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What yield criterion must I use?

Nowadays I am analyzing a study of a hip prosthesis including the bone. I am making the FEA/FEM analysis by Pro/Mechanica software. Metallics materials are titanium alloy (Ti-6Al-4V) and Chromium-cobalt-molybdenum alloy (CoCr29Mo). So they are ductile materials. I understand the main load is the flexural load on the hip.

What yield criterion (Von mises or Tresca) would I use? I have read Von Mises is more precise but lesser conservative. But when I printed the results I could see than the stress distribution is similar in the behaviour but not in the value. The qualitative map is similar but Von Mises criterion stress is greater than Tresca (up to 40 %) which contrasts with the higher value expected for Tresca stress (more conservative). So what yield criterion would I use?

 "Von Mises criterion stress is greater than Tresca (up to 40 %)"

If this is so, something is wrong with your analysis: von Mises  and Tresca cannot differ by more than about 15%. See any good materiial mechanics book on that. 

Furthermore, since your application probably does not allow yield in the metal, do an elastic simulation and check for the yield criterion during post-processing (if you don't do it that way already). Considering the usual scatter in material data, whether you use Mises or Tresca should not make a difference.

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