(Dr. Richard W. Neu, advisor)
"Effect of Notches and Fretting on Fatigue of Steam Turbine Materials at 524°C"
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 (Kt = 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.