Bilinear kinematic curve - bl_ kin

This is a kinematic-hardening bilinear symmetrical curve frequently employed to model idealised elastic-plastic behaviour, semi-rigid connections, lead-rubber bearings, steel hysteretic dampers, and so on. 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