Issue 37

D. Angelova et alii, Frattura ed Integrità Strutturale, 37 (2016) 249-257; DOI: 10.3221/IGF-ESIS.37.33 249 Focused on Fracture Mechanics in Central and East Europe On fatigue behavior of two spring steels. Part I: Wöhler curves and fractured surfaces D.Angelova, R. Yordanova, T. Lazarova, S. Yankova University of Chemical Technology and Metallurgy, Sofia, Bulgaria donkaangelova@abv.bg A BSTRACT . Symmetric fatigue in two spring steels is investigated in three groups of specimens. One of the groups (Steel EN10270-1SH/ DIN 17223C – C 0.82%, Mn 0.76%, Si 0.26%) has experienced rotating-bending fatigue in air, and the other two groups (Steel BS250A53/ DIN 55Si7 – C 0.56%, Mn 0.81%, Si 1.85%), torsion fatigue in-air and corrosion environment. Fatigue life-time data are obtained for both spring steels, and their corresponding Wöhler curves plotted and mathematically described. Surface short fatigue cracks are observed from origin to fracture by using acetate-foil replication technique, and their length, a, measured at the corresponding number of fatigue cycles, N. Those data are presented in plots “Crack lengths, a – Cycles, N ” and a comparison made between both the steels . The fractured surfaces of all specimens have been studied and analyzed by the scanning-electron microscopy methods K EYWORDS . Rotating-Bending Fatigue; Torsion Fatigue; Wöhler curve; Short-Fatigue-Crack Growth; Fatigue- Crack Path I NTRODUCTION pring steel is a low-alloy, medium-carbon steel or high-carbon steel with a very high yield strength, allowing objects made of spring steel to return to their original shape despite significant bending or twisting. Required properties of such a steel vary accordingly. However, whatever its application can be, it is certain that a high stress during cyclic loading and prolonged reliability should be required. The fatigue strength of a spring steel is affected by many factors, such as material, shape, loading, environment. Generally, fatigue failure occurs by propagation of subcritical cracks ranged from several microns to a few hundred microns, [1]. Fatigue crack growth characteristics are normally expressed by the relation between stress intensity factor range and crack growth rate. It is generally recognized that the linear fracture mechanics allows to be defined the threshold condition under which a fatigue crack can propagate as a large crack. In high strength materials as spring steels some physically small cracks can lead to fatigue fracture. But in all cases fatigue failure begins from much smaller cracks known as short cracks originating in one grain or at flows, i.e. as nonmetallic inclusions; these cracks can be surface cracks or below-surface ones and there exist different mathematical models (from linear fracture mechanics ones) of their behavior [1,2]. S