Temperature dependent dynamic response of open-cell polyurethane foams

Abstract

Polyurethane foams have many uses ranging from comfort fitting seats and shoes to protective inserts in helmets and sports equipment. This study aims to analyze the thermomechanical uniaxial compression behavior of two different polyurethane based foam helmet liner pads. These experiments were conducted under strain rates of 10$^2$ s$^{-1}$ and under temperature conditions ranging from -20 to 40\textdegree{C}. This temperature range was chosen to simulate desert and arctic conditions, with a strain rate regime chosen to represent loads that would occur often throughout the life of the helmet, such as drops, bumps from riding in a vehicle, or even falling after a parachute landing. These loads are deemed blunt impacts. Multiple experimental apparatuses were used in this study, including a Shimadzu TCE-N300 thermostatic chamber (used to create the varying temperature environments) and a custom-built drop-test system (used to induce intermediate strain rates). Every experiment was paired with a high-speed camera used for Digital Image Correlation (DIC) to analyze sample deformation. Using the resulting stress-strain curves generated, the foam's mechanical response and energy absorption properties were investigated. Additionally, each foam composition was analyzed with X-ray computed micro-tomography (XCT) to investigate microstructure properties pre- and post-mortem. In depth analysis allowed for accurate modeling of the foam storage properties and the rate dependency behavior as a function of temperature. The XCT was used to probe qualitative microstructure damage. It was found that the energy absorption capability of the low density composition decreased by 48\% as temperature went from 40 to -20\textdegree{C}. The high-density composition saw an inverse response, as energy absorption increased by 53\% as the temperature for the experiment decreased. A comparison between the loading response and the material density characteristics reveal that these particular foam protective properties are heavily dependent on strain rate, as well as temperature.