AbstractThe punching shear failure of reinforced concrete flat plates has puzzled researchers for a long time due to the complexity of the punching shear mechanism and the difficulty of direct measurements of the internal failure mechanism. Measurements from previous punching shear tests cannot provide sufficient experimental evidence for revealing the failure mechanism. In this study, an industrial endoscope and specially designed strain measuring rods were installed inside small holes reserved in slabs around columns to track the occurrence and propagation of internal cracks of five isolated slab-column connections with different longitudinal reinforcement arrangements. The results indicated that critical shear cracks developed from internal diagonal cracks initiated at midheight of the slabs and unevenly distributed around the columns. The propagation of the internal diagonal cracks was independent of flexural cracks initiated at the slab tension faces but closely related to the unloading of radial strains of concrete at the slab compression faces. At the moment of punching failure, the critical shear cracks propagated from compression to tension zones, forming a punching failure cone. Moreover, the failure process was progressive, that is, the continuous weakening of the compression zone by the critical shear crack resulted in the shear splitting of the compression zone, which triggered the final failure of the connections. The experimental evidence obtained can provide a physical background and new ideas for modeling the punching shear mechanism of slab-column connections.