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Definition of Steel Young Modulus 207Gpa
8

Definition of Steel Young Modulus 207Gpa

Definition of Steel Young Modulus 207Gpa

(OP)
Dear all,

Could somebody tell me how was calculated/defined the "standard' value of 207Gpa as Yong Modulus of the steel.
In other words, if I have the stress strain curve as a results of a tensile test (ASTM A370) on a steel sample, then:
1) which part of the curve do I have to consider to calculate the YM. (I mean between which strain limits)
2) which mathematical method do I have to use? (linear regression, chord, etc)

what was historically done to get the value 207 Gpa?http://www.eng-tips.com/

thanks a lot.

RE: Definition of Steel Young Modulus 207Gpa

1.linear portion, i.e. from origin to the yield point.
2. since it is linear, you can calculate YM simply by dividing stress by strain. practically, both engineering and true stress-strain curves for a steel could give you a similar result, however, in theory, you should use engineering curve.

RE: Definition of Steel Young Modulus 207Gpa

207 is a strange number. Most of us use 200.

RE: Definition of Steel Young Modulus 207Gpa

(OP)
Thanks MagBen,

what do you exactly mean for yield point? 0.5% strain? (absolute strain)

Analyzing more than 400 tensile test (ASTM A370) with this method I get an average YM of about 190Gpa with a stdev of 15Gpa. Pretty below the theoretical value of 207Gpa. How is it possible?

RE: Definition of Steel Young Modulus 207Gpa

Depending on the reference, the value at ambient temperature reported for steel is typically 200 GPa. Your average valve and SD seem reasonable.

RE: Definition of Steel Young Modulus 207Gpa

207 GPa is not a theoretical value. while lots of steels have a YM of 200GPa, this value varies depending on what specific steel you are referring to. If you got 190 Gpa, the material could possbily be a stainless steel.

Since YM is related to elastic deformation. For the linear portion, the best regression is naturally linear regression, otherwise it is against the defination of YM. To roughly estimate the YM, one can just calculate one point at 50% yield. If you want to play with statistics, whle still have a low standard devaition, I would recommend using the portion of 20-70% of curve. The closer to yiled, the more linearity the curve could be. In addition, to evaluate the calculation using regression from a statistics viewpoint, S square and P value are more important than SD.

If the accuracy of YM is important to you, donot use an existing stress-strain curve, you may want to desgin an experiment taking the followings into account:
1. use a suitable stain rate
2. increase the length of the specimen at gage section.
3. load 50-60% of yield, unload to 20-30%...repeat couple of times
4. you may want to focus on loads at your interest


p.s. you can use .2% strain as the yiled point,especially when it is not easily define from the curve.

RE: Definition of Steel Young Modulus 207Gpa

p.p.s. Carpenter stainless customer 630 has a YM of 196Gpa, Carpenter stainless Type 420F has a YM of 207Gpa.

RE: Definition of Steel Young Modulus 207Gpa

Young's Modulus does vary with preferred orientation in the material - what is your product form?

Young's Modulus determination requires very accurate strain measurement using a modern extensometer and an accurate force cell. Some force cells have poor accuracy near zero, so you may need to avoid that region. MagBen's suggestion of 20% to 70% of the yield point is a good one.

Section 13.2.1 of ASTM A370 specifically lists 207 GPa for carbon steels. For products like springs, the Young's Modulus is very important. The standard EN 10270-2 for spring wire lists E = 206 GPa.

RE: Definition of Steel Young Modulus 207Gpa

If your application is at anything higher than "room temperature" and if your application does require a very specific value for the Youngs Modulus, you need to either get a stress-strain curve for that alloy at that temperature, or repeat the Youngs modulus test at that temperature

RE: Definition of Steel Young Modulus 207Gpa

3
Young's Modulus of 207GPa is a good typical value for a medium carbon steel.

Pure iron would be slightly higher and alloy steels slightly lower.

The chances of measuring this value with any degree of certainty using a tensile test machine are slim to none.

The basic definition of Young's Modulus E = Ơ/Ɛ implies the use of True Stress and Strain as Engineering Stress is more correctly defined as S and Engineering Strain is e.

I would agree ate very low values of Strain there is little difference between the two values but True Stress needs to be accurately determined and the use of an engineering value will produce an error.

The next problem with using tensile test machines will be due to the Class of Extensometer. A typical extensometer that could be used fro the determination of proof stress just isn't good enough. The type of extensometer needed would be a BS 3846 Grade D and these types of device don't maintain their calibration accuracy for a few days (at least in my experience) Averaging extensometers may help but the class is still important.

The next issue would be machine and test piece alignment. Tensile testers are just not well aligned.

To determine Modulus with any degree of accuracy you would need to achieve levels of alignment around 10 times better that is typical and much more in keeping with machines used for LCF tests.

It would be possible to align a test piece but some high quality specimen holders which allow adjustment.

If you did adjust alignment and used strain gauges attached to the test piece you may get somewhere but the best method to accurately determine the modulus of steels use either resonance or ultrasonic measurement.

