, M. F., Jones, D. R. H., 2005. Engineering Materials: An introduction to microstructures processing and design, Elsevier, Oxford. Bekaert steel cord catalogue, Bekaert S.A., Zwevegem, Belgium, 1982. Berisha, B., Raemy, C., Becker, C., Hora, P., 2015. Multiscale modelling of failure initiation in a ferritic–pearlitic steel. Acta Materialia 100, 11–18. Czarski, A., Skowronek, T., Matusiewicz, P., 2015. Stability of a lamellar structure - Effect of the true interlamellar spacing on the durability of a pearlite colony. Archives of Metallurgy and Materials 60(4), 2499–2503. Feng, F., 2014. Texture inheritance of cold drawn pearlite steel wires after austenitization. Materials Science Engineering 618(A), 14–21. Golis, B., Błażejowski, Z., Pilarczyk, J. W., 1998. Steel wires for tire reinforcement, Publishing house of Faculty of Metallurgy and Materials Science, Czestochowa, Poland. (In Polish). Grygier, D., 2016. Analysis of the causes of damage to the wires of the steel belt of car tires. Interdisciplinary Journal of Engineering Sciences 4(1), 45–49. Grygier, D., Rutkowska-Gorczyca, M., Jasiński, M., Dudziński, D., 2016. The structural and strength changes resulting from modification of heat treatment of high carbon steel. Archives of Metallurgy and Materials 61(2B), 971–976. Grygier, D., Rutkowska-Gorczyca, M., 2015. Influence of operating conditions of the steel cord on the structure and selected mechanical and technological properties of high carbon steel. International Journal of Engineering Research and Science 2(4), 1–6. Grygier, D., Rutkowska-Gorczyca, M., 2016. Influence of operating conditions of the steel cord on the structure and selected mechanical and technological properties of high carbon steel. International Journal of Engineering Research and Science 2, 138–142. Krmela, J., 2017. Tire casings and their material characteristics for computational modelling. The Managers of Quality and Production Association Publishing house, Czestochowa, Poland. Kruzel, R., Suliga, M., 2015. The impact of the steel cord construction of its decline of breaking force after fatigue test in bidirectional bending conditions. Metalurgija 54(1), 214–216. Kruzel, R., Suliga, M., Sosna, S., 2015. Influence of steel cord technology on its working properties, Hutnik-Wiadomości hutnicze, 82, 68–71 (In Polish). Lee, A. B. L., Liu, D. S., Chawla, M., Ulrich, P. C., 1994. Fatigue of cord-rubber composites, Rubber Chemistry and Technology 67(5), 761–774. Massoubre, J., 1984. 35 years of the radial ply tire. Journal of Polymer Science, 39, 129–149. Noor, A. K., Tanner, J. A., 1985. Tire modelling and contact problems: Advances and trends in the development of computational models for tires. Computers and Structures 20, 517–533. Rao, S., Daniel, I. M., Mc Farlane, D., 2013. Fatigue and fracture behavior of a steel cord/rubber composite. Journal of Thermoplastic Composite Materials 14, 2013–224. Tashiro, H., Tarui, T., 2003. State of the art for high tensile strength steel cord, Nippon steel technical report, No. 88. Vedeneev, A. V., 2012. New trends in steel cord development. CIS Iron and Steel Review, 24–29. VERT, 2007. Virtual education in rubber technology. Reinforcing materials in rubber products, FI-04-B-F-PP-160531, The Goodyear Tire and Rubber Company, US.
