A Laboratory Study on the Influence of Guided Drop Tower Carriage Mass and Kinematic Differences to Full-Surrogate Free Falls Toward Enhanced Helmet Certification Methods
Date
2024
Authors
Brice, Aaron
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Abstract
Falling from height presents a significant risk for military personnel due to the frequency at which they perform high exposure maneuvers, such as walking along unstable structures, repelling from buildings or aircrafts, and low altitude egressing. Traumatic brain injury (TBI) resulting from falls from height (FFH) account for approximately 20% of TBIs with a reported cause in the military, despite the presence of protective head gear. This is likely because current certification testing performed on military helmets emphasize protection against ballistic threats over blunt impacts, such as falls. Military personnel have identified the need for the next generation of helmets to provide better protection against blunt impacts. To develop such helmets, a method for helmet evaluation in scenarios that are representative of real-life falls must be established as the new standard for helmet impact testing.
Guided vertical drop towers are a test device commonly used to evaluate the impact attenuating properties of protective headgear in headfirst falls during certification testing. These devices provide a simple, low cost, repeatable means for conducting certification tests over using full-body surrogates to replicate a person experiencing a headfirst fall. However, there are some limitations to the guided drop tower that may limit their ability to properly replicate a fall from height. The most notable limitations are that guided drop towers are constrained to only a single degree of freedom and the impact mass of a drop tower assembly typically only includes the mass of a human head and neck rather than the mass of a full-body. At present there is little work on how these limitations may yield a differing kinematic response between a guided drop tower and that of an actual fall. The objectives of this thesis was to determine if kinematic differences exist between a guided drop tower and a free-falling person, in unhelmeted and helmeted scenarios. The outcomes of this thesis will contribute toward the development of enhanced test standards that evaluate protective headgear in scenarios that are more representative of real-life falls.
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A custom guided drop tower equipped with a Hybrid III head/neck and adjustable weight drop carriage along with a full-body Hybrid III 50th percentile male surrogate, to represent a falling person, were subjected to two experimental series 1) unhelmeted impacts at four angles between 30° and 75° and four impact velocities between 1.50 m/s and 3.00 m/s and, 2) helmeted impacts at 30° and 75° with impact velocities of 3.00 m/s and 4.50m/s. Impacts in both series were conducted onto a rigid impact surface and kinematic measures of head center of gravity linear acceleration, angular acceleration, and angular velocity were measured.
Results of the unhelmeted impact series identified that the drop tower can provide an acceptable approximation of the linear acceleration but not the angular velocity that is likely to be experienced by a person in a headfirst frontal impact. This is due to the angular velocity differing in either the magnitude of the peak angular velocity or direction and time instance of peak measures. Changes to the mass of the drop carriage, to be closer to that of a full dummy, did not bring angular velocity closer to that measured for the full dummy.
The helmeted impact study identified that a drop tower is likely to yield an underestimate of peak kinematics in shallow angle impacts and an overestimate of peak kinematics in steep angle impacts. This suggests that the drop tower, in its current form, provides a varying estimate of the resultant peak kinematics in helmeted impacts which is dependent on impact angle. These differences in response are primarily attributable to variances in helmet liner engagement when comparing the drop tower and a person falling.
The results of this research found that in their current form guided drop towers do not provide a true representation of the kinematic response that is likely to result in a headfirst fall, either unhelmeted or helmeted. Further the addition of mass to the drop carriage in either scenario did not alter the drop tower’s response to a point where it matched the measured response of the falling surrogate .These differences in kinematic responses between the drop tower and what is likely to be experienced by a falling person, specifically in the case of underestimated responses in shallow angle helmeted falls emphasizes the need to further develop testing methods to ensure that future helmets are evaluated in a way that effectively tests the helmet’s impact-attenuating abilities in an actual fall.