Crack growth behavior in an aluminum alloy under very low stress amplitudes
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Tina Kirsten and Martina Zimmermann Institute for Materials Engineering, Technical University of Dresden, Dresden D-01069, Germany
Fatih Buelbuel and Hans-Juergen Christ Institute for Materials Engineering, University of Siegen, Siegen D-57068, Germany (Received 8 May 2017; accepted 20 June 2017)
The near-threshold behavior of long cracks is studied in this paper using precracked flat dogbone specimens of a commercial aluminum alloy in peak-aged and overaged conditions. After introducing the initial crack in compression precracking, the crack was propagated approximately with the constant range of the stress intensity factor at values just above or below the corresponding threshold values. It was found that there were two major mechanisms which kept the crack from continuous extension. First, the crack front was pinned by primary precipitates. This effect was rather pronounced and lead to significant kinking in the crack front and ductile ridges on the fracture surface. The second mechanism was shear-controlled crack extension of very long cracks with plastic zones ahead of the crack tip, very similar to stage-I small cracks. Interaction with primary precipitates deflected the shear-controlled cracks but did not change the crack extension mode.
I. INTRODUCTION
In the very high cycle fatigue (VHCF) regime, interest is generally focused on the crack initiation phase as the life-determining phase (as discussed in the series of VHCF conferences since 1998, e.g., Refs. 1–5). However, in cases in which the crack initiation starts from natural defects such as inclusions, the crack initiation phase may be shortened significantly, and a substantial amount of the lifetime is spent in the crack propagation phase. Long lifetimes may still be achieved, if the loads are small enough to keep the crack in the near threshold regime. Thus attention in VHCF research has to be broadened from the classical VHCF scenario focused on crack initiation to the effect of very small cyclic loads on crack propagation. Small cracks initiated from microstructural defects are known to propagate below threshold in an intermittent way due to interactions of the crack tip field with the microstructure.6,7 This effect is related to the fact that the crack tip field of these cracks is contained within one grain, and that dislocations emitted by the crack tip move along well-defined slip planes. Now, if the stress amplitude is very low, similar effects may happen to long cracks and may lead to unexpected crack extensions phenomena. Contributing Editor: Mathias Göken a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.274
Consequently, it may be worthwhile to study the crack extension behavior in the near-threshold regime. Threshold values and their determination is a classical subject in fracture mechanics, which has been standardized by ASTM E 647.8 The method proposed there relies on the so-called load-shedding procedure in which the range of the stress intensity factor is decreased until the crack
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