Trilinear symmetric curve - trl_sym

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

Initial stiffness (K0)
The default value is 1000

First branch displacement limit (d1)
The default value is 1

Second branch stiffness (K1)
The default value is 10

Second branch displacement limit (d2)
The default value is 5

Third branch stiffness (K2)
The default value is 100

Note: Stiffness values K0, K1 and K2 must be positive. Further, K1 and K2 should always be smaller than K0.