Have you ever wondered why ceramics are hard and brittle while metals tend to be ductile? Why some materials conduct heat or electricity while others are insulators? Why adding just a small amount of carbon to iron results in an alloy that is so much stronger than the base metal? In this course, you will learn how a material’s properties are determined by the microstructure of the material, which is in turn determined by composition and the processing that the material has undergone. This is the first of three Coursera courses that mirror the Introduction to Materials Science class that is taken by most engineering undergrads at Georgia Tech. The aim of the course is to help students better understand the engineering materials that are used in the world around them. This first section covers the fundamentals of materials science including atomic structure and bonding, crystal structure, atomic and microscopic defects, and noncrystalline materials such as glasses, rubbers, and polymers.
1.10B Mechanical Tests Part 2
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En provenance du cours de Georgia Institute of Technology
Material Behavior
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Introduction [Difficulty: Easy || Student Effort: 1hr 30mins]
This module will introduce the core principles of materials science. Topics that will be covered include the different general material types (metal, ceramic, polymer, etc.) and the properties associated with each type, some methods that are used to experimentally determine and quantify a material's properties, and how a materials engineer might go about choosing a suitable material for a simple application. This module also introduces the concept of the microstructure-processing-properties relationship which is at the heart of all materials science.
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Thomas H. Sanders, Jr.Regents' Professor
School of Materials Science and Engineering
When we evaluate materials for mechanical properties, the simplest and
easiest type of test is referred to as a tensile test.
The way the tensile test operates is a specimen is placed in a fixture
where we can apply either a tensile or compressive load to the specimen.
The specimen has attached to it a clip that permits us to measure
how much the material has elongated during the applied stress.
Over to the right are two different types of shapes that are used for
this type of test.
One is around a cross-sectional specimen, and the other is a flat-plate specimen.
So depending upon the particular material that you're using for
a specific application, we'll wind up using a different specimen geometry.
When we apply the load to the material, we wind up describing
a certain portion of this behavior in terms of stress versus strain.
And let's go back and look at what we mean by the concept of stress.
Stress, much like conductivity that we talked about in earlier modules,
is an intensive property.
Intensive meaning that we have eliminated the cross-sectional area.
So now what we do is we take the force that's applied to cause either compression
or tension, divide it by the original cross-sectional area, and
that cross-sectional area then will be divided into the force and
we have a stress parameter.
So on the Y axis we're plotting stress.
With respect to the X axis or the strain axis,
what we're looking at is the change in length divided by the original length.
And when we apply the load, we're, we typically will find ourselves
in the early stages, in a region that we refer to as the elastic regime.
In this particular case, when we look at load versus strain,
what we see is a linear behavior.
The slope of that stress-strain curve defines for
us the elastic modules or the stiffness of the material.
Over to the right what we have,
is the geometry of the specimen that we're looking at.
This is a cylindrical specimen.
It has an initial cross-sectional area, given as a zero.
That helps, then, define the stress as we begin to pull the fixtures apart.
I want to focus this time with regard to tensile testing on the actual specimen,
itself.
When we pull a specimen,
along the direction of stress axis, either compression or tension.
There will be a corresponding change in the material length.
When we look at the material from a cross sectional area point of view,
we find that if we pull the material in tension,
we will see that the cross sectional area will wind up reducing.
So we can talk about the strain in the cross sectional area as being a change
in the diameter, divided by the original diameter of the specimen, and
we can talk about the strain in the direction parallel to the deformation
as being the change in length with respect to the original length.
What we begin to see is as the material becomes deformed,
we see that we ultimately wind up passing through the region that we
refer to as the linear region where we have elastic deformation.
If we continue beyond the elastic range, we see that the curve begins to deviate
from linearity and that region in there is referred to as the plastic regime.
If we begin at point B and unload the specimen.
What will happen is when we go back down to zero load, or
zero stress, we have permanent deformation that's been added to the material.
So the material has been permanently deformed.
And that deformation is referred to as plastic deformation.
When we look at the combination of the stress-strain behavior and
we look at the specimen as it has become deformed, what we see is,
the first on the left we have the length of gage, and as we begin to pull the gage,
we eventually begin to develop something that's referred to as the neck.
