Composite buckling

[victorkemp]

QUAD4, not TRIA3
Symmetric composites only (LAM=SYM)
Shell buckling enabled by default without needing DEBUG,201,1
Not tested and might not work with:

• Offset
• Non-planar elements ((DABS(HBAR) > MXWARP)
• Thermal loading
• Differential stiffness solution type

[zchlrnr]

I like the ordering of the gauss points I think.
Let's try it.

[cristipurdel]

Could you add also the following for non-symmetric composites.
Until a better implementation is found, based on DtildeDMatrix the QUAD4 bending stiffness element matrix could be considered (conservative) as D = D - BAinvB, and I guess the general matrix [(A,B),(B,D)] as [(A,0),(0,D)]?!?
It would be nice to have, since based on my experience, the engineering stacking have a "weak" B matrix for non-symmetric stackings thus a small impact on the eigenvalues