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Very high cycle fatigue of structured sheets

The reduction of aerodynamic drag of modern transport systems by structured sheets is beneficial for both, ecological and economic reasons.

An innovative approach for this are surface riblet structures combined with transversal surface waves with amplitudes of few 1/10 mm at frequencies of some 100 Hz.
The feasibility of this technology is studied in detail in the DFG research group 1779. In this framework, the fatigue behavior of high-strength aluminum sheet material is investigated since fatigue failure may be a technical limit for later application. During the lifetime of a transport system, the frequency of some 100 Hz leads to cycle numbers clearly above 107 cycles, i.e within the very high cycle fatigue (VHCF)-regime.
In the present project the aluminum alloy Al-2024 as a standard material for aircraft fuselage and wings is investigated. The material can be significantly strengthened by precipitation hardening.
It is well known that the Wöhler curve of Al-2024 does not reach an endurance limit for very high numbers of load cycles. Nevertheless, the influence of pure aluminum cladding layers as well as of riblet-like surface structures on the VHCF behavior has not been studied in detail so far. The structuring process leads to work-hardening as well as to near-surface residual stress. Furthermore, the riblet structure causes a notch effect resulting in local stress concentrations which may contribute to early fatigue failure. In addition, the soft cladding layer shows significantly lower fatigue strength than the Al-2024 substrate.

Multiple crack initiation in cladding layer with subsequent crack growth into the substrateMultiple crack initiation in cladding layer with subsequent crack growth into the substrate

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Fatigue experiments are performed using an ultrasonic fatigue testing machine at a frequency of around 20 kHz at room temperature. By in-situ monitoring of relevant parameters for early stage damage detection (harmonic characteristics, damping behavior, etc.) samples can be examined already before, but relatively close to, final failure. After end of lifetime, fracture surfaces are analyzed by scanning electron microscopy in order to investigate the key damage mechanisms.
In addition, 3 point bending tests are performed at a frequency of around 100 Hz since this load type is very close to the final application. The experimental work is supported by elastic FEA simulations.
The obtained results will contribute to the understanding of degradation behavior of structured sheets in the very high cycle regime fatigue in a valuable way.

FOR 1779