# Fretting Fatigue Experiment and Finite Element Analysis for Dovetail Specim at High Temperature
## 2 Experimental Details
### Procedures of Fretting Fatigue Experiment  
The dovetail attachment in an aero-engine is subjected to the fluctuating centrifugal and tangential forces originating from the aerodynamic impact [32]. However, compared with the centrifugal force, the tangential force caused by the airflow impact is much smaller, which is usually simplified, and the fretting fatigue damage is mainly caused by the centrifugal force [20]. Therefore, a uniaxial tensile fatigue machine was used to perform the fretting fatigue test shown in Figure 3. The dovetail configuration tested in the present work is similar to the one used by [PERSON] [19], but the fretting pads are redesigned to better adapt to the testing condition. The assembled attachment structure shown in the blue dashed wireframe of Figure 3 is fixed on the test apparatus through the circular pins on the top and bottom, respectively. The loads applied rely on the DEXKCYQ DPL9010 servo-hydraulic system, providing a maximum tensile force of 100 kN. When the axial fatigue load is applied on the dovetail configuration, both contact surfaces of the dovetail specimen joint on the fretting pads simultaneously. It should be reminded that the contact torque induced by the reaction force can be ignored [12].  
Figure 3: Test equipment for fretting fatigue.  
Figure 2: Dimensions of the specimen. (**a**) Dovetail specimen; (**b**) clamp; (**c**) fretting pad specimen. (Unit: mm).  
The fretting test was carried out at a uniform temperature of 630 \({}^{\circ}\)C and the electromagnetic induction coil is capable of heating an elevated temperature to a maximum of 1000 \({}^{\circ}\)C. To achieve uniform heating of the assembled dovetail specimen, a heating system of electromagnetic induction coil was designed and manufactured marked by the red wireframe in Figure 3. The heater consists of two heating coils, the diameter of which is 20 cm. The real-time temperature is monitored by 4 K-type thermocouples welded on the dovetail specimen and is maintained constant through a feedback adjustment mechanism of the heating system.  
Four load conditions were performed for the test and the specific parameters are shown in Table 3. The load ratio is 0.05 with an alternating load of 1 Hz. Before the dovetail specimen was fixed to the clamp, the surfaces in contact with the fretting pads were cleaned using acetone to remove the oil and impurities. After all the specimens were fixed on the servo-hydraulic system, the electromagnetic heating coil should be turned on firstly to heat the specimen to the target value of temperature. Then the servo-hydraulic system for the fretting fatigue test was turned on and two dovetail specimens were tested at each load condition on the apparatus. The test was paused intermittently once the dovetail specimen failed, then the dovetail specimen and the two fretting pads were replaced.  
After all the specimens were tested, the contact surfaces and fracture of the dovetail specimens were observed with a Scanning Electron Microscope (ZEISS EVO18 Special Edition). The samples prepared for the SEM observation should be machined into small pieces to stick to the observation platform. The characteristics of the worn contact surface and the fretting fatigue on the failed fracture surface can be observed through the SEM. Through the observation of SEM, the results calculated from the finite element model can be compared and verified with the observed and tested results. It needs to be pointed out that the contact pressure of the current test configuration cannot be measured directly. Therefore, the numerical analysis based on the finite element method was carried out to investigate the contact pressure distribution under fretting fatigue load described in the latter section.