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Executive Summary
The performance test proposals in the NPRM constitute a de facto change to the standard which will have unfavorable consequences. Higher quality helmets with superior energy attenuation capability may fail the duration limitation in flat anvil testing at 6.4 meters per second. Redesign and retooling so that helmets meet this new test requirement may be extremely costly for the industry. The duration limitation derives from an inappropriate transition to rigid anvil testing. This was an error in the ASA 1967 standard carried through into the ANSI Z90.1 1971 standard and ultimately into FMVSS 218. The solution to compliance testing is improved test laboratories which must include better quality control, third party confirmation of proper operations and equipment and traceability to national and international standards; not degrading FMVSS 218.
Noncompliance Testing
Measures to simplify verification that helmets in the market comply with FMVSS 218 may be worthwhile but it is essential that these measures do not mistake compliant helmets as being noncompliant. Noncompliance is not simply the converse of compliance. Uncertainties in mechanical testing guarantee that there will always be some helmets for which compliance and noncompliance will be indeterminate. The most reasonable resolution is to establish two sets of test procedures and criteria: one set to demonstrate compliance and the second, slightly less demanding, to demonstrate noncompliance. The differences in severity and criteria between these two would be determined by the test uncertainties so that a given helmet sample found compliant by the first procedure would reasonably never be found noncompliant by the second and vice versa. Noncompliant helmet models would be those for which the manufacturer had not established compliance or for which government testing had subsequently demonstrated noncompliance. Compliant helmets would be those for which compliance testing had been established and for which subsequent government testing had not demonstrated noncompliance.
These procedures should prescribe test and measurement accuracies for compliance testing and recommend them to manufacturers and commercial test houses. But they should dictate test and measurement accuracies to contractors performing noncompliance testing for NHTSA. Since helmet testing severities cannot be finely controlled, they must be measured and these measurements are themselves subject to measurement uncertainty. Both sets of test procedures ought to allow for variability in measured test severity either by varying the test criteria or by discarding the results altogether and repeating the procedure. This sort of provision might have forestalled the quibble raised in the findings for the NeXL® helmets discussed in the NPRM.
FMVSS 218 was developed from a voluntary standard which described test procedures and criteria for determining compliance. Since the standard was voluntary, there were no provisions for enforcement and, therefore, no immediate need for measures to determine noncompliance. FMVSS 218 requires considerable redrafting in order to function effectively as a mandatory standard, even if no changes in helmet performance are sought. However, the changes proposed in the NPRM will not resolve the compliance/noncompliance issues and may even misidentify compliant helmets.
Objections to the Proposed Impact Speed Changes
The changes proposed for the impact attenuation test speeds may cause many helmets which meet the requirements of the current standard to obtain failing results in compliance testing. Some helmets in compliance with the current standard may obtain impact accelerations exceeding 400 G in impact testing under the proposed rules and some may obtain time durations greater than 2 milliseconds for those portions of the response exceeding 200 G.
The 0.4 m/second tolerance proposed for impact against the hemispherical anvil allows impact energies exceeding the ideal value implied in the current standard by more than 15%. The impact performance of helmets tested against the hemisphere is largely a matter of impact energy. Helmets with reasonable flat impact performance capabilities will never fail in hemispherical impact unless the kinetic energy of the impact exceeds a limit which is determined by the helmet configuration. The cause of the failure will be that the helmet wall was compressed to its crush limit and would deform no further. If the head form had any residual velocity at this point, the acceleration levels would be determined by the material properties of the test equipment, particularly, the head form and the anvil itself. The acceleration would spike sharply upward well beyond the 400 G criterion and, likely, well beyond the upper limits of the instrumentation. Remarkably though, so long as the kinetic energy of the impact is even slightly less than this limit value, the impact acceleration trace will remain well below the peak values obtained in flat impacts and well below the test criterion. It’s like walking toward a precipice; everything is fine until that last step when the results become horrendous. Allowing noncompliance testing at energies 15% beyond those demanded in the standard places an unreasonable burden on manufacturers.
The 0.4 m/second tolerance also poses unreasonable burdens for flat impact testing. The flat impact response is such that the crush limits of the helmet rarely come into play. Instead, helmet performance in flat impact depends largely on liner stiffness. The peak G limit is not likely to present much of a problem either. When helmets have trouble with flat impact, it is most likely with the 200 G time duration limit. Unless the acceleration response of the helmet never exceeds 200 G, an increase in test impact velocity is likely to produce a disproportionate increase in the duration of the response at and above 200G. An analysis of time duration versus impact velocity is provided in a separate document[1]. The analysis describes a mathematical model for helmet flat impact behavior and demonstrates the substantial increases in time duration anticipated for increased impact velocities. Allowing noncompliance testing at velocities 6.7% greater than those specified in the standard places an unreasonable burden on helmet manufacturers
Resolutions to the Velocity Tolerance Problem
There are a couple of solutions possible.
