In the Center, we work every day to reduce the risk of injury to vehicle occupants utilizing various tests, analyses, and engineering design tools. Today I would like to welcome you to a virtual tour of one of the Center’s key testing components: the deceleration sled. When you think of crash safety testing, you may think of a full vehicle crashing into a barrier.
While that is important, it is also very expensive. Modern cars are designed such that as the vehicle’s structure crushes, the occupant compartment stays reasonably intact. We can use this idea to test various occupant protection systems within that occupant compartment without crushing a vehicle every time. That’s where our Deceleration Sled comes in. The deceleration sled is designed to carry the vehicle’s occupant compartment. It can be accelerated to a desired speed using a pneumatic system, then “crashed” into a specially designed “shock absorber” that simulates the crush that occurs during an impact.
Test speeds can be up to 42 miles per hour, but are usually a bit lower depending on the intent of the particular test, such as the frontal impact, Federal Motor Vehicle Safety Standard which is conducted at 30 miles per hour. The sled can carry up to 2000 pounds, but of course, more massive payloads mean lower maximum speeds. Because only the relevant interior parts need to be attached to the sled, these tests are highly repeatable, can be more carefully controlled, and are significantly less expensive than full vehicle testing. In a frontal collision, each make, and model of vehicle will crumple and behave differently. We can simulate these differences by tuning the hydraulic “shock absorber” system that stops the sled, called the Decelerator. Because we focus on the collision, we choose a coordinate system that has the sled traveling in the negative direction. But the acceleration that stops the sled is then positive.
You can see in this typical pulse that the maximum acceleration is around 20 to 25 g’s. To work in SI units, we multiply by meters per second squared. That’s about 200 meters per second squared. That’s five times greater than the accelerations your body experiences on a modern roller coaster, and almost 30 times that of a hard stop using the brakes of a car. As you can imagine, human volunteers for this kind of test are not often used—though in the early days pioneering researcher Colonel John Stapp survived incredible accelerations to learn its effect on the body.
Alternatives such as animal testing, cadaver testing, and in-vehicle crash recorders have led to a statistical correlation between measured force, displacement, and acceleration quantities and the likelihood or risk of a particular injury. This allows us to use crash test dummies that are designed not to break but carry sensors that measure physical quantities. These Anthropomorphic Test Devices, or ATD’s, have triaxial accelerometers to measure linear acceleration in x, y, and z, in the head, chest, and pelvis. They have load cells to measure forces and torques in the neck, lower back, and femurs, and linear potentiometers to track chest deformation in frontal impacts and shoulder deflection in side impacts.
One of the frontal impact Federal Safety Standards allows up to 3 inches of chest deflection due to the restraint systems! Some of these measurements can be combined to correlate them to the statistical risk of injury. For example, the x, y, and z components of head linear acceleration from a triaxial accelerometer like this one can be combined to get the vector magnitude. This acceleration magnitude is then used to calculate a quantity called the Head Injury Criterion, or HIC. You can see that not only the peak acceleration would be important in a calculation like this, but also the duration of any significant acceleration.
Because acceleration is related to force, this is tied to the length of time that the force is applied. There is a great deal of fundamental physics in the work that we do every day in vehicle safety. On the simplest level, Newton’s first law explains why restraints like seat belts and airbags are so important: a moving object will continue to move until a force is applied to change that motion. By using restraint systems, we intentionally control that application of force over time.
Without them, it could be the steering wheel, dashboard, windshield, or other parts that apply that force. In any collision, there are really three impacts that engineers consider. First, a car might hit a barrier, and second, the occupant would strike the vehicle interior—we’d like that to be a seat belt or possibly an air bag! The third impact is internal, as the organs inside your body keep moving until forced to stop. For example, the brain would hit the inside of the skull.
We have the most control over the second impact—with careful design the injuries to internal organs can be minimized. Nonetheless, if you are ever in a serious crash, seek medical attention to be sure there are no internal injuries. We have done a wide variety of testing in this facility, from a portion of a Humvee for Army research, to rear impact studies utilizing specialized ATDs designed just for that type of collision.
Our unique mission blends together work in academia, current research in industry, as well as work in the community to spread the word about vehicle safety. Another way we are unique in vehicle occupant safety research has to do with our interest in the smallest occupants. We have partnered with both industry and government agencies to advance the development of child seats that assist to reduce the risk of injury to little ones, not only in front and rear collisions, but also in side impact. Because there is less vehicle structure involved in a near side collision, they tend to have higher rates of serious injuries. Based on some of the work we’ve done in this facility, a number of innovative child seats have been developed and new safety standards are proposed and will be in effect in a few years. As you can see from this brief introduction, fundamental physics is at the heart of what we do, in engineering vehicles for improved occupant crash protection.
We’ll explore some of those ideas in a few challenges for you to work on in a smaller scale in your physics lab at Mechanics, Inc. We appreciate your interest in occupant safety and thank you for working with us to ensure safer vehicles for the future.
Keywords: Auto Crash Safety, Near Side Collision, Serious Crash, Automobile Accident Lawyer, Front and Rear Collisions