Question About Strain-Hardening

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Discussion Overview

The discussion revolves around the concept of strain hardening as presented in Shigley's Design, specifically focusing on its implications for ultimate strength and fatigue strength in materials. Participants explore the relationship between strain hardening, yield strength, and the stress-strain diagram, as well as the educational value of the referenced text.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants note that the ultimate strength of a material remains constant despite strain hardening, as it cannot exceed the material's ultimate strength (Su).
  • Others argue that strain hardening increases the linear portion of the stress-strain curve where yielding occurs, but this may lead to a loss of safety, as over-stressing can result in sudden failure.
  • A participant questions the implications of strain hardening on the stress-strain diagram, specifically whether the slope of the linear region changes or if it simply shifts while retaining the same slope.
  • Some participants suggest that the linear portion of the stress-strain diagram extends further up due to strain hardening.
  • There is a discussion about the educational quality of Shigley's Design, with some participants expressing that it is more of a design reference than a theoretical learning resource.
  • One participant reflects on their prior materials science education and their current exploration of nuances not covered in detail, indicating a desire to understand practical applications better.

Areas of Agreement / Disagreement

Participants express differing views on the implications of strain hardening, particularly regarding the stress-strain diagram and the concept of safety in material failure. There is no consensus on these points, and the discussion remains unresolved.

Contextual Notes

Participants acknowledge limitations in their understanding of the stress-strain diagram and the theoretical aspects of strain hardening, indicating that some assumptions may not be fully explored.

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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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.
 
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.
 
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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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.
 
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.
 
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.
 
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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