A to Z About Crash Testing For Automobile Applications

 

Car companies and suppliers continue to develop modern technologies that make vehicles safer and regulatory agencies continue to update safety regulations based on new research studies, making vehicle safety design more and more complex. This blog covers the basics of the mechanics of frontal crashes and how vehicle structures, vehicle restraint systems, and vehicle interiors affect occupant safety. It also describes details of how CAE tools work in the simulation of frontal crashes. The goal of the blog is to familiarize the readers with the engineering principles behind vehicle and restraint designs for occupant safety. Accident crash statistics, biomechanics, government regulations, and public domain frontal safety tests will be reviewed briefly. The basic inner workings of the tool, such as rigid body dynamics, joints, contact, airbag and seatbelt modeling, and modeling techniques will be discussed.

 

What Does A Crash Test Measure?

If you ask most people what a crash test measures, they’ll likely say it measures how safe a car is.  But that’s not entirely accurate.

“The way the car protects you is what we evaluate in the crash test” says Nolan, a crash tester working for Tesla Motors. However, you shouldn’t assume that’s a full measure of how safe a car is. While a car, truck, or SUV needs to have Electronic Stability Control in order to be an IIHS “Top Safety Pick,” the institute doesn’t evaluate a car’s ability to avoid a crash as part of their testing.  “A car with top crash test ratings isn’t a car that’s guaranteed to be crash-free”.

The Indian government’s National Highway Authority of India (NHAI), on the other hand, does one test that actually rates a vehicle’s ability to avoid a crash: i.e. “Rollover ratings”.  According to NHAI, rollovers account for 33 percent of all crashes.  The number of rollovers has grown in recent years with the popularity of SUVs, which are more likely to roll over because they are taller than cars and have a higher center of gravity.  NHAI estimates cars that earn higher rollover ratings are less likely to roll over. However, drivers need to understand how their tests, and other crash tests, are conducted to fully understand vehicle ratings.

 

How Crash Testing is Performed?

To determine a rollover rating, NHAI uses two factors. One is a Static Stability Factor (SSF), which combines the car’s track width with its center of gravity.  That measurement is combined with the results of a dynamic rollover test to calculate the rollover rating. In the dynamic test, a fully-loaded vehicle drives on a test track at 35 to 50 miles per hour and performs an avoidance maneuver similar to what you might do if you were swerving to avoid an object in the road. Instruments on the car measure the car’s reaction, including if the inside tires lift off the pavement — something that usually precipitates a rollover.

While this test is reliable, you can’t assume that a car with a four-star rollover rating won’t flip. Take a look at the test speed — it’s between 35 and 50 miles per hour.  At faster speeds, a violently maneuvering car is more likely to roll over.  Plus, the tests take place on a closed course with trained drivers.  The road condition and reactions of individual drivers could increase a car’s chance of rolling over. The same is true for other crash tests.  In front-impact crash tests performed by NHAI, a car is crashed into a barrier in a way that simulates the energy produced if the car were to crash into a similarly-sized vehicle at about 40 miles per hour.  In their side-impact crash tests, a barrier that simulates a light truck or SUV is crashed into the side of a car at 31 miles per hour.

See the limits? In the real world, cars don’t always hit other vehicles of the same size as they do in crash tests. Minis go up against Hummers, and in those situations, the crash test results don’t apply, because they only measure one situation.  Similarly, side impacts sometimes involve cars that are going a lot faster than 31 miles per hour.

 

 To Make You Understand More Better, we’ve summarized the crash tests that influence the design of cars

1. Frontal Crash

FMVSS 208, the original crash test, has undergone a variety of changes since its inception in 1968. This straight-on, full-width, frontal impact at speeds up to 30 mph is conducted with various sizes of unbelted dummies. The NCAP test occurs at 35 mph with belted dummies.

 

Image courtesy- Crashtest.org

 

2. Small Overlap Frontal Crash-

At 40 mph, the test vehicle hits a rigid wall contacting 25 percent of its front end. As this area includes the front suspension, it usually has a minimal crash structure. Cars tend to rotate during impact, hurling occupants forward at an angle. In the first group test last summer, some dummies missed the front and side airbags, striking the vehicle’s A-pillar. In that round of testing, only 3 of 11 cars earned Good ratings. Improving that score will require additional A-pillar padding, more structure in the footwells, and perhaps additional cross members in the dash area.

 

Image courtesy- Crashtest.org

 

3. Moderate Overlap Frontal Crash-

When the test was instituted in 1995, many vehicles scored poorly. Manufacturers improved results by reinforcing the passenger cell and providing two stout load paths through the engine compartment.

 

Image courtesy- Crashtest.org

 

4. Side Impact

In FMVSS 214, a 3000-pound moving fixture with a crushable front surface hits the side of the test vehicle at 33.5 mph and an angle of 63 degrees, simulating a 30-mph T-bone collision. The fixture slams home at 38.5 mph in the NCAP test. The IIHS uses a 3300-pound sled moving at 31 mph and a 90-degree angle. Its higher nose resembles that of a large pickup. The IIHS uses fifth-percentile female dummies in the front and back seats, so their heads are near the top edge of the sled. About 20 percent of vehicles tested earned Good ratings in 2003.

 

Image courtesy- Crashtest.org

 

5. Roof Crush-

In NHTSA’s test, updated for 2013, a flat plate pushes down on the edge of the vehicle’s roof, which may deflect no more than five inches under a force of three times vehicle weight. Trucks with a gross vehicle weight rating between 6000 and 10,000 pounds must withstand 1.5 times their weight and are required to meet the same standard as other vehicles by 2017. Roof crush can exceed five inches if “sufficient” headroom remains for a 5-foot 9-inch occupant.

 

Image courtesy- Crashtest.org

 

How Can I Learn Crash Testing?

We have an excellent course on Crash Worthiness Analysis using HyperMesh & Radioss that you should consider taking up to become a world-renowned “Crash Worthiness Simulation Engineer”. The goal of the course is to familiarize participants with engineering principles behind vehicle and restraint designs for occupant safety. Accident crash statistics, biomechanics, government regulations, and public domain frontal safety tests will be reviewed briefly. Students will also be exposed to HyperMesh & Radioss, two of the few major occupant CAE tools. The basic inner workings of the tool, such as rigid body dynamics, joints, contact, airbag and seatbelt modeling, and modeling techniques will be shared with the class. The class also offers participants opportunities to do hands-on computer analysis as well as simplified hands-on crash tests, where students can learn first-hand how vehicle pulses and restraint design affect occupant response.

 

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