Evaluation of J-Integral and Fatigue Crack Growth Rate of Semi-elliptical Crack in Compressor Blade

Abstract

Gas turbine compressor blades are subjected to centrifugal, gas bending and vibratory loads. This repeated loading and unloading can reduce the life of compressor blades. This research aims at the estimation of fatigue crack growth rate of various three-dimensional cracks in a compressor blade considering the J-integral calculations for a semi-elliptical crack, subjected to centrifugal loading. Static stress analysis was carried out to ascertain the critical region or crack zone of the blade. The maximum Von-Mises stress was found at the fillet region near the root of the blade, the predicted state of stress has helped in identifying the region of singularity which may lead to crack initiation. Finite element method was used to evaluate the range of J-integral solutions in the blade with a semi-elliptical flaw. Semi-elliptical crack lengths ranging from 0.5 to 3 mm were considered in the crack zone. J-integral was evaluated for rotational velocity of 10,000 rpm. The J-integral range obtained is employed to predict fatigue crack growth rate by using Paris law. Fracture module in Ansys workbench was used for evaluation of J-integral Mode IІ at the surface interception point varied from 3.6527 to 45.1071 MPa√m. Fatigue crack growth rate at the crack length was determined and it was found that fatigue crack growth rate at the surface interception point increases with increase in crack depth. Fatigue crack length growth rate increased from 1.20015E-4 to 4.8460E-3 m/cycle of b = 0.5 to 3 mm for 10,000 rpm. Also the fatigue crack growth rate estimated may be large, since the effects of crack closure were not considered. The obtained solution is limited by contour integral evaluation for defined crack geometries and its orientation or crack propagation direction. Stress field at the crack tip is a function of crack geometry and orientation. With the rotational velocity of 10,000 rpm, the behaviour of fatigue crack length growth rate was estimated. It was concluded that at the surface crack interception point, fatigue crack growth rate increases with increase in crack depth.

Keywords: Damage tolerance design, fatigue, stress intensity factor, singular element