Dynamic fracture of beam-column specimens containing pre-existing cracks under impact loads

An experimental system of dynamic caustics was applied to carry out fracture experiments for pre-cracked beam-column specimens. The following dynamic fracture behaviors of the specimens under impact loads were discussed: the crack extension tracks, velocities and stress intensity factors at the beam-column joints, the beams and the column ends. The experimental results indicate the following findings. Under impact loads, first, the specimens were cracked at the beam-column joints, and in the process of crack extension, the specimens underwent apparent bending fracture, both of which can explain that the beam-column joints are most vulnerable to damage. The more the pre-cracks, the later the beam-column joints crack, which displays that the less the pre-cracks, the more concentrated and faster the energy can accumulate at the crack tips. For a specimen with pre-existing cracks at the ends, the time interval between the initiations of two cracks is longer than that for one with pre-existing cracks at the beam. However, the second crack, in most cases, tends to occur in the beam. This phenomenon indicates that the fracture of a fixed bearing requires more energy than a simply-supported beam. The bending fracture phenomenon of a beam weakens as the pre-existing cracks increases. This is because the partial energy accumulates at the crack tips of the column ends, accordingly the energy inducing the cracks at the beam to expand, decreases. Due in large part to the cracks on the columns, the tensile stresses and bending moments at the crack tips on the beams decrease, which affect the bending fracture phenomenon of the specimens.