Which statement about the in-plane stiffness of a laminate in which unidirectional plies are stacked crosswise and symmetrically, as in [0/90]ₛ, is correct?

Answer: C. The Young's moduli in the 0° and 90° directions are nearly equal, but in the 45° direction in-plane shear dominates and the modulus is lower

In [0/90]ₛ, loading in the 0° direction is carried along the fibers of the 0° plies and loading in the 90° direction along the fibers of the 90° plies, so the moduli in the two directions are nearly equal. Loaded at 45°, however, the fibers of every ply are 45° off the load direction, and the deformation becomes mainly in-plane shear supported by the resin, so the modulus drops considerably. To approach in-plane isotropy, ±45° plies are added to make a quasi-isotropic layup such as [0/±45/90]ₛ. Saying the 45° direction is the same overlooks this point. In the 90° direction the fibers of the 90° plies carry the load, so the modulus is not that of the resin alone. Note that in any layup the through-thickness direction is supported by the resin, so susceptibility to delamination remains.