Development of the Measurement Technology of Fretting Wear Test Parameters under the Environment of High Temperature and Pressure - Final Report -

This report is dealing with the results of the development procedure of a load cell that could be applied to the fretting wear tester in high temperature and pressure water condition in order to measure the contact force between the nuclear fuel rods and their supports. The strain gage of WK-06-062TT-350 was used with considering the temperature limits and the economical concerns in this study and two rosette pairs were bonded on opposite sides with a 180 .deg. C angle between each other to establish the Wheatstone full bridge circuit. To design bi-axial load cells with good performance in a high temperature and pressure water condition, we should preferentially consider the shape and size of a force sensing element and the waterproofing method at the same time. Because of a limited test section within the autoclave of the fretting wear tester, the load cell should have a relatively small size. In addition, strain gages and their lead wire should be shielded against high temperature water. A simple cylinder type is adopted for the shape of the force sensing element in consideration of easy waterproofing. This force measurement system consists of a force sensing element, a cylinder type cap with a hole, a base case, a force transfer screw, two metallic sealants, tube connector and metallic tube. In this study, a fluoro plastic-coated Nickel-plated Copper wire was used as signal wires for the purpose of the cable shield against the metallic tube in which the wires are inserted. Also, a metallic sealant that was made from Silver-coated Copper was used for waterproofing between the force sensing element and the cylinder type cap in the high temperature and pressure water condition. This enables an easy fabrication without a welding process for waterproofing. From the strain analysis result of the sensing element, the estimated coupling error between the normal and shear component is less than 4%. We determined the location of the strain gages based on the above result. It is determined that the capacity of our bi-axial load cell designed in this study is 50 N in each axis. In this bi-axial load cell, a lower strain of 40 μm/m at the maximum capacity is used to prevent its yielding by excess pressure or crack generation/fracture by fatigue in the high temperature and pressure water condition. From the application result in the high temperature and pressure water condition, both normal (impact) and shear force signals are clearly appeared and it is possible to examine the major mechanism of nuclear fuel fretting phenomenon