Does anyone know where i can get the stress allowable values for diferent materials?
I need this value to compare it to the applied strees when this shaft is subjected to a reversed torsional load.
Thank you.
For aluminum, use The Aluminum Association's design handbook. Note that the allowable stress varies depending on the shape of the section, the load and boundary conditions. If you are looking for Yield Strength, that is easily found and tabulated. Allowable strength is another animal though!!
If you're looking at a shaft with reverse torsional load then you're probably referring to one that is spinning. As well as allowable stresses in terms of satisfying a yield criteria, you should also look at satisfying allowable fatigue stress limits, which can be less than those for the yield criteria of a static structure. The allowable fatigue stress (generally for an infinite life) will be factored by the fact that you have a reverse loading and you should also consider the size of the shaft and its surface quality. Presumably the stress you have calculated will also consider any stress concentration features.