Investigation of the physical mechanism of wall hollowing based on laser acoustic-vibration non-destructive testing

Existing laser vibrometry-based inspection techniques primarily focus on defect localization and vibration visualization, whereas the quantitative determination of internal defect geometry remains challenging. This study develops a physics-based nondestructive evaluation framework by integrating acoustic excitation, scanning laser Doppler vibrometry, and a mechanical impedance model. Three geometric parameters are considered: defect area, overlying-layer thickness, and cavity thickness. The proposed method identifies the equivalent stiffness, mass, and damping of defect regions from measured vibration responses and investigates their relationships with defect geometry, with quantitative inversion primarily focused on defect area. Systematic experiments were conducted on gypsum and C35 concrete specimens containing prefabricated internal cavities with different geometric configurations. The results demonstrate that the proposed framework achieves quantitative inversion of defect area, with mean absolute relative errors of 8.62% for eight gypsum specimens and 7.66% for five selected concrete specimens. The effects of overlying-layer thickness and cavity thickness on resonance behavior were also investigated through theoretical analysis, experiments, and three-dimensional finite element simulations. Compared with conventional SLDV-based defect imaging approaches that mainly rely on vibration amplitude distributions or qualitative interpretation, the proposed method provides a physically interpretable framework for quantitative defect area estimation and the analysis of wall hollowing mechanisms.