To investigate the bonding mechanical properties between polypropylene fiber and concrete interface, this study analyzed the debonding process of the interface through single fiber pull-out microscopic mechanical experiments and numerical simulations. An in-situ scanning observation system was established using micro CT and a self-developed single fiber drawing device to observe the process of pulling out a single polypropylene fiber with indentation from the mortar matrix. The deformation fields of the interface between fiber and matrix was obtained with mechanically regularized global digital volume correlation, and the interface debonding was quantified by calculating the relative displacement of the shared nodes between the fiber and the matrix. A 3D microscopic numerical model reflecting the true shape of fibers and matrix was established based on CT images, and the single fiber drawing process was simulated and analyzed. The results show that the force-displacement curves display multi-peak fluctuations corresponding to the fiber geometry after the peak. The strain fields at interfaces measured by digital volume correlation and numerical simulation show a strain concentration phenomenon related to the geometric shape of the indentation fiber, indicating that the periodic indentation of the fiber increases mechanical interlocking and friction forces between the fiber and the matrix during pullout. The relative displacement at the interface is greatest and decreases along the fiber's axial direction. In the horizontal direction, the variation of relative displacement was correlated with the geometric shape of the fiber. The relative displacement in the vertical direction reflected that the fiber and matrix have completely debonded before the pullout force reaches the peak load.

This study proposes a new algorithm to enhance the accuracy of measuring internal deformation, identify and quantify damage in rocks using digital volume correlation (DVC). This algorithm integrates the multimesh refinement technology into a mechanically regularized global DVC algorithm that incorporates nodal force equilibrium. The accuracy of different DVC algorithms was assessed through scanning experiments on particles with different sizes and digital virtual fracture models. The internal deformation and damage patterns of red sandstone were quantitatively analyzed using in-situ CT scanning during uniaxial compression experiments. The results indicate that the introduction of the mechanical regularization term reduced the measurement uncertainty of the global DVC method by 1-2 orders of magnitude. For the red sandstone specimens, the new algorithm was able to identify sub-voxel microcracks with an aperture of 0.15 voxels, while reducing the computational cost by 85.15%. The damage quantification analysis revealed that newborn cracks with a maximum aperture of 0.32 voxels occurred in the specimen when the axial stress reached 50.58% of its peak value, and these microcracks evolved into voxel-scale fractures when the stress increased to 84.27% of the peak value. The new algorithm not only improves the computational efficiency, but also demonstrates a strong capability in identifying sub-voxel scale newborn cracks, providing a novel method for quantifying internal deformation and damage in rocks.