Elongation in Steel

[cmb042]

Main Question: Is the elongation value of any objective use if the part is designed to operate below yield strength?

I understand that in sheet metal bending or interference fits knowing how much strain a material can tolerate before breaking could be valuable. I have also had it explained to me that elongation is a subjective indication of material toughness, that higher elongation materials means higher fatigue strength. But if my part works in an environment where plastic yield is considered a failure, so the part is designed to never reach yield point (in theory), then should I care what the elongation of the material is?

I have a recent shipment of 303 barstock with certs that match ASTM A582 requirements. UT and YS are near typical values per the ASTM handbook. However the elongation on the cert is half that of the typical value. What is the risk of using this material if the parts made from it are designed to operate below yield strength? Why would this value be so much different than published values?

Thanks.

 

[EdStainless]

Well considering that you selected an alloy with low impact toughness and poor fatigue strength in the first place I don't see why you would care. That is if impact toughness and fatigue properties really play no role in the function of your part.
I would care, as normal strength and low elongation is a sign that either the sulfide inclusions are much coarser than normal or that the material may have been processed wrong (insufficient cold reduction and/or under annealed).
ASTM specs contain no typical values, only spec minimums. On traditional grades such as 3xx stainless it is common for the yield and UTS to be 1.5x the spec min, and for the elong to be >2x the spec min. Newer alloys have been written into specs with minimum values that are more like 90% on typical.
Is this material from a mill that you have used regularly? Do you know who melted and processed it?

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P.E. Metallurgy, Plymouth Tube

 

[mp87]

higher elongation materials means higher fatigue strength

I would rather say the opposite. Ductility is made possible by the presence of several slip systems, which make it easier for extrusions and intrusions to originate on the surface; from which fatigue crack initiation is triggered:

To confirm this, austenitic stainless steels and copper alloys have quite low fatigue strength.

 

[metengr]

Is the elongation value of any objective use if the part is designed to operate below yield strength?

Yes. Examples include forming operations, welding or damage tolerance in service.

 

[cmb042]

Yes. Examples include forming operations, welding or damage tolerance in service.

So how does one quantify the relationship between elongation and damage tolerance? For example lets say my part is a beam, I can determine how much of the beam can be damaged/missing for the typical load to cause stress in the beam to exceed the yield strength. How does that involve the elongation value? Young's modulus is not changing. I have heard it said a few times that elongation matters but I do not see how, or how to account for different elongation values in a static analysis where YS is the failure.

 

[EdStainless]

If your designed system never ever exceeds 33% of the yield strength then it will never fail.
But in the real world there are always 'things' that happen. The first of those is just making the part with no significant defects. If a part has been forged, rolled, stamped, bent, or welded then ductility is critical for it integrity.
First of all what we call yield strength is just and engineering definition. At that load you already have 0.2% permanent deflection. The theoretical yield is when elongation behavior deviates from a true straight line, this is very difficult to measure, and in many alloys it does not really exist.
Secondly loading is not uniform in location or time. We rely on materials 'micro-yielding' when unevenly loaded or impacted. This slight yielding should result in shape changes that allow more uniform distribution of the load and safe service.

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P.E. Metallurgy, Plymouth Tube

 

[mp87]

Damage tolerance assessment is quite a difficult practice, but there are some rules of thumbs that engineers exploit to evaluate the safety of a component or structure.

For example, one can hardly quantify the amount of energy that a structure can withstand upon impact; but reasonably the higher the energy absorbed by a Charpy specimen, the more confident one can be that the same behaviour will be seen during the exercise.

The same applies to toughness. The more plastic deformation the material is able to absorb, the more stresses will be redistributed and plastic energy absorbed before any major failure occurs. A rigorous quantitative assessment would require to evaluate the fracture toughness (KIC or JIC), but it is a complex and delicate test with several constraints to its validity. The elongation at break is easier to obtain and allows to quickly evaluate how much a material can withstand defects, and therefore how much it is on the safe side.

Some empirical relationships were developed to link the two values, see for example that from Hahn and Rosenfield.