PSI - Issue 60

V. Venkatesh et al. / Procedia Structural Integrity 60 (2024) 372–381 V Venkatesh et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Table 1. Results of the semi-quantitative composition analysis of the control rod

Composition, wt.%

Element

Specification

Control rod

Al

5.50-5.75 3.50-4.50 0.30 max 0.20 max 0.05 max Balance

5.8 4.2 0.1

V

Fe

O N

- -

Ti

Balance

Fig. 11. Microstructure of the non-rotating control rod: (a) optical micrograph and (b) secondary electron image

4. Discussion Analysis of the crack origin region revealed that the fatigue crack initiation in the control rod was primarily due stress concentration caused by deep machining marks on the surface. Numerous fatigue cracks were observed to have initiated along the machining marks. Once initiated, the cracks that were favorably oriented with respect to the stress direction have joined together forming a single crack front. This crack front then propagated through the thickness of the bearing housing at the eye-end, eventually resulting in fracture. The fracture propagated along a machining mark over a distance on either side of the crack origin. Incipient cracks were also found at the trough of the remaining part of this machining mark and also in adjacent machining marks. On the unbroken side of the housing, incipient cracks were observed on the fillet surface at similar locations as those on the fractured side. The collar regions of the bearing housing at the eye-end of the control rod experience high bending stress, and these locations are vulnerable to fatigue crack initiation under application load. Fillet radius and surface finish are the two most important parameters that determine the fatigue life of this component. In the present case, the machining of the component was unsatisfactory and resulted in generation of stress concentrators on the surface in the form of deep mark/steps. Examination also revealed flow of material along these deep machining marks. The machining marks and flow of material appears to have occurred because of either use of blunt tool or selection of inappropriate machining parameters. In addition to the machining related factors described above, fretting damage was observed on the bearing housing inner surface at two symmetrically opposite locations. Fretting damage is known to induce premature fatigue crack initiation in engineering components (Pujatti et al., 2014) (Chao, 2019) (Bill, 1983). Although fretting damage was