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Yield Point: Definition, Curve & Elongation

Instructor: Saran Narang

Saran has had significant consulting exposure with a Bachelors degree in Mechanical Engineering.

Ever wondered why some things snap quickly and others can stretch before breaking? In this lesson, you will learn & explore the details that describe the physical phenomenon.

Defining Yield Point

Materials can be anything from concrete to steel to glass. So what exactly is the yield point of a material? Yielding, in our day-to-day language, means to give in to force, pressure or demands. In the case of materials, the idea is the same - a material is said to have yielded when it changes shape after the application of force or when it has 'given in'. For example, when you stretch a rubber band you essentially apply a force on it with your hands and the rubber band yields or deforms. The yield point is the point at which a material's change/deformation has become permanent after the force is removed. If the rubber band had become larger or longer after you stopped pulling it, then you passed the yield point of the rubber band. Yikes! (Sometimes, the yield point is referred to as yield strength)

Stress- Strain Curves

So how do we know when we are close to the yield point of a material? Also, why do some materials break more easily? The answers to these questions lies in something engineers and scientists use, called a stress-strain curve , which is a graphical way of plotting the stress and strain of a material.


Stress-Strain Curve
Stress-Strain_curve


If you see it for the first time and think it looks like rocket science - don't be alarmed! Stress is simply the force you applied on the rubber band, divided by the cross-sectional area. Strain is even simpler, and is just the name given to the measurement of the deformation, or amount of damage your hands just did by pulling on the rubber band! Strain is measured by dividing the change in length of material by the original length. Every material has an entirely unique 'curve' for the same amount of force being applied, since each material is unique at the very fundamental level. The curve for steel will look very different from the curve for concrete or even a very similar material like Iron.

Plastic & Elastic Regions

What if the rubber band returned to the original size and shape after you stopped pulling? If after removing the force, the material returned to the original shape, then we can say that the material was elastically deformed. The stress-strain curve helps us in showing us the region or path till this is true or till where elastic deformation is possible. Since after the yield point the material is permanently deformed or changed, the part of the curve that is before the yield point gives us the elastic region. Permanent changes in the material's shape or size are referred to as plastic deformations. The part of the curve after the yield point gives us the plastic region, in which the material is permanently changed.

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