Abstract: Fracture behaviors of concrete materials exhibit certain nonlinearity due to the existence of a fracture process zone (FPZ) and they play a crucial role in determining the bearing capacity and durability of hydraulic structures. To study the nonlinear fracture behaviors, we conduct three-point bending tests on pre-notched concrete beams and measure their full-field displacements using an electronic speckle pattern interferometry (ESPI). Variations in crack evolution are examined through analysis on the relationships of crack opening displacement (COD) versus crack length a at 15 loading levels. And new formulas are derived for the COD-a relationships based on ESPI measurements to replace the existing formula that is based on linear elastic fracture mechanics (LFEM) and inapplicable near a crack tip.