Stresses in turbine-blade tenons subjected to bending

Abstract

The photoelastic method was used to model large steam turbine tenon-shroud attachments under bending loads. Six models were used to investigate three basic tenon geometries: (a) single round tenon—here two different fillet radius-to-tenon diameter ratios were examined; (b) long narrow tenon—for this geometry the influence of shroud-seating clearance and shroud stiffness was investigated; (c) two separated round tenons. Stress-concentration factors for the tenon fillets were determined based on the nominal bending stress in the tenon using the moment of inertia of the tenon cross section. For the single round tenon, stress-concentration factors of 1.3 and 1.6 were found for fillet radius-to-tenon diameter ratios of 0.41 and 0.19. These compared very well with those values obtained by treating the geometry as a stepped round bar with a shoulder fillet subjected to bending. The long-narrow-tenon geometry showed a higher stress-concentration factor than the two separated round tenons—6.1 compared to 2.9. Increasing the shroud stiffness reduced the stress-concentration factor for the long-narrow-tenon design.