- Brute force: Test more helmets and discard results for which the impact velocities were outside the current specification. This might be modified by allowing passing results for excessive velocities and failing results for insufficient velocities to stand and discarding those results for which an assessment of compliance was ambiguous.
- Improved velocity measurement systems: Better calibrations are possible for single card - single beam systems. Also, some velocity gates use two cards and measure from leading edge to leading edge eliminating light diffraction and photo cell hysteresis effects.
- Improved impact devices: Impact testing imposes inordinate amounts of stress on monorail system bearings[2]. As the bearings wear, frictions increase and so does impact velocity variability. Even if the tester allows for the increased friction, he may not be able to control the test severity sufficiently. The twin wire devices currently used by the Snell Memorial Foundation and other organizations do not have this bearing problem. The devices are much less subject to wear and, for that reason, yield much more reliable test impact velocities and much more repeatable test results.
- More stringent vetting of contractors: Prescribe reliable test equipment and protocols and check for them. Establish reliable proficiency evaluation procedures and apply them. If a test house does not have the requisite equipment or test procedures, or if it cannot maintain current measurement tolerances, find another who can.
- More sophisticated analyses and criteria: It is possible to test helmets at slightly higher velocities and perturb the results to infer a reasonable estimate for their performance at ideal impact velocities. Effectively, high velocity test impact would yield a mathematical force versus crush response model of the helmet which would then be exercised with a hypothetical impact at the appropriate velocity. The helmet would then be evaluated according to its calculated response. As in solution #1, Brute Force, this would call for reworking the impact criteria in FMVSS 218 to enable a more nuanced helmet evaluation that would include a consideration of the test impact velocity.
Further Comments on Time Duration
The flat impact velocity dynamic might also be improved by reconsidering the time duration criteria. These criteria first appeared in the 1967 American Standards Association’s requirements for vehicular helmets. This standard allowed impact testing on either of two substantially different devices: swing away devices in which an impactor struck a stationary helmet and head form which then moved away under the imparted momentum; and guided fall devices essentially identical to the monorail and twin wire devices in current use. Most of the test devices in use at the time were swing away devices.
Although the ASA 1968 standard took pains to structure the impact velocities for guided fall and swing away devices so that the helmet loadings and their peak acceleration responses were reasonably the same, they failed to allow for the substantial differences in the time durations associated with these devices. Instead, a single set of time duration criteria applied to testing done on either device. Since guided fall devices were relatively rare though, the time duration criteria in use had been developed from observations of swing away test results. In general, the swing away device calls for substantially greater impact velocities in order to match the helmet loadings and stresses of guided fall systems. For this reason the impact events for swing away are of much shorter duration. A two millisecond time duration for some portion of the acceleration pulse as measured for a swing away device would be substantially longer for an equivalent test performed on a guided fall device. An analysis of guided fall versus swing away impact test performance is provided in a separate document.[3]
In 1971, the renamed American National Standards Institute revised the ASA 1967 standard and eliminated the swing away device. The guided fall device was the only one allowed for impact testing. However, the time duration criteria worked out for the swing away device were carried along into the revised standard unchanged. When it was noticed that many helmets which passed easily on swing away devices were failing to meet the time duration criteria in guided fall tests, ANSI published a 1973 correction increasing the time duration criteria by 50%. However, by this time, NHTSA had already adopted the requirements of ANSI Z90.1 - 1971 for use in a motorcycle helmet standard to be made mandatory throughout the United States. NHTSA refused to incorporate the corrections imposed in ANSI’s 1973 revision, possibly because the impact criteria were considered temporary. NHTSA had planned to replace the peak acceleration and time duration criteria with a more sophisticated impact performance evaluation based on FMVSS 208 and the Head Injury Criterion (HIC). In fact, though, the ANSI Z90.1- 1971 criteria have remained part of the FMVSS 218 requirements down to this day.
More recently, some research discussed at sessions of the International Workshop on Human Subjects for Biomechanical Research has suggested that the time duration measures do not correlate with other predictors of head injury risk and may be of no real value in helmet evaluation. Although these efforts have not yet been presented formally, it may not be unreasonable to exclude time duration considerations from any NHTSA finding of helmet noncompliance with FMVSS 218, at least until the value of time duration has been established. However, if time duration criteria are not dropped altogether from NHTSA contract testing, they should certainly be increased consistent with the differences associated in swing away and guided fall type testing. At least until new research establishes their value and identifies threshold values for increased injury hazard.
Respectfully
Edward B. Becker
Executive Director/Chief Engineer
© 1996-2012 Snell Memorial Foundation, Inc. Contact info
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