MECHANICAL BEHAVIOUR OF DAMAGED CENTRAL CORE STRAND CONSTITUTING A STEEL WIRE ROPE HOIST UNDER THE EFFECT OF A STATIC LOAD

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1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 7, Issue 3, May June 2016, pp , Article ID: IJMET_07_03_033 Available online at Journal Impact Factor (2016): (Calculated by GISI) ISSN Print: and ISSN Online: IAEME Publication MECHANICAL BEHAVIOUR OF DAMAGED CENTRAL CORE STRAND CONSTITUTING A STEEL WIRE ROPE HOIST UNDER THE EFFECT OF A STATIC LOAD E. Boudlal and M. Barakat ISEM/Higher Institute of Maritims Studies, Laboratory of Mechanics, Km 7 Road El Jadida Casablanca Morocco N. Mouhib, M. Lahlou, and M.El Ghorba Laboratory of Control and Mechanical Characterization of Materials and Structures, National Higher School of Electricity and Mechanics, BP 8118 Oasis, Hassan II University, Casablanca, Morocco H. Ouaomar Faculty of Sciences and Technics, Mghilat, Beni Mellal, Morocco ABSTRACT The main objective of this study is to predict the evolution of damage of the central core (strand) that represent the heart of a wire rope, based on simple experimental tensile test conducted on virgin samples and others artificially damaged by breaking wires constituting the samples at different percentages. The experimental results obtained have allowed us to follow the evolution of the damage and quantify it. Thereafter, it was possible to identify three stages of damage. Therefore, be able to intervene in time for predictive maintenance. This study also includes a correlation between two methods of calculating the damage namely static damage and damage by unified theory and this by analogy to cyclical behavior. The comparison shows good agreement. Keywords: Central Core Strand, Damage, Predictive Maintenance, Tensile Test, Wire Rope. Cite this Article: E. Boudlal, M. Barakat, Mouhib, M. Lahlou, M.El Ghorba and H. Ouaomar, Mechanical Behaviour of Damaged Central Core Strand Constituting A Steel Wire Rope Hoist Under The Effect of A Static Load. International Journal of Mechanical Engineering and Technology, 7(3), 2016, pp editor@iaeme.com

2 E. Boudlal, M. Barakat, Mouhib, M. Lahlou, M.El Ghorba and H. Ouaomar 1. INTRODUCTION Wire ropes are belonging to the class of complex systems due to the large number of components that build them and their complicated processes of operations. It is generally a set of multiple strands wound around a central core strand and each strand is comprised of several wires helically wound [1] (Fig.1). Figure 1 Basic components of a typical wire rope This specific structure permits the wire ropes to resume loads despite the break of one or more wires. Furthermore, they are able to carry loads in the longitudinal direction while being flexible in the lateral direction [2]. Despite all the advantages that this conception represents, it is an accepted fact that wire ropes are consumable with a limited life and it should be replaced before the risk of failure becomes unfortunate. Industrial experience shows that sudden breaking of a large part of wire ropes in service is most often due to the cumulative damage of wires [3][4]. This is particularly insidious because of its hidden nature, which may lead to serious accidents. As part of this problem, this study focuses on the mechanical behaviour of one of the components of the wire rope which is the central core strand of which several lots were artificially damaged at different percentages (14%, 28%, 42%, 57% and 71% broken wires). Based on an experimental tensile tests, the evolution of damage is determined and subsequently the critical life fraction βc is defined. Such a study could be beneficial for manufacturers because of its low cost and speed. 2. MATERIAL & EXPERIMENTAL METHODS 2.1. Material In this study, we consider a Steel Wire Rope of type 19x7 and antigyratory construction (1x7 + 6x7 + 12x7) (Figure 2), with a diameter of 7mm mainly used as a rigging wire rope for all types of cranes and for exploring in the high seas due to its excellent resistance to deformation. The considered central core strand is composed of 7 individual wires, a core wire (core of the central core) and 6 peripheral wires helically arranged around editor@iaeme.com

