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@PHDTHESIS{Stille:202599,
      author       = {Stille, Sebastian},
      title        = {{V}ery {H}igh {C}ycle {F}atigue {B}ehavior of {R}iblet
                      {S}tructured {H}igh {S}trength {A}luminium {A}lloy {T}hin
                      {S}heets},
      volume       = {262},
      school       = {RWTH Aachen},
      type         = {Dr.},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {FZJ-2015-04798},
      series       = {Schriften des Forschungszentrums Jülich Reihe Energie $\&$
                      Umwelt / Energy $\&$ Environment},
      pages        = {XII, 123 S.},
      year         = {2015},
      note         = {RWTH Aachen, Diss., 2015},
      abstract     = {Fatigue testing was performed on two age hardened high
                      strength aluminum alloys (AA 2024 T351 and AA 7075 T6) at
                      ultrasonic frequencies of around 20 kHz in fully reversed
                      axial loading (R = -1). Tests were carried out on flat and
                      riblet structured thin sheets in order to evaluate their
                      usability for a novel technique for aerodynamic drag
                      reduction as well as for gaining further insight into the
                      relevant degradation and failure mechanisms. The studied
                      riblets were of semi-circular geometry and produced by a
                      flat rolling process which was developed at the Institute of
                      Metal Forming (RWTH Aachen University). Important aspects of
                      the present work are the influence of commercially pure CP
                      Al claddings – which are frequently used for the
                      prevention of corrosion – as well as of different riblet
                      dimensions on the fatigue performance. Whereas the bare
                      material shows a continuous transition from high cycle
                      fatigue(HCF) to very high cycle fatigue (VHCF), for clad
                      sheets a sharp transition from HCF failure (up to some
                      10$^{6}$ cycles) to run-outs (at $\geq$ some 10$^{9}$
                      cycles) is observed. Particularly in the megacycle regime,
                      the fatigue life of the structured bare materialis –
                      compared to the non-structured case – significantly
                      reduced by stress concentrations induced by the surface
                      structure. However, the fatigue performance of clad material
                      is not negatively affected by the riblets. In this case, the
                      threshold value at which the transition from HCF failure to
                      run-outs occurs was even higher than in the flat case. The
                      transition stress differs with cladding thickness as well as
                      with riblet geometry. Fatigue cracks are – even in the
                      case of run-outs– always initiated at the surface of the
                      clad layer and grow easily to the substrate. Specimens only
                      fail, if the threshold for further crack growth into the
                      substrate is exceeded. The fatigue limit of both, the flat
                      and riblet structured clad material can thus be described by
                      a fracture mechanics approach using a Kitagawa-Takahashi
                      diagram. In the case of structured clad material, the
                      threshold for fatigue failure is not only directly affected
                      by the remaining thickness of the cladding below the riblet
                      structure. Finite element (FEM) simulations demonstrate that
                      due to plastic deformation a stress redistribution in the CP
                      Al layer occurs which modifies the effective stress at the
                      interface (cladding / substrate). The effective interface
                      stress is thus as well a function of cladding thickness,
                      which therefore, besides the direct effect, also indirectly
                      influences the stress intensity of through-cladding cracks.
                      Further FEM simulations demonstrate that riblets can be
                      optimized with respect to VHCF performance, if the thickness
                      of the clad layer below the riblet valleys is around25\% of
                      the riblet diameter. The failure mechanisms of both tested
                      alloys are similar to each other. Further aspects covered in
                      this work are a detailed analysis of material changes
                      induced by the structuring process and the development of a
                      bending testing setup in which the loading conditions
                      resemble the exposure during use in active drag reduction
                      systems.},
      cin          = {IEK-2},
      cid          = {I:(DE-Juel1)IEK-2-20101013},
      pnm          = {111 - Efficient and Flexible Power Plants (POF3-111) /
                      HITEC - Helmholtz Interdisciplinary Doctoral Training in
                      Energy and Climate Research (HITEC) (HITEC-20170406)},
      pid          = {G:(DE-HGF)POF3-111 / G:(DE-Juel1)HITEC-20170406},
      typ          = {PUB:(DE-HGF)11 / PUB:(DE-HGF)3},
      url          = {https://juser.fz-juelich.de/record/202599},
}