Physics/scissors cutting metal
QUESTION: Hi Steve,
When a pair of scissors cuts a piece of paper, the metal of the scissor blades is a stronger material than the paper - what if the scissors and the paper were made of the same material? Would the ability of the scissors to cut the object then depend on the pressure exerted on the scissors (and the geometry of the scissor blades? cutting angle?) OR would it be impossible for the scissors to cut an object of the same material?
(I have imagined the object being cut is a thin metal rod)
ANSWER: Well, seeing as how I just cut some aluminum foil with aluminum scissors, I'd say yes. The materials are slightly different, given how the foil was rolled, but the scissors were stamped out of rolled aluminum sheet. So it's possible, but you're right, it depends on geometry. Apply enough force and one will give. Those scissors (made for kids, obviously) will give out pretty quick if I use them for anything but paper, I'm sure, but applying enough shearing force to the aluminum foil caused it not to be cut so much as broken. The scissors were just thicker.
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QUESTION: Thanks Steve,
I am interested in the types of material strength that influence this operation. I guess that for the object being cut, it is the Shear strength that determines how easily it will be cut - is this correct?
For the the blades of the scissors, would it be the Compressive strength that determines the ability of the blades to cut the material?
If this is right, would it be safe to assume that, in the case of your scissors, the Compressive strength of the scissor blades was higher than the Shear strength of the aluminum foil that you cut?
That's the basic idea, but shearing a sheet with scissors is surprisingly complex at the small-size scales on which it happens. The scissors in the case of identical materials will be grabbing at the parting sheets. It's like opening a package that's difficult to open until you get a rip started, then it's really easy because you have a long lever arm on each side to pull on, in addition to the scissors meeting point helping cleave the material. There's not a "higher or lower" in the case of different types of strengths, because they refer to different types of action.