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Up Topic Welding Industry / Welding Fundamentals / Calculation of Ultimate Tensile Stress
- - By HJLBX Date 05-23-2016 20:56 Edited 05-23-2016 21:24
In the WIT text, UTS = highest stress [Stress(max)] on the stress-strain curve (before necking).

However, in the WIT formula sheet, UTS = Stress(max)\Area, where Stress(max) = load to break the sample.

Highest stress on the stress-strain curve does not equal the load to break the sample (stress-strain curve endpoint).

So which is to be used ?

Stress(max) = highest stress on stress-strain plot

or

Stress(max) = stress at sample fracture ? [API 1104 uses fracture stress].

Load to break the sample = fracture stress.

Ultimate Tensile Strength is not the same as fracture stress.

What I think is correct is UTS = Stress(max)\Area since it is a stress limit state.

The "Stress(max) = load to break sample" is an error since load to break sample = fracture stress.

On the web I am finding different interchangeably used terminology (TS\UTS) so it is unclear.

From my understanding:

Tensile Strength = fracture stress\area and Ultimate Tensile Strength = Stress(max) from stress-strain curve\area.

Is this correct ?

* * * * *

This is just one of many errors in the AWS study texts.  Someone studies using AWS texts and they fail the exam because of the errors...

:mad:
Parent - By 803056 (*****) Date 05-24-2016 01:21
Good observation. There are differences between API 1104 and the tensile strength calculations and the calculations required to determine the ultimate tensile strength used by AWS D1.1 and ASME Section IX.

One must read the question closely to determine which value is required.

API - Tensile strength = Load at failure / original area

AWS/ASME - UTS = Max Load / original area.

Good luck - Al
Parent - - By js55 (*****) Date 05-24-2016 12:22
I understand that the gobbledygook of language can sometimes be prohibitive. However, ultimate tensile strength is in essence the maximum load achieved prior to necking to failure.
As you look at a stress strain curve you will see the line rise rapidly and straight as stress is increased while strain is not increasing at a rapid rate. This is the elastic zone.
Once the yield point is reached strain begins to increase rapidly, i.e., the curve moves more to the right. This is the beginning of the plastic zone. the need to increase stress is driven by the material experiencing work hardening and a strengthening mechanism, which requires increased load,  until there is no more work hardening to achieve and then strain continues to increase while stress begins to reduce. The failure point is in reality the ultimate tensile point. The curve to the right of this point is simply a phenomenon of measurement as the material exhausts its elongation capacity. To the right of the top of the curve the material is already in failure mode.
Think of it in these terms. If you stopped the test during the rise of stress and strain (to the left of the top of the curve) at a point of stress equal to that which will be shown at the termination point of the curve, there would be no failure. Work hardening would prevent failure. So then, what is the ultimate tensile failure point. The top of the curve.
Parent - - By HJLBX Date 05-24-2016 14:59 Edited 05-24-2016 15:22
The big problem is that in the AWS WIT workbook the formula is given exactly as:

UTS = Stress(max)\Area = load to break sample

Load to break sample = fracture stress (hard materials) or rupture stress (soft materials)

"Load to break sample" is wrong.

It should be stated as "maximum load observed during tensile stress."

* * * * *

The language in the formula can lead someone to make a mistake.  Imagine the guy that follows that language literally... it could result in them failing the test.
Parent - - By js55 (*****) Date 05-24-2016 15:38
What is the maximum stress?
The peak of the curve.

And what is the area?
The area of the tensile sample AS PREPARED.

To measure the area of the sample AFTER testing is a measure of ductility called 'reduction of area'.
Parent - - By HJLBX Date 05-24-2016 15:54 Edited 05-24-2016 15:59
"Load to break sample" is incorrect wording.

"Load to break sample" = fracture or rupture stress -- which is not maximum stress on engineer's (apparent stress-strain plot).

Fracture or rupture stress = stress at endpoint of apparent stress-strain curve.

* * * * *

Unless you look in the WIT text\formula sheet, you won't understand what I am stating.
Parent - - By js55 (*****) Date 05-24-2016 16:05
I can see where you would be confused. You are conflating the equation definition with what you see on the stress strain curve.
The equation stands on its own. The equation is the definition.
Parent - - By HJLBX Date 05-24-2016 16:51 Edited 05-24-2016 16:55
No.  AWS WIT text and formula sheet defines Stress(max) as the "load to break the sample."

