Microscopic Analysis of Degraded Living Hinges of Polypropylene Aerial Cable Closure

Abstract

Living hinges of polypropylene aerial cable closures that had been field-aged for eight years were analyzed in detail. Severe degradation including deep cracking was observed in the hinge central section, which had been subjected to UV light in the outdoor environment and repetitive sharp bending cycles during maintenance. On the other hand, the degradation of the other section was limited to within 10μm from the top surface. A repetitive bending cycle test alone did not reproduce the severe degradation. It was concluded that synergistic effects of solar UV irradiation and repetitive bending stress generated the severe degradation at the hinge section. Micro-cracks initially generated on the UV-degraded brittle top surface were the initial points of major cracks leading to breakage. Microscopic observations revealed craze structures around the major cracks. The crazes, caused by repetitive shear stress during the hinge-bending operations, were considered to be major routes for crack propagation. A test combining UV irradiation and repetitive sharp bending cycles on unused hinge specimens supported the crack generation and propagation mechanism.