Steel02 and strain hardening

[JBlancoB]

Hi everybody:

I'm trying to use steel02 material model for steel bar but I don't know where to find the parameters a1,a2,a3,a4 in order to take into account the strain hardening. Here I make a little description of what I've found and why I can't define these parameters.

According to the source code, this model correspond to the one proposed by Menegotto and Pinto (1973) and it was modified by Filippou in 1983 in order to implement the strain hardening. My doubt is related to the paremeters for the strain hardening. There is no suggestion about them in the manuals and the expression for the stress shifting in source code does not correspond with the one proposed by Filippou et al. (1983).

The expression proposed by Filippou et al. (1983) for the yield stress shifting is:
sigma_st/sigma_y=a3*(emax/ey-a4) (1)
where:
sigma_st is the stress shifting
sigma_y is the yield stress
emax is the maximun strain
ey is the yield strain
a3 and a4 are parameters proposed by Filippou
The parameter a3 and a4 are available in Filippou et al. (1983), but the expression in the opensees source code is:
sigma_st/sigma_y=a3*((emax-emin)/(2*a4ey))**0.8 (2)
where:
emin is the minimun strain achieved during the strain history
** means powered to...

As you can see there is no possible relation between the parameters a3 and a4 in Eqs 1 and 2. Therefore, I can't deduce the parameters a3 and a4 for Eq 2 from the parameters a3 and a4 proposed by Filippou in Eq 1, and I need a direct suggestion for the parameters a3 and a4 in Eq 1.

I'll appreciate any help related to this issue