In the previous lesson we were focusing on metallic materials.

I'd like to have us turn our attention to polymeric materials and

you can see two distinctly different behaviors with respect to polymers.

In one case, what we have is a thermoset or

a thermoplastic which is below its glass transition temperature.

And we'll see more about the glass transition temperature

when we begin to discuss the nature of polymers and

the behavior of polymeric material that are amorphus.

What you see, of course, in this case,

is that there is a relationship between stress and strain.

Very little strain occurs prior to the failure of

the material which is indicated by that point x.

Under some circumstances, with respect to different types of polymers,

we may have a polymeric material that is crystal.

And in the case of a crystalline polymer material, during the deformation process,

energy that is put in the material is elastically and plastically deforming,

and then we reach a region that's referred to as region two or stage two and

we see a constant stress for quite some time as we continue to pull it.

What's happening in this case is that the polymer as part of the crystallization

process has coiled up into a regular array.

And as we go through pulling the material apart there is an unraveling

process associated with those polymer chains.

We ultimately get to region three where the polymers had been stretched out,

and now what we're doing is studying the stress strain behaviour of

a long chain polymer itself.

Until ultimately, the material winds up failing.

Focusing on ceramic material, the first curve to the left is

lustrative of the stress strain behavior in compression.

If we go to the right, what we see is the stress strain behavior in tension.

What we will find is that ceramic materials

tend to fail more readily in tension than they do in compression.

And as a result of this behavior there's an alternative way that we may

view a ceramic material and characterize that material.

And that is a four point bend test.

And what we have illustrated here are the forces that are applied.

There's the stationary force at the bottom, and

then there is a loading force on the top of the slab of material.

When that occurs, what we can begin to think about is what happens

to the bottom and the top surfaces of the material.

On the bottom surface, because we're stretching the material, as a result of

the applied force, what we're doing is creating a surface which is in tension.

On the other hand if we look at the top portion,

the material is now in a state of compression.

Since the compressive strength is higher than the tensile strength, what will

happen is that the failure is going to be occurring along the bottom surface.

And we can use that to our advantage, because we can begin to inspect and

we can look for the sites in which the deformation has initiated,

so with a four point bend test, we can begin to extrapolate and

extract some very important information about the failure

associated with a brittle material like a glass.

The next very simple test I want to describe is the hardness test.

And a hardness test is a way to evaluate the material

without going through the process of machining, and

it can be done very easily on the floor of a manufacturing operation.

The way the hardness test operates is that you have an indenter

which is given as that ball, and

there are a variety of different shapes that that indenter can have.

When you load that indenter up at a controlled rate, and

push it down onto the surface of the material, it creates an indentation.

When we look at that indentation,

the larger the indentation, the more ductility the material is having.

But more than that, the strength of the material is lower

than if we would have a material that had been heat treated or

processed in a way to create a higher strength version.

We would see that when we loaded the specimen up with our hardness tester,

the size of the indentation would become smaller.

Now we can convert the size of the indentation to various units.

One of them that's depicted here is called the Brinell hardness number.

But what's important about that is, what is plotted on the x-axis is the hardness.

And as we go from left to right, the harness is getting larger, and

also the size of the indentation is getting smaller.

Along the y-axis, I've included tensile data that's been

taken on the same materials and the plot

includes a variety of different tests and the scatterband associated

with the tensile strength and the hardness of material that's being tested.

And so what we see is that there exists a good correlation between the hardness,

which is very inexpensive to run, and the tensile test,

which results in design data, but it's a lot

more time and a lot more money associated with that particular test.

So, the hardness then gives us an opportunity to evaluate a material

in a very simple way.

This then ends our discussion of the basic ideas that

are associated with various kinds of tests that we can evaluate.

The tensile, the fracture behaviour of the material,

and the strength of the material.

1.10D Mechanical Tests Part 4

Material Behavior

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