Numerical assessment of blade deflection and elongation for improved monitoring of blade and TBC damage

Abstract

The reliability of turbine blades is largely maintained by damage tolerance approach based on monitoring and pre-set periodic inspections. This can result in unnecessary downtimes, premature part retirement and unforeseeable failures. Therefore, there is growing interest in systems that can reliably detect damages in real‐time. However, many current sensors are based on blade tip clearance and time of arrival. The first primarily correlates with relatively predictable long-term creep deformation and ensuing blade elongation, while the second can be related to blade deflection. Therefore, this research comparatively assesses the two parameters. For this purpose, TBC defects, representative for coating spallation, and notches, representative for blunted blade cracks, are investigated. Overall, the results suggest that the measurement of changes in axial deflection could show higher sensitivity to cracks and TBC defects, and therefore, constitutes a potential alternative for continuous monitoring with respect to unforeseeable rapidly growing blade damage. Moreover, TBC spallation seems more difficult to immediately detect as the ensuing changes in blade tip position are small. However, they cause changes in deflection that can switch from negative to positive as they are located closer to the blade root, which may allow to assess their location during monitoring. In contrast, critical cracks located close to the blade root can cause measurable changes in blade deflexion, potentially making their timely detection and continuous monitoring possible.