Structural integrity assessment of turbine disk on a plastic stress intensity factor basis

Abstract Based on a new fracture mechanics parameter, this study is concerned with assessing the integrity of a cracked steam turbine disk operating under startup-shutdown cyclic loading conditions. Damage accumulation and growth in service occurred on the inner surface of the key slot. To determine the elastic-plastic fracture mechanics parameters, full-size stress-strain state analysis of the turbine disk was performed for semi-elliptical cracks under startup loading conditions. As a result, the distributions of the elastic and plastic stress intensity factors along the crack front in the key slot of the turbine disk were defined according to the surface crack form. An engineering approach for predicting the lifetime of cracked turbine disks is proposed. The predictions of the crack growth rate and residual lifetime of the steam turbine disk are compared for elastic and elastic-plastic solutions. It is shown that the previously proposed elastic crack growth models overestimate the lifetime with respect to the model presented in this research. The expediency of using the plastic stress intensity factor to characterize the fracture resistance as the self-dependent unified parameter for a variety of turbine disk configurations rather than the magnitude of the elastic stress intensity factors alone is also discussed.