Hi

I want to compare the stresses obtained from Pro-mechanica with respect to the material flexural yield stress. Which stress reading (von mises or max principle stress) should I use?

Doesn't it depend on whatever failure criterion is the one your industry demands? Some demand von Mises, some demand max. principal stress, still others demand Tresca max. shear. Use whatever one is appropriate for whatever you are designing.

Remember that a state of stress is a complex organization of normal and shears stress matrix, so talking about principal or von misses stress is a way to see a specifically a state of stress, I mean: Von Misses is a combination of principal and shear stress view as a scalar (magnitude) perhaps principals are the (principals stress) without taking into account those shears stress.

Which one to use?, that depend of the kind of material (ductile -? plastic deformation failure model), Brittle (no plastic deformation basically) etc, you are using in your design.

I recommend you to have a look on:

http://www.efunda.com/formulae/solid_mechanics/failure_criteria/failure_criteria.cfm?search_string=von%20mises

On here you can see the differences between Von Misses, principal stress etc.

Some design codes such asme 8 ( for pressure vessel design) use max shear stress as a design method due to the fact that most of the material used for pressure vessel are made with ductile material, so max shear stress is the one more suitable for such materials.

Hopping that this explanation helps you.

Regards.