3 Mechanical Behaviour of Damaged Central Core Strand Constituting A Steel Wire Rope Hoist Under The Effect of A Static Load Figure 2 Steel Wire Rope of type 19x7 and antigyratory construction (1x7 + 6x7 + 12x7) 2.2. EXPERIMENTAL METHODS As mentioned before, the goal is to follow the damage of central core strand extracted from steel wire rope. For this, static tests were performed on artificially damaged specimens at different levels of damage by cutting some wires (14%, 28%, 42%, 57% and 71% broken wires). To obtain specimens of central core strand, a suitable length of the cable was cut and strands were de-wiring (wiring off). The minimum length of the samples is equal to the length of the test plus the necessary for the mooring. Therefore, a length of 300 mm is anticipated as the length of the test. The measurements tolerance in the length is ± a millimeter for all samples [5]. To break wires manually, a tip was inserted carefully through the number of wires to cut and lift carefully by turning the tip in the direction of wiring then cut using a diagonal cutting pliers. All specimens were tested in tension according to DIN EN with imposed displacement corresponding to a strain rate of 2mm / min. The tests were carried out under the conditions of air and room temperature ( C) on a Zwick Roell type of machine with a force cell ± 10 kn. Figure 3 shows the assembly with a close view of the sample placed between the mooring jaws. The fixation of the samples is performed by means screwed wedges on both ends of the strand in order to prevent sliding of the samples during the tests. Figure 3 Experimental setup of a central core sample extracted from wire rope editor@iaeme.com

4 E. Boudlal, M. Barakat, Mouhib, M. Lahlou, M.El Ghorba and H. Ouaomar 3. RESULTS & DISCUSSION 3.1. Mechanical characterization All the tests leading to the rupture of central core specimens has allowed to trace the shape of tensile curve representing the evolution of stress applied to the virgin specimen strand (MPa) versus strain (%) (Figure4) and subsequently extract the mechanical characteristics summarized in table 1 (the values given are average values). Figure 4 Stress-Strain curve of the extracted virgin central core strand The mechanical properties of the virgin specimen are reported in the table1. Table 1 The mechanical properties of central core strand Mechanical Tensile Elastic Young Poisson s properties strength limit modulus ratio Value 1561 MPa 1367 MPa 189 GPa ν = 0,3 3.2 Tensile tests of tested specimens of the extracted central core strand at different percentages of damage Experimental results according to the number of broken wires (virgin, 14%, 28%, 42%, 57% and 71% broken wires) are given in Figure 5. The curves describe on 3D the evolution of strength (N) versus displacement (mm) editor@iaeme.com

5 Mechanical Behaviour of Damaged Central Core Strand Constituting A Steel Wire Rope Hoist Under The Effect of A Static Load Figure 5 Evolution of the strength (N) versus displacement (mm) for different levels of damage When carrying out the tests and according to the results shown in Figure 5, it was found that the ultimate strength drop depending on the number of broken wires. Therefore, following this reaction of studied strand (central core), it was possible to assess the damage at each level of damage by similarity to the behavior of a material under cyclic loads 3.3. Quantification of static damage The model of static damage (Ds) is to determine the evolutions of forces whose variations are mainly due to damage. Then we quantify the damage by the variable Ds expressed as [6]: Where: F u : Value of the maximum ultimate strength F ur : Value of the ultimate strength F a : Force just before the final break The evolution of central core damage is followed at several levels of degradation starting with its virgin state until failure. This phenomenon is described by the damage parameter Ds Equation (1) by the following limits: In the initial state: F ur = F u D = 0 In the final state: F ur = F a D = 1 The variation of the static damage according to the life fraction is illustrated by the curve in figure 6: (1) editor@iaeme.com

6 E. Boudlal, M. Barakat, Mouhib, M. Lahlou, M.El Ghorba and H. Ouaomar Figure 6 Evolution of the static damage depending on the life fraction Increased damage means the increase in strength loss in static tensile samples. This loss changes when artificial damage becomes more important. The curve in Figure 6. allowed us to identify three stages of damage using the curvature change [7]. The first stage corresponds to its initiation; until β= 2/7 broken wires (28% damage), damage grows relatively slowly. Then there is the stage II which is within the range of β = [28%, 71%] when the damage becomes progressive and predictive maintenance is essential to industrial. The critical life fraction βc =71% is the bridge between the progressive damage of stage II and stage III where the damage is accelerating sharply and the break could be brutal. This means that from 71% of broken wires, the central core strand, the heart of wire rope, is declared in default Quantification of damage using unified theory The damage of the strand being progressive, its variation is influenced by the level of loading. Various representative theories of this damage are given initiated by the linear of Miner law; finding that the damage changes linearly depending on the fraction of life. [8] By analogy with the unified theory, an empirical relationship describing the damage is proposed: (2) Where: β =, γ = and γu = F 0 is the residual endurance limit that could be determined by multiplying the ultimate residual force by a coefficient α (for n = 0; F 0 = α Fu). For a coefficient α = editor@iaeme.com

