The Hybrid III and NOCSAE headforms are two headforms used in impact testing, though their vibrational characteristics are not well understood. They may have different kinematic responses in various impact scenarios if the impact excites any of their natural frequencies; resonance is especially likely to occur in short-duration impacts with a wider frequency spectrum. The same impact on two headforms that perform similarly in blunt impacts can be much different in shorter-duration projectile impacts due to the vibrational responses. The research presented in this thesis had three objectives: to identify the natural frequencies of the Hybrid III and NOCSAE headforms and compare them with published human head values to determine which has a more biofidelic vibrational response; to quantify the frequency response of different baseball catcher's masks and assess their abilities to limit vibrations transferred to the headforms; and to compare kinematic and frequency responses between headforms in different impact scenarios (high-speed, low-mass projectile impacts vs. low-speed, high-mass pendulum impacts) and see how they are affected by various types of head protection. The results show the importance of considering frequency content in impact testing, suggesting that the NOCSAE headform may be more biofidelic in short-duration impacts since its natural frequencies better align with those seen in the human head. The catcher's masks experienced greater vibrational responses than the headforms, but since the NOCSAE's first natural frequency falls within the bandwidth being excited, resonance was seen in this headform's acceleration responses for the projectile baseball impacts. Lastly, while both headforms had higher peak linear accelerations (PLAs) from the short-duration projectile impacts than the pendulum impacts, the projectile impacts caused high frequencies to be excited in the NOCSAE headform, while only exciting low frequencies in the Hybrid III. These results may not be as relevant for long-duration loadings, as indicated by the similar responses between headforms for both the pendulum and helmeted projectile impacts. However, when a wide range of frequencies are being excited with short-duration impacts, these results are important to consider, since natural frequency excitation can influence head injury risk due to higher accelerations. / Master of Science / The Hybrid III and NOCSAE headforms are dummy headforms used in impact testing, but their vibrational characteristics are poorly understood. They may perform differently in certain loading environments due to structural differences; their frequency responses might differ based on impact characteristics. Short-duration impacts excite a wider range of frequencies than longer-duration (padded) impacts. While headforms generally perform similarly during padded impacts where resonant frequencies are avoided, excitation of these frequencies during short-duration impacts can result in different kinematic measurements between headforms. The research presented in this thesis had three objectives: to identify the natural frequencies of the Hybrid III and NOCSAE headforms and compare them with published human values to determine which better represents the head's vibrational response; to quantify the vibrational characteristics of different baseball catcher's masks and assess their abilities to limit vibrations transferred to the headforms; and to compare kinematic and frequency responses between headforms in different impact scenarios (high-speed, low-mass projectile impacts vs. low-speed, high-mass pendulum impacts) and see how they are affected by various helmets. The results show the importance of considering frequency content in impact testing, suggesting that the NOCSAE headform behaves more like the human head in short-duration impacts. Even though the catcher's masks "rang" more than the headforms, the vibrations from the projectile impact were in the appropriate range to excite the NOCSAE's natural frequencies. Thus, there was still an oscillatory response in this headform even when protected with the mask. Lastly, the projectile impacts caused higher accelerations in both headforms than the pendulum impacts. However, high frequencies were only experienced by the NOCSAE headform due to the projectile impacts; for the same impact, the Hybrid III just had low frequencies excited. These results are not as relevant for long-duration impacts, since there were similar responses in both headforms for both the pendulum and helmeted projectile impacts. However, they are very applicable for the short-duration impacts that excite a wide range of frequencies, since natural frequency excitation can increase the risk of head injury due to higher acceleration magnitudes.

Traumatic brain injuries (TBIs) encompass an array of head trauma caused by diverse mechanisms, including falls, vehicular accidents, and sports-related incidents. These injuries vary from concussions to diffuse axonal injuries. TBIs are characterized by the linear and rotational accelerations of the head during an impact, which are influenced by various factors such as the velocity and location of the impact and the contact surface. Consequently, the accuracy of laboratory tests designed to evaluate protective technologies must closely mirror real-world conditions. This dissertation explores the boundary conditions essential for accurately replicating head impacts in laboratory settings. The research aims to improve the reconstruction of head impacts, concentrating on two main areas: 1) examining various aspects of friction during head impacts and 2) biomechanically characterizing the head impacts sustained by older adults during falls. This study provides insights into the overall influence of friction during head impacts. It investigates the friction coefficients between the helmet's shell and the impact surface, as well as between human heads, headforms, and helmets. Additionally, it assesses how these frictional interactions influence oblique impact kinematics. Defining static and dynamic friction coefficients of the human head and headforms is needed to develop more realistic head impact testing methods, define helmet-head boundary conditions for computer-aided simulations, and provide a framework for cross-comparative analysis between studies that use different headforms and headform alterations. This research also introduces and evaluates the accuracy of a model-based image mapping method to measure head impact speeds from single-view videos in un-calibrated environments. This measurement technique advances our comprehension of head impact kinematics derived from uncalibrated video data. By applying this method, videos of falls involving older adults were analyzed to determine head impact speeds and boundary conditions. The resulting data was used to construct headform impacts, capturing linear and rotational head impact kinematics. These reconstructions can inform the development of biomechanical testing protocols tailored to assess protective gear for older adults, with the goal of reducing fall-related head injuries. / Doctor of Philosophy / Traumatic brain injuries (TBIs) are head injuries that can happen in many ways, such as from falling, car accidents, or playing sports. These injuries can range from mild concussions to more severe cases, brain bleeds, or skull fractures. They happen when the head moves quickly or spins because of a hit, which can be affected by the speed of the impact, where on the head the impact happens, or what the head impacts against. Therefore, the accuracy of laboratory reconstruction head impact tests must closely mirror real-world conditions. This dissertation explores the boundary conditions essential for accurately replicating head impacts in laboratory settings. The research aims to improve the reconstruction of head impacts, concentrating on two main areas: 1) examining various aspects of friction during head impacts and 2) biomechanically characterizing the head impacts sustained by older adults during falls. This study provides insights into the overall influence of friction during head impacts. It investigates the friction coefficients between the helmet's shell and the impact surface, as well as between human heads, headforms, and helmets. Additionally, it assesses how these frictional interactions affect head impacts. Understanding how friction influences head impacts is crucial for improving helmet testing methods and allows for more consistent comparisons across various research studies that use different headform models or modifications. This research also introduces and evaluates a method to calculate head impact speeds by analyzing video footage, even if the video was not taken with special equipment or setup. This approach improves our understanding of head movements during accidents by using video clips of falls, particularly those involving older adults, to determine the head speeds and conditions of the impact. The information gathered from these analyses helps to reconstruct these impacts using a headform to measure injury metrics. These reconstructions are crucial for designing tests that can evaluate safety equipment meant to protect older adults from head injuries during falls.