Interpretation ID: 17343-2.pja
Mr. Robert S. Toms
Manager of Engineering
Power Brace
7640 60th Ave.
Kenosha, WI 53142
Dear Mr. Toms:
This responds to your letter requesting an interpretation of whether the elastomeric material your company manufactures could be used to comply with the energy absorption requirements of the National Highway Traffic Safety Administration's (NHTSA's) rear impact protection (underride guard) regulations. 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) very slowly, on the order of approximately 24 hours. As explained below, this material could be used as the active energy absorbing material to pass our compliance tests.
Federal Motor Vehicle Safety Standard No. 223, Rear impact guards (49 CFR 571.223, published on January 24, 1996 at 61 FR 2004) 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. S5.2.2 of the standard states that "[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 test procedures basically require application of a force by a steel block, while recording the force at least ten times per 25 mm, until the guard has been deflected 125 mm. S6.6(c) then requires NHTSA to "[r]educe the force until the guard no longer offers resistance to the force application device. Produce a force vs. deflection diagram . . . Determine the energy absorbed by calculating the . . . area bounded by the curve of the force vs. deflection diagram and the abscissa (x-axis)" (emphasis added). Therefore, the elastic component of energy is not normally counted toward meeting the energy absorption requirements.(1)
One word in S5.2.2 that requires interpretation in this case is the word "plastic." In this context, plastic means capable of being molded, bent, or assuming a new form or shape. Although plastic deformation is normally thought of as permanent, and elastic deformation temporary, there is no time frame explicitly attached to these meanings. Whenever appropriate, NHTSA interprets its regulations consistent with their purposes.(2) The requirement that guards absorb energy was designed to ensure that guards were not too rigid during the onset of force in a crash. The requirement that they absorb the 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. However, any rebound occurring after the crash event, especially slow rebound such as is produced by your elastomer, does not pose any threat to passenger vehicle occupants. Therefore, for real world safety purposes, the time frame within which a material must retain its deformed shape to be considered "plastic" is the duration of a crash event.
The relevant time period for compliance purposes, however, is longer. Standard No. 223 employs a quasi-static test, not a dynamic test, when it tests for compliance with its requirements. NHTSA has no way of determining whether a material would rebound within the time frame of the crash. Therefore, if your elastomer reacts in such a way that it passes the test procedure, it will have passed the requirements, for practical purposes. The critical piece of information you need to determine whether your material will pass the test is when the test ends.
A specific event determines when the test ends. The force application/withdrawal portion of the test procedure is over as soon as the guard no longer offers resistance to the force application device. Since S6.6(c) is a list of steps to be performed, it is reasonable to assume that once a certain step is completed, the next step will be commenced. The step of reducing the force proceeds only "until the guard no longer offers resistance." In practical terms, the guard will generally cease to offer resistance when it loses contact with the force application device. NHTSA has no way of determining any small amount of residual force generated by your elastomer after that point. A properly calibrated load cell (a typical load measuring device) should register zero load, and the force deflection trace should meet the abscissa of the graph upon separation. After that happens, the test itself is completed and all that remains is the computation of the amount of energy absorbed using the area within the force deflection curve.
For a manufacturer to predict the energy absorption of a guard equipped with your elastomer during NHTSA's compliance testing, it needs to have some idea of the rate at which the force application device will be withdrawn. Although the event concluding the test is known, that event can occur at different times, resulting in different amounts of measured energy absorption from the same guard. For example, if the force application device is withdrawn slowly enough, your company's slow-rebounding elastomer may never lose contact with the guard, which could result in a computation showing little apparent energy absorption. On the other hand, if it is withdrawn quickly from the same guard, the force application device would lose contact quickly and more apparent energy would be absorbed.
The rate of withdrawal has not been specified in the regulation or in the test procedures. The equipment that NHTSA and most manufacturers use to test the guards determines, to a large extent, the rate of withdrawal. Force is normally applied using a large hydraulic ram, powered by pumps. Using this system, the ram is withdrawn by reversing the pump. This results in a withdrawal rate about the same as the rate of force application. This relief of force also enhances the safety of the persons performing the test and lengthens the life of the test equipment by reducing the chances of a catastrophic failure of some part of the test device.
Specifying a particular rate of force withdrawal is not necessary or practical. To provide flexibility to the manufacturers, S6.6(a) of the test procedure permits the manufacturer to specify the rate of force application, within a range of 2.0 to 9.0 cm per minute. To accommodate the manufacturer's specified force application rate and guard design, NHTSA may have to vary its test equipment, which might affect the rate at which it can withdraw the force application device. However, it is reasonable to assume that NHTSA can withdraw the force application device at least as quickly as it applied the force.
Therefore, where possible, NHTSA will withdraw the force application device at a rate equal to or slightly faster than the manufacturer's specified force application rate. This gives the manufacturers some control by allowing them to specify the lower bound for the withdrawal rate. If the manufacturer tested its guard by withdrawing the guard at the force application rate, or a little slower, then the amount by which the NHTSA's rate of withdrawal might be higher than the specified rate is not critical, because higher withdrawal rates can only result in earlier separation of the guard from the force application device, and a higher level of computed energy absorption by the guard. In other words, the higher the rate, the more likely the guard is to comply with the energy absorption requirement.
In summary, your elastomer could be used to comply with the standard if it passes the compliance test in Standard No. 223. Based on your description of the behavior of the elastomer, we see no reason why a guard equipped with it would not be able to pass our compliance tests. If you have any further questions, please feel free to contact Paul Atelsek of my staff at this address or by telephone at (202) 366-2992.
Sincerely,
Frank Seales, Jr.
Chief Counsel
ref:224
d.8/4/98
1. These requirements, as stated, are adequate for testing guards made of most materials. Most guards made of steel would exhibit only a small amount of elastic deformation. Therefore, the guard would stay deformed and the force curve would descend to and intersect the abscissa close to the maximum deformation, exhibiting little rebound. Most elastic materials would rebound quickly and completely, following closely the curve produced during force application, descending to the abscissa only at the point of origination. However, your elastomer returns the energy very slowly.
2. When a standard does not specify a particular test condition, we begin with a presumption that the requirements must be met in every test condition in which the test can be conducted. See, for example, NHTSA's October 2, 1990 letter to Mr. S. Kadoya of Mazda. In this case, that would mean that the agency could retract the test device at any rate. However, the agency also looks to the language and purposes of the standard to see if a limitation should be implied. In this case, we find an implied limitation on the retraction speed based on the standard's purposes. Retracting the device at extremely low rates would also only prolong testing.