Mechanochromic molecules as in-situ stress and temperature sensors for high strain-rate loading of

Physics Thesis Presentation by LTJG Patrick Smith, USN

Abstract: We explore the use of mechanochromic molecules as in-situ stress and temperature sensors in heterogeneous polymer composites. These molecules may allow post-mortem analysis of localized hot spots and stress concentrations in plastic-bonded explosives without the need for external gauges and without altering mechanical properties. Our initial focus was on PMA and PMMA systems polymerized directly off spiropyran sensor molecules, a recently developed system which undergoes a color change during stress or temperature excitation. Previous literature has only considered low-rate dogbone testing, and it is unknown if activation will occur under rapid loading since polymer response normally varies dramatically with loading rate. We performed the first high strain-rate mechanical testing of this material using rapid compression from a split Hopkinson pressure bar, and show that indeed considerable spiropyran activation occurs near fracture surfaces and heated regions. Planar impact studies on a light-gas gun revealed that inclusion of the spiropyran does not change the shock response of PMA. To separate the effects of temperature and stress, we also examined a stress-inactive spiropyran/PMA system as well as a new Diels-Alder adduct which responds only to stress. Finally, we report initial analysis of mock explosive formulations embedded with mechanochromic sensors.

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Jul 06, 2013

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