To explore new safety concepts, GM's Safety and Security Initiative team - consisting of individuals from several GM R&D Center departments and the Concept Development Center - put their heads together to modify a Chevrolet Cavalier. The vehicle showcases features designed to enhance crashworthiness, protect occupants, improve the driver's vision, and make the vehicle more conspicuous.
How do they know that these features provide improved performance without crashing the one-of-a-kind car to find out? The team relied on math-based crashworthiness analysis. Fortunately, a fairly complete finite element model of the Cavalier was already available. The modelers first subjected the vehicle to simulated front and side impacts for baseline information, then evaluated the effect of proposed modifications. As the work developed, the entire team put forth ideas, which were implemented first in the model, then in the vehicle itself.
What did the team come up with? About 20 safety features that were eventually included on the Concept 1 car. Three of the most promising are the extendable bumper, cross-car beams, and four-point seat belts.
Extendable bumper - The
extendable bumper relies on the fact that increasing the front
crush space helps keep occupants safer. The team decided to
use Pre-crash Sensors (PCS) to extend the bumper out about
six inches and lock it in place. This transmits crash forces
back into the car. The bumper is connected to two reversible
actuators, so that in a false alarm it can simply be retracted
and set for reuse. The PCS has a time budget of 500 ms, which
is adequate to sense a car approaching at 90 mph - the equivalent
of meeting another vehicle also traveling 45 mph - and extending
the bumper in time when only 20 meters away.
Analysis
of the bumper shows just how effective this solution can
be. GM has received two patents for this innovation.
"The ability to soften the front-end structure of a vehicle suggests that the extendable bumper is one of the few safety features that could improve both the crashworthiness and aggressiveness of a vehicle," says J. T. Wang, Principal Research Engineer and Group Manager of the Vehicle Analysis & Dynamics Lab. In other words, it improves safety both for the occupants of the vehicle with the extendable bumper and for the other vehicle involved in the crash.
Cross-car beams - A feature
often seen in lightweight cars is a fixed front seat. This
is a much lighter way of making the seat, particularly if
both lap and shoulder belts will attach to it. Building the
seat support structure into the car from B-pillar to B-pillar
- horizontally across the middle of the car - achieves the
effect of all-belts-to-seat without the mass penalty. The
modelers designed the cross-car structure not only to resist
intrusion in a side impact, but to collapse in the vehicle
center, which doesn't compromise the safety of the occupant
sitting nearest the impact point.
Analysis
of the cross-car beam demonstrated greatly reduced depth
and rate of intrusion.
Four-point seat belts - The impetus for using four-point belts came from motor-sports crash investigation work. When drivers use double shoulder belts, shoulder or chest injuries are rare even in the most severe frontal crashes. That compares favorably to three-point belts, where a frontal crash sometimes results in broken ribs. Four-point belts also provide symmetric loading, which improves protection in angled impacts.
"One known problem of a four-point belt is that its shoulder belts have a tendency to pull the lap belt upward, increasing the risk of the occupant submarining," says Wang. To alleviate the problem, the researchers used a pair of lap-belt pretensioners in the model, plus a pair of load-limiters to control the shoulder belt load on the upper torso, as shown in Figure 1.
g 1. Interior occupant protection features include hip and shoulder bolsters and the four-point belt system. A computer model was used to evaluate the potential benefits.
By modeling the response with a three-point belt and four-point belt for a 30-mph, frontal-barrier impact crash, the researchers found all injury indices reduced substantially, except neck tension, which increased 13 percent. Later, the team successfully used a small air bag in the model to mitigate the neck tension. GM received a patent for the four-point seat-belt design.
Moving Forward with Safety
"We were encouraged by the potential of these safety features, as demonstrated using math-based tools," says Wang. "Our evaluation focused only on a few impact conditions, however. The next step is to further evaluate these features to address other safety standards and regulations and to analyze their performance in real-world crash events before implementing them in a production vehicle."
Analysis of the bumper
Analysis of the cross-car Beam
