And so if I draw that, this is what it looks like.

I've got my vertical face, I turn it 45 degrees,

90 degrees on my Mohr's circle, or 45 degrees on my stress block,

and I have a positive normal stress of tau and 0 sheer stress.

And then if I go another 180 degrees on my Mohr's circle,

or 90 degrees on my stress block,

I get a normal stress that's in compression and negative.

And so no shear stress, at these are the principal stresses.

Okay, so here is the result, this is my Mohr's circle.

Here is the stress block, and so what I see is that on a 45 degree angle,

I'm going to get a max tension and a max compression.

And so, chalk is strong in compression, but weak in tension.

So, if I take a piece of chalk and

I apply it to pure sheer or pure torsion,

I can see that it'll actually break on a 45-degree angle.

And you can see that, and you can try that on your own.

So you can see the 45-degree helical surface, because it's being

pulled apart by the tension, and the compression is strong and it holds firm.

And so the chalk breaks on a 45-degree helical surface, and

this is typical for any brittle material subject to torsion.

And in fact, here's another example, this is a thin aluminum tube, a hollow tube.

And if I put pure torque on that, I can see that I get in one 45 degree

direction's max compression, and so you can see that it buckles, and

in the other 45 degree direction it gets max tension so that it tears.

And so a perfect example of what happens

when I tprque this pure circular bar.

And so let's look at one more example,

here is a spiral fracture.

I talked about this at the beginning of the course, a spiral fracture of a bone.

If you get your, let's say like your arm caught somewhere and

it gets twisted and broken, that's a torsional break,

we call it a spiral fracture.

And you can see here, here's the bone length, and

here's where the fracture's occurring.

Again, it's about an angle of 45 degrees, and so as predicted by the theory.

So it's quite interesting, and so that's where I'll leave off for

today, and we'll pick up again next module.

[MUSIC]