I would also question why it is a property that needs to be measured as the difference in modulus between to steels of similar composition will be virtually impossible to detect and are unlikely to change much apart from the influence of temperature (10% per 100 degC - approximately)

All of the tensile tester we used to sell had a check during the elastic loading portion of the tensile test.

If the modulus was not within 5% of the expected value we used to unload the machine prior to yield as it was likely there was a problem with the extensometer.

The idea of using Modulus measurement of part of routine QC just doesn't make sense for the majority of steels.



RE: Definition of Steel Young Modulus 207Gpa

Unless you need values at a specific load or temp then using acoustic measurements is the easiest.
In the wire business we did exactly what Ben described.
Long samples, repeated load and unload.
It is important in wire because when you heavily cold work material you change the modulus.
We were drawing high strength steel wire (1.2%C) and we had a strength and modulus range to hit.

I have seen acoustic measurements done using a cell phone with a frequency analyzer app on it.
They were getting values within 3% that way. Using a precision microphone and analyzer you can get <0.5% error.
Steels are nice because the modulus is the same in all directions (unless heavily cold worked).

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

RE: Definition of Steel Young Modulus 207Gpa

FennLane, thanks for correction. By definition, the true stress-strain curve, instead of engineering one, should be used. I meant to true curve, but my fingers failed to follow my brain.

RE: Definition of Steel Young Modulus 207Gpa

(OP)

Dear all,

Thanks a lot for your valuable contribution.

Just some clarification because I understand that I was not very self-explanatory:

1) I am talking about C-Mn steel, Grade X65, the one used for pipeline construction (API 5L or similar)
2) The YM that I refer to was calculated cutting samples in both longitudinal and transversal position. To be honest I didn't recognize a significant difference between the two directions (Long and Transv).
3) Tensile tests were conducted with "standard" extensometers. Tests were performed at room temperature.
4) YM was generally calculated considering the range 10%-50% SMYS and using engineering values.
5) My impression was that the "test procedure" and "test equipment" were not suitable to calculate with a good grade of accuracy the YM
6) Most of the time the operator didn't look to the Stress Strain curve (during test execution) to understand if the test was progressing well or not

We performed similar test on compressive direction, following the ASTM E9 guideline and the test results were 10 times more accurate using the same "standard extensometers", the same calculation method and so on. Therefore I think that 90% of the reason of the reason of the higher spread in the YM (following tensile results) is depending to the test specimen machining and alignment. As normal we take great care for the machining and test piece alignment during compressive test but not during tensile.

Thanks again

RE: Definition of Steel Young Modulus 207Gpa

(OP)

Just one more detail about the sample direction and its influence on the YM:

samples were machined form fabricated pipes in longitudinal and transversal direction.
Pipe was SAW, it means that:
1)we start with a laminated plate (defined direction of lamination=longitudinal respect to pipe axis. Plate was obtained via TMCP from a slab even 10 times thicker than the plate.
2)then we cold worked up the plate to a pipe with UOE or JCOE method (cold expansion ratio max 2% for no more than 3 times.

I didn't noticed significant differences in the YM in long or transversal direction even if based on my limited knowledge if you had significant cold working in one direction the YM in this direction should be slightly higher.

RE: Definition of Steel Young Modulus 207Gpa

In a steel, unless it is heavily cold worked, you should not see a diff in Modulus based on direction. The tensile properties maybe be very different, but not the modulus.

A good tensile machine with self aligning (gimble mounted) jaws and good extensometers will actually yield good data. But you must use long samples, with long range extensometers, or optical ones.
The load and un-load cycles are important to take any 'slack' out of the system.
Temp control is critical, for measurement of load and extension as well as the actual properties.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

RE: Definition of Steel Young Modulus 207Gpa

Ed's statement "tensile maybe different, while Mudulus does not, based on testing directions" appears inconsistent. The inconsistency maybe easily reconciled by noting the emphasis of load and un-load for YM measurement process. Taking away the mechanical "slack" would help the material behavior more isotropically. Remember the tensile strength could be manipulated by changing the strain rate (e.g. the tenisle could increase with increasing strain rate).

I am imagining if the Mudulus could vary a bit at different directions when the sample is a single crystal?

RE: Definition of Steel Young Modulus 207Gpa

In Ti (CP) you have material with a hex crystal structure that develops a texture when rolled.
So Then it gets a final stress relief (CP Ti is not annealed).
The modulus in the three directions in plate or sheet are significantly different. The average is about 15.5kkpsi. The low is about 12.5, and the high is about 17.
All the while the difference in T and L tensile strength can be +/-50%.

There is no dependance of Modulus on strength, they are independent parameters.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

RE: Definition of Steel Young Modulus 207Gpa

Not trying to beat to death, but for a specific material, could it exist a tendency that the higher the yield, the higher the mudulus? If assuming no plastic deformation between zero load and 100% load of yield, the modulus must be proportional to yield strength, i.e. modulus = yield/.2% (perfect linear). The assumption fails, because you can not use the yield point to calculate modulus, the tendency may still exist: the higher the yield, the higher the slopes for most of loads, and so the higher the modulus. The change ratio of yields seems to be higher than that of modulus when the status of mateiral is changed, so even people see a big change in yield, they only see a small change in modulus.

