PSI - Issue 17

D.G. Papageorgiou et al. / Procedia Structural Integrity 17 (2019) 532–538 D.G. Papageorgiou, H. Bravos, C. Medrea / Structural Integrity Procedia 00 (2019) 000 – 000

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rolled. The remained stresses to the parts is raised to specific areas due to cold working during bending and thread rolling (Neves and al. 2014). It was showed that stress concentration inhibits the performance (Tawk and al. 2014), decreases fatigue strength and making fixtures more prone to erosion (Weng and White 1990).

Fig. 2. (a) Detail of the fan blade support system during maintenance, (b) Failed U bold.

Three pieces were analyzed. The first broke to the area where the holder is bolted with the base. (Point 1 on Fig. 3a), after extended plastic deformation. Wear marks were observed due to contact either with the base (point 2 on Fig. 3a) either with the saddle-like aluminum receptor (Point 3 on Fig. 3a), there are indications of extensive vibrations. It is probably caused by an uneven tightening of the bolts during setup. The fracture surface has three distinct areas, characteristic of a fatigue fracture. The crack initiated from a circumferential notch of the thread presenting ratchet marks (Point 1 on Fig. 3b). It was propagated to the inner area of the diameter. Initially, the surface was smooth and gradually propagated with increasing roughness. The detachment area was limited (Point 2 on Fig. 3b). The absence of beach marks suggests fracture of low cycle fatigue. The overload zone located just opposite to the crack initiation point indicates the absence of torsional stresses. It is characteristic pattern of a round bar fracture due to low nominal tensile stresses, under severe circumferential notch (Stein and Makris 1993).

Fig. 3. (a) Images showing the first piece as received (b) General aspect of the fracture surface.

The second part break in two opposite points after limited deformation to the left and to the right side (Fig. 4a). On the left side, the fracture area is quite similar to that of the first holder. The crack initiated to the left side by a “V” point (point 1 on Fig. 4b). Radial ledges were initiated to the diameter inner area and propagated towards the right side of the bar. A relatively small detachment area is observed (point 2 on Fig. 4b). Fracture initiation and the detachment area are on the opposite sides of the round cross section due to low tensile stress (Fig. 4b) (Stein and Makris 1993). Limited beach marks are evident of a fatigue fracture. Just after the breakage of the left side, high stresses were exerted to the right side of the U bolt, exceeding the ultimate strength of the material. A crack was propagated instantly (point 1 on Fig. 4c) which leads to the final detachment of the bar (point 2 on Fig. 4c). The relative positions of crack initiation and the detachment point are in acute angle, which is characteristic of the presence of torsional stresses.