4.2. Fan blades fatigue test
The fatigue tests were carried out in the form of trapezoidal wave with a loading frequency of 0.04 HZ, as shown in Fig. S3, under normal temperature and atmospheric conditions. Stress ratio (R =0) was conducted in all tests. The stress range measured by the test system was 0~5000 Mpa, and the range of rising speed at this frequency was 0~3000r/min. The actual working condition of the fan was simulated to the maximum extent. In order to ensure that each revolving speed range could be accelerated to the specified speed, the differences of accelerations at different speeds were taken into account.
Experiments were carried out based on the predicted model, and fatigue life tests were conducted on fan blades under four different levels of constant stress. The starting and stopping test was carried out at 1600 rpm firstly, and then the 1800, 2000 and 2200 rpm tests were done. There were at least four sample fans were used for each set of fatigue life test. Then, cumulative fatigue tests were carried out at four levels of 1600~2200 speed and at least 4 sample fans were used in the fatigue life test. In order to discover the generation of fatigue crack, the response of each critical section and dangerous part was monitored and observed at different test time during fatigue test. The fatigue crack propagation was observed and recorded. A crack with a length of 6mm appears on the surface near the root of the connecting end of blade and impeller, indicating impeller failure, as shown in Fig. S4.