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@PHDTHESIS{Kovacs:141080,
      author       = {Kovacs, Stephan},
      title        = {{L}ebensdauer und {S}chädigungsentwicklung martensitischer
                      {S}tähle für {N}iederdruck-{D}ampfturbinenschaufeln bei
                      {E}rmüdungsbeanspruchung im {VHCF}-{B}ereich},
      volume       = {214},
      school       = {RWTH Aachen},
      type         = {Dissertation},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {FZJ-2013-06282},
      isbn         = {978-3-89336-959-1},
      series       = {Schriften des Forschungszentrums Jülich Reihe Energie $\&$
                      Umwelt / Energy $\&$ Environment},
      pages        = {123 S.},
      year         = {2014},
      note         = {Dissertation, RWTH Aachen, 2010},
      abstract     = {Low-pressure steam turbine blades are usually made of
                      martensitic steels with Cr contents between 9 and $12\%,$
                      which combine good corrosion resistance, high mechanical
                      strength and sufficient ductility. The inhomogeneous flow
                      field behind the vanes generates high-frequency oscillations
                      above 1 kHz. In addition, the blades with lengths up to 1.5
                      m are operated at rotational speeds up to 3000 rpm,
                      resulting in large centrifugal forces leading to the
                      superposition of extremely high mean stresses. Also
                      resonance oscillations during start-up and shutdown cannot
                      be completely excluded. Currently, the components are
                      designed using high safety factors against S-N curves with
                      an assumed asymptotic fatigue limit above 107 load cycles.
                      Nevertheless, fatigue cracks are observed even at high
                      number of cycles, starting fromthe blade root without
                      pre-damage by erosion or steam droplet impingement. While
                      fatigue failure usually occurs at the surface, fatigue
                      cracks at very high number of cycles (> 108) initiate at
                      oxides or intermetallic inclusions below the surface. This
                      transition between both failure mechanisms in the Very
                      High-Cycle Fatigue (VHCF) regime is in the focus of numerous
                      current research activities, because numbers of cycles above
                      108 can be attained in a viable period of time using the
                      recently developed high-frequency testing techniques
                      operated at 20 kHz. Also for wind turbines, gas turbines,
                      bearings, springs, etc. VHCF issues become increasingly
                      important.Within this work, the fatigue life and damage
                      behavior of a martensitic Cr-steel during fatigue loading
                      with and without high mean stresses at number of cycles to
                      failure above 108 was analyzed. On the one hand, the studies
                      gave insights into the relation between fatigue life and
                      fatigue damage evolution of the investigated group of
                      high-strength steels in the very high cycle fatigue regime
                      (up to 2∙109). In particular, the influence of high mean
                      stresses on the VHCF behavior (fracture origin, crack
                      growth, fatigue life) which was not investigated in detail
                      before is studied and the crack initiation and propagation
                      mechanisms are analyzed by electron microscopy (SEM, TEM /
                      FIB). With this, the work contributes to the reliable design
                      of future low-pressure steam turbines. The results show that
                      in particular non-metallic inclusionsin the steel cause
                      fracture by fatigue cracks initiated in the volume under
                      very high cycle fatigue conditions. This fatigue behavior
                      can be described very well by means of fracture mechanics
                      approaches over a wide range of load ratios.},
      keywords     = {Dissertation (GND)},
      cin          = {IEK-2},
      cid          = {I:(DE-Juel1)IEK-2-20101013},
      pnm          = {122 - Power Plants (POF2-122)},
      pid          = {G:(DE-HGF)POF2-122},
      typ          = {PUB:(DE-HGF)3 / PUB:(DE-HGF)11},
      url          = {https://juser.fz-juelich.de/record/141080},
}