Application of acceleration frequency domain factors in the analysis of headform impacts / by Evan Walsh.
Walsh, Evan S.
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This research thesis was composed of two studies. The first study was a pilot investigation to show evidence of the validity and reliability of the instruments and methodology used in the second, main investigation. The principal investigation explored the possible expansion of head injury tolerance curve information based on vibration and the frequency domain of impacts in order to improve predictive models. The frequency domain uses a mathematical transform to change time domain measures of time and acceleration in the abscissa and ordinate to frequency and power of vibration (Smith, 2002). The scope of this study was delimited to: 1. The use of a spherical cast urethane headform with high biofidelity was used to represent the human head. It was assumed the head would react in a similar manner without endangering human participants or requiring postmortem human subjects. 2. The inclusion of two drop heights, 0.2m and 0.4m. Fleights used in helmet testing of greater than one meter would be unnecessarily high for testing with an unhelmeted headform and would result in extreme impacts. 3. The inclusion of direct impacts only. Indirect causes of head acceleration, such as body impacts causing the type of injuries often associated with whiplash, were not considered in this study. The following limitations were identified: 1. The headform was constructed of cast urethane and may not possess the same level of biofidelity that is now standard on instruments currently used in helmet testing such as the Hybrid III anthropometric test device or similar models. This limitation did not affect the validity of outcomes from the present study; however results may not be generalized to live subject head injury prediction models. Differences among headforms have been noted by researchers with respect to relevance of impact measurement comparison between studies and testing procedures (Bishop, Norman, & Kozey, 1984). 2. The headform was not free to rotate; therefore, angular measures were extrapolated from kinematic and kinetic equations. 3. Mechanical wear to the Headform Impact Measurement Device and headform was expected to occur, however, the impact of this deformation was accepted to be minimal and inconsequential to the study’s outcome. 4. Due to the manual release of the mechanical switch supporting the headform, minor errors in drop height were present. These drop height variances were kept within one m illimeter (±0.001 m) and were not expected to affect the study’s dependent measures significantly. -- Introduction