Bilinear steel model - stl_bl

This is a uniaxial bilinear stress-strain model with kinematic strain hardening, whereby the elastic range remains constant throughout the various loading stages, and the kinematic hardening rule for the yield surface is assumed as a linear function of the increment of plastic strain. This simple model is also characterised by easily identifiable calibrating parameters and by its computational efficiency. It can be used in the modelling of both steel structures, where mild steel is usually employed, as well as reinforced concrete models, where worked steel is commonly utilised.

Five model calibrating parameters must be defined in order to fully describe the mechanical characteristics of the material:

Modulus of elasticity - E
This is the initial elastic stiffness of the material. Its value usually oscillates between 200 and 210 GPa. The default value is 200 GPa.

Yield strength - fy
This is the stress at yield. Its value typically varies from 230 MPa up to 650 MPa. The default value is 500 MPa.

Strain hardening parameter -
This is the ratio between the post-yield stiffness (Esp) and the initial elastic stiffness (Es) of the material. The former is defined as Esp=(fult-fy)/(-fy/Es), where fult and represent the ultimate or maximum stress and strain capacity of the material, respectively. Its value commonly ranges from 0.005 to 0.015. The default value is 0.005.

Fracture/buckling strain -
This is the strain at which fracture or buckling occurs. The default value is 0.1 (this may be a reasonable value for steel rebars in reinforced concrete sections, but rather inappropriate for steel profiles - users should thus set it with care (even using an infinitely large value when no fracture/buckling modelling is desired)).

Specific weight -
This is the specific weight of the material. The default value is 78 kN/m3.