So how they test for fatigue is they take something called a rotating beam specimen.

Now this specimen's a shaft, they put it in four point bending.

So it's actually going to look, if you exaggerated

the distortion, it would be something like this.

And so, they're going to take the shaft,

they put it in the four point bending machine and then they start to rotate.

And what happens is the filament along the top of the shaft is first in compression.

And then it rotates down and

it goes through the neutral axis where it experiences no stress.

And then, it rotates down again, and

it goes into the bottom of the beam where it is going to experience tensile stress.

And then, it rotates back through the neutral axis and then,

back into compression and that's typically one cycle.

So you've removed and reapplied the load in one cycle.

This is called a fully reversible load and that means you're going

from 0 up to 20, down to negative 20, and then back to 0.

Or maybe you're going from 0 up to 100 down to negative 100 back to 0 and

so on and so forth.

So in fully reversible loading, your stresses are equal in magnitude.

And you go from tensile to compression to tensile to compression.

So, SN diagrams hold for this fully reversible loading condition,

the strength life diagrams that we're going to look at.

If I looked at the stresses, what would happen is if I started at the neutral axis

and I rotate down, I'm going to end up in tension, so my stress goes up to 20 ksi.

I'm going to go down to 0, as I go through the neutral axis.

I'm going to go down to -20 as the filament comes up into compression and

then I'm going to go back to 0 as the filament comes back to the neutral axis.

So it's seeing a fully reversible stress.

And what you can do is you can run tests, so

if they gather a whole bunch of these rotating beam specimens

made of the material that your part or component is going to be made out of,

they can run what we call an RR Moore rotating beam specimen test.

And it gives you something called an SN, or

strength life, diagram and this is what that diagram looks like.

So on the x-axis, we have Number of Stress cycles, N, to failure.

So how many cycles or how many rotations can you go through before that beam fails?

On the y-axis we have Fatigue Strength, which is capital S, subscript f and ksi.

So what is the strength that your component can see, for

how many cycles before it fails?

And typically this is done in a log, log graph.