Fretting Fatigue Testing Methodology Incorporating Independent Slip and Fatigue Stress Control

摘要

A fretting fatigue apparatus was designed, built and tested incorporating independent computer control of fretting fatigue slip distance and fatigue stress. This was accomplished through the utilization of two coaxial servo-hydraulic test actuators controlled in real time by computer. The central hydraulic actuator applies the fatigue load to the test specimen, while the outer concentric hydraulic actuator applies the fatigue load to the test specimen, while the outer concentric hydraulic actuator moves the fretting pin carrier apparatus. Independent control of slip displacement is achieved with the use of a capacitance displacement gage attached to the specimen fret pin carrier in such a manner that relative displacements of <5 mu m can be controlled. Capacitance gage measurements indicate the relative motion of the fatigue specimen surface caused by loading with respect to the fret pins. The fret pin carrier is subsequently moved to accommodate this motion plus its own-programmed motion. Load cells are provided both above and below the fatigue specimen allowing for measurement, by difference, of the forces applied by the fretting pins. These forces can be used to calculate the dynamic coefficient of friction during test operation. Finally, a 3-D finite element analysis model was constructed of the fatigue specimen and the fret pins to determine analytically the slip occurring at the fatigue specimen surface within the bounds of the test operation.