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Structural Integrity Procedia 00 (2018) 000–000 Available online www.sciencedirect.com Available online at at www.sciencedirect.com Structural Integrity Procedia 00 (2018) 000–000
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Procedia Structural Integrity 13 00 (2018) 1626–1631 Structural Integrity Procedia (2016) 000–000
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ECF22 - Loading and Environmental effects on Structural Integrity ECF22 - Loading and Environmental effects on Structural Integrity
Analysis of fatigue life of the steel cord used in tires in Analysis of fatigue life of the steel cord used in tires in unidirectional bending XV Portuguese Conference on Fracture,and PCFbidirectional 2016, 10-12 February 2016, Paço de Arcos, Portugal unidirectional and bidirectional bending a Robert Kruzelaa, of Malgorzata Ulewicz Thermo-mechanical modeling a high pressure Robert Kruzel , Malgorzata Ulewicza turbine blade of an Czestochowa University of Technology, Faculty of Civil Engineering, ul. Akademicka 3, 42-201 Częstochowa, POLAND airplane gas turbine engine Czestochowa University of Technology, Faculty of Civil Engineering, ul. Akademicka 3, 42-201 Częstochowa, POLAND a a
P. Brandãoa, V. Infanteb, A.M. Deusc*
Abstract AbstractaDepartment of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, The bresults of fatigue life tests of steel cord used in constructionPortugal machinery tires are presented in this paper. Steel wire used in tires, IDMEC,ofDepartment oftests Mechanical Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, The results life of steelEngineering, cord for usedtransferring inInstituto construction machinery are presented in this paper. Steel wire used tires, called the steelfatigue cord, is largely responsible largePortugal loads, but tires unfortunately it quickly undergoes fatigue. The in fatigue called cord, is largely responsible for transferring large loads, but unfortunately it quickly undergoes fatigue. The fatigue life cCeFEMA, of the thissteel steel cord is influenced by many factors related to its construction, workmanship quality, as well as the method of Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, life of this steel each cord is influenced by In many factors related toPortugal its construction, workmanship as well as the method of of bending during working cycle. laboratory tests, the fatigue life of steel cords wasquality, compared in the conditions bending during working bending, cycle. Inonlaboratory fatigue fatigue life of testing steel cords wasforcompared in Inthe conditions of unidirectional andeach bidirectional a speciallytests, built,the innovative machine steel cords. fatigue life tests, unidirectional bidirectional bending, on a specially built, innovative fatigueFor testing machine for steel cords.the Innumber fatigue life tests, a fundamental and difference is visible in the number of cycles leading to breaking. one-way bending process, of cycles Abstract difference a fundamental is visible the numberbending. of cyclesInstead leadingoftothe breaking. For one-way process, the number of cycles is significantly greater than for the in bidirectional wire fracture process,bending a steel cord elongation process takes is significantly greater for the bending. Instead of the wire fracture process,ofa failures steel cord takes place that increases thethan number of bidirectional fatigue cycles, which is reflected directly in the number in elongation the tires ofprocess construction During their operation, modern aircraftcycles, engine components aredirectly subjected to number increasingly demanding operating conditions, place that increases the number of fatigue which is reflected in the of failures in the tires of construction machines especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent machines one of Published which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict © degradation, 2018 The Authors. by Elsevier B.V. © 2018 The Authors. Published by Elsevier B.V. the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation © 2018 The under Authors. Published by B.V. Peer-review responsibility of Elsevier the ECF22 organizers. Peer-review of the ECF22 organizers. company,under wereresponsibility used to obtain and organizers. mechanical data for three different flight cycles. In order to create the 3D model Peer-review under responsibility ofthermal the ECF22 needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were Keywords: fatigue strength; steel cord; crack propagation; unidirectional bending; bidirectional bending obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D Keywords: fatigue strength; steel cord; crack propagation; unidirectional bending; bidirectional bending rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a 1. model Introduction can be useful in the goal of predicting turbine blade life, given a set of FDR data.
