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. 2000;44:133–148.

Studies of Vehicular Padding Materials

Anthony Sances Jr 1, Fred H Carlin 1, Brian Herbst 2, Steve Forrest 2, Steve Meyer 2, Anil Khadilkar 3, Keith Friedman 4, Jack Bish 4
PMCID: PMC3217370  PMID: 11558079

Abstract

The Federal Motor Vehicle Safety Standard 571.201 discusses occupant protection with interior impacts of vehicles. Rule making by the National Highway Traffic Safety Administration (NHTSA) has identified padding for potential injury reduction in vehicles. In these studies, head injury mitigation with padding on vehicular roll bars and brush bars was evaluated. Studies were conducted with free falling Hybrid 50% male head form drops on the fore head and side of the head and a 5% female head. Marked reductions in angular acceleration, as well as Head Injury Criterions (HIC), were observed when compared to unpadded roll bars and brush bars.

PADDING has been used for energy management in helmets, sports equipment, protective structures, packaging, comfort and support systems for decades. The interior of motor vehicles has been identified as an area where injury can occur in frontal, side, rear, roll over, or oblique impacts. Head impacts with the roof, pillars and support structures have been studied by Fan (1998), Monk (1986) and Partyka (1988). Recent rule making by the National Highway Traffic Safety Administration has identified padding as a potential mechanism for reduction of head injury. Federal regulations for occupational protection in interior impacts is given in the Federal Motor Vehicle Safety Standards rule 571.201. The regulation applies to interior velocity components including headers, pillars, side rails, roll bars, and braces and stiffeners. Standards and studies of helmets for energy management have been reviewed by Becker (1998) and Myers (1993).

To better delineate the head injury mechanism during the direct impact, it has been shown that the quantification of angular acceleration along with Head Injury Criteria, and linear acceleration is important. Newman (1998) has criticized Head Injury Criteria (HIC) and reviewed angular acceleration as an injury mechanism. Studies of Thibault (1990) Margulies (1992), Meaney (1998) and Ommaya (1984) have associated angular acceleration and angular velocity with concussion, acute subdural hematoma, and diffuse axonal injury. Living human studies by Pincemaille (1989) in amateur boxers have also identified injury levels of angular acceleration and angular velocity. Ewing (1975) has conducted studies on volunteer Navy pilots and determined non-injurious levels of angular acceleration. McLean (1996) has suggested that brain injury does not occur without head impact. Recent studies analyzing the occurrence of diffuse axonal injury with direct impacts and translational accelerations in contrast to rotation has been advanced by Mishimoto (1998).

Although it is well known that the padding decreases impact accelerations, to the best of our knowledge, a comprehensive study from the perspective of head injury reduction including angular acceleration is lacking. The existing standard to determine the effect of padding is based on the one-dimensional spring-mass impact system (Federal Register, March 1998). This approach is applicable to calculate the peak acceleration. It is, however, not possible to directly quantify the angular acceleration and velocity, and Head Injury Criteria. Therefore, the determination of these responses during various padding conditions is necessary to address the biomechanical effects of padding. Furthermore, such quantification permits the determination of the relative effect of padding on the biomechanical responses (e.g., reduction of angular acceleration versus HIC). This is our impetus for the study. Consequently, the present study was designed to evaluate the variations of head injury biomechanical responses due to various padding materials under different impact velocities.

METHODS

The first study was conducted with three different types of padding examined. All padding was approximately 25 mm thick. The baseline structure was an unpadded production brush bar. The first type of foam, Simpson roll bar padding, was directly attached to the baseline 32 mm diameter steel structure.

Simpson (1999) roll bar padding is a “BeadAll” foam manufactured by DuPont. It is similar to the foam used in their helmet liners. The padding was replaced for each test and approximately 20 to 30 minutes were allowed between each test.

The 0.9 mm thick cold-rolled steel metal air gap padding was placed onto a secondary structure with the same dimensions as the unpadded brush bar and welded to the existing set up to keep the test configuration the same.

A production brush bar with production foam padding was also tested. Each brush bar was secured in place to prevent movement and aligned with an approximate 1.93 m long lightweight thin wall poly vinyl chloride (PVC) tube pendulum. The 5th percentile Hybrid III female head and neck was mounted at the end of the pendulum. The height of the pendulum was varied to obtain different impact velocities from 3.4 to 6.7 meters/second (m/s). The impact was offset from the head center of gravity (CG) by approximately 25 mm. The head was instrumented with a three orthogonal uni-axial IC 500G sensors model 3031 that measured the accelerations in the x, y and z directions and an ATA angular rate sensor mounted at the center of gravity of the head form. The angular acceleration was derived from the recorded angular rate by differentiation. All data were collected at a sample rate of 10 kilohertz. The acceleration data was filtered at SA.E. Filter Class 1000. The HIC was calculated over a 36 ms interval. The ATA data was filtered at SAE Class 180.

