Steel - Hardening behavior after cycles - Stress/strain Curve

[Henry0726]

Hello, all!

I was looking at an article and I found these results:

These are the results for some samples with the same material and all under total reverse cycle load.
I don't understand the reason of Yield stress reduce after receive some cycles. I had in mind that there is a hardening behavior in the grains, which increases the strength of material. The same that happen when apply shot peening, for example, which material receive a residual stress with hardening surface.

Could, please, somebody explain me better the reason of the yield stress reduce after some cycles in this case?

Thank you.

 

[EdStainless]

Internal redistribution of strain.
In the wire business if we had wire that was too strong, we would "kill" it by pulling zig-zag around some rollers.
The plastic strain cycles would drop the Yield.

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P.E. Metallurgy, consulting work welcomed

 

[MFAMET]

Those stress-strain curves could come from the same material but under different cold work conditions. The top curve represents a material which was cold worked fully with a high strength, low strain exponent and lower formability. The bottom cure was from an annealed one with a higher strain exponent which will have a better formability.

 

[EdStainless]

I don't think so MFAMET, I think that all of this was done on the same sample.
Not just samples of the same material, but the very same piece of material.
I have done this with wire.
Even though for most spring wire there is no requirement for elongation or Yield we had in-house limits.
If you send someone wire with similar UTS but very different Yield then it will form differently
This is why we would deliberately over-strain some lots of wire to lower their yield.
This is even more pronounced with higher strength material, such as 300ksi spring wire.

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P.E. Metallurgy, consulting work welcomed

 

[MagBen]

Noticed lots of people were confused with "mechanical" stress relief (myself was not excluded at the beginning) given that the mechanical cold work would induce working harden leading to a higher strength.
In this case, the cyclic loading did the opposite: stress relief.

Basically the cycle load re-distributes the internal stress and reduces localized stress concentration. the plastic deformation can also rearrange the structure and promote relaxation. so, this can normally be only applied to stressed material. if the material is fully annealed, you will see cycles could increase yield strength.

my personal experience was with straight-and-cut operation (from coil to bar). The strength of the wire sample from straightened bar decreased obviously, and what's more, the diameter increased! we have to set a smaller final drawing diameter to compensate the increase.

In mentioned shot peening situation, it has a similar mechanism. it induces compressive stresses on the surface of the material, which can help to counteract tensile stresses and improve fatigue resistance.

 

[IngeniousQuest]

The way metal deforms plastically is mainly influenced by the movement of dislocations within its crystal structure. It's widely accepted that the yield stress of metallic materials increases as the dislocation density increases.

However, during reverse cycle loading, dislocations within the material may rearrange, annihilate, and interact differently compared to monotonic loading. This could decrease the overall dislocation density, making the material less resistant to plastic deformation. As a result, we may see a decrease in yield stress.