Metals 101-7 Tensile Testing and the Stress Strain Diagram

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Video Transcript

A tensile test is a great way to see how strong a material is.  Let’s take a look a what a tensile test will tell us.

A tensile test is a way to see how strong a material is by pulling on it. To perform a tensile test, we first need a sample of the material we are interested in testing.  These samples are called “test specimens” and they usually look like little barbells. These specimens are often called “dog bones” because they are wider at the ends.  This helps the specimen break in a predictable and repeatable way. The test pieces can be round or square or even tubular or other shapes.

Next we need a machine that is strong enough to pull the test specimen apart.  We’ll use this old government surplus tester today. As this machine pulls the specimen, and it tells us how much force it is pulling with. When i turn the crank on this tensile tester it turns a gear that slowly moves the grips apart.. It’s hard to see so I’ll set this indicator on it to show you.   Now you can see that this grip slowly moves away from the other by as I turn the crank.

When we do a tensile test, we want to see how much stress builds up in our sample.  Remember that stress is a force divided by a cross sectional area.  The gauge on our tester gives us values of force in pounds. To get stress values, we’ll divide the pounds by the cross-sectional area of the sample.  The sample we are going to test is .450 inches wide by .134 inches thick. So the cross sectional area is .0603 square inches.

We’ll want to know about strain, too.  This device, called an extensometer, will measure how much the specimen is stretched.  We will set it so that these two knife-edge prongs are two inches apart. We call this the gauge length, and this will be our original length.. Then we will measure the amount the dial moves and divide by our two inch gauge length.  Remember that strain is the change in length divided by the original length. [drawing for this]

Now as i begin turning the crank you can see that the value on the force gauge dial quickly rises while the dial on the extensometer barely moves.  … but as I keep cranking, the force gauge dial almost stops increasing and the extensometer dial starts moving more and more. Eventually, the dial on the force gauge drops down a little bit.  The red drag pointer shows where the force reached its maximum. This drop tells me the sample is about to break, so I will remove the extensometer so it won’t get damaged. As I keep cranking I can see the sample start to neck down, and….it breaks.

Let’s plot this on a diagram. We’ll put strain along this axis.  We will label it with this lower-case epsilon (that’s a Greek letter e). And we’ll put stress on this axis, and we’ll label it with this lower-case sigma (that’s a Greek letter s).

Now let’s take some values from our test to see the relationship between stress and strain.  

When we first began turning the crank to pull the sample apart the stress built up quickly and there was very little strain.  Notice that the graph for this region makes a straight line.

But as we kept cranking, the part started stretching, but the stress only increased by a small amount.

Eventually, the stress started dropping, and a small neck became visible on the part.  We removed the extensometer and kept cranking. The part necked even more and suddenly broke.  That was the end of our test. This necking behavior happens in ductile materials. I’ve got an older video of a brass part necking down.  Here the necking is really easy to see. If a material doesn’t neck down like this in a tensile test, you know it won’t bend very much without breaking.

Now let’s take a closer look at that diagram.  You can clearly see two different behaviors here.  At first, the relationship between stress and strain is proportional (or linear).  Do you remember elastic deformation? That is what is happening here. The atoms of metal are just being stretched apart, but their structure is not changed.

But eventually this will reach a limit and the atoms will be moved out of their position in the lattice.  This is called the yield point. Beyond the yield point the metal stretches without generating much more stress.  It begins to elongate and it is permanently deformed. The type of deformation happening here, you may remember, is called plastic deformation.  During plastic deformation the rows of atoms slide past each other, and will never go back to the way they were before.

Eventually the material flowed so much that this neck formed.  As the neck formed on the sample, the stress part of our diagram started falling back down.  Eventually the specimen broke, and that was the end of our test.

We can learn about a material from performing a tensile test and generating its stress-strain diagram.  With this diagram we can start talking about strengths of materials in a much more meaningful way. We can also see how easy or hard it would be to shape a material, and we can see the different ways that a material could be useful or some of the limitations it might have.