The second study was conducted which incorporated a 25 mm CG offset side head impact. A tri-axial sensor accelerometer array was mounted in a 50% Hybrid III male head form along with an angular rate ATA sensor. Freefalling side head drops at approximately 5.4 meters per second were made onto a 47 mm diameter bare steel roll bar rigidly supported at each end. The roll bar was padded with standard production roll bar foam and two different commercial after market foams of 20 to 25 mm thickness and a comparable thickness metal air gap padding. All data were sampled at 10 kilohertz. The accelerations were filtered at SAE class 1000 and the angular velocities at SAE class 180. The 36 ms HIC was calculated from the resultant tri-axial acceleration and the angular acceleration was derived from the angular rate sensor.

The third study was conducted with a free falling Hybrid III 50% male head form at speeds of 5.4 m/s. An Entran EGE-73BQ-2000 tri-axial accelerometer array and an angular rate ATA sensor was incorporated within the head form. The head form was dropped onto the forehead with a 25 mm offset from the CG onto a 47 mm bare steel bar. Seven mm standard vinyl and foam padding, 25 mm commercial racing roll bar padding, 25 mm Simpson racing roll bar padding, and 25 mm metal air gap padding were tested. All data were recorded at 10 kHz and filtered at SAE class 1000. The 36 ms HIC was calculated based upon the resultant acceleration. Angular accelerations were derived from the angular velocity data, which was filtered at SAE Class 180. In all tests, the dummy’s head was impacted with the geometrical center of the bar.

RESULTS

The first study demonstrated up to a 78% reduction in the HIC and a 72% reduction in the angular acceleration at impact speeds of 6.7 m/s. (Table 1). The stock production approximately 25 mm thick brush bar padding gave an approximate 44% reduction in the angular acceleration and a 55% reduction in the HIC compared to the baseline production roll bar. All padding reduced the HIC, linear acceleration, peak angular velocity, and peak angular acceleration compared to the baseline production brush bar. The greatest percent reductions occurred at 6.7 m/s.

Table 1.

Pendulum Head Side Impact with 25 mm offset from CG 5th percentile female Hybrid III Head and Neck

Test Impact Speed m/sec HIC Linear Accel G Peak Angular Velocity rad/sec Peak Angular Accel rad/sec2
Production Bar 3.4 202 135 28 4650
Production Bar 4.5 454 196 38 5300
Production Bar 4.5 582 245 34 7307
Production Bar 6.7 1966 500 49 12473
Simpson Padding 3.4 90 49 24 1966
Simpson Padding 4.5 200 71 31 2685
Simpson Padding 4.5 239 73 32 2812
Simpson Padding 6.7 710 167 44 6562
Simpson Padding 6.7 667 121 48 4660
Metal Air Gap 3.4 126 71 27 2737
Metal Air Gap 4.5 150 81 33 2330
Metal Air Gap 6.7 423 141 38 3460
Padded Production Brush Bar 3.4 105 68 24 2463
Padded Production Brush Bar 4.5 239 92 34 3565
Padded Production Brush Bar 6.7 883 196 47 6921
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The second experimental side-head impact study showed up to a 90% reduction in the HIC and an over 80% reduction in the angular acceleration at impact speeds of 5.4 m/s (Table 2). The stock vinyl and foam covering on the roll bar gave an approximate 17% reduction in the angular acceleration and a 13% reduction in the HIC compared to the baseline roll bar. All padding reduced the HIC and angular acceleration compared to the baseline roll bar.

Table 2.

Free Fall Head Side Impacts 50% male Hybrid III Side head impact 5.4 m/s and 25 mm CG offset

Impact Description HIC Linear Accel G Peak Angular Velocity rad/sec Angular Accel rad/sec2
Baseline Roll Bar Impact 2626 458 33 12271
Stock Vinyl and Foam on Roll Bar 2268 414 32 10162
Commercial Padding on Roll Bar 1130 248 27 6032
Simpson Padding on Roll Bar 558 144 21 2681
Metal Air Gap on Roll Bar 240 68 23 1672
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The third series of forehead impact experimental studies demonstrated up to a 90% reduction in the HIC and an 80% reduction in the angular acceleration at impact speeds of 4.5 m/s (Table 3). The stock vinyl and foam covering on the roll bar gave an approximate 15% reduction in the angular acceleration and a 28% reduction in the HIC compared to the baseline bar roll bar. All padding reduced the HIC and angular acceleration compared to the baseline roll bar.

Table 3.

