13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Fretting fatigue analysis on nuclear fuel cladding tubes Lichen Tang, Shurong Ding, Yongzhong Huo* Department of Mechanics and Engineering Science, Fudan University. 220 Handan Road, Shanghai 200433, China. * Corresponding author: yzhuo@fudan.edu.cn Abstract Grid-to-rod fretting failure due to fuel rod vibration remains as a significant cause of nuclear fuel failure in pressurized water reactors (PWRs). In order to carry out fretting fatigue tests to tube shaped specimens, a special designed apparatus was developed. The strain-life curve of the Zr-4 tube specimen under some given fretting contact pressure is obtained. It is found that the fatigue life of the Zr-4 tube under fretting contact conditions can be substantially lower than its life without fretting. The fretting wear scars and fatigue cracks were analyzed by microscope. The results show that the initial crack does not occur at the middle of the contact area where the wear depth reached the maximum, but at the edge of the initial contact area. where the amplitude of stress is confirmed to reach the maximum by FEM simulation. The position of crack nucleation can be predicted by half-length of the initial contact region and the slip displacement. Keywords: Fretting fatigue test, Tube specimen, Nuclear fuel cladding tube, Fretting fatigue nucleation. 1. Introduction Fretting is a special wear process that occurs at the contact area between two materials under loads and subject to minute relative motions by vibrations or some other forces. Fretting decreases the fatigue strength of materials operating under cycling stresses [1, 2]. The fretting fatigue phenomenon is very complex because there are too many factors that can affect the fretting behavior of materials [3]. A number of studies were taken to analyze the effects of various parameters on the fretting damage, such as the contact pressure [4-6], the amplitude of slip displacement [4], the coefficient of friction [5] and the nature of contact [7, 8]. For various materials, both experimental and numerical methods are taken to analyze the fretting damage [6, 9-11, 15]. Bridge-type fretting pads were used in many fretting fatigue experiments to produce contact in two places on each side of the specimen [6, 13, 14]. The pair of bridges and its load cell had to be hung on the specimen during the test by friction, thus a large clamping force and a flat contact were a must. Petiot [9] bolted one foot of the bridge to the end of the specimen and reduced the clamping force to a value of 100N. Similar apparatus were developed for the cases of point contact, such as cylinder-cross-cylinder and sphere-on-flat. The advantages of two arrangements of specimens and fretting bridges were discussed by Waterhouse [17]. Grid-to-rod fretting failure due to fuel rod vibration remains as a significant cause of nuclear fuel failure in pressurized water reactors (PWRs) [16]. The fuel rods are supported by the friction forces between the fuel rods and the springs or dimples of the grids in a fuel assembly as shown in Fig.1. When the reactor operates, the coolant flows through the surface of the fuel rods, and causes a flow-induced vibration (FIV) phenomenon, which is considered as the main driving force for fretting damage of the fuel rods. Fretting wear experiments of nuclear fuel cladding tubes were carried out by Kim [7] to study the effects of contact forces and slip displacements on fretting wear
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