Acrylic Plastic Spherical Shell Windows Under Point Impact Loading

Abstract

Acrylic plastic spherical shell sector windows with 117-deg included angle and outside radius of 24-in. (61 mm), have been impacted at their center, with a 12,500 lb (5662 kg) weight, in a simulated ocean environment. Velocities of impacts ranged from 0.205 to 10.702 ft (0.06 to 3.26 m) per second. It has been found that fracture of windows is initiated by tensile stresses on the concave surface of the window, directly below the point of impact. Compressive stresses, generated by external hydrostatic pressure, decrease the destructive effect of tensile stresses introduced by point impact loading. For 2.25 and 4.0-in. (57 and 101 mm) thick windows the critical impact velocities were found to fall into the 1.5 to 3 ft (0.45 to 0.91 m) per second range, the exact value being a function of window thickness and external hydrostatic pressure. A finite element analysis was found to agree rather well with the experimental. This analysis can be employed to predict, with a reasonable degree of confidence, the critical impact velocities for acrylic plastic spherical windows in the bows of submersibles.