I assume you’re talking about laminated composite shells...
For a laminated composite, you probably want to look at ply level results. You can get these from Nastran if you use PCOMP cards. You define the properties of a single ply on a MAT8 card (2D orthotropic) and the ply stacking sequence on the PCOMP.
The ply strains in the element coordinate system would be eps-xx, eps-yy, gam-xy. For a failure analysis, you really want the strains in the material coordinate system (which can be different for each ply). These would be eps-11, eps-22, gam-12, and are used for failure analysis since the allowables are determined by testing in these directions. Note that Nastran uses the engineering definition of shear strain.
In addition, Nastran can compute a “failure index” if you use PCOMPs. A failure index of greater than one indicates failure, less than one is OK. You have your choice of failure criteria (Max Strain, Tsai-Wu, etc), and you have to input the allowable stresses or strains on the MAT8 card.
This is just an overview. For details, you must read the Nastran documentation, especially for PCOMP and MAT8.
