Engineering fracture mechanics as applied to onion peeling

Pearl onions are a pain in the ass, but engineering can help you peel them.

I'm making a side dish that calls for peas and whole peeled pearl onions. With big onions, you can usually just flake off that outer layer, but it doesn't work with pearl onions. I have no idea why. The solution is to carefully cut from pole to pole one layer farther down - through the outermost edible layer of the onion. Remove that, and you're golden. But even that edible layer will balk at being peeled off a sphere. You can't just pull it outwards like an orange rind or it'll break.

What does this have to do with fracture mechanics, you ask? What you're really doing is propagating a crack. There are three ways to crack something:
Mode 1: "tension". You pull the two sides apart - like an orange peel.
Mode 2: "in-plane shear". You grab the two sides, push one away from you, and pull the other towards you.
Mode 3: "out-of-plane shear". You grab the two sides and pull one along the crack in one direction while pulling the other along the crack in the other direction. Sort of like twisting off a jar lid.
The magic way to peel an onion is Mode 2, illustrated in the photo above. You lift the edge of that edible layer, creating a crack between the layer and the onion beneath. Then you push that edible layer sideways on the onion (counterclockwise in the photo). It's counterintuitive that the best way to separate a layer from what's underneath it is to slide it sideways, but it keeps it whole so it comes off neatly.

And that's your engineering lesson for the day.

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