This is incorrect.  Both the WIT and formula sheet are wrong.

If exam taker follows the AWS defined formula, then they will be calculating API ultimate tensile strength instead of AWS\ASME ultimate tensile strength.

You have to look at the WIT text and formula sheet to see the errors.
Parent - - By js55 (*****) Date 05-24-2016 18:07
No. You are operating off of YOUR understanding of what the terminology means NOT engineering understanding.
Parent - - By HJLBX Date 05-24-2016 18:40
I am not going to argue with you.

The WIT formula sheet states Stress(max) = "load to break sample."

This is incorrect.

Breakage does not occur until after Stress(max); it occurs at the fracture\rupture stress.

Breakage occurs at the very end-point of the tensile test; Stress(max) occurs in the region between strain hardening and necking.

The above definitions and facts are straight out of engineering texts.

* * * * *

The AWS WIT text and formula sheet terminology are wrong.
Parent - By js55 (*****) Date 05-24-2016 18:48
:lol:
- - By welderbrent (*****) Date 05-24-2016 22:37
He's not going to argue with you, he's just going to stand his ignorant ground that the text book is wrong and thus so are you.
Parent - - By js55 (*****) Date 05-25-2016 12:30
Brent,
I still think the confusion lies in conflating the stress strain curve indications with what is actually happening. The curve, subsequent to the peak, shows stress being reduced. However, stress is not synonymous with load. The load, the actual available pulling force of the machine, is not being reduced. Load and stress are not synonymous. They coincide on the left side of the curve. But due to necking they do not coincide on the right side of the curve.
We can conjecture that if there were some secondary work hardening that kicked in at some point in the necking regime the curve would show the stress rise again rapidly.
If I were to guess this is actually what happens in some materials when immediately subsequent to the yield point there is actually a continuous rise and fall in the stress strain curve similar to a sinewave, ~~~~, prior to the rise in stress resuming.
Parent - - By HJLBX Date 05-25-2016 16:34
Different language and terminology in different sources = confusion (or I should say -- at least on my part).

My confusion was "load to break sample" = "breaking load."  These are two different values that are not fully explained in the AWS WIT.

"Breaking load" in some texts is defined and used in problems sets as the "load at point of fracture."

In other texts, "breaking load" is not used, but instead "breaking force" which is also defined as the applied force at the point of fracture.

Finally, in all the texts that I used, "load" = force (weight), but the UTS is stress (force per unit area).

I had to dig through various engineering texts and finally found one with clear explanations that doesn't inter-mix terminology\language.

* * * * *

All of the below is within the context of a tensile test:

What is the maximum stress a material can carry to resist complete failure = Ultimate Tensile Stress.

What is the minimum stress that must be applied to a material to bring it to complete failure = Ultimate Tensile Stress.

Ultimate Tensile Stress is a "threshold" stress; if the applied load per unit area always remains below this threshold value, then the material will not fail.

So, according to the above definitions and explanations, I now can see how "load required to break sample" can be used to mean = Stress(max) = Ultimate Tensile Stress - even though, technically, it is the "stress required to break sample."

In one U.S. text I found the key explanation that made it clear.  Since UTS is always reported as load (weight) per square inches with a coefficient of 1, in engineering practice it is often simply, but erroneously, called a "load."  The text recommends not to refer to UTS as a load, but instead a stress -- why ? -- to avoid confusion !

