PSI - Issue 14

Sudheer Patri et al. / Procedia Structural Integrity 14 (2019) 688–695

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Sudheer patri et. al./ Structural Integrity Procedia 00 (2018) 000–000

assembly was not smooth and over travel was also observed. Upon examination by borescope, failure of all the screwed fasteners of the joint was observed. The failed joint could be accessed after dismantling of upper housing of the mechanism; the photograph of failed joint is shown in Fig. 2. Some screw washers can also be seen in the figure. The total upper plate got twisted with respect to the bottom support plate. 4 nos. of screws fell into the upper part and got trapped in it. Dismantling of the mechanism was required for retrieving all the failed screws. The failed joint is located along the load transfer path of the mobile assembly to the upper part support structure and is loaded mainly in compression. Thus, during normal operation, the screws are subjected to bolt preloads only. The joint is also subjected to nominal torque during the rotation of the drive screw, which would be resisted by the friction between screw heads and the mating plate.

Fig. 2. Photograph of the failed joint of SNDHM

4. Failure analysis The photograph of one of the failed screw is shown in Fig. 3. The fracture surface is inclined by ~45° to the axis and accompanied by plastic deformation. All the screws fractured in the threaded region and the axial location at which the failure occurred is same for all the screws.

Fig. 3. Photograph of the failed screw