Projectile Impact on Glass and Polymeric Ophthalmic Lenses and Circular Plates

The ballistic limits and fracture behavior of six piano ophthalmic lenses, composed of glass, CR‐39 and polycarbonate, and of two types of disks composed of glass and CR‐39, respectively, were determined experimentally. Projectiles consisted of 4.5 min diameter lead spheres and slugs fired at velocities ranging from 38 to 185 m/sec. The targets were examined by means of a 24‐spark Cran‐Scbardin camera at interframe times as low as I μsec, using ordinary cinematographic, shadow‐optic and birefringent techiques. A ballistic limit of about 100 m/sec was established for the two CR‐39 lenses, while a limit of about 50 m/sec was found for all but one of the glass specimens and for the CR‐39 flat plate. The German glass lens tested shattered at the lowest achievable velocity. Large deviations in material properties were noted in presumably identical samples. Edge flaws in the specimens, but sometimes not the precise manner of the support, appeared to have a significant influence on the impact phenomenon. The presence of a membrane on the rear surface of various samples did not influence the ballistic linnit, but did occasionally affect the fracture pattern. Characteristic star and spiderweb crack configurations were observed when fracture was initiated at the impact point, irregular distributions were noted when failure was initiated at the edge, and a closely‐spaced, fine fan of curved cracks occurred when fracture commenced at some interior point.