Fatigue Failure in High-Temperature Single Crystal Superalloy Turbine Blades

Abstract

Fatigue failures in PWA1480/1493 single crystal nickel-base superalloy turbine blades used in the Space Shuttle Main Engine fuel turbopump are discussed. Many blades in the turbopump had small HCF cracks initiating from the leading edge radius and extending across the airfoil tip on the pressure side. Stage II non-crystallographic fatigue cracks with multiple origins were present at the core leading edge radius and extending down the airfoil span along the core surface. The longer cracks transitioned from stage II fatigue to crystallographic stage I fatigue propagation, on octahedral planes. Reasons that contributed to initiation and growth of tip cracks were the small leading edge fillet radius, the non-uniform wall thickness on pressure and suction sides, the high mean stress, and consequently low allowable alternating stress. Many blades also showed fatigue damage in the attachment regions leading to fretting induced cracking with multiple origins with stage II cracks. An investigation of crack depths on the population of blades as a function of secondary crystallographic orientation revealed that for β = 45+/- 15 degrees tip cracks arrested after some growth or did not initiate at all. Detailed 3D FE stress analysis of AHPFTP/AT SSME single crystal turbine blades subjected to rotational, aerodynamic, and thermal loads was conducted. Evaluation of stress response at the blade tip and attachment regions, as a function of primary and secondary crystal orientation, revealed that there are preferential β orientations for which crack growth is minimized at the blade tip. The FE analysis results give further evidence to the conclusion that control of secondary and primary crystallographic orientation has the potential to significantly increase a component's resistance to fatigue crack growth without adding additional weight or cost. Fretting contact stresses in the attachment region were seen to reach peak values at locations where fretting cracks have been observed. Contact stresses at critical attachment regions also varied significantly as a function of crystal orientation alone. The tangential normal stress σ X in the attachment region increased with increasing coefficient of friction at the critical contact location. This is of practical interest since cracks are thought to initiate at locations where σ x reaches a maximum tensile value, and hence high temperature metal coatings applied to the attachment region, to reduce the coefficient of friction, will be beneficial in enhancing fretting fatigue life. Additional fretting damage parameters need to be evaluated in detail for single crystal materials.