The effect of coupled wear and creep during grid-to-rod fretting

The initial stages of grid-to-rod fretting (GTRF) are associated with stress-relaxation. Both creep and wear contribute to this process. These mechanisms act in concert, and are influenced by each other. The development of a strategy to couple creep and wear is important for the numerical modeling of gap formation in a pressurized-water reactor (PWR). However, the characteristic time scales for the two processes are very different, which can cause numerical problems. In this paper, an approach is presented to develop reasonably efficient, yet accurate, models that couple the two processes. This approach is used for a numerical analysis of gap formation during grid-to-rod fretting. In this analysis, the effects of wear are integrated with the effects of creep relaxation, providing insight into the relative roles of the two mechanisms in this particular application. During the early stages of gap formation, the fluid-induced excitation forces are not large enough to cause macroscopic slip and wear across the entire interface. As a result, creep, rather than wear, is the dominant mechanism that dictates the evolution of the contact stress. Creep contributes to stress relaxation both through the creep-down of the cladding onto the fuel, and by local stress relaxation at the contacts. Although localized wear always occurs at the edge of the contacts, the effects are so small that this can be considered to be an incubation period for wear. Eventually, the contact force relaxes to such an extent that slip occurs over the entire contact between the rod and grid, and wear becomes the dominant relaxation mechanism. The simulations demonstrating these concepts explored different pressure fluctuations, friction coefficients, wear coefficients, and initial interference, to show how these different parameters affect the wear profile and the time at which contact is lost between the rod and grid. (C) 2017 Elsevier B.V. All rights reserved.