Very High Cycle Fatigue Behavior of Riblet Structured High Strength Aluminum Alloy Thin Sheets
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Beschreibung
(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 106 cycles) to run-outs (at some 109 cycles) is observed.
Particularly in the megacycle regime, the fatigue life of the structured bare material
is – 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
Autor*in
Sebastian Stille