Under what conditions would you expect the material to fail in brittle and in ductile fracture?

Question

Assume that you can determine the theoretical strength of a crystal for failure in tension and in shear. Under what conditions would you expect the material to fail in brittle and in ductile fracture.

Answer 1

I am not a metallurgist but I can give some examples:
brittle fracture: fatigue, hydrogen embrittlement, stress corrosion
ductile: excessive stress

Answer 2

Heres what it says in my Mech D textbook Ductile materials are normally classified such that Ef>=.05 and have an identifiable yield strength that is often the same in compression as in tension (Syt = Syc =Sy). Brittle materials , Ef<.05 do not exhibit an identifiable yield strength, and are typically classified by ULTIMATE tensile and compressive strengths, Sut and Suc, respectively (Where Suc is given as a positive quantity) basically a brittle material wont be classified by a yield strength because they don't yield they just snap or break.