And so, using the midrange stress and the alternating stress,

we can fully quantify a fluctuating stress.

Now, in the next lesson,

we're going to learn something called the Modified Goodman equation.

And that's going to help us use these midrange and

alternating stresses to determine how much life is remaining in the part and

if it's in an infinite life or a finite life range.

Some things to keep in mind where students often get confused in midrange and

alternating stresses, is if you have any of type

of discontinuity where you're changing diameters near the fillet radius or

you have a hole or maybe you have a key in a shaft.

You're going to need to use a fatigue stress concentration factor.

And so students get it confused.

Should I apply the stress concentration factor to just the midrange stress or

just the alternating stress?

And in this class, we're going to simplify life a little bit and

we're going to assume that there's no plastic strain at the notch.

And in that case, you can apply the stress concentration factor

to both the alternating and the mid-range stresses.

Now, if you're designing in a way where the plastic strain at the notch can't be

avoided, so you're going to get some sort of plastic deformation at that stress

concentration factor.

Then you can go ahead, if you're going to act conservatively,

you can assume that you just apply the stress concentration factor to

the alternating stress and not to the midrange stress because the midrange

stress is kind of a static component.

But there's some other methodologies that you can also utilize in this case that

can be found in Shigley's machine design textbook.

However, here in this course, we're going to simplify life and we're going to

assume that there's no plastic strain at the notch and so you apply the stress

concentration factor to both the alternating and the mid range stresses.

Now ideally, again, what we're doing with these equations is we're modeling

a material and we're trying to predict its behavior using models, right?

The golden standard or best practice is always to test and

to test with your actual geometry configuration.

So here in Mil-Handbook 5J you can see they've run fatigue cycle tests and

it says right here that the Kt, so their stress concentration factor is 2.

And then they give you the actual geometry of that stress concentration factor

up here and so this is really nice because your stress concentration factor is

already built into the load.

If you can test with a fluctuating load, that's even better, so

if you're testing with your actual loading and the actual number of cycles.

And here the stress ratio of -1.00 means it's fully reversed, but they also

have a stress ratio of 0.54, which would be more of a fluctuating situation.

And you can see for this material, this is 4340 steel,

they've tested both a KT of 2 and 3.

So, sometimes you can go and

find test data with your actual stress concentration factors,

built into the test and if you can find it and use it, that's the best way to go.

A lot of times you won't be able to find that data, or maybe you're trying to run

analysis to see if your part is going to survive the testing,

which will come later.

In which case you can calculate with the stress concentration factor.

So, that's it for today's module.