Fragmentation of a sphere initiated by hypervelocity impact with a thin sheet

Selected results of tests in which 9.53-mm-diameter, 2017-T4 aluminum spheres impacted 0.25-mm- to 4.80-mm-thick, 6061-T6 aluminum sheets are presented. Impact velocities for these tests ranged from 1.98 km/s to 7.38 km/s. Flash x-rays were used to view the debris clouds produced by the impacts. As impact velocity was increased, failure of the aluminum sphere progressed through the following stages of fracture and fragmentation: (1) formation of a spall failure at its rear surface, (2) development of a detached shell of spall fragments, and (3) complete disintegration of the sphere. The threshold impact velocity for development of the spall failure in the sphere was observed to be a function of the bumper-thickness-to-projectile-diameter ratio (t/D), and to increase as the t/D ratio decreased. When the debris cloud was fully developed, the disintegrated projectile formed its dominant feature--an internal structure, composed of a front, center, and rear element, located at the front of the debris cloud. The front element was small and consisted of finely-divided projectile and bumper material. The bulk of the fragmented projectile was contained in the center element, a disc-like structure made up of a large central fragment surrounded by numerous smaller fragments. A shell of fragments, spalled from the rear of the sphere, formed the rear element. Radiographs of the debris clouds were analyzed to determine the size and size distribution of certain fragments within the cloud. The size of the large fragment was shown to be dependent on impact velocity and t/D ratio. The smaller fragments in the center element were several times larger than the fragments in the shell of spall fragments forming the rear element. Detailed analyses of fragments in the shell of spall fragments were made. The analyses indicated their median Martin's statistical diameter exhibited an orderly dependence on impact velocity and t/D ratio.