Investigation of membrane tearing characterization and healing
Zignego, Donald Lee
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As man's curiosity for deep space exploration increases, so does the demand for long duration space systems. To be able to explore deeper into space, a system is required in which little or no fuel is required for propulsion. Solar sails met this challenge by using solar photon momentum exchange for propulsion. Solar sails are large gossamer structures spanning as long as three football fields in width and length, but only microns in thickness. The main photon catching structure is constructed of thin polymeric membranes, which are quite susceptible to damage. Other applications for these polymeric membranes include flex circuits, and thermal control membranes. The tearing behaviors of these materials are vital in the design of these structures. In this study, the tear behaviors of two popular thin polymeric materials used in the space industry are explored; biaxially-oriented polyethylene terephthalate (Mylar) and Kapton. Tearing properties for the Mylar were experimentally determined using essential work of fracture methodology. Experiments were performed on DENT specimens with varying ligament lengths, and on three different thicknesses of Mylar: 25.4 micron (1 mil), 50.8 micron (2 mil), and 76.2 micron (3 mil). Specific essential work of fracture values were determined to be 37.027 kJ/m ², 36.974 kJ/m ², 36.853 kJ/m ² for the 25.4 micron, 50.8 micron, and 76.2 micron thick Mylar specimens, respectively. The specific essential work of fracture values were nearly identical for the three separate thicknesses, and therefore can be concluded that it is indeed a material property of the Mylar itself. Using biology as inspiration, the possibilities of self-healing in both the Mylar and Kapton are also analyzed. Analysis of the possibilities of self-healing was studied using an ultrasonic transducer. Specimens with an initial crack were created and strain or fracture energy values computed with and without healing. The Mylar's ability to resist tearing increased as much as 1300% in the healed specimens, and likewise as much as 950% increase for the Kapton. Tear tests were also performed at sub-zero temperatures to analyze the effects at space equivalent temperatures.