PSI - Issue 17

Shahnawaz Ahmad et al. / Procedia Structural Integrity 17 (2019) 758–765 Shahnawaz Ahmad/ Structural Integrity Procedia 00 (2019) 000 – 000

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specimens extracted from turbine disc of transport aircraft. The structural analysis shows very high stresses at blade roots. The centrifugal stresses were found to be 243 MPa at 10400 rpm and 340 MPa at 12300 rpm, while the stresses due to the steady component of the flow field were found to be 70 MPa (Figure 7).

Figure 5Accelerated Mission Test cycle for the Gas Turbine Engine Figure 7Centrifugal and steady flow stress (at blade root) During acceleration from an initial speed to a final speed the rotor may pass through several critical speeds. While under acceleration the flow path forcing function can be expressed as, ( )   2 2 0 0 6 0 { }cos { }sin 2 2 aerodynamic m s m s m m t t F t F F mn t F mn t     +     = + + + +           (4) where m is the harmonic number (1-6 generally), 0  is the initial rotating frequency (in rad/s), and  is the acceleration of the rotor (in rad/s 2 ). This unsteady transient loading with the harmonic components that were obtained from the flow path excitation force evaluation, was applied on the blades, in ANSYS Graphical user interface environment. Acceleration values were obtained from the operating cycle shown in Figure 5. Stresses computed for this transient numerical problem with accelerating periodic forcing are shown in Figure 8. Figure 6Zoomed view of the take-off cycle

Figure 8 Alternating Stress Amplitude for the operating cycle

Figure 9 von Mises stress at Blade root as rotor goes from 0 to 10400 rpm

Figure 10 von Mises Stress at Blade root as rotor goes from 10400 to 12300 rpm

Stress amplitudes at the blade root near the fir-tree section, at critical speeds that the rotor passes as it accelerates from rest to its operating speed can be seen for 0-10400 rpm in Figure 9 and for 10400-12300 rpm in Figure 10. These stresses are used in the life estimation algorithm. 2.3. Life prediction algorithm The life estimation algorithm is developed based on the combination approach by Dowling [7]. It combines the local strain concepts to predict the crack initiation life and fracture mechanics principles to predict the propagation life. The combinational approach based on strain-life concepts for prediction of initiation life avoids the inherent difficulty that would have been faced in the application of LEFM (Linear Elastic Fracture Mechanics) to describe the short crack behavior at the defect root. The total life of a component consists of two parts- initiation life and propagation life.