Question About Strain-Hardening

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In summary, the book "Shigley's Design" briefly discusses strain hardening in chapter 2. The author explains that strain hardening can increase the ultimate load in tension for a bar, but it also decreases the "safety" of the material. The linear portion of the stress-strain diagram can extend further up, leading to permanent deformation and a higher yield strength. Additionally, the book mentions that strain hardening can also improve fatigue strength, as it is correlated with the local ultimate strengths of the material. However, the book is not recommended for learning the theory of materials science and is better suited for practical design applications.
  • #1
Saladsamurai
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In Shigley's Design there is a brief discussion in chapter 2 about strain hardening. At one point he says,

A little thought will reveal that a bar will have the same ultimate load in tension after being strain-hardened in tension as it had before...

Is this just because you cannot stress the bar beyond Su and thus cannot improve upon that number?He also says that
the new strength is of interest...because the fatigue strength improves--since fatigue strengths are correlated with the local ultimate strengths

What does this mean? What are "local ultimate strengths?" And how are they correlated to fatigue strengths?

Any thoughts are appreciated
 
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  • #2
The ultimate strength is the ultimate strength. It's as good as the material can do. When you strain harden the bar, you increase the linear portion where yeilding starts to occur, but you loose 'safety'. In other words, if you over stress the metal, it will yield. Next time you get to that stress level, it will remain linear. Go past that level, and you strain harden it further. You can keep on doing this, until you get to the point where the material has no more forgiveness. Instead of yeilding when you over stress the bar, it will reach Su and suddenly fail quite dangerously like cast iron.
 
  • #3
Cyrus said:
The ultimate strength is the ultimate strength. It's as good as the material can do. When you strain harden the bar, you increase the linear portion where yeilding starts to occur, but you loose 'safety'. In other words, if you over stress the metal, it will yield. Next time you get to that stress level, it will remain linear. Go past that level, and you strain harden it further. You can keep on doing this, until you get to the point where the material has no more forgiveness. Instead of yeilding when you over stress the bar, it will reach Su and suddenly fail quite dangerously like cast iron.

Thanks Cyrus, this makes sense.

When you say
you increase the linear portion where yeilding starts to occur, but you loose 'safety'

what does that implicate for the stress-strain diagram? That is, does the slope of the linear region change? Or does it 'shift' while retaining the same slope?

I am not sure if I am wording my question correctly.
 
  • #4
Saladsamurai said:
Thanks Cyrus, this makes sense.

When you say


what does that implicate for the stress-strain diagram? That is, does the slope of the linear region change? Or does it 'shift' while retaining the same slope?

I am not sure if I am wording my question correctly.

I believe it just makes the linear portion extend further up.

PS: I hope you are not using that book to learn. It's really a design book and not a 'learning' book. It's terrible for the theory. It's great for, "I want to do THIS". Look it up, do it.
 
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  • #5
Cyrus said:
I believe it just makes the linear portion extend further up.

PS: I hope you are not using that book to learn. It's really a design book and not a 'learning' book. It's terrible for the theory. It's great for, "I want to do THIS". Look it up, do it.

No. I already took materials science, I am just exploring some of the nuances that we didn't go into with much detail.

It's for my ME Design course that I am taking. Hope I get some use out of this book; at $200 USD, it's the most expensive one I have purchased yet.
 
  • #6
Saladsamurai said:
No. I already took materials science, I am just exploring some of the nuances that we didn't go into with much detail.

It's for my ME Design course that I am taking. Hope I get some use out of this book; at $200 USD, it's the most expensive one I have purchased yet.

I had to look into making springs a few months ago. This book is very good for looking up how to do things. I didn't care about the theory of how you make springs. I just wanted to make one. This is what the book is strong for. "I want to make car brakes" ...go to chapter 16.
 
  • #7
Cyrus said:
I believe it just makes the linear portion extend further up.

For the most part correct. When you pass the yield point, you reach areas of permanent deformation. When you then unload the part, even though you are past the proportional (linear) area, you unload at the Elastic Modulus. That is, when you get down to zero stress, you have a non-zero value for strain, that is your permanent deformation.

As Cy mentioned, because the linear line is now "shifted" over, it increases the yield strength...I'm rambling...I need a cup of coffee.
 
  • #8
minger said:
For the most part correct. When you pass the yield point, you reach areas of permanent deformation. When you then unload the part, even though you are past the proportional (linear) area, you unload at the Elastic Modulus. That is, when you get down to zero stress, you have a non-zero value for strain, that is your permanent deformation.

As Cy mentioned, because the linear line is now "shifted" over, it increases the yield strength...I'm rambling...I need a cup of coffee.

Yeah, I saw that in the chart but I didn't mention it because I didn't want to say something incorrect about it.
 
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1. What is strain-hardening?

Strain-hardening, also known as work hardening, is a process in which a material becomes stronger and more resistant to deformation as it is deformed or strained. This occurs due to the rearrangement and entanglement of the material's atomic structure.

2. How does strain-hardening affect the properties of a material?

Strain-hardening can significantly increase the strength and hardness of a material, making it more difficult to deform or break. However, it can also reduce the ductility and toughness of a material, making it more prone to cracking or fracturing under stress.

3. What are some common examples of strain-hardening?

Strain-hardening can be observed in many materials, such as metals, plastics, and ceramics. For example, when a metal wire is repeatedly bent or stretched, it becomes harder and less flexible due to strain-hardening. Another example is the kneading and shaping of clay, which increases its strength and makes it less malleable.

4. Can strain-hardening be reversed?

Yes, strain-hardening can be reversed through a process called annealing. This involves heating the material to a high temperature and then slowly cooling it down. This allows the atomic structure to relax and return to its original state, reducing the effects of strain-hardening.

5. How is strain-hardening measured and quantified?

Strain-hardening is typically measured and quantified using the strain-hardening exponent (n-value) and the yield strength. The n-value is a measure of the material's ability to strain-harden, with higher values indicating a greater increase in strength. The yield strength is the amount of stress required to permanently deform a material, and it increases as strain-hardening occurs.

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