Not to pretend to be an expert, myself actually doesnot have first hand experience in measuring modulus, it is to me a learning expereince! I really appreciated and enjoyed reading different opinion from all different perspectives.

RE: Definition of Steel Young Modulus 207Gpa

With stainless alloy steels low amounts of cold work (<25%) don't change the modulus a detectable amount.
In a SS this level of cold work could raise the tensile 50% and double the yield strength.
At high amounts of cold work (>50%) the metals have no distinct yield as they have only a few % elongation at failure. These cases can have distinctly elevated Modulus, though the way that you do stress relief is important also. You don't want to be measuring residual stress and calling something else.

You can't use the engineering yield point for modulus calculation.
Once the load/strain line deviates from straight you are probably at 0.1% or less elongation.
With a 10" sample this would be 0.010" total elong.
Our optical extensometers would measure 0.0005" change on a 10" sample.
Going clear to 0.2% is almost always way beyond true elastic behavior.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

RE: Definition of Steel Young Modulus 207Gpa

(OP)

Ed, Mag,

I just can confirm that once the load/strain line deviates from straight (YM change) you are probably at 0.1% or less elongation.

this was the reason why we decided to use the SS curve up to 50% of the SMYS to calculate the YM.
In our case 50% of SMYS is about 225Mpa that correspond to about 0.1% strain (assuming YM=210Gpa).


Hey guys I really appreciated all your contribute but one of my initial question remained nor=t replied.

What was historically done to get the value 207 Gpa? I mean how the value of 207Gpa or 200Gpa was calculated. I mean considering which steel and which SS limits?

RE: Definition of Steel Young Modulus 207Gpa

I agree with the comments about modulus depending on interatomic bonding and the way it dominates behaviour.

The definition of yield point has always been the subject of much debate and argument with regard to the correct definition.

With steels that exhibit discontinuous yield it is quite straightforward apart form do you take the Upper or Lower Yield points.

The use of a 0.2% proof stress has been historically used because it was relatively easy to measure and quite repeatable.

It is also a similar level of strain to the discontinuous yield point of a low carbon steel.

Expressions such as limit of proportionality and the like were all developed in the days when strain measurement accuracy was poor and it reality we are potentially arguing about the difference in the thickness of the line produced by an old fashioned X-Y Recorder.

The reality becomes what we really mean by yield point for materials that exhibit a 'continuous' yield and there is no clear 'point'.

Do we mean when one dislocation slips by one Burgers Vector or may be 100 dislocations of may be 1000.

The definition of yield point could be considered to be very closely related to the accuracy of strain measurement and for most conventional materials changing the point we use will only serve to penalise a material and potentially increase cost.

For expedient reasons 0.2% strain was chosen and this has only become an issue with the development of high strength materials that are brittle elastic in nature and mean that 'yields' need to measured at very low strain levels.

If we design using a simple strength of material approach for this type of material we run the risk of experiencing catastrophic failure as the design approach used for materials that are more ductile and can work harden may not be good enough for very strong materials.

As Brittle Elastic materials don't have much work hardening capability this will seriously impact on their ability to tolerate defects and I would imagine that an LEFM approach may be better and evaluating these types of material and KIC may be a better guide to is performance.

I realise this is off topic but yield point measurement is only part of the picture.

RE: Definition of Steel Young Modulus 207Gpa

What seemingly learned from this thread is that, unlike lots of resources state, steels do NOT exhibit linear-elastic behavior even under a load much lower than .2% yield.

Due to more or less plastic deformation and dislocation movement, modulus seems to decrease with increasing loads (refer to metengr's link). To accurately determine the physics modulus, a regression may be applied, and the load is extrapolated to zero to get the interception as the “true” modulus.

A question arises, who cares about this “true” modulus in engineering world?

RE: Definition of Steel Young Modulus 207Gpa

Even a regression analysis will be problematic as is there is a measurement error in the basic loading line you will just be fitting incorrect data.

There will always be some small degree of hysteresis in a loading/unloading curve even in a notionally elastic region although this is commonly due to extensometer behaviour.

A traditional extensometer will rock on its knife edges and have some movement and even non-contacting devices can have backlash in their drives.

I haven't used a speckle interferometry laser device so I couldn't comment about this style of extensometer but this may be an improvement on more common devices.

I think accurate determination of E for a typical category of material is important to know and would always use ultrasound or resonance methods of its determination.

To measure E on a day to day basis as part of a QA test just seems a waste of time as it will never be correct to better than a few percentage points.

As was pointed out by Maui there will be little change due to the majority of variables so as long as we have a useable number for design work all should be good.

I was involved in reviewing a programme of JIC Single Specimen tests carried out on A533B specimens some years ago and the crack lengths were calculated using specimen compliance based on an E value of 190GPa which seemed a little on the low side.

Modulus had been measured using an old Hounsfield Tensometer with a Mercury Cell and crosshead deflection and was, IMHO, a complete waste of time.

It took quite some time to re-calculate the results of more than 200 tests with more appropriate values.

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