1. Introduction ©With 2016 the Thegrowth Authors. by Elsevier B.V. an increase in demand for more advanced tires is observed, which would ofPublished the automotive industry, With thetransfer growth of the automotive an increaseof inand demand more advanced tires is observed, which would Peer-review under the responsibility of the industry, Scientific Committee PCF 2016.for effectively generated overloads to the pavement assure safe operation. Currently, for tire manufacturing effectively transfer the generated overloads to the pavement and assure safe operation. Currently, for tire manufacturing exists a complex combination of rubber mixes, steel wires and textile fibers, such as nylon or polyester, which are joined Keywords: High Pressure Turbine of Blade; Creep; Finitesteel Element Method; Model; Simulation. exists a complex combination rubber mixes, wires and 3D textile fibers, such as(1984). nylon or polyester, are joined together in the vulcanization process, Vedeneev (2012), VERT (2007), Massoubre The greatest which influence on the together in the vulcanization process, Vedeneev (2012), VERT (2007), Massoubre (1984). The greatest influence on the life and load capacity of tires, used in all types of vehicles is shown by the internal structure of the tire and the life and load capacity of tires, used in all types of vehicles is shown by the internal structure of the tire and the reinforcement used. For reinforcing tires, either steel wire or its products in the form of strands or ropes are used, called reinforcement used. For reinforcing tires, either steel wire or its products in the form of strands or ropes are used, called the steel cord, Tashiro and Tarui (2003), Bekaert catalogue (1982), Krmela (2017), Noor and Tanner (1985), which, the steel cord, Tashiro and Tarui (2003), to Bekaert (1982), more Krmela (2017), and Tanner (1985), which, embedded in an elastomer, are intended make catalogue the construction rigid, whileNoor imparting to it the appropriate embedded in an elastomer, are intended to make the construction more rigid, while imparting to it the appropriate 2452-3216 © 2018 The Authors. Published by Elsevier B.V. 2452-3216 © 2018 Authors. Published Elsevier B.V. Peer-review underThe responsibility of theby ECF22 organizers. * Corresponding Tel.: +351of218419991. Peer-review underauthor. responsibility the ECF22 organizers. E-mail address: [email protected] 2452-3216 © 2016 The Authors. Published by Elsevier B.V.
Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the ECF22 organizers. 10.1016/j.prostr.2018.12.342
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resistance to tensile forces. Single steel wires have a diameter ranging from 0.15 to 0.38 mm and are the most often manufactured as brass- or zinc-coated wires, Golis, Błażejowski and Pilarczyk (1998). The reliability of operation of steel cord used in the construction of tires or conveyor belts guarantees the safety of operation of the rubber product. The service properties of steel cord are significantly influenced by the chemical composition of steel used, as well as the wire drawing and cord winding technologies. Wires intended for the production of steel cord, made of unalloyed pearlitic steel, containing from 0.70 to 0.95% of carbon, belong to a group of unalloyed steels of a quality class designed for drawing or cold rolling, Ashby and Jones (2005), Czarski, Skowronek and Matusiewicz (2015), Grygier et al. (2016). The fatigue strength of those wires largely depends on the metallurgical purity of the material, and particularly on the contents of oxygen, silicon and Sulphur. The presence of non-metallic inclusions in steel strongly reduces its fatigue strength, because a high stress concentration occurs around impurities, leading, as a consequence, to material fracture, Golis, Błażejowski and Pilarczyk (1998), Ashby and Jones (2005), Berisha et al. (2015), Grygier (2016). The problem of the fracture of products made of pearlitic steel being subjected to plastic working or being in operation, has been arising interest among many researchers for a number of years now, since the fatigue strength of steel cord affects the life of tires and, as a consequence, the safety of their operation, Golis, Błażejowski and Pilarczyk (1998), Grygier (2016), Feng (2015), Kruzel and Suliga (2015), Rao, Daniel and Mc Farlane (2001), Grygier and Rutkowska-Gorczyca (2015), Kruzel, Suliga and Sosna, (2015), Grygier and Rutkowska-Gorczyca (2016), Lee et al. (1994).. Considering the fact that the fatigue properties of cord have a decisive effect on the cord behavior in different working