What Does the Takata Airbag Case Say About Accelerated Test?

The widening probe of Takata Corp.’s airbag failures raises an important and largely overlooked question for design engineers: What do the ruptured airbag inflators say about accelerated life testing?

Accelerated test is relevant in the Takata case because the vast majority of affected vehicles were roughly a decade old and operated in hot, humid climates. One question for investigators is whether any short-term accelerated procedure could have identified a problem that surfaced once in every 340,000 or so vehicles, and even then only after years of use.

More than 30 million older vehicles made by 10 different automakers have been recalled to have inflator mechanisms replaced in Takata airbags. (Source: Design News)

”It’s a particularly appropriate question right now, given the speed with which new technology is being introduced,” David Cole, chairman emeritus of the Center for Automotive Research, told Design News. “We’re pushing technology into products at a tremendous rate. And we need to know -- what are the age-related issues we aren’t addressing with accelerated testing?”

The focal point of the Takata case is the airbag’s inflator. The inflator, a metal cartridge packed with ammonium nitrate propellant, has reportedly ruptured in some cases. The ruptures are said to be highly dangerous, largely because “metal shards from the airbag can be sprayed throughout the cabin,” Consumer Reports wrote in a recent description of the problem. To date, the problem has been linked to more than 100 injuries and eight deaths.

The cause of the ruptures is still unclear. But if the propellant wafers broke down over time in high humidity -- as many experts suspect -- excessive pressure inside the inflator body might have resulted. Then, the body would have ruptured.

And therein lies the connection to accelerated test. An engineer can test a product to excess -- higher temperatures, humidity, stresses, and strains -- for a short, intense time period. But there’s no guarantee that months of excess will accurately model reality over ten or 15 years.

That’s not to say that accelerated test is at fault in the Takata case. Nor does it mean that such procedures shouldn’t be used. They should be. Without accelerated test, innovation would slow to a snail’s pace. "If you had to test everything over its entire expected life, we’d all be driving Model As," Cole said.

But experts say that some products lend themselves to accelerated test better than others. “It isn’t a big issue with metals,” Cole told us. “But with chemicals, the properties can change over time.”

Cole recalled the case of a pickup truck in which the electrical insulation deteriorated over years. The problem wasn’t exposed during accelerated tests, but in real life, the deterioration happened, and it caused fires. Ultimately, the automaker lost tens of millions of dollars in recalls while it nervously waited for customers to bring their trucks back to their dealerships for fixes.

To be sure, Takata’s airbag dilemma may ultimately have nothing to do with age, deterioration, or testing procedures. Last week, two US senators demanded all vehicles containing Takata’s airbags be recalled, after a side airbag inflator on a 2015 Volkswagen Tiguan ruptured. The Tigaun scenario could be an important twist: If one-year-old vehicles eventually get recalled, it might suggest that age isn’t the ultimate culprit.

But the Takata situation nevertheless highlights an issue that engineers need to discuss. It raises a set of questions that haven’t been fully dealt with up to now. Today’s vehicles are incorporating new technologies -- electronics, software, materials -- at record rates. The pace of innovation is changing. The rules are changing.

Maybe it’s time for engineers to step back and take a hard look at the old methods of accelerated test. "You don’t hear much discussion about this,” Cole said. “But the issues need to be raised.”

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Senior technical editor Chuck Murray has been writing about technology for 31 years. He joined Design News in 1987, and has covered electronics, automation, fluid power, and autos.