Evaluation of Impact Test Accelerations: A Damage Index for the Head and Torso

The head Severity Index concept has attracted widespread attention in the automotive industry. This index is intended to estimate human survivability in a systematic way without relying on judgment values. It is employed for evaluating the probability of internal head injury for those indeterminate conditions where the human tolerance limits are not clearly defined.

This paper discusses a damage index which is believed to be superior to the current Severity Index in several respects:

1. 1.
   The concept is applicable to internal injuries of the torso as well as the head.
2. 2.
   It is felt to describe the actual damage mechanism more directly.
3. 3.
   It fits the Wayne State head tolerance curve better than the Severity Index.
4. 4.
   It is suitable for analyzing impact pulses of any time duration. Examples cited in this paper include rocket sled exposures (250 ms duration) down to severe head impacts (5 ms duration).
5. 5.
   It is more convenient to employ. The index is inherently insensitive to long, low amplitude pulses; it is also largely independent of signal filtering characteristics.

This damage index is an extension of proposals made by other investigators. It employs a spring-mass model of the particular human organ under consideration; the input pulse is used as the forcing function and the displacement of the organ is calculated. This organ displacement is used as the parameter of damage. The displacement limit is set by employing input pulses which are known to approach the human tolerance limit. The analysis presented in this paper indicates that the displacements calculated in this manner are not overly sensitive to small errors in the determination of spring-mass values; thus the basic biomechanical data need not be highly refined. Future medical progress will ultimately determine the true suitability of this proposed index. It is felt, however, to be the most realistic basis for appraisals based on current knowledge.