conditions, the understanding and determination of these properties is crucial, especially as the cord constitutes up to 25% of the tire weight. Steel cords used for production of passenger car tires have splices of two to five fibers (e.g. 2 x 0.25 + 2 x 0.25; 2 x 0.25 + 1 x 0.30 - the first number means the number of middle wires and the second number of braided wires with adequate their diameter), while cords used for tires intended for trucks and special construction machines are of multiwire constructions (e.g. 2 x 0.30; 3 x 0.30 + 6 x 0.30). Currently, wishing to reduce their tire manufacturing cost, companies, instead of steel cords of large sizes (such as 4 x 0.28), increasingly often use cords with a smaller number of wires (e.g. 2 x 0.3). Such an approach is acceptable, provided that the cord reinforcement has the appropriate strength parameters. In connection with the above, the present study has made an attempt to evaluate the effect of the fatigue of steel cord on its strength properties, depending on its construction and the type of steel used. 2. Materials and methods The starting material for testing were steel cords of the following constructions: 3 x 0.25 + 6 x 0.35 (cord A); 2 x 0.25 +2 x 0.25 (cord B) and 2 x 0.30 (cord C) all made of steel D76, whose chemical composition is given in Table 1. Steel cords of the 2 x 0.30 construction (cords D, E, E, G), made of different steel grades, were also subjected to testing. All the cords tested were made in the Cord Production Plan (Železárny a drátovny Bohumín) in the Czech Republic. Table 1. Chemical composition of steel used for cords A-C. Material type D76
C
Mn
Si
K
0.76
0.61
0.19
0.007
Content of the chemical element, % S Cr Ni Cu 0.011
0.025
0.025
0.025
Al
Mo
N
0.002
0.005
0.025
The cords were subjected to testing for the force of breaking the cord in a whole (on a ZWICK Z/100 testing machine adapted specially for this purpose) in accordance with standard PN-EN ISO 6892-1:2010, after having previously been fatigued on a universal testing machine constructed at the Czestochowa University of Technology (Fig. 1). The above-mentioned machine enabled tests to be conducted in either unidirectional or bidirectional cord bending conditions, and its principle of operation is depicted schematically in Fig. 2 (R - bending roller radius, bending arc length, A, B - beginning and end of specimen contact with the bending roller, hm - test specimen length). The complete unidirectional bending on a testing machine is understood as bending the cord from its straightened state to a bent state and then back to the initial state, while the complete bidirectional bending is understood as transition from a bent state to a straightened state, and then to an oppositely bent state. The frequency of variations in fatigue machine loading is the number of cycles, N, per minute, i.e. the complete motions of the cord to and from.
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Table 2. Chemical composition of steel used for cords D-G. Material type (cord) C86D2 (cord D) C82D2 (cord E) C80K (cord F) C72DP (cord G)
Content of the chemical element, % C
Mn
Si
K
S
Cr
Ni
Cu
Al
Mo
N
0.87 0.81 0.79 0.73
0.66 0.62 0.67 0.59
0.21 0.18 0.22 0.28
0.006 0.007 0.006 0.005
0.015 0.021 0.019 0.016
0.052 0.064 0.056 0.071
0.043 0.039 0.056 0.049
0.041 0.054 0.087 0.077
0.004 0.006 0.009 0.005
0.007 0.009 0.006 0.008
0.015 0.016 0.018 0.026
Fig. 1. The fatigue testing machine of the authors' design.
(a)
(b) Fig. 2. Schematic diagram of the machine for cord fatigue testing: (a) unidirectional bending conditions; (b) bidirectional bending conditions.
An analysis of the microstructure of wires used for cords, made in compliance with the recommendations of standard PN-EN 10323:2005, was also made within the study using an Axiovert 25 optical microscope. The microstructures were observed on Nital-etched micro-sections. The microscopic examination did not show any significant differences in the structure of wire specimens tested. Figure 3 shows a sample microstructure image obtained for wire of steel D76. The microstructure depicted in the photograph, composed mainly of pearlite, shows cold work effects after performed plastic working. The material was not subjected to recrystallizing annealing, as evidenced by the oblong, heavily deformed structure. All of the wires tested were made of unalloyed pearlitic steel and the employed cold drawing process yielded a large degree of their plastic deformation. A similar deformation degree (in the range of 80–90%) has been observed for wires of eutectoid steel, Grygier and Rutkowska-Gorczyca (2016).
