Evolution of the FPZ in steel fiber-reinforced concrete under dynamic mixed-mode loading

This work studies the fracture process zone (FPZ) evolution of steel fiber-reinforced concrete under dynamic mixed-mode loading. We made beams with fiber contents of 0%, 0.4%, and 0.8% in volume and tested them in three-point bending at low displacement rates (2.2 μm/s and 22 mm/s) and impact rates (1.77 m/s and 3.55 m/s). All beams had a notch in the center of the half-span to facilitate a mixed-mode process zone extended from its tip. We recorded the entire crack propagation process in all the tests with a high-speed camera. Using the digital image correlation (DIC) measurements, we locate the FPZ tip from the tensile strain field and determine the stress-free crack tip from the displacement jumps. In this way, we capture the entire process of FPZ evolution. The FPZ stretched with crack propagation until a maximum —full FPZ length— and shortened afterward. The FPZ length at peak load and its maximum value increased with the loading rate. The loading rate and the fiber content have opposite effects on the full FPZ length. The faster the loading rate, the longer the full FPZ length. The more fiber content, the shorter the full FPZ length. In addition, the fiber addition helped to generate multiple FPZs and to cause more distributive damage in the matrix. Under impact loading, the cracked matrix and the steel fibers work in a synergetic way to dissipate the excessive energy input due to impact.