NHTSA is proposing injury criteria and performance limits that it believes are appropriate for each size dummy. The agency is placing in the public docket a technical paper which explains the basis for each of the proposed injury criteria, and for the proposed performance limits. The title of the paper is "Development of Improved Injury Criteria for the Assessment of Advanced Automotive Restraint Systems."
Standard No. 208 currently specifies five injury criteria for the Hybrid III 50th percentile adult male dummy in barrier crash tests: (1) dummy containment--all portions of the dummy must be contained in the vehicle passenger compartment throughout the test, (2) HIC (Head Injury Criterion) must not exceed 1,000, (3) chest acceleration must not exceed 60 g's, (4) chest deflection must not exceed 76 mm (3 inches), and (5) upper leg forces must not exceed 2250 pounds.
Under today's proposal, NHTSA would generally apply these and certain additional injury criteria to all of the dummies covered by the proposal. However, the criteria would be adjusted to maintain consistency with respect to the injury risks faced by different size occupants. Also, with respect to some types of injuries, the agency is considering alternative injury criteria.
For chest injury, NHTSA is considering two alternatives. Under the first, or primary, alternative, the agency would add a new criterion, Combined Thoracic Index (CTI), which was recently developed by the agency. New analyses of cadaver test data using a variety of restraint system combinations indicate that thoracic injury prediction can be improved by considering a linear combination of chest deflection and chest acceleration rather than solely by considering the criteria independently. CTI links the combined effect of both parameters with the risk of injury.
In proposing to add CTI, the agency has considered whether to adjust the existing limits on chest deflection and/or chest acceleration. In the absence of the existing injury criteria, the proposed CTI limit (CTI = 1) would permit (for the Hybrid III 50th percentile adult male dummy) chest deflection to exceed 76 mm (3 inches) when acceleration is very low, and acceleration to exceed 60 g's when chest deflection is very low.
NHTSA notes that, in the case of chest deflection, the current 76 mm (3 inch) limit is very close to the limit capable of being measured by the Hybrid III 50th percentile adult male dummy. Therefore, it does not appear to be possible to adjust this parameter in a meaningful way. In the case of chest acceleration, the agency notes that it does not have any cadaver data concerning injury risk associated with very low deflection and chest acceleration above 60 g's. The agency requests comments on this issue. NHTSA is especially interested in data and/or analyses concerning the risk of injury associated with low deflection and high acceleration.
As the second alternative for chest injury, the agency would simply continue to maintain separate limits on chest acceleration and chest deflection.
NHTSA is also proposing to add neck injury criteria. The agency notes that it added neck injury criteria as part of the temporary sled test alternative, although the standard does not otherwise specify neck injury criteria. The neck injury criteria for the sled test alternative include separate limits on flexion, extension, tension, compression and shear.
NHTSA has recently developed an improved neck injury criterion, called Nij. The agency believes that a disadvantage associated with specifying separate limits for flexion, extension, tension, compression, and shear is that it does not account for the superposition of loads and moments, and the additive effects on injury risk. The agency developed Nij to take account of these effects.
NHTSA is considering two alternatives with respect to neck injury criteria. Under the first, or primary alternative, the agency would add Nij to Standard No. 208. In terms of performance limits, the agency is requesting comments on Nij=1.4 and on Nij=1. As discussed in the technical paper concerning injury criteria, Nij=1 reflects certain critical values developed using biomechanical data. However, based on concerns about practicability, particularly with respect to tests specifying use of the 5th percentile adult female dummy, as well as concerns about correlations between biomechanical data and real-world crash data, the agency believes that Nij=1.4 might be a more appropriate performance limit. NHTSA requests comments on this issue.
As an alternative to Nij, NHTSA is also requesting comments on establishing separate limits on flexion, extension, tension, compression and shear, i.e., the approach adopted for the sled test alternative. The proposed regulatory text includes this second alternative as well as Nij.
As indicated earlier in this section, NHTSA is generally proposing to apply the same injury criteria to all of the dummies covered by today's proposal, adjusted to maintain consistency with respect to the injury risks faced by different size occupants. There are, however, some exceptions to this. The agency is not proposing to apply the dummy containment injury criterion to the 12 month old CRABI dummy since that criterion does not appear to be relevant to the low risk deployment test using that dummy. The agency is not proposing chest deflection or CTI requirements for the 12 month old CRABI dummy because that dummy does not measure chest deflection. (As indicated above, chest deflection is needed to calculate CTI.)
The agency requests comments on the proposed injury criteria, on how they are calculated, and on the proposed performance limits. To help facilitate focused comments, the agency is including specific values for each performance limit in the proposed regulatory text. However, NHTSA is considering a range of limits above and below each specified value. Depending on the public comments, the agency may adopt for the final rule values higher or lower than the ones included in the proposed regulatory text. The agency requests commenters to address what values should be selected for the final rule, their rationale for their recommendation, and the implications of adopting lower or higher values than those specified in the proposed regulatory text.
