| In this dissertation, I examine the behavior of polyurea and of cellular solids in biomechanical impacts in order to improve existing protective gear. The hyperelastic behavior of polyurea is used to explain its superior impact resistant properties over other materials. The ratios of the hyperelastic constants using the Ogden hyperelastic model demonstrate the ability of polyurea to resist compression in the loading direction. Furthermore, polyurea is able to withstand a level of energy at a lower strain than Viton and Treloar's (natural black) rubber (two hyperelastic materials chosen for comparison), which makes it more favorable in impact management. A thin layer of polyurea was able to reduce the likelihood of injury in each of the three biomechanical situations examined.;For a helmeted football player, a 0.8 mm layer of polyurea located on the inside of the helmet is able to reduce the force transmitted to the player's head by 17%. This in turn results in a reduction of the likelihood that this player would receive a concussion by nearly 20%. Furthermore, the location of the polyurea was found to be important, as a layer placed on the outside of the helmet actually increases the peak transmitted force, as it causes the helmet shell to stiffen.;A rear-foot strike runner can benefit from a thin layer of polyurea with a reduction of up to 13% in tibial axial acceleration, depending on the speed and mass of the runner. A modification of the midsole that includes a polyurea-filled hole below the calcaneus can reduce the tibial axial acceleration by about 30%. A drop-weight experiment that mimics a heel strike was performed that reduced the peak transmitted force by 14% and 12% for the 1.83 mm polyurea insole and the polyurea-filled hole, respectively. A finite element simulation provides numerical verification of the experimental results, as well as a dynamic model for designing new sneaker materials based on experimental data, rather than on measured velocities in static simulations. However, more testing in this impact scenario is suggested to ensure repeatability, including the use of more human subjects.;During a simulated lateral fall, a person can benefit from a 0.45 mm-thick layer of polyurea (approximately the thickness of five sheets of paper) adhered to the outside of a protective pad, which results in a reduction in peak force of 17%. The polyurea layer should be placed on the outside of the pad, on the side that is adhered to the pouch. Furthermore, analyzing both the rise time, as well as the peak force, as independent parameters in the design of protective padding is important. As components of the impulse, or momentum, the reduction of peak force and the increase in rise time are diluted, since the area under the force-time curve measured at the greater trochanter is constant. Thus, they must be analyzed as separate parameters in order to best understand the behavior of the pads during an impact. |
