Abstract
Head injuries are common in motor vehicle collisions, with differences in head injury risk varying with occupant characteristics. The mechanics of the neck ultimately determine the motion of the head, affecting the head’s interaction with interior vehicle structures and subsequent injury risk. Variations of the neck, across sex, anthropometry, and muscle tensing ability are likely to affect overall head and neck motions and loads. It is critical that occupant models used to assess vehicle safety, like physical crash test dummies and computational models, capture the human-like neck response. These occupant models often represent certain portions of the population, like small females and midsize males. The neck flexion response for midsize male volunteers, which includes the effect of muscle tensing, has been quantified previously at the Naval Biodynamics Laboratory (NBDL), but there is not an analogous dataset for small female volunteer neck biomechanics.
Biofidelity corridors quantifying the neck flexion response in frontal impacts for small female volunteers are needed. Direct data collection on small female volunteers outside low severity impacts is not possible, so alternative methods were explored to estimate the volunteer-like neck biofidelity targets for small females across a range of impact levels, specifically 3-g (Δv 20 km/h), 8-g (Δv 43 km/h), and 15-g (Δv 61 km/h). These methods include leveraging experimental data from both post-mortem human subjects (PMHS) testing and volunteer subjects, analytical comparisons across sex and anthropometry, and simulating impacts with existing occupant models.
Small female (approximately 5th percentile) and midsize male (approximately 50th percentile) PMHS were tested in matched conditions that reflected the original NBDL methodology at 3-g and 8-g. Head excursion, rotation, and moment at the base of the neck were larger for the midsize male PMHS compared to the small female PMHS; the motion measures were larger for the midsize male PMHS compared to the original NBDL testing with midsize male volunteers, while the moment was smaller for midsize male PMHS compared midsize male NBDL volunteers. In a retrospective analysis of volunteer testing performed by the Air Force in the 1980s through 2000s across impact severities from 4-g to 10-g, sex was the most significant predictor of head excursion and rotation while accounting for differences in whole body height and weight. The relationship observed between the midsize male and small female PMHS was reflected in the Air Force volunteer testing analysis, where males have larger magnitude kinematics. These experimental data highlight the expected directional shift in magnitude from a midsize male to a small female in neck biofidelity targets in subject types with and without muscle contraction (e.g., volunteers and PMHS).
To estimate how much neck flexion biofidelity targets shift between volunteers of different sizes (in the NBDL testing condition), scaling was applied to original NBDL midsize male volunteer motions and moments to estimate small female volunteer motions and moments. Using a scaling factor based on neck length provides a reasonable estimation of the small female volunteer response. However, based on initial assessments in scaling PMHS data, the scaled estimate from NBDL volunteer data is likely an underprediction of actual small female targets and provides a lower bound for small female volunteer head excursion, rotation, and moment. Because experimental data showed that the small female response was smaller in magnitude compared the midsize male response and scaling provides a lower bound, a feasible region for the small female response is bounded by the outer corridor of the midsize male response and inner corridor of the scaled small female response. By combining observations from the PMHS tests, prior volunteer tests, and scaling analysis, this dissertation identifies this region of feasibility for a small female response (i.e., the region bounded between the upper limit of the NBDL midsize male volunteer and the scaled small female volunteer responses), which can be used as biofidelity corridors to assess small female dummies and human body models.
Two small female occupant surrogates were exercised in an NBDL-like environment at 3-g, 8-g, and 15-g and compared to the region of feasibility for small female volunteer response. A small female crash test dummy, the THOR-05F, predicted appropriate head motion lower impact levels, but overpredicted motion at higher impact levels. A small female computational model, the GHBMC-F05, yielded similar results to the dummy for linear motion, though provided an improved estimate of head rotation.
This dissertation provides corridors for the biomechanical response of the small female neck, while considering how muscle tensing could contribute. The presented corridors are based on evidence both PMHS and volunteers, and both females and males. Two use cases of these corridors are explored by assessing two surrogates used in automotive safety testing against the newly developed and estimated neck flexion biofidelity targets for small female volunteers. These corridors can be used to address future efforts in predicting head motion in frontal impacts for small females.
