Biomechanics justification for empirical head tolerance criteria

Criticisms of existing tolerance criteria are assessed briefly in the context of their relevance and importance in the design of protective systems. A biomechanics analysis is given for head impact, with emphasis on the general form of derived tolerance criteria rather than on their detailed evaluation. Head materials are assumed to be inhomogeneous, nonlinear and viscoelastic, and related materials failure criteria are used to derive appropriate head injury criteria. It is shown that the Gadd Severity Index and the Head Injury Criterion are plausible and fundamentally correct forms of criteria, in contrast to criticisms which have appeared in the literature. The present analysis gives credibility to these empirical criteria, indicates their limitations and the assumptions upon which they are based, and enables them to be seen in the context of other proposed head tolerance criteria.