In cases where a monotonically increasing curve such as pressure_vs_leakage in *AIRBAG or effective_stress_vs_effective_strain in plasticity models in *MAT are to be parametrically identified by an optimization software such as LS-OPT to match against a physical test, it is important to ensure that the points identified by LS-OPT satisfy the non-negative slope. To satisfy this, it is common to place constraints on the points such that the f(Xi+1) is greater than or equal to the f(Xi). This method is less desired as LS-OPT has inherent difficulties in removing points that do not satisfy the constraint.
An alternative method suggested in this post eliminates the need to have constraints by using *PARAMETER_EXPRESSION keyword such that LS-OPT only computes an incremental value of the function whose lower bound is ZERO to yield a minimum slope of zero that is permitted by LS-DYNA.
The method is illustrated here.