Structural Analysis of Interlocked Low-Pressure Turbine Blades: Influence of Contact Conditions and Pretwist Angles

The dynamic response of interlocked low-pressure turbine blades is significantly affected by the modelling of the contact region. The influence of the various hypothesis regarding the contact forces is still not well understood. Therefore, this study analyses the effect of different contact conditions on the equilibrium position and the mode shapes. Three main contact assumptions are used for the calculation of the linear mode shapes at different nodal diameters: Cantilever, where there are no contact forces in the interlock and the blades oscillate independently; Free slip, where the contact surfaces are together in their normal direction but there is no tangential force; Full-stuck, with bonded motion of the interlock contact between neighbouring sectors. The cantilever modes do not provide an accurate description of the motion of the structure, whereas a full-stuck analysis indicates that, due to the increased stiffness caused by the shroud, modal families can present a transition from edge modes (low nodal diameter) to torsion modes (high nodal diameter). For three different pretwist angles (0.5°, 1°, and 1.5°), the static equilibrium position is computed using a friction law of Coulomb type with stick/slip and gap states. The variation of the normal force with the pretwist angle is approximately linear across all considered cases. On the other hand, assuming that the contacts are bonded (full-stuck) for the static analysis, a nonlinear relationship with the pretwist angles is found, leading to unrealistic outcomes.