Date of Award

Fall 12-11-2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Polymers and High Performance Materials

Committee Chair

Dr. James Rawlins

Committee Chair Department

Polymers and High Performance Materials

Committee Member 2

Dr. Scott Piland

Committee Member 2 Department

Human Performance and Recreation

Committee Member 3

Dr. Trent Gould

Committee Member 3 Department

Human Performance and Recreation

Committee Member 4

Dr. Jeffery Wiggins

Committee Member 4 Department

Polymers and High Performance Materials

Committee Member 5

Dr. Sergei Nazarenko

Committee Member 5 Department

Polymers and High Performance Materials

Abstract

The purpose of this dissertation is to gain a greater scientific understanding of the changes in functional material properties and impact performance of an American football helmet outer shell material under expected service life exposures. The research goals are to (i) quantify chemical, physical, thermal, and mechanical degradation of an American football outer shell material under expected environmental conditions and (ii) develop a linear drop test impact protocol to employ expected on-field impact conditions to American football helmet components and a plaque-foam (i.e., shell-liner) surrogate. Overall, a step-wise progression of analysis was demonstrated to concurrently quantify and understand changes in material properties at the molecular, microscopic, and macroscopic levels. Changes across chemical, physical, thermal, and mechanical properties were evaluated following laboratory exposures to 480 hours of accelerated weathering, increasing intensities of n-Butyl acetate solvent, and 12 repetitive linear plaque-foam impacts.

In Chapter II, an instrumented drop test setup was substantiated to investigate linear impact attenuation performance. In Chapter III, laboratory exposure to UV light, oxygen, moisture, and elevated temperatures induced molecular degradative bi-products and physical aging up to ~1% into the plaque thickness which led to altered aesthetic properties, chemi-crystallization, and an increased resistance to surface indentation and tensile deformation. In Chapter IV, solvent-induced plasticization, crystallization, and stress-cracking of up to ~3% into the plaque thickness led to an increase in surface porosity which scattered light and decreased tensile properties. In Chapter V, impact exposure induced rubber-toughener (RT) cavitation that generated voids via delamination at the RT-matrix interface at which led to rings of stress-whitening, strain-induced crystallization, increased butadiene RT density, and shifts surface modulus and tensile properties. This dissertation preliminarily substantiated (i) a drop test setup attempting to accurately replicate on-field impact conditions to investigate linear impact attenuation performance, and (ii) polymer techniques and protocols that could elucidate the rates and degrees of material degradation.

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