PSI - Issue 13

Robert Kruzel et al. / Procedia Structural Integrity 13 (2018) 1626–1631 Kruzel and Ulewicz / Structural Integrity Procedia 00 (2018) 000 – 000

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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 multi wire 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

Content of the chemical element, %

C

Mn

Si

K

S

Cr

Ni

Cu

Al

Mo

N

D76

0.76

0.61

0.19

0.007

0.011

0.025

0.025

0.025

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, h m - 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.