PSI - Issue 17

Maricely De Abreu et al. / Procedia Structural Integrity 17 (2019) 618–623 M. De Abreu et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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These are gradually replaced by well-defined fatigue striations in the crack propagation stage, where signs of secondary longitudinal cracking by localized decohesion at the austenite-ferrite interphase are visible at higher magnification view (Fig. 4e). According to Figs. 4f and 4g, the macroscopic fracture characteristics of ES wires broken in the F-QL tests are very similar to that of LDS wires. The cracking process also begins at one end of the notch produced by the transverse loading and the propagation takes place following an inclined plane with respect to the axis of the wire axis, but at an angle, somewhat greater than that found in case of LDS. The morphology between crack initiation and growth hardly differs in this case (Fig. 3h). The crack advances through the pearlitic colonies without secondary microcracking, by the progressive breaking of the cementite lamellae and the subsequent growth of micro voids in the ferrite phase (Fig. 5e). This explains the higher angle of the fatigue propagation plane and the more abrupt stepped striations detected in the ES wires. The axial tensile fatigue tests under static transverse load (F-QL) shows that currently used cold-drawn eutectoid steel (ES) wires for fabrication of high-strength structural strands maintain the fatigue resistance levels required by the technical codes for prestressing steel, and the FIB recommendations for the strand-tendons of cable-stayed bridges. The transverse load has to exceed a threshold close to 40% of the resistant capacity of the wire in simple tension and the maximum axial fatigue loads to surpass 70% of it to reduce the fatigue limit below the required 200 MPa stress range. Analogous F-QL tests carried out with cold-drawn wires of equal strength and diameter, made of lean duplex stainless steel (LDS), show that the performance of these wires relative to the fatigue resistance under transverse loading surpass that of conventional eutectoid steel wires used for strand manufacturing. The designed testing method not only allows the comparison of the tensile fatigue resistance of different high strength wires regarding their sensitivity to transversal loading but it also permit to determine the service conditions of the wires involving transverse loading that increase the risk of fatigue failure. According to the behavior of the tested eutectoid and lean duplex stainless steels wires this occurs for transverse loads roughly exceeding a half of the tensile bearing capacity of the wires. 4. Conclusions

Acknowledgements

The authors gratefully acknowledge the financial support obtained from the Spanish Ministry of Science and Innovation through the project BIA 2014-53314 – R and the collaboration with INOXFIL S.A. who kindly provided the high-strength, lean and duplex steel wires.

References

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