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Experimental characterization of nonlinear structures usually focuses on fundamental resonances. However, there is useful information about the structure to be gained at frequencies far away from those resonances. For instance, non-fundamental harmonics in the system's response can trigger secondary resonances, including superharmonic resonances. Using the recently-introduced definition of phase resonance nonlinear modes, a phase-locked loop feedback control is used to identify the backbones of even and odd superharmonic resonances, as well as the nonlinear frequency response curve in the vicinity of such resonances. When the backbones of two resonances (either fundamental or superharmonic) cross, modal interactions make the phase-locked loop unable to stabilize some orbits. Control-based continuation can thus be used in conjunction with phase-locked loop testing to stabilize the orbits of interest. The proposed experimental method is demonstrated on a beam with artificial cubic stiffness exhibiting complex resonant behavior. For instance, the frequency response around the third superharmonic resonance of the third mode exhibits a loop, the fifth superharmonic resonance of the fourth mode interacts with the fundamental resonance of the second mode, and the second superharmonic resonance of the third mode exhibits a branch-point bifurcation and interacts with the fourth superharmonic resonance of the fourth mode.

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Nonlinear normal modes (NNMs) are often used to predict the backbone of resonance peaks in nonlinear frequency response functions. Regardless of the definition considered, one important limitation remains, i.e., the NNMs require a multi-point, multi-harmonic external forcing to be excited. To address this limitation, the present study proposes a new definition of NNMs, termed phase quadrature backbone curve (PQBC). The advantage of PQBC is that an actual solution of the nonlinear frequency response obtained under mono-point, mono-harmonic external forcing is followed for which phase quadrature of a selected degree of freedom is achieved. Additionally, super and subharmonic resonance peaks can be captured by adapting the phase quadrature condition. Finally, no post-processing is required to get amplitude-forcing relations of the NNMs. Isolated responses can, in turn, be predicted from these relations.

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