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In the present paper, vibration responses of the healthy and cracked blade under the influence of stress-stiffening and spin-softening have been studied. The three-dimensional finite element-based numerical analysis is carried out to study the blade's mode shape and natural frequencies and are validated with the experimental modal analysis test results. In the case of spin softening, for healthy blade at the turbine’s speed over 1000 RPM, natural frequencies increase significantly in both first flap-wise and chordwise bending modes. In both cases of STF and SOF, for the cracked blade, the deviation from the healthy blade in the natural frequency increases with increasing crack size, maximal in a crack size of 90%. It is also observed that irrespective of crack size, deviation in natural frequencies as compared to healthy blade increases as spin speed increases in the cases of chordwise bending mode (Mode II) and higher-order overlapped mode IV, and in the case of fundamental torsion mode (Mode III), higher-order overlapped mode V and VI natural frequencies decrease in both STF and SOF cases.

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In the present study, the dynamic behaviour of the last stage low-pressure steam turbine blade with Fir Tree root at different conditions of blade root flank faces and their interfaces with rotor groove have been analyzed. Modal analysis has been done using a finite element approach to evaluate natural frequencies and evaluation of Campbell diagram generated under these conditions. For this, both healthy and defective blade have been taken. Since, the variable crack size of fir tree root flank has been taken, hence the excitation pattern has been evaluated due to stiffness variation of the cracked blade. This analysis provides the basis of excitation pattern of cracked blades due to inherent character and critical stressed zone. The outcome of this study forms the guidelines and checks during the fitting of blades in rotor assembly and its checks during health audit, overhaul, Overspeed Balancing test and frequency turning.

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