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Stress Strain Curve: Definition & Yield Point

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  • 0:04 The Stress-Strain Curve
  • 1:41 Brittle Stress-Strain Curve
  • 1:53 Ductile Stress-Strain Curve
  • 2:47 Ultimate Strength
  • 3:21 Lesson Summary
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Lesson Transcript
Instructor: David Redmond

David has been an mechanical engineering manager, designing products for 20 years. He has a Bachelor's Degree in Mechanical Engineer and is trained in Design for Six Sigma

In this lesson, you will learn what a stress-strain curve is, how to interpret the curve and understand what the yield point is and what it indicates about the material properties at that point.

The Stress-Strain Curve

Why can you bend a paper clip into any shape you want? Why does a glass pane break? The material properties of the paper clip and the glass pane are different. One way to visualize the difference in the material properties is a stress-strain curve.

A stress-strain curve is a graphical representation of the behavior of a material when it's subjected to a load or force. The two characteristics that are plotted are stress on the y-axis and strain on the x-axis. Stress is the ratio of the load or force to the cross-sectional area of the material to which the load is applied. The standard units of measure for stress are pounds per square inch or Newtons per square meter squared.

Strain, on the other hand, is a measure of the deformation of the material as a result of the force applied. Deformation is a change in the shape or form of the material. For example, a person standing on the end of a diving board causes it to deform or bend as result of the weight or the force. There is no unit of measure for strain since it's a ratio of the deformation over the initial length. If for example, the strain measured is 0.05, this means that there are 0.05 inches of deformation for every inch of length.

Materials fall into two basic categories: brittle materials and ductile materials. Brittle materials, like glass, will break or fracture without bending when a large enough force is applied. Ductile materials, like steel or aluminum, will bend when a force is applied. If the force is large enough, the material will permanently deform and not return to its original shape. Let's look at a few typical stress-strain curves.

Brittle Stress-Strain Curve

In the brittle material curve, a yield strength or yield point is the same as the fracture point. The brittle material curve reveals that the material fractures or breaks instead of bending when the force is high enough.

Ductile Stress-Strain Curve

Ductile Material Curve

In this ductile material curve, you can see a point labeled yield strength, also known as yield point. The dip in the curve at this point is an indication that the material has yielded or deformed. After the load is removed, this deformation will be permanent. Before this point, the material is elastic. Elastic materials deform but return to the original shape after the load is removed.

In the picture on your screen below, you'll notice that the figure on the left shows the behavior of an elastic material.

Deformation

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