Getting a grip on airbags
A student project at Kettering University utilized the institution's Crash Safety Center to gain a full appreciation of the power behind the deployment of vehicle air bags.
One day, without realizing it, a passenger in a car could accidentally put their foot into their mouth, or involuntarily take a bite out of their steering wheel.
Sometimes the temptation is simply too great, particularly after several hours on the road: people sit in the passenger seat of a car on a hot, dusty drive across the country during the summer, and instinctively lift their legs and rest them on the instrument panel (IP), commonly referred to as the dash. Other times drivers are tired from stretching their arms to the steering wheel and decide to pull the seat closer.
But is it safe to rest legs on the dash at anytime during a drive, or to sit too close to the steering wheel?
For three Kettering students, the answer to both of these questions is a resounding NO. The students-seniors Kyle Phillips of Newfane, N.Y., Rob Kleinhardt of Sterling Heights, Mich., and Sean Martinez of Tuscon, Ariz.-participated in Dr. Henry Kowalski's MECH 514: Experimental Mechanics course this past spring term. In this course, which introduces students to strain measurement techniques and the importance of identifying and measuring physical quantities such as displacement, weight, load and temperatures during product testing and validation, teams must propose and administer a capstone project during the term. The title of their project is "Airbag Deployment Force," an initiative that measures the amount of force exerted on passengers when the airbag is deployed.
Their preliminary thoughts about conducting this project hinged on several assumptions, one of which is that people sit too close to the steering wheel and front dash in a vehicle. The goal of this project was to determine if there is a potential hazard to occupants when sitting in close proximity to the steering wheel, based on a measurement of deployment force at varying distances from the airbag. Although they had hoped to gauge the force of an airbag deployment upon a passenger's legs when resting on the IP, they realized the scope of this would require a passenger compartment from a vehicle, as well as an crash dummy that could be sacrificed, all of which were simply too costly to secure.
But their research did reveal some important conclusions about the force exerted by an airbag upon a driver. To fully study this idea, they used Kettering's Crash Safety Center under the direction of Dr. Janet Brelin-Fornari and Dr. Kowalski's high speed digital video camera to video the deployment of airbags and the amount of force they create. The camera, a FASTCAM-PCI Model 1KC, can shoot digital video in full color and capture the integrity of various items, such as the explosion of an airbag from its housing.
The team constructed a rigid fixture that would allow them to vary the airbag deployment distance and properly measure the deployment force. The fixture uses a jack stand to allow adjustment to the deployment distance and attachment to the crash lab sled, and represents a surface that is much more stronger than, say, human bones.
The fixture allowed the team "the capability to measure acceptable levels of strain," explained Phillips. They also attached strain gages on the top and bottom of the fixture beam ends, and then calibrated the measurement gages in preparation for the test. But all of this simply didn't take place overnight, noted Martinez. "It took several weeks to prepare the fixture and insure that it would operate correctly when a load was applied," he said, a feat that underscored the students' interest and enthusiasm for this project.
Furthermore, the cost of obtaining new airbags was prohibitive. Kleinhardt said that the team decided to visit several scrap yards and found a number of airbags used previously in Oldsmobile vehicles from the 1980s from Mike's Auto Salvage of Flint, Mich.
They then took the airbags back to the testing facility and began the process of setting up the data acquisition equipment for the recording of measurements once airbags deployed. They determined several data points, which represented various distances from the airbag face to the chest plate situated on the fixture. These points ranged from 1.75 inches to 10 inches. Then they ran the tests and recorded some interesting results.
For example, at a distance of four inches from the airbag face to the chest plate, the deploying airbag exerted a maximum load of 912 pounds when released. In the slow motion video clip captured by Dr. Kowalski's high speed camera, one can clearly see the chest plate on the fixture bow upward as the airbag pushes against it (click here to see a slow motion video of this experiment). At a distance of 10 inches, the maximum load is only 91 pounds.
The students reached some intriguing conclusions regarding this airbag deployment test and quickly noted that the newer airbags now used in cars deploy safely and quickly. But for this test, their findings indicate several important observations:
- airbags require a certain distance to reach their maximum force and thereby provide an appropriate level of safety during a collision;
- once the critical distance is reached, the force of the deployed airbag decreases and the airbag is at the maximum deployment; and
- a potential hazard exists if an occupant sits too closely to the steering wheel or dashboard, especially when they rest their legs on the dash.
For Kowalski, this study revealed the enthusiasm and dedication students have toward their education at Kettering. "With the advent of high speed digital photography and data acquisition systems, senior students in the experimental mechanics course are proposing and completing projects that are not only challenging but informative," he said, adding that the air bag deployment project team "is a prime example. Their inquiry into what effect a deployed air bag has on an occupant led to the creation of an instrumented frame and air bag in close proximity. The test results clearly demonstrate that the resulting force exerted by an air bag is huge. The proverbial saying 'putting a foot in your mouth' immediately comes to mind when considering what the consequences of an passenger air bag deploying while resting your feet on the IP."
Written by Gary Erwin