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.

Assessment of Substrate and TBC Damage Effects on Resonance Frequencies for Blade Health Monitoring

“…At this same temperature however, the natural frequency of mode 5 decreases by approximately 1810 Hz as the notch size progresses to a depth of 4 mm at midspan. This is the largest notch size considered in this study, with deeper midspan notches resulting in infinite deflection indicating potential blade fracture (van Dyke & Nganbe, 2021). The drop brings the mode 5 natural frequency to 18086 Hz, representing about 9.1%, still 1136 Hz higher than the estimated excitation frequency.…”

“…Similarly, notches with 1.5 mm tip radius are introduced on the pressure side of the blade. This required a further refining of meshes around the cracks as described in an earlier publication (van Dyke & Nganbe, 2021). As seen in Table 1 for a 3 mm deep crack for example, the estimated natural frequency values are stable beyond 21909 nodes, which is used in this work.…”

“…Threedimensional damages in the form of notches with circular tip are then also introduced. They are created with varying depths and positioned at different distances from the blade root using the cut revolve feature in Abaqus as described earlier (van Dyke & Nganbe, 2021). Finer meshes are used around notches similar to the two-dimensional models, leading to increased element counts depending on the type and size of the defects.…”

“…Its inter-turbine temperature before the power stage is approximately 785°C and the exhaust temperature is between 565-600°C. Its further characteristics and main operation parameters are described in detail in the 2000 Pratt and Whitney Canada Large PW100 Series Training Manual and in earlier publications (van Dyke & Nganbe, 2021;Hosking et al, 1999;Saravanamuttoo, 1987). Cruise loading condition at 85% of the maximum propeller shaft rotation speed of 1200 rpm is taken as basis for the simulation.…”