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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Figure 15FFT of Healthy Blade

Figure 17 FFT for blade with 2.5 mm deep crack

Figure 16 FFT for blade with 1.5 mm deep crack

Table 5Variation of Fundamental Natural Frequency with Crack Size in Hz (& % change) Natural Frequency (Hz) Healthy 1.5mm Crack 2.5mm Crack Experimental 2565 2531 (1.33%) 2490 (2.92%) Corrected 2504.8 2471.6 (1.33%) 2431.6 (2.92%) FE SIMULATION 2502.7 2476.2 (1.05%) 2426.9 (3.09) Table 6 Mode 2 Damping Ratio for Healthy and Cracked Blade Damping Ratio Beam Initial Value Iteration No. 1 2 Healthy 1.00 E-3 1.72 E-4 1.73 E-4 Cracked 1.00 E-3 1.64 E-4 1.64 E-4

5. Conclusion

A workable structural integrity analysis and life estimation procedure for aero-engine blades and bladed discs has been implemented. The stress map under flow path excitation and centrifugal stresses has been constructed for a typical AMT (Accelerated Mission Test) to test the integrity of the engine during service. The maximum stress has been observed at the blade root region, which can serve as a crack nucleation site. The life of the bladed disc was estimated using strain-life approach (initiation life) using Massing’s and Morrows hypothesis and Paris’s law (propagation life). The linear damage accumulation model by Miner was used to find the damage done to the system in one AMT cycle, which is then used to estimate the life of the bladed disc. The total life has been estimated to be 3409 operating cycles of the engine, or 10438 hours of flight time for initiating and propagating a flaw size of 0.1 mm at blade root. The variation in the natural frequency is approximately of the order of 0.25 % for each percent of crack depth/blade width ratio. These changes were estimated using FEA software COMSOL and further verified through more sophisticated measurement by using Laser Doppler Vibrometer setup. Crack size greater than 1 mm could be detected by using the vibration signature of the blade and therefore could be a good technique to detect larger cracks. However, in case of tight cracks, the shift in the natural frequency may be smaller and would require a better controlled test environment. Damping ratio calculated for healthy and cracked blade show a difference of 5.20% for the second mode. Modal damping apparently bears good correlation with blade structural integrity can be further explored as a viable tool for damage assessment.

6. Acknowledgement

The authors are thankful to Director DMRL for giving opportunity to carry out this work. They express their gratitude to officers and staff of MBG for timely help whenever required. Authors are also thankful to IIT Kanpur