Fretting stresses in single crystal superalloy turbine blade attachments

Single crystal nickel base superalloy turbine blades are being utilized in rocket engine turbopumps and turbine engines because of their superior cree p, stress rupture, melt resistance, and thermomechanical fatigue capabiliti es over polycrystalline alloys. High cycle fatigue induced failures in airc raft gas turbine and rocket engine turbopump blades is a pervasive problem. Blade attachment regions are prone to fretting fatigue failures. Single cr ystal nickel base superalloy turbine blades are especially prone to frettin g damage because the subsurface shear stresses induced by fretting action a t the attachment regions ran result in crystallographic initiation and crac k growth along octahedral planes. This paper presents contact stress evalua tion in the attachment region for single crystal turbine blades used in the NASA alternate advanced high pressure fuel turbo pump for the space shuttl e main engine. Single crystal materials have highly anisotropic properties making the position of the crystal lattice relative to the part geometry a significant factor in the overall analysis. Blades and the attachment regio n are modeled using a large-scale three-dimensional finite element model ca pable of accounting for contact friction, material anisotropy, and variatio n in primary and secondary crystal orientation. Contact stress analysis in the blade attachment regions is presented as a function of coefficient of f riction and primary and secondary crystal orientation, Fretting stresses at the attachment region are seen to vary significantly as a function of crys tal orientation. The stress variation as a function of crystal orientation is a direct consequence of the elastic anisotropy of the material. Fatigue life calculations and fatigue failures are discussed for the airfoil and th e blade attachment regions.