Failure Analysis of Wire Ropes Used in Multi-Wire Machines for Cutting Blocks of Stone

This paper reports the analyses carried out with the company Pedrini SpA ad unico socio, located in Carobbio Degli Bergamo (IT). Wire ropes with diamond beads, used as cutting tools in multi-wire machines for cutting blocks of stone, were considered and a failure analysis of the wire ropes was carried out. The aim of the paper is to highlight the damage mechanisms of the wire ropes to increase service life of these cutting tools. Microscope observations and the penetrating liquids method were used to analyze the damaged wire ropes. Fatigue, corrosion and contact fatigue problems were observed and the effect of the centering of the beads on the wire rope was studied.


Introduction
Wire ropes with diamond beads used in machines for cutting blocks of stone are subjected to fatigue, contact fatigue, corrosion and corrosion-fatigue loads in an aggressive environment.
As shown in Figure 1, 1-3 multi-wire machines for cutting blocks of stone are made of two structural main components: the supporting structure, fixed with flanged bolts to the ground, (1 in Figure 1), and a vertical moving part 4 ( Figure 1). Several wire ropes with diamond beads are put in motion by a driven drum (2 and 7 in Figure 1). The tensioning mechanical system (9 in Figure 1) allows to apply a tension to the wire ropes with diamond beads while the machine is cutting the stone blocks. Several pulleys guide the wire ropes; up to 80 wire ropes can be used and mounted in parallel on the structural component 4 and on several pulleys. The motorized drum is the component 3 in Figure 1 and a threephase asynchronous electric motor is mounted on the machine and puts the drum and the wires in motion. Wire ropes with diamond beads are the cutting tools of the machine and the designer must take care of such components when mounted on the machine. It is well known that the structural behavior of steel wire ropes, composed of several strands, is complex and multiaxial stresses, along with contact fretting stresses, must be managed. Working conditions of the wire ropes have to be strictly controlled and checked periodically.
Notwithstanding there are many literature references on the study of the damage of wire ropes, few research references can be found, as far as the author knows, that would allow to understand their structural behavior in terms of damage or failure analyses. [4][5][6][7][8][9][10][11] In 4 Authors report a study on the diamond wire cutting of concrete materials. Wire cutting with diamond technique was used in the United States until the early 1980s and allowed to cut reinforced concrete structures, regardless of thickness and reinforcement content. In 5 an innovative and optimized design of automatic adjustment system for beaded rope of new diamond wire sawing machine is reported, while in 6 the mechanics of sawing granite with diamond wire is considered. Research on cutting performance optimization of diamond wire saw is deepened in. 7 In these papers the structural design of the wire rope with diamond beads is introduced and the mechanical structure and control of the adjusting device of the diamond wire saw are described. Working parameters are transmitted via wireless signals to achieve remote control. Mechanics of cutting procedure is deepened and mechanical simulation and optimization models of the wires with diamond beads are proposed. Many references are available on the study of wire ropes without diamond beads; [8][9][10][11] such references allow to understand the mechanisms of failure in case of absence of the beads: unfortunately, the Author of this paper found that the structural fatigue and corrosion-contactfatigue behavior of the wire rope is highly influenced by the presence of the diamond beads.

Fig.1: Multi-wire machine for cutting blocks of natural or artificial stone
This paper contains the results of the observation of surface damage of wires used in multi-wire machines for cutting blocks of stone and the optical analysis of beads for 2.35 mm cables. The cables are used as a support for pearls equipped with diamond inserts for cutting stones (beads) (Figure 2).

Fig. 2: a) wire with beads and b)bead
The analyses and observations under the microscope refer to wires and ropes used by Pedrini multi-wire machines for cutting hard stones (http://www. pedrini-italia.it/). The wires and ropes are produced by different specialized companies, along with the insertion of the diamond beads.
The objective of the observations is to evaluate the surface state and the surface damage of wires and ropes starting from production, use and damage. The wires with beads work hard in a very aggressive environment and the surface state of the same greatly influences the fatigue resistance and could be the cause of premature failure.
The samples were taken from wire ropes having 2,35 mm diameter. The wire rope is composed of 7 strands wires, one of which is located at the centre of the wire rope ("soul"). Each strand contains 7 single wires having 0.3 mm wires diameter ( Figure 5).

