Stress strain and tensile strength diagram

In summary: A draw ratio is the ratio of the diameter of the necked region to the average diameter of the rest of the sample.
  • #1
akkshaya
4
0
why does stress decrease after it reaches its maximum tensile strength after its plastic range ?
 
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  • #2
Hi akkshaya, welcome to PF. What are your thoughts? We don't simply provide answers here, because that wouldn't help anybody learn; you have to present your own attempt at a solution.
 
  • #3
Look up necking and strain hardening, that should give you the information to understand why stress reduces after UTS.
 
  • #4
all i know is that its due to the necking or the neck formation .when i sarched the info , it said about two tangents and "draw ratio".. what is a draw ratio??. necking is when large amount of strain is experienced if a little stress is applied .. is this right ? am not clear in this necking concept .. can you help me out ?
 
  • #5
I agree that it corresponds to necking. How does necking affect the assumptions of an engineering stress - engineering strain curve? (Hint: compare with a true stress - true strain curve.)
 
  • #6
necking means your sample gets much thinner at some point than the rest. Imagine most of the cylindrical sample diameter is say 1 inch, length is 5 inches, there's a region which is 1 inch wide in the middle that's only half an inch in diameter. that region is said to show necking, deformation is not uniform in your tensile test sample.
 
  • #7
thanks for the necking concept ... and i saw that true stress-true strain curve and its explanations... i still don't uderstand how the load falls off if the area of cross section decreases after the application of a lot of stress..
 
  • #8
akkshaya said:
i still don't uderstand how the load falls off if the area of cross section decreases after the application of a lot of stress..

[STRIKE]The load didn't fall off! That's absolutely not what's plotted on the y-axis of an engineering stress-strain diagram.[/STRIKE]

The load decreases from its maximum point, but what is happening to the true stress (vs. the engineering stress) in the material?

EDIT: Corrected my mistake.
 
Last edited:
  • #9
The load decreases if the engineering stress-strain curve y coordinate decreases.
 
  • #10
nvn said:
The load decreases if the engineering stress-strain curve y coordinate decreases.

Yep, I misspoke there, since the engineering stress is the load normalized to a constant value. Revised to draw a comparison between the engineering stress and the true stress.
 
  • #11
The load falls off because the amount of area reduction in the necking area is faster than the work hardening in the material. So although stress increases, the load still drops. Engineering stress doesn't take the area decrease into account so it shows a decrease whereas true stress relates to the actual cross section area, so it always increases until fracture.
 
  • #12
oh k now i get it .. thanx.. and also what is a "draw ratio"?
 

1. What is a stress-strain diagram and why is it important?

A stress-strain diagram is a graphical representation of the relationship between stress and strain in a material. It shows how a material responds to external forces and provides valuable information about its strength, ductility, and toughness. This diagram is important because it helps engineers and scientists understand the mechanical properties of a material, which is crucial for designing and predicting the behavior of structures and products.

2. How is stress and strain measured in a stress-strain diagram?

Stress is measured in units of force per unit area, such as pounds per square inch (psi) or Newtons per square meter (N/m^2). It represents the amount of force applied to a material per unit of its cross-sectional area. Strain, on the other hand, is a measure of the deformation of a material and is typically expressed as a percentage change in length or volume. It is calculated by dividing the change in length or volume by the original length or volume of the material.

3. What is the difference between elastic and plastic deformation in a stress-strain diagram?

Elastic deformation occurs when a material is stretched or compressed but returns to its original shape once the external forces are removed. This is represented by the linear portion of the stress-strain curve, known as the elastic region. Plastic deformation, on the other hand, occurs when the material is permanently deformed and does not return to its original shape. This is seen in the curved portion of the stress-strain curve, known as the plastic region.

4. How is tensile strength determined from a stress-strain diagram?

Tensile strength is the maximum stress that a material can withstand before it breaks. It is typically determined by finding the point on the stress-strain curve where the material begins to fail, known as the ultimate tensile strength. This is represented by the highest point on the curve and is an important factor in determining the suitability of a material for a specific application.

5. Can a stress-strain diagram be used to compare different materials?

Yes, a stress-strain diagram can be used to compare the mechanical properties of different materials. By analyzing the shape of the curve, engineers and scientists can determine which material is stronger, more ductile, or more brittle. This information is crucial in selecting the appropriate material for a specific application, such as building construction or manufacturing of products.

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