PSI - Issue 13

Ulewicz R. et al. / Procedia Structural Integrity 13 (2018) 2164–2169 Ulewicz R., Nov ý F., Novák P., Palček P. / Structural Integrity Procedia 00 (2018) 000 – 000

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3

Table 3 Mechanical properties of the draw hook prescribed by the standard TNŽ 28 2612

YS 0.2 offset [MPa]

Elongation [%]

K U [J]

Steel 47 Mn standard TNŽ 28 2612

UTS [MPa]

735 - 845

-

11

29

2.2. Failure characterization Characterization of the failed draw hook was carried out in two steps. Firstly, a visual inspection was conducted and subsequently, the fracture surface was characterized by the fractography analysis using the scanning electron microscopy (SEM). The failure occurred after fracture from the pre-existed fatigue cracks, as is clearly shown in Fig. 2. The visual inspection revealed that the failure occurred in the transition region, where the rectangular cross-section is fluently changed to the cylindrical shape. The TNŽ 28 2612 standard prescribes transition radius of r = 2.5 mm for this area. Measurement of mentioned radius carried out on the profile projector showed, that the radius in the examined draw hook was not larger than r = 1.5-1.65 mm. Figs. 2c and 2d show two independent primary fatigue cracks, propagating from the transition radius. For both primary fatigue cracks, several initiation places were observed. The character and the position of the fatigue crack initiation places suggest that the examined draw hook was not loaded in pure tension loading conditions and this fact is examined in more details in the following sections. The fractography analysis revealed transcrystalline manner of the fatigue crack propagation (Fig. 3a) with occasional occurrence of the intercrystalline facets. Area of the final fracture exhibited mostly the transcrystalline cleavage facets, with a small fraction of intercrystalline facets. The fracture surface did not show any evidence of the strong oxidation, which implies that the operation time, between the fatigue crack initiation and the final failure, was not long. Examination of the whole draw hook assembly revealed the heavy wear damage of the housing of a linear casing. Surprisingly, the upper part of the linear casing was heavily worn, while the bottom part was almost undamaged (Fig. 4). The further analysis explained the observed damage as a result of the long time operation of the cars with the wheels turned to a minimum allowed diameter (984 mm). A sequence of the cars with the lowest and the highest allowed wheels diameters caused that the vector of the force, acting on the draw hook, was not coaxial with the axis of the draw hook's assembly. This explained the observed strong wear of the upper part of the linear casing and the suggested bending loading, as well.

a)

b)

c) d) Fig. 2 a, b) Overall view of the fractured draw hook. c) View of the fracture surface, the arrows indicate several places of the fat igue crack initiation. d) One of the primary fatigue cracks in the detailed view.