Hey everybody, I went through a discussion with a colleague today about Finite element modeling of composite structures and how to interpret the stress analysis. I understand that for isotropic materials, principal stresses could be used against the allowable stresses to see if failure will occur. (or von mises vs yield) For composites, would there be a preference of using the in plane principal stress vs using the normal stress? Some research online tells me that there will be times when both of them will be the same value, but sometimes it won't be- when it is not equal, what value should be used? Also, when the orientation of the plies in composite materials is not well known, as well as the fiber type (unidirectional, etc), would using principal stress be more accurate or normal stress be more accurate? Thanks everybody in advance for your insights.