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Repetitive impact exposure and characterization of stress-whitening of an American football helmet outer shell material
Affiliation:1. School of Polymers and High Performance Materials, The University of Southern Mississippi, Hattiesburg, MS, USA;2. School of Kinesiology, The University of Southern Mississippi, Hattiesburg, MS, USA;1. SMC Corporation, College Station, TX 77845, USA;2. Solid State Ionics and Surface Sciences Lab, Department of Physics, Southern University and A&M College, Baton Rouge, LA 70813, USA;3. Department of Physics, National Institute of Technology, Warangal, India;4. The University of Southern Mississippi, College of Science and Technology, 730 E Beach Blvd, Long Beach, MS 39560, USA;5. Institut für Chemie, AG Elektrochemie, Technische Universität Chemnitz, D-09107 Chemnitz, Germany;1. Concordia Centre for Composites (CONCOM), Department of Mechanical and Industrial Engineering, Concordia University, 1455 De Maisonneuve Blvd. W., Montreal, Quebec, H3G1M8, Canada;2. Center for Applied Research on Polymers and Composites (CREPEC), Canada;1. Faculty of Civil Engineering and Mechanics, Jiangsu University, Zhenjiang, Jiangsu, 212013, China;2. Beijing Key Laboratory of Aeronautical Materials Testing and Evaluation, Beijing Institute of Aeronautical Materials, Beijing, 100095, China;1. Department of Mathematics, The University of Southern Mississippi, Hattiesburg, MS 39406-5045, USA;2. Department of Mathematics and Statistics, University of Missouri – Kansas City, Kansas City, MO 64110-2499, USA;1. The School of Kinesiology and Nutrition, The University of Southern Mississippi, Hattiesburg, MS, United States;2. The School of Polymer Science and Engineering, The University of Southern Mississippi, Hattiesburg, MS, United States;3. College of Education and Human Development, George Mason University, Fairfax, VA, United States
Abstract:Mechanical stress exerted upon impact-modified polycarbonate (PC) and poly(ethylene terephthalate) (PET) blends has been reported to generate microscopic voids via rubber-toughener (RT) particle cavitation which can macroscopically manifest to visibly whiten the material. Previous work has revealed a whitening phenomenon in collegiate American football helmet outer shells after a single season and in helmet-grade plaques following linear impact testing. The purpose of this research was to quantify the effects of repetitive linear drop exposures on the (i) impact performance; (ii) physical and thermal properties; and (iii) surface and tensile mechanical properties of a stress-whitened American football helmet outer shell material. Statistically significant changes in plaque impact performance corresponded to substantial stress-whitening that penetrated up to 40–45% into the plaque thickness and elicited shifts in surface and tensile mechanical properties. Nanoscale microscopy revealed elongation of the PC/PET matrix and delamination at the RT-matrix interface. Thermal property analysis suggested the concomitant occurrence of RT cavitation and strain-induced PET crystallization. Overall, the research identified a battery of diagnostic tools to characterize material property changes of stress-whitening in rubber-toughened helmet outer shell materials.
Keywords:Stress-whitening  Rubber-toughening  American football helmet  Outer shell degradation  Impact performance
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