Injuries in Rear Crashes, Seat Structure and Product Liability Issues
TASA ID: 3190
Questions of adequate seatback strength, or the lack thereof, have been raised in many product liability claims against automobile manufacturers over the years. What role do the structures of the automobile seat and seatback have in injuries to vehicle occupants? What is the current state of knowledge?
Here are some facts based on my structural and statistical analyses, as well as the relevant published research.
Product Liability Issues
The Claims: In the product liability arena, the claims generally have involved the lack of adequate strength in the seatback structure, with allegations of this having led to large excursions of the occupant, resulting in injury-producing contacts with parts of the vehicle or with rear occupant. Claims have also been made that stronger structure for seatbacks would likely reduce or eliminate these injuries.
The product liability debate has focused on 'stiff' seatbacks versus 'yielding' seatbacks and the advantages of each. It should be noted that seatbacks are 'distributed structures' and thus cannot be scientifically described by a single parameter or by one simple term ('stiff', 'yielding' etc.) These non-technical descriptions of a complex structure are used to convey the notion that one seat ('stiff') would have undergone smaller deformation under a controlled laboratory test than another (yielding') seat in exactly the same test.
The Statistics
Big Picture: The average number of occupants involved in crashes every year in the U.S., as estimated from the NASS-CDS database of the National Highway Traffic Safety Administration, is shown here (Ref 1).
Comparison of this 'exposure' data with the recorded injuries in the above database for the years 1996-2007 shows that rear impacts are associated with comparatively lower rates of injuries than other crash modes. The observed number of injuries where the recorded maximum AIS (which stands for 'Abbreviated Injury Severity') was two or greater is shown here and illustrates that the ratio 'number of occupants with maximum AIS of 2 or greater' to 'number of rear crashes' is the smallest of the categories shown here. Similar conclusions (Ref 2) are drawn when injury rates are examined for the category of passenger cars struck in the rear by light truck vehicles (which include SUVs, small pickups and vans). In this category, rear crashes account for 29% of all cases but only 8% of the total 'societal harm.'
Who is Involved in Rear Crashes? The estimated annual number (from NASS-CDS data) of automobile occupants involved in rear crashes for the year 2007 shows the age distribution to be fairly widespread across the driving age population.
Risk Ratio versus Age: The cross-relationship between the number of occupants involved in the crashes versus the number of those with 'K' or 'A' injuries was examined and is shown here for the year 2007 as a percentage. The peaks in the
plot are due to the small number of cases available. The injury risk is observed to be fairly uniform across the age spectrum but with higher risk for older drivers.
Past Research
Simple relationships between the DV (change in velocity of the vehicle) and injury parameters have been proposed to explain the complex phenomena of injury causation in rear crashes. For example, one paper (SAE paper 2003-01-2205) concluded that 'stiff' seatbacks provide equal or better protection than 'yielding' seatbacks and defined 'serious injury zone' and 'no serious injury zone' based on the mass of the occupant.
A subsequent publication (SAE paper 2007-01-0708) presented data contradicting the validity of the above findings. Other papers have analyzed the effect of variables such as seatback recline angle (SAE paper 2009-01-1200).
The Structural Mechanics Of Seats
Do Seatbacks Fail? : All structures deform (i.e. yield under loading). Seatback deformations may consist of several components:
- Compression of the padding on the seatback
- Bending and deformation of the seatback frame
- Rotation of the seatback around the hinges
- Deformation of recliner structure and resulting rotation of seatback
- Motion and deformation of the seat bottom structure
- Deformation of the vehicle floor and resulting motion of seat and seatback
- Deformations in the joints and connections
- etc.
Structural Design: The design practice is to provide adequate structure in the seat and the seatback to support the expected operational loads, while meeting other constraints of aesthetic, comfort and performance criteria. Designs are also dependent on features, e.g. recliner mechanisms, electric motors, attachment points of seatbelts, etc. A photograph of a modern seatback structure is shown here.
Design for Crash Safety: In a rear crash, the loading on a seatback consists principally of the occupant's mass multiplied by the vehicle's forward acceleration. In other crash modes (e.g. frontal and oblique crashes), seatbelt forces also need to be considered, since they may be a significant part of the loading on the seat and the seatback.