Mechanical Features of the Wire Ropes
With and without Zn galvanized coating were studied. The mechanical features of the galvanized wires are reported in Figure 3 and Table 1.

Experimental Setup
The study was conducted by means of microscopic analyses and tests with penetrating liquids. For  Significant sections of the beaded wire as shown in Figure 4 were investigated. Section X.1 was not considered but we focused on the evaluation of the centering of the cable in the beads. The sections were obtained using a metallographic cutting machine. The wire ropes with beads were also observed by unwinding the strands and the core both by opening the individual strands and by releasing the individual wires before proceeding with the observation. Figure 5 shows two examples of preparation of a stranded cable and single strands and wires. Figure 6 shows two examples of the cracked surfaces of the wires. Those cracks greatly affect the fatigue resistance of the whole wire rope with beads. Figure 7 shows an example of the cracked surfaces of the rope. To evaluate the effect of the environment on the wire rope, tests were carried out with penetrating liquids (blue ink). Liquids were poured onto the flexed sample to simulate operational behavior. Figure 8 shows the penetrating liquids experimental test.

Microscopic Observations and Results
The centering of the wire rope in the beads was checked too (Figure 9).

Analysis of the Results and Discussion Fatigue Resistance of the Wire Ropes
The wire ropes studied in this work are designed with low fatigue resistance safety factors (2)(3). Previous analyses helped in reaching some useful conclusions. [3][4][5][6][7][8][9][10] Considering a breaking load R m =2000 MPa, the alternating bending fatigue limit is equal to: If you are considering a pulsating cycle from scratch: We obtain σ m,lim = σ a,lim = 160 MPa.
The wire rope can therefore withstand very low fatigue loads. Contact between the wire rope and the beads was observed (Figure 9). Beads and the wire rope are The entire microscopic analysis revealed: These conclusions can be drawn: • The strands can be in contact or not depending on the axial position in the bead, including the core. • Strands and core rotate on themselves, and the strands rotate around the core as well. • The strands often encounter the metal base. • Possible detachments are observed between the metal base and the diamond paste. • The diamond paste has no uniform thickness, with a maximum ratio that can reach 2:1. Where the thickness is smaller, a lower density of diamonds is observed. This could be the cause of the ovality of the bead.

Discussion of the Results
Observations and analysis of the damaged wire ropes allowed to highlight that the beads have no continuous side surface and at the discontinuity the finish is very poor. Moreover the insertion of the diamond chips is not uniform. The insertion of the splinters causes localized lifting of the material. This could cause premature detachment of some of them. Wire ropes are mechanical components that work in a complex stress state with contact loads, wear, corrosion and fatigue resistance problems. The presence of the diamond beads is a further stress concentration, with corrosion and contact wear fatigue problems if the beads come into contact with the wire rope during assembly or in working condition.
The advice is to product wire ropes with beads in which the centering of the cable with respect to the bead is carefully controlled. No contact between rope and bead should occur. According to the results and observations this is the most important advice for producers of the ropes with diamond beads.

Conclusions
This paper reports the failure analysis of the damage mechanisms of wire ropes with diamond beads mounted in machines for cutting stones. Wire ropes with diamond bead are cutting tools subjected to fatigue, corrosion-contact-fatigue stresses. Cracks and defects are present in the strands of the wire ropes, generated during the production process: these cracks are further sources of stress concentrations. The observation at the microscope, and the penetrating liquids analyses, highlighted that the most important advice to give to the producers of the wire ropes with diamond beads is to product components in which the centering of the cable with respect to the bead is carefully controlled.

Acknowledgments
The author would like to thank Pedrini Spa ad unico socio, located in Carobbio Degli Angeli, Bergamo (IT), for the fructuous collaboration and permission to publish this paper.
With almost 60 years of experience in the stone field, Pedrini is an international leader in the manufacturing of machines for the processing of The experience achieved through continuous investments in technology, research and innovation enabled Pedrini to improve the quality and reliability of its machinery thus becoming a valid reference point on the worldwide market.
Pedrini's technical department can count on about 20 professionals, among which engineers and specialized technicians, operating with the most advanced instruments and software for mechanic and electronic design.

Funding
The research was funded by PedriniSpA ad unico socio, located in CarobbiodegliAngeli, Bergamo (IT).