Designed to Fail

How many of you have had this conversation?

“I remember my grandma’s old Buick. That thing was built like a tank. You could hit a tree and barely leave a dent and drive away.  These new cars just fall to pieces.” In my case, I usually hear this from my father-in-law, when defending his own elderly Buick ownership.

And to some degree, he is correct. Newer cars can be totaled at lower speeds, but it is on purpose. For the 50th anniversary of the creation of the Insurance Institute for Highway Safety, they staged a test to demonstrate how much car crashes have changed in that time (Warning: a very lovely car was destroyed to make this point).

Insurance Institue for Highway Safety Photoshoot from Billy Hunt on Vimeo.

Watching the video, your nostalgic friend/relative seems right about those old cars. The Bel Air comes out of the collision looking much more intact than the Malibu. The dummies, on the other hand, tell the real story. The Malibu driver gets out of the wreck with a broken ankle. The Bel Air driver is mostly likely dead, or permanently disabled. The failure of the car is what saves the driver’s life, with the energy of the crash largely expending deforming the metal, rather than being transfered to the driver.

Cars are hardly the only thing designed to fail. Certain styles of safety valves are designed to rupture before a pressurized tank would explode. For things like liquid nitrogen tanks, there is a standard release valve, like you’d find on a hot water heater, but there’s also a secondary burst disk in case the release valve isn’t working correctly. If you manage to prevent safe pressure release, the results could look something like this episode of Mythbusters:

Mythbuster’s Exploding Water Heater

On the other hand, with a faulty relief valve and a burst disk, you instead get a large pop, a very loud hissing until the tank has evacuated enough pressure, and you get to keep your building intact (though you may severely startle the grad students in the next lab over…).

When else is failure a good thing?


Thanks to bsabarnowl for the Buick picture!

6 responses to “Designed to Fail”

  1. GEARS

    I’m going to cherry pick the easiest answer. Failure is a good thing when it teaches you something. That said, if the thing that failed kills you, you’ve got a problem.

  2. Jim Green

    Electrical fuses come to mind.

  3. EricJuve

    How about shear pins on shafts/pulleys, freeze plugs on gas engines (not the core plugs).

  4. billswift

    What GEARS said. Otherwise failure is NOT a good thing. In your examples you aren’t comparing failure and non-failure, you are comparing a controlled failure with uncontrolled failure; of course a controlled failure is better, that does not make it a good thing though.

  5. Moiety

    Mmmh, would I cause a bursting disk or a pressure relief valve failing, a failure? Not necessarily. There is a subtle but important difference between component failure (as described) and the failure of the process to stay within its operating limits. If I were to describe the safety valve as failing, I would conclude my hazard studies and fault tress on the valve and not on the actual cause.

    In any case not too many items are designed to break. Middle management comes to mind but that is always broken. I always think certain check valves are designed to fail.

  6. Jacob

    I was expecting a post titled “Designed to Fail” would be about engineered obsolescence. But this was interesting too.