If I use von mises theory (VMT) and max principle stress theory (MPST) for the same geometry the maximum allowable stress obtained from VMT is two times of MPST. Hence if i select the brittle material i can select the material with less yield strength. (ie) If in VMT the allowable stress obtained is 90MPa but for the same geometry and loading in MPST we get 45MPa. Hence i can choose the brittle material with 70MPa. Is it right?
Perhaps I'm misunderstanding your question, but why should the maximum principal stress always have a fiixed ratio to the von Mises stress? For example, in a state of hydrostatic tension, the von Mises stress is zero, but the maximum principal stress can be large.
Furthermore, if your geometry contains notches, be careful in evaluation local stresses - maxima in contour plots are totally unreliable (use values at integration points instead), and the results will strongly depend on the mesh density (since the stress at a sharp notch is computationally infinite).
Yes, the von mises stress is not always two times of max principle stress. But in most of the case it happens or in my geometry it happened. So if i don't have notches can i choose brittle material as per max principle stress theory. Since, the allowable stress is less in max principle stress theory compared to von mises.
"ut in most of the case it happens"
This must be either pure luck or a very simple geometry. (Looking at a Mohr's stress circle, I would actually expect the opposite: The (Tresca) shear stress should be half of the principal stress in uniaxial tension.)
please also note that brittle materials typically have different strength under tension and compression
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