Symmetric bilinear curve - bl_sym

This is a curve frequently employed to model idealised symmetric elastic-plastic behaviour. An isotropic hardening rule is adopted and three parameters need to be defined in order to fully characterise this response curve:

Initial stiffness (K0)
The default value is 20000

Yield force (Fy)
The default value is 1000

Post-yield hardening ratio (r)
The default value is 0.005

Note: Evidently, in those (relatively common) cases where the post-yield stiffness is not very high and the maximum force does not thus reach a value that is twice its yield counterpart, this response curve will behave in the same manner as curve bl_kin.