I just came across the 2014 comedy Neighbors starring Seth Rogen and Zach Efron. One of my favorite scenes is when the fraternity next door begins subjecting Rogen’s character, Mac, to a series of escalating pranks after he calls the police on their house party. Mac knows the frat brothers have broken into his wife’s car and stolen the airbags, but he can’t figure out why until he sits down on a booby-trapped office chair.
The airbag launches Mac into the air, slamming him into the drop ceiling before crashing back down onto his cubicle desk. Somehow, he even loses his shoes as he’s tossed into the air like a rag doll.
Now, what would engineers do if they were working from home and had access to simulation software? (Note: Four second video of simulation results at the link.)
...how high we can launch a virtual Seth Rogen? We set out to find out what would really happen if you sat directly on top of an airbag and deployed it.
We placed the airbag on the seat of a chair where our 200-pound adult human model would sit. For this study, we used the Humanetics Hybrid II 50th Aero FE Dummy Model along with Altair RadiossTM, a solver for highly non-linear problems under dynamic loadings. An aerospace dummy was used rather than an automotive model since it allowed us to observe the axial spinal thrust caused by the initial airbag interaction and forces applied to the neck purely in the Z direction.
...the simulated physics of a surprise airbag-induced flight match up quite well with the movie special effects of Neighbors, but the injury risk data extracted from our crash simulation show how dangerous this experiment could be in practice...don’t try this at home bros.
...our virtual Seth Rogen (saw) head acceleration reaching over 250 g and the head injury criterion (HIC) value reaching over 3500. In automotive crash tests, engineers typically flag head acceleration values of more than 80 g over a duration of three milliseconds. The maximum HIC value allowable in automotive crash testing is 1000, which also greatly exceeds industry safety guidance.
We also observed forces on the spine reaching nearly 8 Kilonewtons (KN). Though the peak force is only reached for a few microseconds, 8 KN is the equivalent of close to 1,800 pounds of force (over 800 kg).
Finally, a neck injury criteria (Nij) is a formula to express injury potential combining four possible modes of neck loading – tension, compression, forward (flexion) bending, and rearward (extension) bending. For good safety ratings, engineers aim to achieve a Nij number below 0.9, but our data shows a Nij number of 1.2, a very high number which equates to elevated risk of serious, possibly even life-threatening injury.