Mode III fracture performance of impact-damaged marble under different impact cycles and pressures

To investigate the influence of such disturbances on Mode III fracture behavior, ENDB specimens with varying degrees of impact-induced damage were prepared and tested. The results indicate that both increasing impact cycles and higher air pressure promote the initiation and linkage of internal microdefects, resulting in cumulative and irreversible damage. Compared with intact specimens, the Mode III fracture toughness (KIII) decreases by 15.2%, 22.2%, 40.4%, and 53.1% after 1 to 4 impact cycles, respectively. A similar reduction trend is observed as the impact pressure increases. In addition, prior impact damage significantly affects crack propagation behavior. The average crack growth velocity declines continuously with increasing impact cycle. After a single impact, the propagation speed drops from 193.62 m/s to 85.47 m/s, corresponding to a reduction of 56%, while a total decrease of 91.8% is observed after four impacts. A pronounced reduction in crack velocity is also observed with increasing impact pressure, with a decrease of 92.4% as pressure rises from 0.35 MPa to 0.45 MPa. Fracture surface analysis further reveals that both higher impact cycle and pressure lead to a broader distribution of surface height variations and a noticeable increase in fractal dimension. These changes indicate a progressive enhancement in fracture surface roughness under stronger dynamic disturbance conditions.