plasticity buckling

[cristipurdel]

Would be nice to have a metallic plastic buckling as follows:

• in the buckling section, would add a true/false button, which should be taken into account only if: Ex = Ey (e.g. wo Poisson Mebrane for quasi-isotropic materials) and BCx=BCy=0
• after running normal elastic buckling, consider the nx and nxy allowables (only for first minimum positive eigenvalue), convert them to stresses and use HSB 52100-01E case 4 and 2 respectively from §3.3
• use an non-linear algorithm (e.g. Newton-Raphson) to get the new plastic stresses (without considering cladding)
• find new buckling ratios for (bi-axial and shear individual loads) and use them in HSB 51200-01G eq. 3.2.8 (without bending and considering the +- sign for compression)