Engineering Stress: Definition & Equation

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• 0:04 What Causes Stress?
• 1:19 Types of Stress
• 2:01 Compression & Tension Stress
• 2:42 Shear & Bending Stress
• 4:06 Torsion & Fatigue Stess
• 5:11 Lesson Summary
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Lesson Transcript
Instructor: John Parsons

John has taught physics to both engineers and non-engineers, and has a master's degree in Civil and Environmental Engineering.

Stress is one of the most common causes of structural failure. This lesson contains the definitions of the major types of stress, what causes them, and the equations needed to compute an object's stress.

What Causes Stress?

Stress is the result of internal forces, or forces that result when internal particles react to each other. Force is the measure of the amount of energy that's applied to an object. These internal forces are caused when a load is applied to an object. The most common loading types include longitudinally-loaded objects, axially-loaded objects, and torsional-loaded, or twisted, objects.

To be able to look at the internal forces that cause stress you have to cut the object in question. Cutting means you make a hypothetical cut through the object and create a diagram looking at those forces. Here is an example of a cut object with the three most common internal forces applied to the object:

Once these internal forces are computed, to convert them into stress, divide the force by the cross-sectional area. Dividing the force by the object's cross-sectional area, which is the area of the object when you look along the cut, allows us to normalize the forces and take into account that a larger object will be able to hold more forces for no other reason than the object is bigger. Here is the most common equation to compute stress:

Types of Stress

There are six major types of stress that cause failures. Each of these different stresses is caused by a unique situation, and it's the way the object is loaded that dictates what type of stress the object experiences. The two main ways forces can be applied to an object are axially or longitudinally. When an object is axially loaded, the forces are applied in line with the major axis of the object. With longitudinally-loaded structures, forces are applied so they are perpendicular to the major axis.

The six major types of stress are:

1. Compression
2. Tension
3. Shear
4. Bending
5. Torsion
6. Fatigue

Let's look at each type in more detail.

Compression & Tension Stress

Compression stress is the result of axially-loaded forces pointing towards the center of an object. There are two major issues with compression stress: Compression forces can cause an object to shorten, or they can cause an object to buckle. When an object buckles, it bends in such a way that it can no longer hold the load, even though structurally speaking, the object can hold more stress than is applied to it.

Tension stress is caused when axially-loaded forces are pulling away from an object's center, and perpendicular to the object's surface. Tension stress can cause lengthening of an object. There are several materials, concrete for example, where the object can only withstand a fraction of the stress when the object is in compression.

Shear & Bending Stress

Shear stress is caused when the forces applied to an object are parallel to the object's cross-section. This stress can cause the object to deform and, in some cases, pull apart. As the object deforms, it changes. The shape of the object can change, which can affect how the object withstands other forces.

Bending stress is seen in longitudinally-loaded objects. The forces cause the object to bend, usually in a downward direction. The farther away from the object's fixed supports, the greater the bending stress as is seen in the diagram here:

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