Interpretation ID: GF006103

Larry C. Dickinson, Ph.D.
Manager, Engineering & Technology
Martin Marietta Composites
PO Box 30013
Raleigh, NC 27622

Dear Mr. Dickinson:

This responds to your phone conversation with George Feygin of my staff and subsequent e-mail asking whether Federal Motor Vehicle Safety Standard No. 223, Rear impact guards (FMVSS No. 223), S5.2.2 allows for "elastic deformation" as opposed to plastic deformation. Specifically, you ask whether the requirement that the energy absorption be accomplished by plastic deformation would preclude a material that returns to its original shape (i.e., elastic material).

By way of background, the National Highway Traffic Safety Administration (NHTSA) does not provide approvals of motor vehicles or motor vehicle equipment. Under 49 U.S.C. Chapter 301, manufacturers are required to certify that their vehicles and equipment meet applicable requirements. The issues raised by your letter are addressed below.

FMVSS No. 223 requires that underride guards fitted to new trailers and semitrailers provide a certain amount of energy absorption to lessen the crash forces on a passenger vehicle colliding from the rear. In creating FMVSS No. 223, the agency sought to balance concerns associated with rear impact guards being overly rigid and non-yielding in rear impact crashes against those associated with the guards being insufficiently rigid to prevent intrusion of a trailer into a vehicle occupant compartment. Accordingly, the standard requires that a certain minimum amount of force be absorbed through "permanent yielding" or plastic deformation of the guard. [1] S5.2.2 of the standard states:

" [a] guard  . . .shall absorb by plastic deformation within the first 125 mm of deflection at least 5,650 J of energy at each test location P3."
(emphasis added).

The standards energy absorption requirement cannot be met by elastic deformation for two reasons. First, the language of the standard calls for plastic deformation. Second, the prescribed method of measuring the required energy absorption greatly restricts any elastic qualities of the rear impact guard. Specifically, any energy that the guard returns to the force application device when the load is removed (i.e., the elastic component of the deformation) is subtracted from the total energy absorption for purposes of meeting the requirement.

The requirement that guards absorb energy by plastic deformation was to ensure that the guard did not subsequently return the absorbed energy to the colliding vehicle, because that energy return could increase the chance of death or injury to the occupants. Any immediate rebound occurring after the crash event could pose a threat to passenger vehicle occupants.

In an August 4, 1998, letter to Mr. Toms of Power Brace (copy enclosed), NHTSA stated that the plastic deformation requirement of S5.2.2 does not preclude use of certain elastic materials that return to their original shape very slowly (approximately 24 hours). Based on your conversation with Mr. Feygin, it is our understanding that your question concerned materials that returned to their original shape immediately after impact.

I hope this information is helpful. If you need further assistance, please contact George Feygin of my staff at this address or at (202) 366-2992.

Sincerely,

Jacqueline Glassman
Chief Counsel

Enclosure
ref:223
d.10/21/03