"Proof stress"

[defg]

Dear Sir,
I have come across one phenomena that,we have carriedout some tests on high yield steel grade 460,and unfortunately they failed the yield strength requirements.(have no definite yield)Does the proof stress approximates the value to the yield stress in materials not exhibiting a definite yield point?(By plotting the stress-strain curve for the steel on order to be able to define the proof stress that will cause a permanent extension of a tensile test piece,commonly the stress to produce 0.2% extension)
If the stated material pass this criteria,will it be a gurantee of acceptance?

 

[metengr]

Your post is somewhat confusing. Proof stress, by definition, is the stress to cause some level of permanent deformation.

Yield strength is the stress where a material exhibits a specified permanent deformation (by definition 0.2% offset on a stress-strain curve).

You need to decide the level of permanent deformation to report - 0.2% or 1% or 2%, etc., for the material. If the stress to cause permanent deformation is above the proof or yield strength, the material should be acceptable for use.

 

[Mattest]

Metengr - I think you are confusing Proof stress and Yield stress. These are two different properties of metal as defined by EN10002. There is also Upper Yield point (ReH) and Lower yield point (ReL). Everyone should be talking about the same property.

defg : If the material exhibits no yield point when it should then I would estimate that you have a heat treatment issue.

 

[metengr]

Mattest;
I hope didn't confuse anyone. Proof stress can be any value at or above where permanent deformation occurs.

 

[EdStainless]

Does the material meet the Ultimate tensile strength and elongation requirements??
I doubt it if there is no distinct yield.
Sounds like someone missed the heat treatment.

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[gfbotha]

It is not clear whether a distinc yield point is indeed expected from the steel you tested. So, from a designer's point of view I comment as following:

For a particular steel that shows a definite yield point, the yield strength is normally higher than the 0.2% proof strength (sometimes called "offset yield strength").

So, if the 0.2 proof strength is higher than some required yield strength, I would say yes you are OK regarding the yield requirement. But, I agree with EdStainless that you now make sure about elongation in particular...

 

[Mattest]

We are forgetting the original clue:

"unfortunately they failed the yield strength requirements.(have no definite yield)"

If defg expects a yield point and the material does not exhibit this then there is a problem with the material, and it will probably not be suitable for its purpose.

He is asking for a guarantee that a proof stress (offset yield) is equivalent to a Yield stress, so it is important not to assume and give wrong feedback (in my opinion).

 

[metengr]

From ASM;

....The yield strength is the stress required to produce a small specified amount of plastic deformation. The usual definition of this property is the offset yield strength determined by the stress corresponding to the intersection of the stress-strain curve offset by a specified strain. In the United States, the offset is usually specified as a strain of 0.2 or 0.1% (e = 0.002 or 0.001)

Offset yield strength determination requires a specimen that has been loaded to its 0.2% offset yield strength and unloaded so that it is 0.2% longer than before the test. The offset yield strength is often referred to in Great Britain as the proof stress, where offset values are either 0.1 or 0.5%. The yield strength obtained by an offset method is commonly used for design and specification purposes, because it avoids the practical difficulties of measuring the elastic limit or proportional limit.

 

[Mattest]

metengr,

I still think we are not talking about offset yield or proof stress. We are talking about upper yield strength. this is a point where the gradient of the stress strain curve is zero for a defined strain. See Figure 2 on the following link.

http://www.aut.ac.ir/departments/mech/staff/solidmechanics/alizadeh/Tensile Testing.htm

 

[metengr]

Mattest;
Understood. The OP is still confusing to me because the poster does not seem to grasp that high yield strength materials exhibit very high yield/tensile ratios. In my opinion, this is the issue related to the OP.

I have come across one phenomena that,we have carriedout some tests on high yield steel grade 460,and unfortunately they failed the yield strength requirements.(have no definite yield)

The poster never indicated what type of test, was it a standard tensile test (ASTM E 8) or some other test method in an attempt to determine YS, UTS, etc?? From my exposure to high strength steel data, there should be a yield point before final fracture unless it was missed (which is what I suspect). The poster should review the actual stress-strain data and plot it. Personally, I don't like the term proof stress because it conjures up different meanings to different people.

 

[gfbotha]

My feeling is still we need some more clarification from the original poster. We might be assuming the wrong things? Nevertheless it is good to revisit the different definitions!

Some higher strength materials do not show a definite yield point in the pure sense that the extension continues for some distance whilst the load stabilises or even drops. Then of course, the offset yield strength becomes very useful.

Regarding a possible heat treatment problem: do you mean missing normalising or harden & temper? (Just want to be sure I follow where you guys are steering because I’m not sure about the equivalent of this material spec. either.)