Free Fall Forehead Impacts 50% male Hybrid III Forehead impact 5.4 m/s and 25 mm CG offset

Impact Description HIC Linear Accel G Peak Angular Velocity rad/sec Angular Accel rad/sec2
Baseline Roll Bar Impact 1871 324 48 8948
Stock Vinyl and Foam on Roll Bar 1348 279 47 8293
Commercial Padding on Roll Bar 794 208 44 6700
Simpson Padding on Roll Bar 383 128 43 5362
Metal Air gap on Roll Bar 192 64 36 4750
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DISCUSSION

The purpose of this study is to determine the biomechanical effects of various padding materials. The commonly used biomechanical models include human cadaveric, anthropometric physical and mathematical models. Although it is ideal to conduct experiments using cadaveric models to quantify the biomechanical effects of padding, the limited availability and associated costs restricted its use in this study. Another potential tool was the mathematical computer model (e.g., finite element technique). However, the experimental validation data required to develop the computer model warrants a separate testing using physical models. Consequently, anthropometric physical models were directly used in this study.

The present work was designed to provide comprehensive data to address the biomechanical effects of padding by quantifying the angular acceleration, HIC and linear acceleration. For this purpose, tests were conducted using two types of dummy models (5th percentile female and 50th percentile male) under wide ranges of velocities (3.4 to 6.7 m/sec). Compared to our preliminary presentation (Sances 1999), the present study provides more detailed results with additional experiments using the 5th percentile female Hybrid III dummy. Repeated tests were conducted using different padding materials and velocities to ensure the quality of the data collection. It was found that variations among the data were minimal. Since the controlled experiments were conducted using the anthropometric test devices, tests were not repeated for all velocities.

The rationale for quantifying the linear acceleration, HIC, angular velocity and acceleration and to evaluate the biomechanical effects of padding materials stems from the following studies. In 1966 Gadd proposed a severity index, SI, based upon the Wayne State Tolerance Curve (WSTC). It was followed by the Head Injury Criterion (HIC) proposed by Versace (1971) that attempted to incorporate the observation that a higher peak acceleration was tolerable if presented in a shorter period of time. Versace emphasized that the WSTC represented only the boundary between acceptable and unacceptable levels and that it did not provide a scaling for injury. In 1984, Mertz emphasized that injury assessment values should only be used to interpret the response of dummies and thereafter only as a guide to design. Nevertheless, a graph relating HIC to percent population expected to experience life threatening brain injury was advanced.

The importance of the contribution of rotational motion in the production of brain injury has long been advanced [Holbourne 1943]. Hirsch and Ommaya (1970) observed that concussion as well as visible hemorrhages and contusions at the brain surface could be produced during “whiplash” without direct head impact thus demonstrating that direct impact is unnecessary in the production of severe brain injury. Studies by Fan (1998) have indicated that suitable padding of interior vehicle surfaces can reduce the HIC by 50% or more at head contact speeds up to 8.9 m/s. Early studies by Monk (1986) showed similar results. Sances (1986) has reviewed early studies of head injury. Partyka (1988) has shown similar results.

Various authors have cited human head injury limits or non-injurious levels of head angular acceleration. Lowenheim (1975) conducted studies on cadavers and mathematical analyses and concluded that bridging veins and head injury can occur at angular acceleration levels of 4500 rad/sec2. Ewing (1975) studied volunteer Navy pilots on the HyGe sled and found no brain injury for angular accelerations up to 1700 rad/sec2. Studies with human boxers fitted with accelerometers on their heads were conducted by Pincemaille (1989). The studies led to concussion ranges in the living human of 13600 to 16000 rad/sec2. Ommaya (1984) conducted studies with the non-human primate and scaled a brain injury level in the human of approximately 4500 rad/sec2. A review of brain injury models by Newman (1998) concludes that HIC may not predict injury correctly. He also extrapolates from the non-human primate studies of Thibault and the proposed scaling to the human to suggest that an angular acceleration of 8000 rad/sec2 is consistent with the lower tolerance limits of acute subdural hematoma and 12500 rad/sec2 for concussion and 15000 rad/sec2 for diffuse axonal injury. Margulies (1992) and Meany (1998) have developed various models supported by animal studies to predict human tolerance levels to angular acceleration. Recent studies by Mishimoto (1998) have been done to establish the hypothesis that diffuse axonal injury can occur with translational acceleration. McLean (1996) has shown through extensive analysis of vehicular injuries that brain injury does not occur without impact.

