13th International Conference on Fracture June 16–21, 2013, Beijing, China deformations during the blunting of the crack tip. After the fracture initiation at JR=Jc the plastic dissipation decreases leading to a decaying R-curve. For a prescribed loading J=Jc this point is unstable so that a dynamic crack propagation initiates. For σc/σy=3.07 and σc/σy=3.10 the crack begins to propagate with the ductile mechanism until cleavage initiates and the propagation becomes dynamic, too. The distribution of the void growth behind the current crack tip is depicted in Fig. 8 showing a so-called stretch zone. Whilst the crack propagates only by cleavage for σc/σy=3.07, the crack arrests again for σc/σy=3.10 followed by ductile propagation. Such a behavior accords to experimental results, compare Fig. 1. Fig. 8: Stretch zone at initial crack tip (σc/σy=3.07) In order to investigate the effect of the crack tip constraint, now the crack propagation in a shallow-crack three-point bending specimen of width W=100 lnl is simulated with a FE-mesh like that in Fig. 6. The relative crack depth amounts to a/W=0.15, a value corresponding to the experimental data which will be used below for a comparison. The computed R-curves are depicted in Fig. 9 for the lower ductile-brittle transition region. Fig. 9: R-curves for shallow-cracked specimen (a/W=0.15) The results show that initially the crack grows by cleavage but nevertheless stably until the crack propagation becomes dynamic at the unstable point. Typically, in the context of brittle damage this point is used for the definition of the fracture toughness Jc. However, because of the initially stable crack propagation a Jc defined this way does not depend only on the crack tip constraint at the initial crack tip. Merely, it is also significantly influenced by the evolution of the crack tip constraint with ongoing crack growth. -6-
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