Is there a criterion for ductility that says something like this: If a material has an elastic modulus above such-and-such it is considered brittle

Or something to do with percent elongation?

I'm wondering if there is an accepted number at which the transition occurs so I can put in my program something like "If this, then use brittle failure criterion, if not, use ductile failure criterion"

Thanks,
Tony

Ductility is linked to plastic behaviour, hence not with the elastic modulus. No practical relationship to it neither.

Unfortunately, there is no usable theory of ductile behaviour. Ductility is measured by dozens of methods, all contradictory, none leading to practical forecasts of a part's behaviour. And the limit isn't sharp neither.

Elongation at break relates with ductility, yes.
K1c as well, and many more.

If we wish to decide through elongation at break - for no good reason:
Under 1%, most people would say "Brittle".
Over 5%, most people would say "Ductile".
At 3% like brass can be, many people would say "a bit brittle".
But this is much more than most ceramics, which break with no elongation at all.

It's not as simple as 'if a material has this, then it's brittle' there are so many factors affecting the ductility.

Temperature the obvious one, which I think is what you're talking about, but, the strain rate on the material is also a factor that can change the ductile to brittle transition temperature.

Also it's not as simple as to say, given a certain strain it's a brittle failure, due to the huge variaty of strains at failure.

Brittle or ductile failure is usually determined by examining the fracture surface of a specimen, the shinyness and surface fceatures often give information on the failure mechanism (river marks, progration lines etc)

Jim