Experimental investigation on the concrete fracture process zone using electronic speckle pattern interferometry

In order to advance the understanding of fracture failure mechanisms in concrete, a series of three‐point bend tests of pre‐notched specimens were conducted to study the characteristics of the fracture process zone. Electronic speckle pattern interferometry (ESPI) was used to determine the location and size of the cracks. The experimental results have shown that crack in the pre‐notched concrete beams will initiate when the load increases to 40% of the peak load (Pmax) and expands unstably after reaching the peak load. When the load drops to about 25% Pmax in the load drop section, a complete fracture process zone (FPZ) was developed, and the corresponding crack tip opening displacement is about 7.1 times to the critical crack tip opening displacement. The analysis of surface strain of concrete specimen showed that microcracks will initiate when the surface tensile strain of concrete reaches about 2 × 10−4; when the surface tensile strain value of concrete reaches 4 × 10−2, the FPZ will move forward; the length of the FPZ of concrete essentially remains the same in the entire fracture process, which is about 71% of the ligament height. The analysis of experimental data also shows that the size of the FPZ is not significantly affected by the cubic compressive strength of concrete. In addition, the tension‐softening curves of the specimens were finally determined by using the incremental displacement collocation method, and it is concluded that the critical width of the FPZ is related to the cubic compressive strength of the concrete.