Discussion of initial and subsequent crack growth

The J-integral criterion for initial crack growth is discussed. It is pointed out that an essential requirement for the validity of this criterion is autonomy of the end-region near the crack-tip. This autonomy ought to exist even at large scale yielding if there is a substantial strain hardening, in contrast to the autonomy of the plastic region, which does not even hold in the whole range of linear fracture mechanics. Finally, stable and unstable crack extension is described from an energy point of view.