PSI - Issue 4

Zoran Odanovic / Procedia Structural Integrity 4 (2017) 56–63

61

Author name / Structural Integrity Procedia 00 (2017) 000 – 000

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the axle. The final axle fracture surface accounts for approx. 30 – 40% of the cross-sectional area and is shown as light-colored surface in Fig. 4 labeled as zone D. This type of the fracture could be defined as a ductile. Generally, it can be concluded that the crack initiations formed at the surface of the critical radius of the axle. The newly formed cracks then propagated by the fatigue mechanism, until the final axle fracture happened when the remaining ligament of the axle could not withstand the bending stress of the loaded axle. Macrostructure and presence of the defects and imperfections in the axle material was analyzed using sulphur print method (Baumann method). The axle cross section next to fractured surface was tested and the result is presented in Fig. 5. Only small-scale heterogeneities were observed in the central zone of the cross section. However, dark etching strings of inclusion between the marked white lines at the mid radius of the cross section were observed. By comparing Figs. 4 and 5, one can notice matching between zones labeled as A-B-C- D and A’ - B’ - C’ - D’. This observation could clarify and additionally explain possible mechanism of axle fracture. The initial cracks from the axle surface likely propagated through the cross section in the transversal direction of the axle by a different mechanism, as explained before, until reaching the strings of inclusions in zone between white lines in Fig. 5. Orientation of these inclusions are perpendicular to the direction of the cracks propagation, and one can assume that they represents a temporary barrier to the further crack propagation in the radial direction. Remaining axle ligament, labeled as zone D or D’ in Figs. 4 and 5, carried the load until the moment when the axle could not resist the bending stress any more leading to the final fracture of the axle.

AR

Fig. 4. Fracture surface with zones characterized by different types of fracture mechanism

Fig. 5. Macrostructure of the axle cross section obtained by sulphur print (Baumann method)

Fig. 6. Lamellar perlite and ferrite in the axle material microstructure (etched in 3 % Nital)

Microstructure was analyzed using LOM, SEM and EDX analysis at magnifications up to 500 times. Testing was performed on the samples extracted from the axle near the axle fracture as labeled in Fig. 3. Sample preparation was performed by the classical methods of grinding and polishing. Etching was done with 3% Nital solution. The axle material has a lamellar ferritic-perlitic microstructure, banded in the longitudinal direction as shown in Fig. 6. The content of non-metallic inclusions was determined by the comparison with reference charts, according to the standard ISO 4967. The presence of non-metallic inclusions of the A, B, C and D type was detected. Their sum for A, B and C type was not greater than specified by standard EN 13261:2003. But in the samples from locations near the axle surface and from axle center, inclusions of type C (silicate) and type D (globular oxide) were identified in the amounts above allowed specifications. Corrosion pits were observed in the material surface layer in the locations of the transition radius zone labeled as AR in Fig. 4, in the vicinity of the cross-section where the axle fracture occurred. Characteristic corrosion pits are presented in Fig. 7. Cross-sectional shape of analysed pits could be defined, according the standard ISO 11463:1995, as wide and shallow in Fig. 7a and elliptical in Fig. 7b. Size of registered corrosion pits ranged from approx. 25 to