Defining Fatigue

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Materials Science: 10 Things Every Engineer Should Know

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University of California, Davis

Materials Science: 10 Things Every Engineer Should Know

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We explore “10 things” that range from the menu of materials available to engineers in their profession to the many mechanical and electrical properties of materials important to their use in various engineering fields. We also discuss the principles behind the manufacturing of those materials. By the end of the course, you will be able to: * Recognize the important aspects of the materials used in modern engineering applications, * Explain the underlying principle of materials science: “structure leads to properties,” * Identify the role of thermally activated processes in many of these important “things” – as illustrated by the Arrhenius relationship. * Relate each of these topics to issues that have arisen (or potentially could arise) in your life and work. If you would like to explore the topic in more depth you may purchase Dr. Shackelford's Textbook: J.F. Shackelford, Introduction to Materials Science for Engineers, Eighth Edition, Pearson Prentice-Hall, Upper Saddle River, NJ, 2015

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Fracture Toughness / Fatigue

Welcome to week 4! In lesson seven we will examine the concept of critical flaws. We’ll define fracture toughness and critical flaw size with the design plot. We’ll also distinguish how we break things in good and bad ways. Lesson eight explores the concept of fatigue in engineering materials. We’ll define fatigue and examine the fatigue curve and fatigue strength. We’ll also identify mechanisms of fatigue.

Introduction to Fatigue1:59

Defining Fatigue6:07

The Fatigue Curve and Fatigue Strength5:22

Mechanism of Fatigue5:58

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James Shackelford
Distinguished Professor Emeritus
Department of Chemical Engineering and Materials Science

And now we're going to go to a video lecture and introduce the definition

of fatigue in a formal way, and see how that has an interesting

relationship to the concept of fracture toughness that we introduced earlier.

So now let's look at another type of test,

which is a variation on our traditional tensile test.

And that's involved in the definition of fatigue.

Just like ourselves, late in the day, we'll feel tired.

Materials get tired, too.

They fatigue in the following and specific way.

So it's simply saying that after several cycles of use,

that a material that we would expect it to bear that load

application under normal circumstances, under a single stress application.

After many, many cycles, suddenly, shall we say, gets tired and fails.

And this can happen well below the tinsel stress which we

tend to associate with the maximum stress that engineering

material can withstand under the definition of

the traditional parameters associated with the tinsel test.

So of those big four parameters, the tensile stress is again that maximum or

ultimate tensile stress that is maintained over the course of the stress application.

So let's again do as we did before.

Remind ourselves again we now know this by heart, but

let's just draw again a stress-strain curve.

The elastic portion, the plastic portion and ultimate failure.

So now let's just colorize this.

And now, consider what happens if we do this experiment,

but stop well below the tinsel stress,

again remember out tinsel stress was up here.

Let's say that we now simply load the material way below the tensile stress,

maybe halfway up to the yield stress.

So we're very safely unambiguously within the elastic portion of the material.

We'll also draw in the yield strength here.

So tensile strength yield strength, we're going to stop

the load application right there about half way up to the yield strength so

it should be absolutely no problem.

The only thing different now is that

as we learned in the earliest discussion of the tensile the test and

the nature of elastic deformation, which is temporary deformation.

Plastic deformation is permanent deformation.

Elastic deformation is temporary deformation.

So if we do this loading up to this very benign level,

In which expect no damage at all.

We've just disturbed the material with a very,

very small amount of elastic extension and

then we take the load off and we just go right back to the origin again.

So again, no harm whatsoever.

But let's say we wait a few minutes and we decide to do that test again and for

whatever reason, we decide again to stop at the midpoint.

And we only have, again, loss of destination,

we remove the load and it goes back to the origin again.

Again, pretty boring stuff.

But now, let's do the following.

We'll add another color here just to round this out.

Lets now consider the origin and if we're gonna keep track of this,

lets consider now a time access that comes out of the board.

And so now let's look at the stress-time axis.

And that's basically what the sketch is here, originally on the screen.

So what we have is the stress,

relative to the tensile strength of the material, over a long period of time.

And so our point is, just as we looked at a fresh way at

the stress-strain curve when we're talking about creep deformation back in our

discussion about mechanical properties, now we're looking at the stress-stain

curve in a novel way, in this case by looking at a third axis,

at a time axis coming out of the plane of the stress-stain curve.

So now we have plane of the, again the stress versus time plane, that curve.

Well, again as I had suggested if we took a few seconds to

stress the material up to, now our suggestion about half the yield strength.

Release the stress,

again the strain on the material would have gone back to zero and

then just do this sawtooth effort all along and

perhaps a few million times we do this.

All of a sudden we have something that reminds us of this bad story.

The catastrophic failure that we

talked about in the discussion of fracture toughness.

Now we have a catastrophic brittle fracture

of this material that we thought was pretty ductile and would have had a very

benign general yielding type of behavior that would have been desirable.

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