function second_derivative = diff2_potential(chi,sigma_inf,sigma_y,delta,K)

% This function returns the second derivative of the isotropic hardening
% potential in terms of chi, with constant values of sigma_inf (asimptotic
% value of the stresses), sigma_y (elastic limit), delta and K (isotropic
% hardening modulus). To be used in the implementation of the nonlinear
% isotropic hardening model.

second_derivative = (sigma_inf - sigma_y)*(delta * exp(-delta*chi)) + K ;