This work explores a means whereby multi-harmonic excitation can be used to isolate a NNM.

The theory shows that a plot of the input force versus response velocity can be used to monitor the phase condition of the structural response in real time, as well as understand which harmonics should be added to the input force to minimize the phase condition.

This is numerically demonstrated using a representative nonlinear reduced order model with modal damping.

The numerical results show that this metric is more useful than the phase of the response and its harmonics, the phase being found to be highly sensitive to perturbations.

The methods are examined experimentally by measuring two NNMs of a flat clamped–clamped beam.

Modal interactions from internal resonances are detected experimentally and isolated to the level permitted with the current setup.