Hi,
i.m.h.o. it's the original question that doesn't make any sense: yeld strength of the materials, as mihaiupb says, MUST be known by the designer before starting any calculation, be it FE or analytical!!! I agree that maraia's problem is that some of the material involved don't have a yeld point but rather a continuous transition, but even in this case there are other (normated...) ways: deviation from proportionality, etc.
Then, once the strength limits (both for tensile and shear) are known, one can input them into the appropriate dialog in ANSYS (Failure Criteria) in order for the program to give plots of the stresses relative to the limits (i.e. a safety factor distribution).
Regards