Complete operating deflection shapes and model updating for an excited structure in thermal environments via an optimized continuously scanning laser Doppler vibrometer with a two-dimension scan scheme

Abstract

In order to improve the efficiency and accuracy of operational modal analysis (OMA) and model updating with structures in thermal environments, an optimized continuously scanning laser Doppler vibrometer (CSLDV) with a two-dimension scan (2D-S) scheme (CSLDV 2D-S) system was first utilized to estimate modal parameters for complete operating deflection shapes (ODSs). The experiment was carried out on a free-hanging plate made of aluminum. Before this work, the manual-moving laser Doppler vibrometer as a non-contact measurement method replaced heat resistant accelerometers and was used in thermal vibrational measurements to extract only a few dozens of measured points across the grid. The CSLDV is capable of efficient and spatially dense vibration measurements by sweeping its laser spot along a scan path on the surface of a structure. Compared with a manual-moving laser Doppler vibrometer in thermal vibrational measurements, an optimized CSLDV 2D-S system is able to measure thousands of vibration points in a few seconds and reconstitute precise multi-ODSs through the demodulation method and arranging each single ODS according to the position of scan paths on the measured structure. Hence, the complete ODSs can be meshed after interpolation, instead of rough mode shapes formed by the LDV. In terms of model updating considering thermal environments (MUCTE), the target responses composed of measured natural frequencies and selected complete ODSs, which are more physical and correspond to the FE model nodes one by one, replace the past practice, whose ODSs are inaccurate and extracted by frequency response function and commercial software. Even simplifying the cumbersome process of reconstructing a model from response data, the selected complete ODSs are capable of improving model accuracy by a factor of twenty. The model updating based on sensitivity analysis is implemented with the parameter choice of Young's and shear moduli. After MUCTE, the difference between updated and measured natural frequencies is reduced three times as much as those with the past method. The updated target parameters accord with the physical properties of the material in thermal environments and the updated FE model is capable of predicting the first twelve modes of the structure in a uniform 300 °C field.