Size scale effects on brittle to ductile transition

In the present paper the influence of a specific microstructural feature on the brittle to ductile behaviour of semi-brittle materials is emphasised. The dislocation free zone size (formerly introduced to account for the behaviour of these materials) for which incomplete crack tip shielding is expected, is a function of the dislocation sources activity. This is a first scaling factor, it is required but not sufficient. A second factor has to be introduced along the third dimension : the spacing between dislocation sources. For initially dislocation-free crystals a sharp transition is observed when its value is not negligible compared to the sample's thickness. For materials containing bulk sources a soft transition is always noticed : we think that "stimulated emission" rather than spontaneous nucleation is likely to occur at crack tip, under a low stress level. This soft transition results from the by-passing of the high-energy cost spontaneous emission