Concept of Stress: Why stress is needed?

To make lazy students work and explain what they mean?

 

<time travel back 30 years to my Advanced Machine Design class in graduate school....bzzzzzt! >

I vaguely recall that the concept of stress was developed to describe a field of force applied to particle: dF/dA. It integrated to F/A. It was a useful concept because it allowed one to derive high-order equations that provided a uniform description of how force was applied to objects.

Hope this helps. Like I said, it was long time ago. I also learned that most of the equations used by engineers in common practice are chock-full of estimations that eliminate the higher-order equation components. What's a few percent of error between friends anyway?

<back to future/present day...bzzzzt! :tongue: >

 

I'm not sure why you call stress a hypothetical quantity. It's no more hypothetical than pressure (and the units are the same, of course).

Stress is useful for describing how different materials behave, in a way that is independent of the shape of particular objects made form the materials. For example Young's modulus (stress / strain ) describes how rigid or "stretchy" different materials are. If the stress is higher than the elastic limit for a material, it will not return to its original shape when you remove the loads that caused the stress. If the stress is higher than the "ultimate stress", the material will crack or break.

As a simple example, the force needed to break a piece of wire depends on the material it is made from, and also on the thickness of the wire, but the stress needed to break it only depends on the material. The same stress level will also break a cantilever beam, or a plate, or anything else made from the same material.

 

From Wikipedia:

Instead of looking at it as force per unit area, look at it as force per molecule. Stress is useful to determine how strong a material is. When a part breaks under a certain force, we actually break the molecular bonds between the particles. The greater the area of the fracture, the higher the number of molecules, the greater the force needed.

 

I like to think of stress as way of comparing how different geometries and materials act under the applied load. This lets me decide on the best geometry and type of material to use for the intended application.

 

How does that make it hypothetical? Considering the definition of the word, it doesn't make any sense to me.

You might not see the affects of the load on the material, but you can see what's applying a force to it and the material itself. What about Reynolds number and the lift coefficient, do you think they're hypothetical too?

 

What my experience says is Stress is the resistant provided by the material. Hence the name. It something like the stress/tension that is present in our body/mind too. Just because you cant see or measure it does it mean its not present. I just gave a rough analogy here.
Hope this helped :) :shy:

 

@koolraj

You said stress = force/area..
What force? what area?

When you first start considering internal forces within a body it is tempting to try to continue to work in force terms only , as you did with forces between bodies.

However try to explain why an auto tyre (which is round off the vehicle) has the shape it does and describe what forces are acting and over what areas in terms of forces.

Then you will see the need for the stress concept.