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Strain Hardening. What can I say about Strain Hardening?

To understand how strain hardening works, you have to understand the way that metals are stuck together.

When you take a bunch of atoms of a metal, they're going to clump together. Otherwise they'd be a fluid.

Imagine a bunch of oranges or something, stacked up. Each row of oranges is a plane. Most metals are going to naturally form into something stacked along those lines. They usually tend to minimize the volume taken up by the atoms. Why? They just do.

Anyhow, the exact stacking pattern of the material doesn't really matter, but usually it's either going to be a Body Centered Cubic, a Face Centered Cubic, or a Hexagonal Close Packed. Why? Just because they do. Go ask the Philosophers.

So anyways, when there's a stress applied to these planes, they can slip relative to each other. This is usually a bad thing. Most people don't like their bridges sagging in the middle.

Now imagine that these planes are like a bunch of sheets of paper. It's really easy to make 'em slide. But they're not quite flat perfect planes. Every plane will have a bunch of imperfections in them. Missing atoms, extra atoms pushing the others out of place, atoms of another element that's either larger or smaller than the rest of them. Perhaps there's a plane that just ends, forcing the plane above and below it to stretch and fill up the gap. There's others.

Anyhow, all these things happen all the time in metals. And they're going to act just like folding or sticking tacks into sheets of paper will. They'll make it harder for the planes to slide relative to each other, thus making it harder to stretch the metal.

But what happens when you do manage to pull hard enough to stretch the material? Well, first, as it stretches, it's going to decrease the cross sectional area of the part being stretched. The same volume of metal now needs to fill a larger length, so area decreases.

Also, you're going to cause more of these imperfections. Entropy increases and such.

So, the more you stretch something, the harder it gets to stretch. Fun huh? That's Strain Hardening, so called because the amount something is stretched compared to its original length is called strain. And because it gets harder.

So YEAH. There's a side effect too. The more you strain harden (also called work hardening, and sometimes cold working) something, the less ductile it becomes. Keep on doing so, and it'll experience brittle failure. This can contribute to Structural Fatigue.

It's fairly easy to test this effect. Take a paper clip. Bend it. Keep bending it. You should notice that the paperclip gets harder to keep on bending, until it finally experiences brittle failure (breaks).

Strain hardening is often used because it's a good way to increase the strength of a material with little effect on the chemical or electrical properties of it.

Of course, it also does make stuff more brittle, and stresses from the working process can sometimes build up in the piece being made. This can increase the likelyhood of failure. If this is a bad thing, the effects of Strain Hardening can be eliminated by annealing it. Basically you heat it up past the point where the crystals of metal start growing together, and then let it cool.

The effects of Strain Hardening are most pronounced in stuff that is made by banging it out. Forging and such. Take a piece of metal, squeeze it between two casts, and you get a differently shaped piece of metal.

I've got this aluminum ashtray that we made in my Manufacturing Engineering class. We just took small sheet of aluminum, and stuck it between the dies and turned on the hydraulic press, and waited. The original sheet was easy to bend. The ashtray is not. I like it, since it's got University of Calgary Mechanical Engineering stamped into it.

Anyhow, I'm rambling now, and this is a factual writeup. I should stop.


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Sources: Class Notes, ENMF417 and ENME421, University of Calgary
Materials Science and Engineering, William D. Callister, 5th Edition

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