Isotropic hardening law

Hi all,

I have come across the two relations where aim to describe the isotropic hardening of a material

Power law:

R = Kεpn R is the variation in stress from initial yield, εp is the plastic strain where K is the strenght coefficient and n is the strain hardening exponent as observed in Ramberg Osgood equations.

Exponential law:

R = R∞ [1-e(-bεp)] where R∞ is the saturated value of the R variation, b is the rate at which the sauration is reached.

From all the literature I have read on combined hardening I have seen that the authors have used the exponential law for the isotropic hardening when considering cycic plasticity. Does anyone know the reasoning of this?

Looking forward to your comments