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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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.