Experimental investigation of long-term fretting wear of multi-span steam generator tubes with U-bend sections

Accurate prediction of the growth rate of fretting wear damage of steam gen erator (SG) and heat exchanger (HX) tubes is needed for safe, reliable and economical operation of the nuclear power plant. The methodology currently used to predict the long-term fretting wear behaviour of the tube is based on combining the short-term wear data base with non-linear finite element i mpact simulation of the tube/support dynamic interaction. The main objectiv es of this investigation are: (a) to generate long-term experimental data t hat will be used to verify this methodology, (b) to investigate whether the fretting wear of a multi-span SG tube with a U-bend section is self-limiti ng, and (c) to examine the dynamic interactions between adjacent tube suppo rts. An experimental set-up has been developed to simulate the fretting wea r behaviour of a full scale multi-span SG tube with U-bend section. Straigh t and U-bend tube supports were instrumented for in situ measurement of the tube orbital motion and impact forces, On-line calculation of the energy d issipated at the tube/support interface in the axial and out-of-plane direc tions was performed. Analysis of the results indicated that the increase in the tube/support clearance results in a non-monotonic change in the rate o f energy dissipated in the wear process (work rate). This behaviour, which suggests that the fretting wear behaviour of a multi-span tube could be sel f-limiting when the forcing function is invariant, is in agreement with com puter-simulation predictions. The results indicated also the strong dynamic interaction between adjacent supports. It has also been concluded that the specific wear rate coefficient is not constant, and depends on the magnitu de of the work fate. For the work; rate range investigated, the wear rate c oefficient at the U-bend section decreases and approaches an asymptotic val ue, as the work rate increases. Analysis of the results also showed that th e ratio between the maximum and average wear depths, R-h = h(max)/h(av), de creases with the increase in h(av). (C) 1999 Published by Elsevier Science S.A. All rights reserved.