7 Mechanical Behaviour of Damaged Central Core Strand Constituting A Steel Wire Rope Hoist Under The Effect of A Static Load The variation of the damage according to β with γ as a parameter and that of the linear Miner rule is shown in Figure7. Each curve is associated to a loading level. Figure 7 Evolution of Damage by unified theory and Miner law in function of the life fraction It is noted that the damage curve approaches gradually the bisector (the linear Miner law) versus β for high levels of loading. Arguably Miner law provides greater safety and simplicity for the user that the unified theory. It is for this reason that many researchers adopt this law for damage study of wire ropes Comparison of the two methods of damage calculation The correlation between the damage calculated from equation (1) of static damage and that of the equation (3) of the unified theory appears on the curves in Figure 8. Figure 8 Comparison of damage according to the unified theory and the Miner law with static damage editor@iaeme.com

8 E. Boudlal, M. Barakat, Mouhib, M. Lahlou, M.El Ghorba and H. Ouaomar Comparing the damage curves of two calculation methods and the curve of the cumulative damage (Miner law), we see that the curve of static damage is similar to that of the damage according to the theory unifies for loading levels = 2.26 and =2.14 in the predefined stage I (β = [0%, 28%]). However, it is clear that the curve deviates from static damage in stage II (β = [28%, 71%] ) and stage III (β = [71, 1%]) to lie beyond Miner. 4. CONCLUSION Concerning the study of central core strand extracted from wire rope hoist, it was possible to follow the evolution of damage to each percentage of damage based solely on easy tensile tests. Two damage quantification methods were used for this study: the method of calculation of static damage and the method of calculation by unified theory. Comparisons of results have shown good agreement. Three stages of damage were determined; Stage I [0, 28%] corresponding to the initiation of the damage, stage II [28%, 71%] for the progressive damage that requires predictive maintenance and stage III [71%, 1] where the damage is brutal, so the strand (central core) is declared in default. Furthermore, a study of the behavior of an entire wire rope is being established with data the cable geometry and the damage of the coiled strand and the central core. REFERENCES [1] Canadian Centre for Occupational Health and Safety, Wire rope lifting in February [2] Yujie Yu, Finite element study of behavior and interface force conditions of seven-wire strand under axial and lateral loading Construction and Building Materials66 (2014) [3] Brevet. P : Pathologie des suspensions. la corrosion - la fatigue. Rapport technique PB/MMC , Laboratoire Central des Ponts et Chaussées, [4] Ponthiaux. P., Wenger. F. et Richard. C.: Tribocorrosion. Techniques de l'ingénieur, COR 60:1_22, Dec [5] EN : Metallic materials Tensile testing-part 1: Method of test at ambient temperature : European Standard: 2001 has the status of a DIN Standard. [6] Mouhib. N., Ouaomar. H., Lahlou. M., El Ghorba. M, Characterization of residual energy loss and Damage Prediction of 7-wire strand extracted from a steel wire rope and subjected to a static test, International Journal of Research 2(06), June [7] Mouhib. N., Etude multi-échelles du comportement mécanique d'un câble métallique de levage de type antigiratoire (19x7) soumis à des chargements statiques et prédiction de sa durée de vie: expérimentale, analytique et numérique. Phd thesis, ENSEM. MOROCCO [8] C. Bathias, J. Bailon, La fatigue des matériaux et des structures, pp [9] P. Govindarao, Dr. P. Srinivasarao, Dr. A. Gopalakrishna and C V Sriram, Improvement of Tensile Strength of Butt Welded Joints Prepared by Vibratory Welding Process. International Journal of Mechanical Engineering and Technology, 4(4), 2013, pp editor@iaeme.com