3.2. Stress analysis
Modal analysis was carried out to examine the dynamic behavior of blades in service. Natural frequencies of the system and relevant mode shapes were calculated. The analysis showed that the first natural frequency was 57.1Hz; the other three frequencies are 57.4Hz, 89.8Hz and 285.5Hz. The first natural frequency of the blade was higher than the blade’s rotation frequency of 26.7Hz (1600 rpm), so it would not cause resonance. The other three revolutions also carried out modal analysis, but did not resonate.
The finite element method was used to calculate centrifugal stress. In this study, the stress value were 214.12 Mpa, 235.20 Mpa, 258.30 Mpa, 282.41 Mpa and its corresponding X-direction stress, revolving speed were shown in Table 3. The aerodynamic stresses were normally very low, and the model was used for approximate aerodynamic analysis of blade root section. To estimate the pressure along the blades at 1600 rpm, the aerodynamic stress of the blade was calculated. Matching the maximum obtained stress value of 214.12 Mpa, at the root area of blades where failure had occurred as shown in Fig.5. According to analytical and the finite element model calculated, the magnitude of aerodynamic stress were 4.3 and 4.4 Mpa, respectively.