Full-field measurement of lightweight nonlinear structures using 3D SLDV

Modern non-contact measurement techniques such as the Three-dimensional Scanning Laser Doppler Vibrometry 12 (3D SLDV) are advantageous in measuring vibrations of lightweight, thin-walled aerospace structures, which were 13 conventionally deemed as difficult or not feasible to apply using attached transducers. Nevertheless, the full-field 14 measurements using 3D SLDV are still limited to extracting modal properties of linear structures, while measurements 15 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 (MISO) vibration controller to deal with nonlinear 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 structures. An advanced test strategy is introduced, which is capable of obtaining amplitude-dependent resonant frequencies, modal damping ratios and full-field, multi-harmonic mode shapes of nonlinear normal modes (NNMs). Conflicting parameters such as the frequency resolution and measurement time are optimised 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 behaviours. 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.