It's a language thing...
Parent - - By js55 (*****) Date 05-25-2016 17:55
The way I have always looked at it is that:
Load is the energy imposed upon the specimen by the machine. The actual physical mechanical pull.
Stress is the resistance of the specimen to the load, measured by the machine and shown by the stress/strain curve. This is why the 'stress', the 'curve', can reduce even though the 'load' has not. This is why in some materials the sinewave yield phenomena certainly does not represent an alternating reduction in load.
And you are right, there is difficulty in the terminology.
The two best resources I have found are:
ASM Handbook Volume 8 Mechanical Testing
and
Mechanical Metallurgy by George Dieter
Parent - By js55 (*****) Date 05-25-2016 18:11
A stress strain curve is actually a fascinating history of a testing process. As the curve approaches the UTS point, which is in essence unmeasurable in exactness (as is the EXACT yield point), elongation increases while work hardening decreases. This is why the curve flattens out.
And since impact toughness is a mechanical phenomena that combines strength and toughness the area under the curve can be measured to determine a calculation of impact toughness. Though at room temperature to be sure, since that is predominantly where tensiles are tested, except for specific hot tensile tests and B31.3 type cryo notched tensiles for austenitics.
Parent - - By welderbrent (*****) Date 05-25-2016 22:24
I generally find that 'confusion' is indeed the culprit in language, terms and definitions disagreements.  Often, it takes going through the seminar with an instructor slowly walking the students through the various terms and definitions and coming to an understanding of the applications being used. 

Going through engineering manuals will only confuse the issue further.  They may be correct for their application but they are not looking at it the same way that the welding inspector, especially in a lab, does. 

At what point is it considered 'broken'?  That is academic to the base question being considered.  One can always twist the definition to make it fit one's one theory and application to try to understand UTS and the curve. 

Brent
Parent - - By js55 (*****) Date 05-26-2016 12:33
I think the first step towards a clear understanding is realizing that the phenomenon, the real physical activity, such as tensile testing, is an entirely different thing than either the words we use about it or the representations of it in graphs. Words and graphs are sloppy approximations, though they may be the best we have.
Parent - - By HJLBX Date 06-17-2016 04:40
I checked into this topic further.

The AWS WIT text and formula sheet are both WRONG.

Both the above define

P(max)
_____   =  "Load required to break the sample"

Area

The above definition is absolutely dead wrong.

The unit of load is force (weight) - and not [ force /  area] which is a  unit of pressure - as given in the WIT text.

The "Load required to break the sample"  =  The maximum, total applied force required to break the sample  =  The maximum, total applied weight required to break the sample.

* * * * *

P(max)
_____  =  Ultimate Tensile Strength, where Area = Cross Sectional Area of Test Sample;  it does NOT equal the "Load required to break the sample."

Area

* * * * *

The "Load required to break the sample" =  Ultimate Tensile Strength (UTS) x Total Cross Sectional Area  = 

P(max)
_____   X   Total Cross Secional Area  =  Weight Required to Break the Sample  = Load Required to Break the Sample

Area

* * * * *

For example, a tensile specimen has a UTS of 32 ksi.

            P(max)        32.000 lbs
UTS  =  _____    =   ________

             Area            1  sq. in.

What is the loadrequired to break a 1 sq. in. sample ?

UTS  x  Total Cross Sectional Area =  Weight\Force\Load Required to Break Sample

32 ksi  x  1  sq. in.  =  32,000 lbs  =  Load to Break a 1 sq.  in. sample

What is the load required to break a 5 sq. in. sample ?

32 ksi  x  5 sq. in.  =  160,000 lbs  =  Load to Break a 5 sq. in. sample

The above cross section values are simply used for ease of calculation.

* * * * *

If someone literally applies the definition given in the WIT text and formula sheet, then they probably will answer certain questions incorrectly.

A person might state that "Load required to break the sample" is the same for both a 1 sq. in. and 5 sq. in. samples - because - according to the WIT text and formula sheet, this is how it is defined.

Both the WIT text and formula sheet definitions of P(max) / Area are WRONG.

Go look it up in any basic\introductory engineering statics text... or even the AISC Steel Construction Manual...
Parent - - By Texglide Date 07-01-2016 19:24
HJLBX,   The texts I've seen simply instruct the reader to use the maximum value recorded, which is not necessarily the force indicated at the actual breaking point.  ie. UTS  Seems pretty simple?
Parent - By HJLBX Date 07-02-2016 04:29
I showed both the WIT text and formula sheet to a licensed (PE) structural engineer.  Even he says the way the formula is defined is incorrect.

It doesn't really matter.  All that matters is that I know what is correct and not correct.

Both the WIT text and formula sheet states" UTS =  "the force required to break the sample."

That is absolutely wrong...
Up Topic Welding Industry / Welding Fundamentals / Calculation of Ultimate Tensile Stress

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