Introducing the Concept of Critical Flaws

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來自 University of California, Davis 的課程

Materials Science: 10 Things Every Engineer Should Know

961 個評分

University of California, Davis

Materials Science: 10 Things Every Engineer Should Know

961 個評分

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

從本節課中

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.

Introducing the Concept of Critical Flaws3:13

Fracture Toughness and the Design Plot8:26

Critical Flaw Size and the Design Plot5:12

A Play of Good versus Evil!4:54

與講師見面

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

Welcome back once again to the teaching laboratories here

in Kemper Hall on the UC Davis campus.

In this segment, we're gonna talk about fracture

a concept that indicates how important it is to be aware of

small defects in the material, defects on a microscopic and

even millimeter scale that can lead to catastrophic fracture.

[MUSIC]

In introducing fracture toughness to our discussion of important things that

we should know about material science, we've really added a fifth very important

parameter on top of those big four that we got from the stress strain curve.

In this case, Fracture toughness gives us

a way to quantitatively determine when a flaw size is critical.

And as I said at the opening,

these are flaws that can be fairly substantial in size.

A few millimeters deep crack on the surface of a material.

But we probably can think of no more iconic example of defective material than

the Liberty Bell.

Which for the lifetime of the United States has represented liberty.

But also has a very famous and

large crack that is visible on the surface.

And in fact that crack is one that

was a concern when the Liberty Bell was moved in 1976 on the occasion of

the bicentennial of the American Republic.

And in order to make the Liberty Bell more accessible to a large number of

potential tourists in Philadelphia, it had to be moved to a new location.

And in moving, there was substantial concern that that,

crack could be a critical one, and by the concept of fracture toughness it

could have completely destroyed this great iconic American image.

So as a result sophisticated acoustical equipment was

used to monitor any potential growth of that crack,

and fortunately no such crack growth did occur.

So, from something as famous as the Liberty Bell to something as

routine as everyday structural components in engineering designs, we have to be very

sensitive to the possibility of the motion of such cracks, how they might begin,

and how they might grow until they become what we call a critical flaw size.

So now, let's look at exactly what these terms mean and

exactly what defines a critical flaw that can lead to catastrophic failure.

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