2.4 | Test conditions and CCF test system
The conditions of CCF tests are listed in and the test load spectrum is shown in Figure 5(a). Since it is difficult to determine the value of the high-cycle loads, CCF tests are carried out at several vibration amplitudes in this study. According to the experimental data under different vibration amplitudes, the σH-N curve (in which σH represents the vibration stress and N represents the CCF life expressed by high-cycle fatigue cycles) can be obtained. The last column of Table 1 shows the number of specimens under different load conditions. 3 or 4 blades are tested to consider the dispersion at some specific vibration amplitudes. All the blades are tested until fracture.
One CCF test system shown in Figure 5(b) is established. It mainly consists of the servo-electric fatigue tester, clamp, electromagnetic induction heating equipment and electromagnetic vibration exciter. Figure 5(c) and (d) show the clamp which can apply the LCF load and HCF load to the blades simultaneously without interfering with each other. The low-cycle load is not applied to the blades directly. Instead, it is applied to the top of the outer clamp and transfers to the blades through the inner clamp along the load path (the red arrow in Figure 5(c)). The electromagnetic vibration exciter, which generates the high-cycle vibration load, is connected to the inner clamp. There are two bearings between the outer clamp and inner clamp in order to reduce friction and ensure that the inner clamp can vibrate freely.
In addition, it is significant to ensure the relationship between the vibration stresses of the critical position and vibration amplitudes. This aim is to control the vibration stresses of the critical position by controlling the vibration amplitudes during the CCF tests. The FE method is applied to calculate the vibration stress distribution of the critical position at different vibration amplitudes. Figure 6 indicates that there is a linear relationship between the vibration stresses of the critical position and vibration amplitudes.