Dual-side amplitude-fluctuation ESPI for simultaneous vibration-mode characterization of composite panels

For composite panels exhibiting dissimilar geometric characteristics or asymmetric damage distributions on opposing surfaces, distinct vibration modes may emerge on each surface at the same excitation frequency. To address this challenge, this Letter proposes a dual-side amplitude-fluctuation electronic speckle interferometry (DAF-ESPI) system capable of simultaneously characterizing the vibration behavior on both surfaces of a panel. The proposed system consists of two Michelson-type interferometric configurations symmetrically arranged on opposite sides of the specimen, enabling synchronous acquisition of the vibration modes on both surfaces. The feasibility of the system is first validated through experiments on an aluminum alloy panel and subsequently demonstrated on honeycomb sandwich panels containing various internal defects. Experimental results confirm that the DAF-ESPI system provides a comprehensive understanding of the panel's vibration characteristics, highlighting its potential as a practical tool for internal defect detection in composite structures.