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Transition of crack closure mechanism in Ti-6Al-4V
Arne Kriegsmann and Clemens Müller
During cyclic loading, fatigue crack closure occurs when crack faces are in contact before
minimum load. It is assumed that no crack-tip damage occurs while crack faces are in
contact, so this portion of the load cycle is ineffective for fatigue crack growth. Therefore,
the effective stress intensity range ∆Keff at the crack tip and thereby the crack propagation
rate decreases.
Fatigue crack closure can result from residual plastic deformations remaining in the wake
of an advancing crack, fracture surface roughness or corrosion deposits in the crack wake.
These three closure mechanisms are called plasticity induced closure, roughness induced
closure and oxide induced closure, respectively.
500 µm
25 µm
Fig. 1a: Lamellar microstructure Fig. 1b: Equiaxed microstructure
Fig. 1c: Transition from coarse lamellar (left) to equiaxed (right) microstructure
At higher values of ∆K, material at the crack tip experiences large tensile plastic strains
which are not reversed upon unloading. Tensile residual displacements left in the crack
wake cause plasticity induced crack closure. At low stress intensity levels crack growth
proceeds locally in one slip system, leading to a kinked crack and finally to roughness
induced crack closure. Thus, when determining the crack propagation threshold a change
from plasticity induced to roughness induced crack closure takes place for most
microstructures. This transition and its consequences on crack closure level is still not fully
understood.
Ti-6Al-4V lamellar microstructures (Fig. 1a) cause roughness induced crack closure even
at high ∆K values. At the same ∆K, equiaxed microstructures (Fig. 1b) lead to plasticity
induced crack closure resulting in lower Kop values. In this investigation a heat treatment
using a temperature gradient was used to produce specimens showing a sharp transition
(Fig. 1c) from lamellar to equiaxed microstructure. During fatigue crack growth this
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