Results indicated that all padding material reduced the HIC, peak linear acceleration, angular velocity, and angular acceleration. The reduction of the above head injury parameters was higher at higher velocity (6.7 m/sec) compared to lower velocities. In general, a similar observation was noted between 5th percentile female and 50th percentile male dummies. The reduction of HIC and angular acceleration was slightly higher in 50th percentile male dummy. The present results of reduction in head injury parameters coincide with previous literature. The evidence of others shows that padding can reduce the potential of severe head injury in everyday activity and sporting equipment (Becker 1993) [Myers 1993]. Studies by Fan (1998), Monk (1986) and Partyka (1988) show marked decreases in HIC with head impact on padded surfaces in vehicular interior surfaces. A review of skull fracture and facial injury has been given elsewhere [Sances 1986, 1996] [Yoganandan 1989, 1995].

In summary, a comprehensive experimental study was conducted to evaluate the biomechanical effects of various padding materials at different velocities using anthropometric 5th percentile female and 50th percentile male test devices. The HIC and angular acceleration were markedly reduced with the addition of padding materials. This observation was true for all padding materials and velocities. The present findings delineate the biomechanical importance of padding materials in the mitigation of head injuries.

(Presenter: Anthony Sances, Jr.)

Jim Raddin: Since one of the parameters that affects the performance of padding would be its thickness, it appeared, unless I missed something, that you were using the foam thicknesses at 25mm and the metal air gap padding at 31mm. If you had used similar thickness pads, would you not have expected differences in your results?

A. Sances: Perhaps, somewhat, but not a significant difference, I don’t think. It was really difficult for us to fabricate it. We did the best we could with fabrication. I don’t think significant differences would have been attained.

Murray Mackay: One slightly oblique comment specific to A pillars. Those of us who have done indepth crash investigation I think will see in many of the studies that the size of the cone of vision restriction of the A pillar is a significant factor in causation terms, particularly for intersection crashes. And one can’t help noticing that A pillars these days seems to be getting wider and wider, probably as a result of the sort of concerns that you’re suggesting. And I do think that there is this sort of second order effect that needs to be looked at specifically in the context of vision restrictions on A and possibly on B pillars as well before we go enthusiastically for bigger structures. Now one can, of course, with clever engineering limit that, but it is a concern that we see in doing causation studies that needs to be looked at.

A. Sances: I think that’s a very good comment, but some of the industry has been embracing padding for some years and seem to have gotten around that problem, but I’ll leave that for people like yourselves to deal with because that’s really not my area.

Richard Kent: This is not really my area of study and so I don’t know much about it and I apologize if this question is simple. I know from looking at side impacts, sometimes the addition of padding is a tricky thing and it may do things you don’t understand in the real world. I wonder if, in keeping with Murray Mackay’s oblique comment, you’ve looked at oblique impacts where the angle of impact is maybe not 90°. Are there issues with the neck loading and angular accelerations and angular velocities of the head perhaps being higher with padding than without?

A. Sances: Are you saying that angular velocities would be higher with padding than without padding? Is that your question?

R. Kent: I’m wondering if you’ve looked at that or has it ever been studied? I just don’t know.

A. Sances: I doubt that because the padding reduces the energy once you have energy absorption. I would suspect that you would not enhance the angular acceleration because we see that the angular acceleration appears to be associated with the marked rebound that we see and padding ameliorates that to a great extent. I think the angle of approach is important, I have to agree with you.

Jeff Crandall: Can I expand on that last question. There’s a body of research at Duke University that suggests that potential pocketing related to padding, while it may protect the head, may actually put the neck at a greater risk and I wonder if you can comment on that.

A. Sances: That is a potential area of interest and I think that the body of biomechanics is concerned with that and that has been studied to some extent by NHTSA, particularly with respect to glazing. When they were looking into the glazing materials and they found in their latest studies, in their 1999 glazing studies, that the head injury criterion went down and they felt that the parameters that they evaluated in the neck were still within a reasonable limit and below injuries, they didn’t concern themselves with that issue. But obviously since we have more head injuries and we have to deal with this and since the Federal Government has advanced this hypothesis, it is important for us to look at all the means for head injury.

J. Crandall: I believe the Duke data does definitely show an increased potential risk of injury for the neck. And I think that for the [FMVSS] 201 test there is no neck measurements at all; it’s just a head form. So they’re not being evaluated simultaneously. There is the potential, while improving head injury, to increase the potential risk of neck injury.

A. Sances: That’s possible. We’ve done some studies in our laboratory, and the laboratory studies that I’ve done so far with a torso and the head and neck have not suggested that there has been a serious problem with the neck situation.

Figure 1.

Figure 1

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Illustration of overall test configuration

Figure 2.

Figure 2

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HIC for various tests grouped by impact velocity. The tests with error bars signify repeated tests

Figure 3.

Figure 3

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Translational acceleration for various tests grouped by impact velocity

Figure 4.

Figure 4

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Angular acceleration for various tests grouped by impact velocity

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