Hi, sorry for bother! I build a kriging model, and based on the kriging I use Performance measure approach, Single loop approach, Sequential optimization and reliability assessment for RBDO.

Plot for RBDO - Convergence of the Constraints are obtained.

But what’s the meaning of G for vertical axis in the plot?

In these plots, G is the value of the limit-state evaluated at the minimum performance target point (MPTP). Please have a look at Section 1.3.1.3 of the RBDO manual for more details on what this point means. It is basically used to transform the reliability problem into a deterministic one.

When the constraints are saturated, that is P_f(d) = Pf_target, then the G(MPTTP(d)) = 0. The plots therefore say that, at convergence, all constraints are satisfied and saturated.