Each year, millions of mild traumatic brain injuries (mTBIs), or concussions, occur as a result of sports and recreational activities. These injuries produce physical and chemical changes within the brain resulting in physical, emotional and financial complications. Compared to other sports or recreational activities, incidence rates of concussion are especially high in football across all levels of play. In response, the NFL has released several finite element analysis (FEA) models of popular football helmets which researchers may use to innovate and evaluate new helmet technologies.

There are several key metrics applied in the analysis of football helmet performance including the head injury criterion (HIC), which is a measure of resultant linear acceleration and time, and diffuse axonal multi-axis general evaluation (DAMAGE), which is a direction specific measure of rotational acceleration. It is important to recognize that no singular metric can adequately capture all mechanics of an impact with may influence severity scores. Moreover, there are some mechanics of impact, such as displacement, which are typically not feasible to measure experimentally. As such, it is beneficial to use FEA, a computational method of analyzing complex structures, to evaluate impacts with greater granularity than experimental analysis. Given the relationship between displacement and acceleration, it was hypothesized that helmets which maximize displacement without densification over the duration of an impact would produce lower severity metrics scores.

First, a design of experiments was conducted subjecting 3 popular football helmet models to the NFL helmet testing protocol in LS-Dyna, a finite element analysis solver. Helmet performance was evaluated using several metrics including HIC, DAMAGE, peak resultant linear acceleration and peak resultant angular acceleration. A novel computational tool for determining the normal and tangential components of displacement of a pad under impact was presented and used to evaluate relationships between displacement and impact severity metrics. It was found that correlations arise between normal displacement and linear acceleration-based metrics in critical locations. For example, the strongest correlation between normal displacement and HIC (R² = 0.8) was found at the side upper (SU) location which has been shown to correlate most strongly with the probability of concussion. Correlations were not observed with respect to tangential displacement.

Considering the results of this work, namely the relationship between normal displacement and linear acceleration-based metrics, a novel FE model of LCE for impact simulations in presented and subjected to drop test load cases. It was hypothesized that, due to the rate dependence of LCEs, there exists some optimal kinetic energy input resulting in a peak displacement correlated to a minimum peak acceleration. Moreover, it was hypothesized that the lowest severity metric scores would be observed at the optimal impact conditions. For this effort, the LCE model was defined in 2 geometries and subjected to drop impacts with varying kinetic energy inputs. It was observed for each geometry that there was an optimal kinetic energy input that produced the lowest severity metric scores. It was also shown that for displacements both less than and greater than the optimal, severity metric scores increased. Minimum acceleration increased by an average of 60% between drop heights. Optimal displacement varied by less than 1% on average between drop heights and was observed to fall between 16 and 17 mm.

In the context of helmet design, there are a variety of materials which have been used to attenuate impact energy. As such, this research also presented a model for a non rate dependent (NR) material designed to be an analogue to traditional helmet foams and a model possessing increased rate dependence (IR) with respect to the LCE. The goal of this effort was to assess the performance, and thus potential for use in helmet impact applications, of LCE compared to potential alternative helmet pad materials. As expected, results indicated a singular optimal impact condition for the NR material with HIC increasing linearly with displacement at impact energies greater than that condition. The NR model resulted in greater HIC value than the LCE or IR models across nearly all impact conditions.

In general, the optimal displacement for the LCE and IR models was nearly identical. However, the LCE produced lower severity metric scores at lower kinetic energy inputs. Under optimal input conditions, the performance between the 2 models was nearly identical while the IR model produced lower severity metrics at input energies above the optimal. The greatest difference in minimum acceleration observed between the LCE and IR models was 8%. The results of this work indicate improved performance of LCE over the NR material and comparable performance with respect to the IR material which possesses 20% greater rate dependence. Given the similarity in performance between LCE and IR materials and the manufacturability of LCE, it was concluded that LCE is a great option for use in helmet impact applications.

The primary goal of this research was to further explicate the mechanics of impact which influence severity metrics. It is the first to present a computer code capable of explicating the normal and tangential components of displacement of a pad under impact. Drawing on the understanding gained from the application of this code, this research also sought to investigate the performance of an LCE material with respect to metrics used in the evaluation of overall football helmet performance. This work is also the first to present an FE model of LCE for impact applications. Results of this work have also presented a means of predicting displacement and HIC outcomes of a given simulation using the known inputs of kinetic energy and volume. The value of these insights is that they expand on the performance of individual materials which may be used in helmet pad structures. The performance of individual materials inherently influences the performance of the larger pad structure and thus the helmet as a whole. Therefore, the results of this research have demonstrated a framework which researchers may employ in the investigation of novel football helmet technologies.