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Fig. 3. Microstructure of wire (steel D76) used for cord A-C; the longitudinal section.
3. Results and discussion At the first stage of the investigation, steel cords A, B and C were cut into 800 mm-long sections and appropriate loads were selected to them (Table 3). For the cord A, a load of 0.40 kN was applied; for the cord B, a load of 0.30 kN; while for the cord C a load of 0.20 kN. After mounting a respective load, steel cord was put in a universal fatigue life testing machine that enabled a unidirectional and a bidirectional bending processes to be conducted. As shown by preliminary tests, the basic difference between the unidirectional and bidirectional bending processes is visible in the number of cycles leading to a cord rupture. For the unidirectional bending process, the number of cycles is significantly greater than for bidirectional bending. In the preliminary tests, when conducting the unidirectional bending process for 24 hrs, the cord wire fatigue level necessary for a specimen rupture was not attained. In view of the fact that for unidirectional bending tests the number of cycles would be too large and fatigue tests for those cord types would take a long time, it was decided to conduct further tests under bidirectional bending conditions, where a specimen undergoes very large plastic deformations in two directions, alternately, in a single cycle. The fatigue effect is then obtained in a shorter time and the comparative results are correct and equally good. So, each steel cord was put to a fatigue test that lasted, respectively, 15, 30, 60 and 120 minutes. During the course of tests, the number of cycles was determined, which is to be understood as the number of complete bidirectional bends. During the tests, the number of bends was counted several times in relation to an assumed time unit, namely 60 s, to obtain a result of 45 complete bends/minute. The cord was bent bi-directionally with cyclically varying tensile loads and the rope rotating around its own axis. The obtained test results are provided in Table 4. During the bending, variable (tensile, bending and compressive) cyclic stresses occurred in the cord, which caused a cord fatigue that manifested itself in wire cracking with a characteristic fatigue fracture surface and growing specimen elongation. As shown by the data in Table 4, for test cords made of high-carbon steel D76, regardless of the cord construction, a reduction in cord tensile strength was observed with increasing number of bends. The decrease of steel cord strength under the effect of variable bending stress is not a linear relationship. After 675 fatigue cycles, regardless of the examined cord construction, a slight, statistically insignificant decrease in breaking force by approx. 0.22% was observed, compared to the control specimen. After 5400 fatigue cycles, the greatest drop in cord strength (by 12.95%) relative to the control specimen was observed for the cord C (of construction 2 x 0.30) in spite of applying the smallest load, while the least, for the cord A (a decrease by 5.60%). The obtained results confirm that the cord winding method, as well as the number of wires and their diameter, have a major effect on the fatigue resistance of cord. The fatigue life of steel cord is also influenced by splice making technology (either rotor or rotorless), workmanship and the material grade used. The process of steel cord elongation proceeds differently from the wire crack build-up process. For cords of a construction having a larger number of wires, a slight elongation of specimens subjected to fatigue testing was observed. Specimens of the A type cord with an initial length of 800 mm and construction 3 + 6 after the bidirectional bending process had a length of about 802 mm, which constitutes an increment in length by 0.2%, relative to specimens with a smaller number of wires in their constructions. For the C type cords of construction 2 x 0.30, no increment in total specimen length was observed, which remained at 800 mm. This was probably caused by the wires arranging relative to one another in multi-wire constructions, stretching of complete cords and a slight elongation of splices in the cords, or their straightening.
