Experimental study on the fragment characteristics of marble spheres under repeated impacts

Impact-induced fragmentation of rock blocks is a frequent phenomenon during rockfall hazards, which significantly increases the safety risk to local community and infrastructure. This paper aims to thoroughly investigate the fragment characteristics of rockfall caused by multiple impacts, including the degree of fragmentation and the fragment size distribution. Forty-seven sets of repeated impact tests were conducted using a new impact device. Marble spheres of different sizes repeatedly collided with three types of target plates under various velocities. The mass of each fragment was measured separately after the sphere was broken. It was determined that the fracture patterns of the marble spheres may change with an increase in the impact velocity, which causes a critical velocity to divide the degree of fragmentation of marble spheres into two phases. Above the critical velocity, the degree of fragmentation rose with an increase in the impact velocity. However, below the critical velocity the degree of fragmentation remained steady. In addition, the sphere size and the target plate material also affected the degree of fragmentation for the large spheres with high impact energy. The fragment size distribution depended on the degree of fragmentation, and the ratio of the largest fragment mass to the initial sphere mass decreased with an increase in the degree of fragmentation. A new simple model containing a single parameter with a clear physical meaning was established to describe the fragment size distribution with great accuracy.