In 1997 McLean et al. (1997) demonstrated that energy absorbing headwear for car occupants might be effective in reducing the numbers of head injuries sustained by car occupants. The estimated benefits were greater than the estimated benefits of padding of the upper interior of vehicles to the requirements of the US Federal Motor Vehicle Safety Standard 201. This report investigates the suitability of selected materials for head protection, in the form of a headband that could be worn by car occupants.
The study is divided into three phases. Phase 1 surveys materials with a range of properties and impact behaviours. Impact tests provided the data by which assessments were made of the materials' effectiveness. The tests in this phase showed that a range of materials were able to attenuate the severity of the impact to a reasonable degree.
The materials identified in Phase 1 were tested further in Phase 2. Prototype headbands were constructed and attached to instrumented headforms which were dropped onto standard helmet testing anvils. The purpose of these tests was to examine the prototypes' response to concentrated loading. Several prototypes showed themselves to be unable to perform adequately in these tests; the anvils split or shattered the headband. Several prototype designs did perform well in Phase 2. These designs were tested in simulated head strikes with vehicle structures in Phase 3.
Phase 3 consisted of a series of preliminary tests in which a headform, protected by the prototype headband, was fired toward an interior structure that commonly causes head injury to car occupants in crashes.
Two prototype concepts appear worthy of further investigation. A headband constructed of polyurethane foam and a headband consisting of a cardboard honeycomb liner encased in a hard shell both significantly reduced the severity of impacts with the car structures. However, further investigation into optimising the selection of materials for their impact absorbing qualities and their comfort and durability in normal use is warranted.
These tests demonstrate that a headband for car occupants could significantly reduce the severity of certain head impacts in a crash. The best prototype headband reduced the HIC and peak acceleration values by over 60 percent in a standard test with the interior of the car. The reduced impact was approximately equivalent in severity to an unprotected impact with the structure at half the speed.
Conclusions and recommendations
The results from Phase 3 indicate that a headband can greatly reduce the severity of an impact to the head. HIC was reduced by 25 percent with the use of 25 mm of BB-38 polyurethane, and 67 percent with the honeycomb cardboard prototype, when compared with an impact with no headband. It is also noteworthy that the peak force produced in the test using the honeycomb headband was less than half the force produced by the headform alone. The honeycomb cardboard absorbed around three quarters of the impact energy before it began to bottom out.
The tests indicate that a crushable material, such as honeycomb, has the most effective characteristics for a headband. The ideal material would be one which
- Limits the peak force applied to the head
- Does so at a constant level from the initiation of the deformation
- Returns little energy to the head
- Does not bottom out
Practical considerations limit the thickness of the headband, so the challenge is to absorb the maximum amount of energy while limiting the peak loads transferred to the head of the wearer. In this way, the maximum amount of energy can be absorbed before the material bottoms out. Honeycomb is stiff initially when loaded, compared to polymer foams, but the peak load is limited by its inherent properties. The material stores little elastic energy, so the head of the wearer would be unlikely to rebound as severely as with some other materials.
One concern we had with the honeycomb cardboard is its durability. The material may deteriorate dut to environmental factors. There are several alternatives to paper, however, for the construction of a honeycomb structure. These include aluminium, polymers, and coated paper. These materials would give the same benefits as the honeycomb cardboard: energy absorption, force limiting characteristics, lightweight structure, but with the benefits of water resistance, and durability, in storage and handling.
The polyurethane headband also performed reasonably well in all phases of the tests. The BB-38 grade was the best performer of the polyurethanes. It may be possible to formulate a polyurethane with improved properties. However, at this time we have not seen a polyurethane which can match the honeycomb material in its behaviour.
We recommend that further investigation is made into materials of a honeycomb structure to find a material of the correct crushing strength and durability. We also recommend that prototypes be developed further to be included in a testing program that would include other vehicle structures tested over a range of velocities.
Type: Research and Analysis Report
Sub Type: Consultant Report
Author(s): R Anderson, K White, J McLean
ISBN: 0 642 25502 4
Topics: Head, Occ protection
Publication Date: 01/01/00