Confusion about the Strain Energy when loading/unloading a sample

In summary, the conversation discusses the difference between work done on and by a sample during loading and unloading. The area under the loading curve represents work done on the sample, while the shaded area represents work done by the sample when unloaded. The white sliver represents energy lost. The difference in terminology between "on" and "by" is a matter of perspective and whether energy is being put into or taken out of the system.
  • #1
KevMilan
12
0
The answer says B, it does makes sense, but why D is regarded as incorrect. If the Elastic Potential energy when loading is the area under the loading curve, so why not with the unloading?
20190314_053026.jpeg
 

Attachments

  • 20190314_053026.jpeg
    20190314_053026.jpeg
    28.4 KB · Views: 3,296
Physics news on Phys.org
  • #2
D is the work done by the sample while unloading.
 
  • #3
What about the area under the loading curve? Is it the workdone on the sample or by the sample?
 
  • #4
KevMilan said:
What about the area under the loading curve? Is it the workdone on the sample or by the sample?
Like I said, it is the work done by the sample. The force exerted by the sample on its surroundings is directed toward the sample, and the displacement at the boundary where the work is being done is directed toward the sample. So the work done by the sample is positive. The work done on the sample is negative.
 
  • #5
For the area under the loading curve, shouldn't it be the workdone ON the sample? Because we are the one putting force/stress on it? Could you clarify? Thankyou.
 
  • #6
KevMilan said:
For the area under the loading curve, shouldn't it be the workdone ON the sample?
Because we are the one putting force/stress on it? Could you clarify? Thankyou.
Yes. The force we apply and the displacement are in the same direction.
 
  • #7
KevMilan said:
What about the area under the loading curve? Is it the workdone on the sample or by the sample?
Work done ON the sample, but that area is the sum of the shaded area and the "white" sliver.
Area under top line = work done on the sample, Energy Input
Shaded area = = work done BY the sample when unloaded - Energy returned
The white sliver represents in Energy Lost (Put in, but not returned, ie Option A).
 
  • #8
One more thing, why is it the workdone BY the sample (not ON)? (Is it because we're not putting any force/tension on the sample when unloading?) Any simple explanation would be very helpful!
 
  • #9
KevMilan said:
One more thing, why is it the workdone BY the sample (not ON)? (Is it because we're not putting any force/tension on the sample when unloading?) Any simple explanation would be very helpful!

I suspect you would get similar load/unload curves for a trampoline, and I am happy to consider that the trampoline is doing work on me (work done by the trampoline) when it is flinging me back into the air after I have jumped on it and pushed the mat down (I did work on the Trampoline).
 
  • #10
I see. Thanks a lot man [emoji106]
 
  • #11
KevMilan said:
One more thing, why is it the workdone BY the sample (not ON)? (Is it because we're not putting any force/tension on the sample when unloading?)
We are applying a force on the sample during unloading. But that force is opposite to the displacement so we are doing negative work. If you interpret the dark area as negative, then it represents the work done ON the sample during unloading.
 
  • #12
KevMilan said:
One more thing, why is it the workdone BY the sample (not ON)?
'On' or 'By' is not necessarily obvious and it's actually a bit anthropomorphic, imo.
It may be better to think in terms of the system as a whole and whether energy is being put in or taken out. During displacement from the equilibrium position, Energy is put in (the direction of the arrow on the diagram is an indication of this). As the system returns to equilibrium position, Energy is taken out.
 

What is strain energy?

Strain energy is the potential energy stored in a material when it is deformed or stretched. It is the work done on the material to cause the deformation and is usually measured in joules (J).

How is strain energy related to loading and unloading a sample?

When a sample is loaded, strain energy is stored in the material as it is deformed. When the load is removed and the sample is unloaded, the stored strain energy is released. The amount of strain energy stored and released depends on the material's properties and the amount of deformation.

Why is there confusion about strain energy when loading and unloading a sample?

There may be confusion because strain energy is often not directly measured but is instead calculated using other parameters such as stress and strain. Additionally, the amount of strain energy stored and released can vary depending on the testing conditions and the material's behavior.

How does strain energy affect a material's behavior?

Strain energy is an important factor in understanding a material's behavior under loading and unloading. It can affect the material's strength, stiffness, and ability to withstand repeated loading cycles. Materials with higher strain energy may be more prone to failure or fatigue, while materials with lower strain energy may be more resilient.

What are some practical applications of understanding strain energy in materials?

Understanding strain energy is crucial in designing and testing materials for various applications. It can help engineers determine the maximum load a material can withstand without failure, predict the material's behavior under different loading conditions, and optimize the material's properties for specific uses. Strain energy is also important in fields such as biomechanics, where it is used to study the behavior of biological tissues and structures.

Similar threads

Replies
16
Views
2K
  • Introductory Physics Homework Help
Replies
7
Views
2K
Replies
5
Views
1K
  • Mechanical Engineering
Replies
2
Views
786
Replies
8
Views
1K
  • Mechanics
Replies
2
Views
216
Replies
1
Views
1K
  • Introductory Physics Homework Help
Replies
4
Views
369
  • Introductory Physics Homework Help
Replies
5
Views
438
  • Materials and Chemical Engineering
Replies
4
Views
5K
Back
Top