(Dr. Richard W. Neu, advisor)

"Effect of Notches and Fretting on Fatigue of Steam Turbine Materials at 524°C"

Abstract

The fatigue behavior of two steam turbine steels, 12 Cr stainless steel used for buckets (blades) and CrMoV steel used for wheels (or rotors), was investigated.  Fatigue failures may originate at the attachment point between the bucket and wheel, where the stress concentrations due to the geometry and fretting may be significant.  This study was aimed at determining the relative sensitivity of these materials to notches and fretting using simplified laboratory tests at a temperature that is similar to that of the attachment point.  A key task of the current investigation was to design and implement a fretting fatigue apparatus capable of producing fretting fatigue damage at an elevated temperature.  Uniaxial fatigue experiments were conducted at 524°C on smooth and notched specimens as well as under fretting fatigue conditions.  The smooth specimen fatigue strength of 12 Cr is greater than that of CrMoV.  However, the notched specimen strengths were nearly the same, due to the higher notch sensitivity of 12 Cr.  Fatigue lives of 12 Cr steel under fretting were lower than that of smooth fatigue tests conducted, but greater than that of notched (K_t = 2.18) specimens.  The implications for life prediction modeling are discussed noting that fretting fatigue can be treated as a notch problem, but having a relatively larger stress concentration and a smaller highly stressed volume.