The loading pattern in a purely rear impact will be a rearward loading on the seatback, as well as a bending moment at the base of the seatback. The resultant force on the occupant may be visualized as one force component tending to slide the occupant upward and rearward along the seat, and the second component causing compression of the seat with resultant rearward displacement of the occupant. The total excursion of the occupant is the vector sum of all the components.
From this simplified analytical model, observations may be made as follows:
- Injuries to the occupant may result from contacts with various parts of the vehicle including the seatback, headrest, roof, pillars, rear seats, etc. (This assumes that the occupant remains contained within the vehicle).
- The magnitude of injury is dependent on the structural property of the contacted component, the rate of acceleration (or deceleration) of the body segment, and its injury tolerance threshold.
- The larger the seatback recline angle, the larger the force tending to slide the occupant (upward and rearward) out of the seat, and the smaller the force causing seat compression.
- The dynamic friction between the seatback and the occupant is the principal component resisting the sliding motion. The contact area between the occupant and the seat/ seatback is one of the factors influencing the dynamic friction.
- For a properly positioned occupant, a 'softer' seatback structure may lead to a larger contact area because the resulting compression of the seatback causes more of the occupant's torso to be in contact.
- For a properly positioned occupant, a 'softer' seatback structure may lead to smaller (as compared to the case below) 'sliding excursion' and larger (as compared to the case below) 'compression excursion.'
- Conversely, for a properly positioned occupant, a 'stiffer' seatback structure will likely lead to larger (as compared to the case above) 'sliding excursion' and smaller (as compared to the case above) 'compression excursion.'
- For occupants that are 'out of position' (such as leaning sideways and/or in a fully- reclined seat, etc.), the resulting motion in a rear crash may not utilize significant portion of the seat structure properties and the available dynamic friction.
In order to apply the above general observations to specific incidents and to reliably evaluate the likely causes of injuries, it is necessary to examine the event and the vehicle structure in detail, establish the forces on the occupant, and then evaluate the contributions of the seatback structure, as well as of any other vehicle components.
Results from this finite element simulation confirm the conclusions from the earlier simplified structural analysis. For the case where the seatback and the supporting structure have been stiffened to eliminate structural displacement and deformation, the 'seat compression excursion' is eliminated, but a large amount of 'sliding excursion' of the occupant may result and lead to multiple possible contacts between the occupant and the vehicle's interior, such as torso-to-seatback, head-to-roof, neck-to-headrest and head-to-headrest.
A study of the kinematics of a 95th percentile ATD (Anthromorphic Test Device) in a rigid seat in the same vehicle was also conducted, and the results are shown here. It is observed that for this case of a rear crash, the contacts between the occupant's body segments and the interior parts of the automobile occur at different instances of time and involve a different set of kinematics. Although not presented here, the impact velocities of the body segments and the vehicle interior were also different in this case than that for the 50th percentile ATD.
Other Factors in Design of Front Seatback
Safety of Rear Seat Occupants in Frontal Crash: Since the design of an automobile should comprehend likely operating conditions and crash scenarios, the role of the seatback structure of the front seat should also be evaluated for the case when a rear seat occupant is present and the vehicle is involved in a frontal crash. In this case, the rear occupant's impact will be with the back of the front seat. Simulation of a mid-sized sedan in a 30 mph frontal crash was conducted with a 50th percentile male ATD in the rear seat, restrained by a three-point seatbelt with pyrotechnic pretensioners. The front seat and some other parts of the vehicle are shown as planes in order to visualize the contacts clearly. The front seat position was approximately the position that a driver (of the size of the 50th percentile male ATD) would have set.
CONCLUSIONS
The interactions between a vehicle occupant and the structures of the seat and the seatback depend on several inter-related factors, such as direction and severity of crash, resulting vehicle deformation and dynamics, occupant sizes and postures, seat position and recline, structural geometry and dynamic properties, etc. In examining crash performance of a vehicle's seat and seatback during a crash, it is necessary to evaluate all these factors and assess the effect of any proposed design changes across the spectrum of operating conditions and the crash scenario.
REFERENCES:
1. "Evaluation of Pre-Crash Sensing and Restraint Systems Effectiveness," SAE Paper 2010-01-1042
2. "Perspectives On Vehicle Crash Compatibility and Relationship to Other Safety Criteria," Enhanced Safety Vehicles Conference, Paper 2003- 412
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