13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- The prestressing steel wire exhibits a Paris curve below that of the hot rolled bar (lower parameter C) [11], thus producing a retardation in fatigue crack propagation with the cold drawing process. In addition the fracture toughness is also higher in the cold drawn wire than in the hot rolled bar [23]. This indicates that cold drawing is beneficial since it improves both the fatigue and the fracture performance by dropping the Paris law [11] and elevating the fracture toughness [10], a clear implication for structural engineers. Moreover, the prestressing steel wire is again the best option on the basis of a clear reduction of the size of the surface defects (acting as crack initiators) and the microstructural changes induced by the drawing process (e.g., orientation of the cementite layers acting as barriers to dislocational movement). Both characteristics, small surface defects and special microstructural arrangement, contribute to a delay of the initiation of fatigue crack growth. 5. Conclusions The following conclusions may be drawn from the experimental results of fatigue crack growth from surface defects in pearlitic steel: (i) Fatigue cracks in pearlitic steels are initiated at the wire surface starting from small defects. In the hot rolled bar the fatigue initiators are mainly the surface defects (material losses) while in the prestressing steel wire such initiators are principally the voids created by the existence of particles near the wire’s surface. (ii) Fatigue cracks created from defects exhibit a fractographic appearance consisting of ductile microtearing events which can be classified as tearing topography surface or TTS, and exhibit a spacing remarkably lower in the prestressing steel wire than in the hot rolled bar. (iii) The number of cycles necessary for fatigue crack initiation in the prestressing steel wire is quite higher than that of the hot rolled bar (for Δσ=σY/2) and thus changes in surface defects and microstructural arrangement produced by the drawing process considerably improves its fatigue performance. Acknowledgements The authors wish to acknowledge the financial support provided by the following Spanish Institutions: Ministry for Science and Technology (MCYT; Grant MAT2002-01831), Ministry for Education and Science (MEC; Grant BIA2005-08965), Ministry for Science and Innovation (MICINN; Grant BIA2008-06810 and BIA2011-27870), Junta de Castilla y León (JCyL; Grants SA067A05, SA111A07 and SA039A08), and the steel supplied by EMESA TREFILERÍA (La Coruña, Spain). References [1] I. Verpoest, E. Aernoudt, A. Deruyttere, M. de Bondt, The fatigue threshold, surface condition and fatigue limit of steel wire. Int J Fatigue, 7 (1985) 199–214. [2] J. Llorca, V. Sánchez-Gálvez, Fatigue threshold determination in high strength cold drawn eutectoid steel wires. Eng Fract Mech, 26 (1987) 869–882. [3] S. Beretta, S. Matteazzi, Short crack propagation in eutectoid steel wires. Int J Fatigue, 18 (1996) 451–456. [4] S. Beretta, M. Boniardi, Fatigue strength and surface quality of eutectoid steel wires. Int J Fatigue, 21 (1999) 329–335. [5] T. Shinohara, K. Yoshida, Deformation analysis of surface flaws in stainless steel wire drawing. J Mater Process Technol, 162–163 (2005) 579–584. [6] N. Singh, V. Sánchez-Gálvez, Effect of Ca(OH)2+NaCl environment corrosion fatigue crack growth in cold drawn eutectoid steel rods. Br Corros J, 26 (1991) 117–121.
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