Issue 37

D. Angelova et alii, Frattura ed Integrità Strutturale, 37 (2016) 258-264; DOI: 10.3221/IGF-ESIS.37.34 258 Focused on Fracture Mechanics in Central and East Europe On fatigue behaviour of two spring steels. Part II: Mathematical models D.Angelova, R. Yordanova, T. Lazarova, S. Yankova University of Chemical Technology and Metallurgy, Bulgaria donkaangelova@abv.bg, r.yordanova@uctm.edu 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. All experiments include testing to fracture, applying acetate-foil replication technique, replica monitoring of short crack surface growth, length measuring of propagating cracks, a, at the corresponding number of fatigue cycles, N. Data obtained from replica monitoring are presented in plots “Crack lengths, a – Cycles, N”, and used for calculating fatigue crack growth rates, da/dN, and graphical presentations “Crack growth rates, da/dN – Crack lengths, a”. A mathematical description of da/dN – a is presented by introducing a parabolic-linear model in different versions for each of the steels. The model versions are verified through comparing the experimental fatigue lifetimes with those calculated by the proposed model version. K EYWORDS . Rotating-Bending Fatigue; Torsion fatigue; Short Fatigue-Cracks; Modeling of Crack-Growth rate. I NTRODUCTION t is well known that fatigue life of technical components made of high-strength materials, and subjected to high-cycle fatigue, can be dominated up to 90% by stages of initiation and propagation of microstructurally short cracks. This supports the significance of quantitative analyses of all physical mechanisms being responsible for local stress concentration, plastic deformation and, eventually, the initiation and propagation of short fatigue cracks which are very different from traditionally investigated long fatigue cracks [1, 2]. The early propagation mechanisms of short fatigue cracks are strongly related to the local microstructural features. Only when the sizes of these features become small than the crack length, predictions of crack propagation rates can be made on the basis of linear-elastic fracture mechanics. Fatigue long/short crack growth characteristics are normally expressed by the relation between crack growth rate and stress intensity factor range/short crack length, which involves two very different methods (by nature) for data obtaining based on different kind of specimens when observing and registering different physical cracks - long cracks, and physically small and short cracks [2, 3], also the two methods concerning long and short cracks use different technical standards [4]. E XPERIMENTAL WORK Material and specimens. Testing. n the present study, fatigue in two spring steels are investigated: (i) Steel EN10270-1SH/ DIN 17223C (Steel A) for conducting own fatigue experiments, [5, 6], and proposing new mathematical models of short fatigue crack growth; and (ii) BS250A53/DIN 55Si7 (Steel B) for using some already published results, [7], describing those results by the I I