What are the proposed changes for M2010 Draft 02-07
The Changes
Most all the differences between the draft M2010 and the current M2005 standard flow from one basic change in test philosophy: the drop masses in the impact testing will depend on head form circumference. The significant changes are:
Data provided by Dr. Ching of the University of Washington suggest that head mass correlates well with the cube of head circumference. The impact masses specified in ECE 22-05 follow such a relationship and these head forms are geometrically identical to the ISO head forms currently in use at Snell.
Since the changes in head form mass will affect the kinetic specifications, the impact severities in the draft M2010 standard have been restated in terms of velocity. The velocity for first impacts has been set to 7.75 m/sec which is, effectively, the same as the current M2005 first impact velocity. However, current helmet materials and technology indicate that smaller helmets, when tested on lighter head forms will be able to withstand substantially greater second impact velocities than larger helmets tested on heavier head forms. For this reason, the second impact velocities set for smaller head forms are appreciably greater than those set for the larger head forms. M2010 seeks to demand all the protective performance reasonably possible for each different head size rather than select a single, uniform second impact velocity and, by doing so, limit helmet protection for all sizes to that achievable for the largest helmet sizes.
The change in drop mass specifications also imposes changes in impact testing. The 5.0 kg drop mass specification for all head sizes in previous Snell standards virtually assured that a helmet meeting impact requirements on the largest appropriate head form would also meet those requirements when tested on smaller head forms. The new mass specifications remove that assurance. For M2010, helmets which meet requirements on a particular head form may fail if tested on a different head form. Moving up to a larger, heavier head form will lead to greater stresses in the liner and shell and may cause the helmet to fail catastrophically, particularly against the hemispherical anvil. And moving down to a smaller, lighter head form will lead to increased peak accelerations and may cause the helmet to fail by exceeding the peak criterion, particularly against the flat anvil. Therefore, any helmet model submitted for certification must reasonably meet requirements on every head form appropriate to its intended size range. Testing on the largest and smallest appropriate head forms should provide a reasonable demonstration of a helmet’s capabilities to meet requirements on all intermediate head forms sizes.
The Snell lab currently has an objective procedure to determine the largest head form appropriate to a particular helmet but I know of no reliable method to determine the smallest appropriate head form. Instead, we will require manufacturers to declare the intended range of head circumferences for each helmet configuration submitted for certification and to label each unit distributed for sale with the maximum and minimum head circumferences for which that helmet is deemed suitable. It is permissible to label the helmet with a narrower range of circumferences than that declared for certification but the minimum circumference on the label must never be less than the minimum declared originally and the largest never larger than the original certification.
The ISO C head form has been added to the A, E, J, M and O head forms currently in use. This head form bridges the large gap in circumference and head form mass between the A and E head forms.
Other Changes –
Curiosities
The mass of the anvil support block in Snell standards had
previously been required to be no less than 135 kg. In fact, the blocks at the California lab and
at
Although this draft calls out impact velocity rather than
energy, some adjustments may be necessary to allow for variances in the mass of
the drop assembly. Equipment breaks
under testing stresses and must be repaired or replaced. The total mass of our assemblies fluctuates
with every repair and with every component change. Informed opinion here has it that we cannot
conduct an efficient helmet test operation if we require that drop assembly
mass tolerances be any tighter than plus or minus 100 grams. Senior personnel at other test facilities
agree. This 100 gram tolerance amounts
to plus or minus 3.2% for the A head form which implies that without some
adjustment for drop mass, some helmets could conceivably be impacted 6% more
severely than others tested on a different day.
However, since we can measure this mass much more accurately than we can
control it, this standard requires that impact velocity be “adjusted” to allow
for the difference between the measured mass of the head form assembly and the
ideal mass.
Potential Problems
M2005 helmets sized for the ISO J head form may continue to meet the requirements set forth in this draft M2010 but helmets intended for smaller head sizes may have difficulty in flat impact testing and helmets for larger head sizes may have difficulty with hemispherical impact testing. The lighter test head form masses imply higher peak accelerations and may lead to problems with the 300 g criterion. Although the heavier head forms imply lower peak accelerations for some impacts they also imply significantly higher impact energies suggesting problems in testing against the hemispherical anvil.
Many current helmet configurations are intended for a range of head circumferences and could reasonably tested on two or more different head forms. But the constant 5.00 kg drop mass specified in M2005 demands the same energy management and approximately the same shock attenuation regardless of head form size. M2010, however, demands significantly greater levels of energy management for larger head forms and greater shock attenuation for smaller head forms. Helmets subject to testing on two or more head forms must provide the energy attenuation demanded by the largest head form in its range and the shock attenuation demanded by the smallest. For this reason, helmets intended for broader ranges of head circumference may have to be bulkier, heavier and, ultimately, less appealing than helmet intended for narrower ranges.
Potential Advantages
This draft M2010 may be much more compatible with the requirements of DOT and ECE 22-05. Although testing suggests that larger sized Snell M2005 helmets might do well in ECE 22-05 type testing. Smaller sizes could not simultaneously satisfy the attenuations demanded by ECE and the energy management demanded by Snell. Since M2010 will call out head forms with a cubic relationship between circumference and drop mass just as do DOT and ECE 22-05, the impact attenuations demanded are much more similar. It is expected that manufacturers will be able to qualify their Snell certified models to DOT and to ECE 22-05 readily and demonstrate compliance with European and United States requirements. Furthermore, the fact of Snell compliance will demonstrate a substantial protective benefit over and above that demanded by either US or European authorities.
Request to All
Interested Parties
Please review the details of this standard carefully. Although I have done my best to estimate reasonable impact severities, particularly for second impacts, my confidence in these estimates is far from absolute. For this and for every other aspect of the draft, I will be grateful for all constructive advice and criticism.