Sometimes this reason is referred to as the onset of plastic instability.
And we begin to see then on the stress-strain curve a bending down, or
what might appear as the reduction in the strength of the material.
However, what's actually happening is not that the material is getting weaker.
What is happening at this point beyond the region where it is necking,
is that the cross sectional area of the specimen is significantly decreasing,
which means that if we were to compensate for the change in cross sectional area,
we would be able to see the actual true stress and
strain behavior of the material.
What we do then is we come up with another parameter that will describe
the tensile behavior, and we'll talk about that in the next slide.
But here what we do is, we define the yield strength or the point at which
the elastic behavior ends, and we would refer to that then as sigma YS.
When we talk about the point of plastic and stability,
it occurs on the maximum of the stress-strain curve.
And that's referred to as the ultimate tensile strength or sigma UTS.
And the corresponding points with respect to strain.
Then tells us about the end of the elastic range with respect to strain.
The point at which the plastic instability occurs.
And then in terms of the final failure or the fracture strain.
The point at which the material goes into two parts.
Depending upon whether or not you are using the material and
the particular application is for a structure.
Generally speaking what we do is we consider something
we refer to as the engineering stress.
When we talk about the engineering stress
what we're doing is describing the stress-strain behavior
when the stress is calculated on the original cross sectional area.
Now, the reason we use this in design is because,
generally speaking, we try to avoid having the material change
its shape as a result of the load in a particular structure.
So, generally,
we are down below the yield point when the material is actually used in application.
So when we talk about engineering stress,
we're talking about the behavior at relatively small amounts of strain.
Now if we're using the material ultimately for
an application like container products for beverages.
We're interested in the onset of that plastic instability for
the purpose of making sure that during the deformation process,
the material is deforming in a uniform way.
Beyond the point of instability, the material begins to deform locally and
the cross sectional area is changing.
So in order to evaluate a material in terms of the physics of the material.
What we often do beyond the yield point,
we talk about a parameter that's referred to as the true stress.
And in the case of true stress, what we do is to compensate for
the change in cross sectional area we constantly update the cross
sectional area as we go through the stress-strain curve.
So that will take into account those regions in the material where
the instability has occurred and the cross sectional area has been reduced.
So hence we have a true stress and engineering stress.
Another parameter that comes directly out of a stress-strain test is
the fracture or the strain at failure.
And those are given as the Xs on each one of these curves.
In the first curve what we have is a material that is behaving
in a way that is brittle in nature, that is it's deforming elastically.
It reaches a maximum point and then it breaks.
This is characteristic oftentimes of ceramic materials that tend to be
very brittle and do not allow for any plastic deformation.
On the other hand when we look at metallic materials we know that,
that not only can they deform elastically but they can deform plastically.
And the third curve is one that is characteristic of many polymeric or
foam materials.
That is what happens is it starts off slow the deformation, the stress and
the strain are related to one another.
And then there's a region, a flat region called the plateau region
where either the material is unraveling or in the case of foams,
the spaces that are between the ligaments in the foam begin to collapse.
And then eventually, it moves into a location where everything has collapsed,
and what we have is the final failure of the material, again, given that at that X.
We can begin to talk about the parameter called the material toughness, and
that ultimately winds up being related to the area of under the stress-strain curve.
So material one would have a low toughness even though it has a high strength,
it has a low toughness because the area is small.
We look at the metallic material, it's a tough material because it has a good
combination of the strength as well as the area under the stress-strain curve.
So, that material is tough.
When we look at material behavior three, where we have that
nice long plateau region, and ultimate failure, when we look at the area
under that stress-strain curve, the toughness of the material is high.
And generally speaking, what we would like to do is to
have a material response like that in specimen three
where we're interested in the material that is going to absorb energy.
When we look at the stress-strain curve behavior, something jumps out immediately.
We're going to describe another term.
And that term here with respect to the stress-strain curve
is called brittle behavior.
So in this particular case, the material only deforms in the elastic region,
whereas a ductile material is a material which deforms elastically and plastically.
In the next lesson, we want to focus on
the behavior of the material itself during a tensile test.
And we'll be able to describe how the material changes its
overall appearance when the behavior of the material is brittle,
as opposed to one which is ductile.
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