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Upper interior head impact safety is an important consideration in vehicle design and is covered under FMVSS 201. This standard generally requires that HIC(d) should not exceed 1000 when a legitimate target in the upper interior of a vehicle is impacted with a featureless Hybrid III headform at a velocity of 15 mph (6.7 m/s). As HIC and therefore HIC(d) is based on translational deceleration experienced at the CG of a test headform, its applicability is often doubted in protection against injury that can be caused due to rotational acceleration of head during impact. A study is carried out here using an improved lumped parameter model (LPM) representing headform impact for cases in which moderate to significant headform rotation may be present primarily due to the geometric configuration of targets.

The Head Injury Criterion (HIC) in FMVSS 208 for evaluating the potential head injury requires maximization of a mathematical expression, involving the time-average acceleration, by varying the limits (t1, t2) of the time interval over which the average is calculated. This paper describes the HIC behavior through the analysis of a function of two independent variables t1 and t2. The analysis is carried out for any arbitrary acceleration profile a(t). It is found that maximization requires that a(t1) = a(t2). Also, for the unique values of t1 and t2 that maximize HIC, the average acceleration between t1 and t2 is 5/3 times the acceleration at t1 or t2. Illustrative examples are provided by applying this condition to simple pulses. Numerical results are presented in tables and graphs.