Phase-based vibration imaging for structural dynamics applications: Marker-free full-field displacement measurements with confidence measures

Inspired by the concept of thermal imaging, we develop a phase-based vibration imaging tech-nique using marker-free full-field displacement measurements, which aims to determine vibra-tional energy distribution and possibly vibration parameters as the thermal imaging determining thermal energy distribution using colormap. In this study, accurate full-field measurements are conducted at the sub-pixel level by performing a full-field phase-based optical flow with three novel progresses. First, an image-preprocessing Gaussian smoothing gradient is applied to reduce the nonlinear phase caused by homogenous patterns and image noise. Second, two confidence measures (phase nonlinearity and filter response) are adopted to obtain reliable displacement measurements estimated from the multi-scale and multi-direction phases. Third, the combination of vector average and the intersection of constraints ensure the accuracy of integrating every pixel's displacement components. The vibrations can then be imaged by analyzing the acquired full-field displacements using techniques such as mean absolute deviation and frequency analyses. The capability of the proposed technique is demonstrated through numerical experiments using artificial patterns. After then, experimental validation is performed, which includes full-field operational deflection shape extraction on a 5-story structure and vibration imaging of an in-service hydrogen charging station, further proving the effectiveness of the proposed vibration imaging technique.