Full-Field Measurement of Lightweight Nonlinear Structures Using Three-Dimensional Scanning Laser Doppler Vibrometry

Modern noncontact measurement techniques such as the three-dimensional scanning laser Doppler vibrometry (3D SLDV) are advantageous in measuring vibrations of lightweight, thin-walled aerospace structures, which were conventionally deemed as difficult or not feasible to apply using attached transducers. Nevertheless, the full-field measurements using 3D SLDV are still limited to extracting modal properties of linear structures, while measurements of complex nonlinear structures are rarely reported. This paper aims to extend the full-field measurement capability of 3D SLDV and combines it with a multiple-input–single-output vibration controller to deal with nonlinear structures. An advanced test strategy is introduced, which is capable of obtaining amplitude-dependent resonant frequencies, modal damping ratios, and full-field, multiharmonic mode shapes of nonlinear normal modes (NNMs). Conflicting parameters such as the frequency resolution and measurement time are optimized by combining phase separation and phase resonance testing techniques in a coherent strategy. The capabilities of the proposed nonlinear modal testing strategy are demonstrated on a realistic, large-scale fan blade that exhibits softening behaviors. Two of its NNMs were investigated at larger vibration amplitudes. Its nonlinear modal parameters were successfully extracted and validated, highlighting the time efficiency and data accuracy of the proposed strategy for measuring industrial-scale, lightweight nonlinear structures.