Calibration of a multi-scale DIC system using a stochastic phase target

Digital image correlation (DIC) is widely recognized as a powerful experimental method capable of providing highly accurate full-field data at different scales. Although the integration of multiple stereo DIC systems operating at different resolutions into a multi-scale framework has great potential, its practical implementation is limited by the difficulty of using standard calibration methods for global registration of data into a unified coordinate frame. To address this problem, this paper presents a novel method for calibrating a multi-scale DIC (MS-DIC) system using a stochastic phase pattern generated from a series of fringes displayed on a monitor placed in the defocused near-field of the cameras. The main advantage of the proposed approach is that a single pose of the monitor is sufficient to obtain an accurate initial estimate of both the intrinsic and global extrinsic parameters of all the cameras of the MS-DIC system. An additional optimization step using a single image of a speckle pattern in the in-focus measurement region is used to refine the calibration. The method is validated by estimating its metrological performance in shape and deformation measurements using a synthetic speckle pattern displayed on a monitor at different poses. As an illustrative example of application, the displacement and strain fields are measured at two different magnification factors for a plate with a central rectangular notch subjected to out-of-plane bending.