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I made a suggestion to my boss to cut a thin, shallow slot on a metal sheet on the side we wanna make 90°, so that the machine could bend it. However, the outcome was "The metal broke".

My boss was disappointed. I believe other people cut a slot to help bending too. I want to find some theories to support my idea that it was not incorrect. Please help if you're interested...

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Look up stress concentration and specifically en.wikipedia.org/wiki/Three_point_flexural_test –  Martin Beckett Aug 31 '12 at 4:41
    
@MartinBeckett Thanks for your suggestion. I just read the link, the picture "Single edge notch bending specimen for fracture toughness testing" is a bit difference with my case. In my case the force from the v-die of a bending machine was in the same side. –  Marco Aug 31 '12 at 5:04
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Bending a material causes plastic deformation in it. More ductile the material, more easier it is to bend. If you cut a slot in it, the stress concentration will increase in the region around the slot, which in turn decreases the ductility. So, bending after cutting a slot will cause the material to break.

I believe other people cut a slot to help bending too.

Yes, cutting a slot does make the bending easier because the material has "space" to bend. But the right way to do this is not by cutting a slot.

Ideally you would want to make the same slot by other methods, which do not increase the stress concentration (forming, etc.)

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