Beyond the Tensile Strength

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來自 University of California, Davis 的課程

Materials Science: 10 Things Every Engineer Should Know

963 個評分

University of California, Davis

Materials Science: 10 Things Every Engineer Should Know

963 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

Dislocations Explain Plastic Deformation / Stress vs. Strain -The “Big Four” Mechanical Properties

Welcome to week 2! In lesson three we will discover how dislocations at the atomic-level structure of materials explain plastic (permanent) deformation. You will learn to define a linear defect and see how materials deform through dislocation motion. Lesson four compares stress versus strain, and introduces the “Big Four” mechanical properties of elasticity, yield strength, tensile strength, and ductility. You’ll assess what happens beyond the tensile strength of an object. And you’ll learn about a fifth important property – toughness.

The Stress versus Strain (Tensile) Test3:54

The “Big Four” Mechanical Properties3:25

Focusing on Strength and Stiffness4:04

Beyond the Tensile Strength4:09

Focusing on Ductility1:47

A Fifth Parameter – Toughness2:14

與講師見面

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

So, now let's look in more detail at what's happening beyond the tensile

strength, and the fact that the material is not getting weaker even though

the engineering stress is dropping off.

And this sort of narrowing of the diameter occurs.

Again, if we recall that up in this area, even with Poisson's ratio

we're taking into account, the cross-sectional area here is approximately

still equal to that initial cross-sectional area.

That is the ongoing, even though we're distorting this material now,

and again, deforming it severely at this point, we're going

to show you this stress concentration in the center, in the neck down.

We're still defining the engineering stress as the load over A0.

We're continuing to define the strain as the Delta L over l knot.

This A knot continues to be our normalizing factor.

It's not taking into account the true cross-section areas.

What's happening at this point is that the actual cross-sectional area at this point,

A, is significantly less than A naught.

So, as a practical matter the true stress,

Is now significantly greater

than the engineering stress.

And so if we were to plot, maybe we'll color code this.

If we were to plot the true stress.

I'll just add my little red line here as the true stress.

That would really be about the same up to this point, but will continue to go up.

So the material is really not becoming any weaker or

softer as the engineering stress strength plot implies.

In fact would continue to show higher strength if you will.

More and more resistance to deformation right up to the point of failure.

But again we were convenienced historically,

have defined engineering stress and engineering strain in this way.

And so we live with this fact, but nonetheless,

it is a practical consideration, because again, we're designing these systems.

You mechanical engineers, you civil engineers, are defining systems in

terms of the initial dimensions of the system.

And these gross degrees of deformation are,

if you will, worse case scenarios in which it is not

the anticipated dimension of the system in the original design.

So it's not inappropriate that we do monitor stresses and

strains in that sense, so when we do get into the overload

condition where we are stretching the material to extensive

degrees of plastic defamation, then this tensile strength

Or T.S., is a very useful parameter to indicate,

within the dimensions of the original design, this is the maximum load,

the maximum stress that can be withstood by this particular material.

So it becomes a very important kind of ultimate strength indicator

in the context of the dimensions of the overall part.

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