Figures of the experimental system: (a) the 3D figure of the experimental system, (b) the schematic of the loading section, (c) the field photo of experiment. In panel (a), 1: X type lifting platform, 2: S type force transducer for normal force, 3: PMMA specimens consisted of three blocks, 4: S type force transducer for tangential force, 5: tangential cylinder, 6: motor driver, 7: LCD displayer, 8: data acquisition devices, 9: camera.