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Table 3. Fatigue testing parameters for cords A, B and C Cord
Cord construction
A
3+6
B
2+2 x 0.25
C
2x0.30
Number of cycles 675 1350 2700 5400 675 1350 2700 5400 675 1350 2700 5400
Length of sample, mm 800 800 800 800 800 800 800 800 800 800 800 800
Load, kN 0.40 0.40 0.40 0.40 0.30 0.30 0.30 0.30 0.20 0.20 0.20 0.20
Table 4. Results of measurements of tensile strength, Rm, with indicated number of fatigue cycles, N of steel used for cords D-G. Cord A B C
0* 1665.1 938.9 443.1
Number of cycles, N 675 1350 2700 1661.4 1655.8 1632.1 936.8 928.1 902.1 442.1 432.5 402.8
5400 1571.9 870.7 385.7
The fatigue strength of steel wire depends largely on the metallurgical purity of the material, and especially on its contents of non-metallic inclusions (silicon, sulphur and nitrogen), because it is around those impurities that high stress concentrations form, which result in wire cracking. Therefore, in the subsequent part of the investigation, the strength parameters were determined for cords made of different steel grades (cords D-G) of construction 2 x 0.30, while considering the fact that for this type of construction (cord C), the greatest difference in strength decrease after the bending process was observed. Subjected to testing was wire of a diameter of 1.60 mm, from which, after the drawing process, 0.30 mm - diameter wires were used for splicing the cord under consideration are obtained. The mechanical properties of the wire are given in Table 5. The bidirectional bending test was performed in accordance with the standard PN-ISO 7801: 1996 applicable to determining the resistance of wire to plastic deformation; whereas, the fatigue properties of cords made from the above wires are shown in Table 6. Table 5. The mechanical properties of respective 1.60 mm-diameter wires after heat treatment Strength [MPa]
Cord D E F G
Rm-average 1335 1285 1255 1175
Number of twists [100×d] Min.-Max. 1300-1340 1280-1300 1240-1260 1150-1190
Average 33 30 40 31
Min.-Max. 26-39 24-36 32-44 28-36
Number of bends 5 [mm] Average 10 10 10 10
Min.-Max. 8-12 8-12 9-11 9-11
Table 6. The mechanical and fatigue properties of finished 2 x 0.30 construction cords Cord D E F G
Breaking force, [N] Average 410 400 410 390
Min.-Max. 405-415 395-405 400-420 385-400
Time [min.]
Number of cycles
Length of sample, [mm]
Load, F [kN]
290 280 270 250
13824 13200 12076 11904
1270 1300 1320 1330
0.20 0.20 0.20 0.20
As demonstrated by the data in Table 7, cords D-G of construction 2 x 0.30 with the use of load of a magnitude of 0.20 kN showed a gradual decrease in strength after subjecting them to a specified fatigue. After 675 fatigue cycles, a slight (< 0.3%) decrease in breaking force was observed for all of the examined cords (D-G). By contrast, after 5400 fatigue cycles, the highest percentage drop (12.73 %) in tensile strength was observed for cord D, and the lowest (5.43
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%), for cord G. Cord D contained higher contents of non-metallic inclusions (0.87 % C, 0.021 % Si and 0.015 % S), compared to cord G ( 0.73% C, 0.28 % Si and 0.016 % S). The presence of such a large amount of non-metallic inclusions, especially in the form of oxides, may cause a reduction in material ductility, thereby making technological processes difficult and, in extreme cases, even leading to wires rupturing during the operation of the product. Table 7. The decrease in the strength of 2 x 0.30 construction steel cords D – G, depending on the number of fatigue cycles applied. Cord D E F
% /675 0.28 0.22 0.19
G
0.14
Percentage decrease in the strength/number of cycles % /1350 % / 2700 2.35 8.86 1.88 4.76 1.22 3.81 0.51
1.93
% /5400 12.73 10.21 7.29 5.43
