I always understood that a symmetric laminate had A16=A26=0 as Brian said, and that uncouples direct loads and twisting, but a balanced laminate had to have D16=D26=0 and that uncouples bending and twisting. I also understood that to get A16, A26 to be zero requires lamina of the same orientation each side of the mid plane (symmetric), but to get D16, D26 to be zero required lamina of the opposite orientation (ie. + orientation matched by - orientation) to have a balanced laminate. These requirements can not both be met. However my understanding was that because the D terms depend on the third power of distance from the mid-plane the coupling between bending and twisting diminished rapidly. Hence laminates should be symmetric. Am I wrong?
Blakmax