PSI - Issue 60

Keshav Mohta et al. / Procedia Structural Integrity 60 (2024) 402–410 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

405

4

neighborhood come into contact. Thus, the wear is not contained to a point and spreads to nearby area locally over a period. Hence, the wear damage is assumed uniform over the entire bearing pad surface and average work rate at a bearing pad is calculated for wear assessment. Steps used to calculate the work rate from FEA results are shown in Fig. 3. For a selected bearing pad, the incremental normal work for a smaller time interval is calculated by summing up element level calculated incremental normal work. Total normal work at a bearing pad is calculated by summing up incremental work for time intervals. Average work rate per element is then evaluated after accounting for the number of finite elements on the bearing pad and duration of stabilized time history. The volumetric wear rate is evaluated based on the normal work rate as per Archard’s wear law. It may be noted that the normal work rate evaluated above is based on a small duration run, and more appropriately be called as initial normal work rate. This is because under continuous fretting, the crests of the interacting surfaces wear out gradually resulting into the larger areas from the neighborhood coming into contact, which leads to spreading of wear over the bearing pad area. In addition, it may also lead to reduced interaction between the surfaces due to increased gap. As a result of wear, the contact force between the bearing pad and PT undergo changes/ redistribution resulting in reduced wear rate. Irradiation effects on fuel also lead to increased fuel pin stiffness and smaller sliding amplitudes. Therefore, the normal work rate decreases continuously and fret depth saturates over a period. A number of analytical and semi analytical methods to account for the contact geometry adaptation during the fretting wear process have been reviewed by Argatov and Chai (2020). However, for present analysis, the gradually decaying wear rate curve as developed by Norsworthy and Ditschum (1995) has been used for its simplicity and the similarity to the application. 2.4. Estimation of Fretting Wear from Normal Work Rate

Fig. 1. Contact force and sliding at bearing pad-pressure tube (BP-PT) contact interface