4. Conclusions From the investigation carried out it can be concluded that the main signs of the fatigue wear of steel cords made of high-carbon steel is a reduction in their strength, cracking of wires and an elongation of the test specimen section. The distribution of fatigue cracks depends on the number of bends and the method of bending the rope section tested. Unidirectional bending on a testing machine causes lesser material fatigue, compared to bidirectional bending. The fatigue of steel cord caused by bidirectional bending results in a decrease in mechanical properties of the steel cord, regardless of the cord construction. The greatest drop in cord strength relative to the number of fatigue cycles occurred in the case of cord of construction 2 x 0.30 and a lower carbon concentration in the steel (0.73 %). By contrast, the least effect on the cord strength was shown by a test performed on the 3+6 construction cord, which confirms that the cord winding method, as well as the number of wires and their diameter have a key effect on the tension of steel cord. References Ashby, M. F., Jones, D. R. H., 2005. Engineering Materials: An introduction to microstructures processing and design, Elsevier, Oxford. Bekaert steel cord catalogue, Bekaert S.A., Zwevegem, Belgium, 1982. Berisha, B., Raemy, C., Becker, C., Hora, P., 2015. Multiscale modelling of failure initiation in a ferritic–pearlitic steel. Acta Materialia 100, 11–18. Czarski, A., Skowronek, T., Matusiewicz, P., 2015. Stability of a lamellar structure - Effect of the true interlamellar spacing on the durability of a pearlite colony. Archives of Metallurgy and Materials 60(4), 2499–2503. Feng, F., 2014. Texture inheritance of cold drawn pearlite steel wires after austenitization. Materials Science Engineering 618(A), 14–21. Golis, B., Błażejowski, Z., Pilarczyk, J. W., 1998. Steel wires for tire reinforcement, Publishing house of Faculty of Metallurgy and Materials Science, Czestochowa, Poland. (In Polish). Grygier, D., 2016. Analysis of the causes of damage to the wires of the steel belt of car tires. Interdisciplinary Journal of Engineering Sciences 4(1), 45–49. Grygier, D., Rutkowska-Gorczyca, M., Jasiński, M., Dudziński, D., 2016. The structural and strength changes resulting from modification of heat treatment of high carbon steel. Archives of Metallurgy and Materials 61(2B), 971–976. Grygier, D., Rutkowska-Gorczyca, M., 2015. Influence of operating conditions of the steel cord on the structure and selected mechanical and technological properties of high carbon steel. International Journal of Engineering Research and Science 2(4), 1–6. Grygier, D., Rutkowska-Gorczyca, M., 2016. Influence of operating conditions of the steel cord on the structure and selected mechanical and technological properties of high carbon steel. International Journal of Engineering Research and Science 2, 138–142. Krmela, J., 2017. Tire casings and their material characteristics for computational modelling. The Managers of Quality and Production Association Publishing house, Czestochowa, Poland. Kruzel, R., Suliga, M., 2015. The impact of the steel cord construction of its decline of breaking force after fatigue test in bidirectional bending conditions. Metalurgija 54(1), 214–216. Kruzel, R., Suliga, M., Sosna, S., 2015. Influence of steel cord technology on its working properties, Hutnik-Wiadomości hutnicze, 82, 68–71 (In Polish). Lee, A. B. L., Liu, D. S., Chawla, M., Ulrich, P. C., 1994. Fatigue of cord-rubber composites, Rubber Chemistry and Technology 67(5), 761–774. Massoubre, J., 1984. 35 years of the radial ply tire. Journal of Polymer Science, 39, 129–149. Noor, A. K., Tanner, J. A., 1985. Tire modelling and contact problems: Advances and trends in the development of computational models for tires. Computers and Structures 20, 517–533. Rao, S., Daniel, I. M., Mc Farlane, D., 2013. Fatigue and fracture behavior of a steel cord/rubber composite. Journal of Thermoplastic Composite Materials 14, 2013–224. Tashiro, H., Tarui, T., 2003. State of the art for high tensile strength steel cord, Nippon steel technical report, No. 88. Vedeneev, A. V., 2012. New trends in steel cord development. CIS Iron and Steel Review, 24–29. VERT, 2007. Virtual education in rubber technology. Reinforcing materials in rubber products, FI-04-B-F-PP-160531, The Goodyear Tire and Rubber Company, US.