Structural Integrity Analysis and Life Estimation of a Gas Turbine Bladed-Disc

Abstract Turbine blades in an aero-engine are subjected to severe conditions of high temperature and pressure, which cause high levels of stress leading to crack formation and subsequent failure in service. We have investigated the influence of crack on vibration parameters of a typical aero-engine gas turbine blade and have described a life assessment approach for blades and bladed discs. A typical transport aircraft AMT (Accelerated Mission Test) cycle has been utilized for getting operating parameters. Material data is taken from tests conducted on specimens extracted from turbine disc of a transport aircraft. Initial studies are carried out on idealizations involving cantilever beams with uniform cross-section; the procedures are then extended to free-standing turbine blades with asymmetric airfoil cross section mounted at a stagger angle on a rotating disc. Dynamic characteristics of the blade are estimated and free vibrations analysis has been carried out for healthy blades and those with cracks of different sizes. Influence of crack size on natural frequencies and mode shapes is studied. Results show a difference of less than 1% in frequency for cracks less than 1mm in length; for larger crack lengths the frequency shifts are higher. Analytical results are compared with experimental tests on a Laser Doppler Vibrometer set-up. Subsequently, forced vibration analysis is performed and a methodology, using Lazan’s law, is developed to extract modal damping ratios from the strain energy of the blade under nozzle excitation pressure fluctuations. Modal damping ratios, thus obtained, are indicative the energy dissipation in the component under such stress conditions. The ratios show differences of the order of 5% between healthy and cracked blades for the second mode. These observations lead illustrate that modal damping has strong correlation with blade structural integrity. The possibility of employing these modal damping ratios as indicators for the presence of cracks / defects is discussed.