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.