PSI - Issue 4

Zoran Odanovic / Procedia Structural Integrity 4 (2017) 56–63 Author name / Structural Integrity Procedia 00 (2017) 000 – 000

60

5

based on the measured results for the similar steel from literature Odanovic (2011). Equation (2) was then modified to: K Ic 2 =0.27236 * KU * R e - 0.00578 * R e 2 (3) Where KU is impact energy in [J] and R e is yield strength in [MPa]. Applying equation (3) using the values for impact energies and yield strength from standard EN 13261, and test values shown in Table 2 and 3, values of the strain fracture toughness K Ic were calculated and presented in Table 5.

Table 5. Calculated plane strain fracture toughness K Ic

Impact energy KU 5/300 (J)

Yield strength, Re (MPa)

Test specimens position Longitudinal Transverse Longitudinal Transverse

Plane strain fracture toughness K Ic (MPa.m

0. 5 )

min. 30 J min. 25 J

min. 320

Standard EN 13261 Measured

44.97 39.83 32.80 20.13

21.8

235 233

11.33

values

Table 5 shows that in the longitudinal direction values for strain fracture toughness K Ic were approximately 25% lower than the minimum impact energies and yield strength values required by standard EN 13261. The transverse direction values were approximately 50% lower than those required by the standard. Such low values of the strain fracture toughness K Ic , especially in transverse direction, indicate low resistivity to crack propagation of the crack initials from the axle surface. Summarizing the results, it is evident that the steel used to manufacture investigated axle is with respect to its chemical composition in accordance with the requirements of the corresponding standard. On the other hand, mechanical properties of applied material are below the standard requirements. This suggests that the investigated axle is susceptible to formation of the initiations for different kind of damages as a result of the exploitation conditions. Also, resistivity to propagation of the initial cracks is below the required resistivity. Low values of mechanical properties may be a result of insufficient reduction rate in hot rolling/forging process, or inadequate heat treatment process during the axle production. The fractured surface in a cross section of the railway axle is shown in Fig. 4. The following characteristic zones could be distinguished on the fractured surface. The first one labeled as A in Fig. 4 is the zone around the circumference of the fractured surface with tooth-like numerous initial cracks of different sizes. This zone covers about half of the radius of the axle. The size of these initial cracks ranges from 10 to 20 millimeters in depth, and from 2 to 20 millimeters in width. These cracks were probably initiated from the numerous initial parallel cracks on the outer surface of the critical axle radius, labeled as AR in Fig. 4. One can assume that these cracks were initiated by corrosion. Due to the high stress concentration they spread parallel to the axle cross section or they connected and integrated with parallel initial cracks of similar kind. In this process they formed, tooth-like initial cracks having a specific shape of ratchet. These marks show a presence of corrosion products. The following characteristic zones are labeled in Fig. 4 as zone B and C. Appearance of these zones is similar in morphology, but different in shape. It can be concluded that they were initiated at the ratchet marks and propagated by the fatigue mechanism. One can notice in these regions fatigue zones resulting from the slow fatigue crack propagation. These zones are highly oxidised which indicates long duration of axle exploitation. Zone B and C are separated by a radial crack. One can conclude based on their depth that these zones formed around the same period of exploitation, but in parallel cross sections of 3.4. Macro and microstructure analysis of the fractured railway axle In order to identify the cause of the axle fracture, macro-fractography was performed and the fractured surface of the axle journal was analysed. Also, segregation testing was conducted by macroscopic method using sulphur print - Baumann method, Schumann (1989). Results are presented in Figs. 4 and 5.