Real life seemingly not matching material properties 1

[Marmight]

I was doing some informal "testing" of two material samples. They were material sample cards from protolabs, one of Delrin homopolyer and one of Valox 420 PBT with 30% glass fill. Each card is about 3" x 6" and 1/8" thick.

The Delrin has a rated yield strength of 71MPa on prospector, and some other sources listing a break strength of ~90ish MPa. The Valox with 30 glass is listed as having a yield strength of 120MPa.

With that being said, in my informal desk side strength test, I was able to break the Valox card one handed bending it with my thumb on the desk. (fingers on top edge, thumb pressing in the middle and bottom edge on the desk)

With the delrin card, I was unable to make the card break with this method. Nor with that method two handed. Indeed it wasn't until I brought it close to my chest and compressed it between my palms with lots of leverage was I able to make it break.

To me this doesn't fit the story told by the material properties sheets.

What am I missing?

 

[IRstuff]

Were these samples even intended for this type of testing? What you did doesn't appear to be a valid yield strength test

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[3DDave]

Valox 420 PBT looks to be very brittle, elongation of 1-2%. That isn't much curvature. In the matweb sheet tensile yield and ultimate strength were listed on one page as identical, indicating the material will suddenly rupture once the yield limit is reached. It also has a low tensile modulus, so it should not bend much before it breaks.

It's possible the Delrin stretched to limit the tensile load to be below the ultimate strength. Delrin is a tradename for dozens of versions of the material.

It's better if you link to the source of the properties and to the description of what exactly protolabs was sending.

matweb:

http://www.matweb.com/search/datasheet.aspx?matguid=f336bc508fde4b10839b711657426aaa&ckck=1

for SABIC Innovative Plastics Valox® 420 PBT, 30% glass fill.

 

[rb1957]

maybe the delrin was over-spec ?

another day in paradise, or is paradise one day closer ?

 

[LittleInch]

Errr your "informal desk side strength test" is nothing of the sort. What it is is an informal elongation to failure test

This appears to be as scientific as saying I had a thin piece of glass and thin piece of cardboard. The glass broke when I applied an excessive bending force to it as it was brittle and the cardboard was much more ductile and just bent. Which is the "stronger"??

Yield stress is but one parameter which needs to be addressed to determine which is the best material for your use.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Marmight]

Below is an image of the sample cards that were used.

Link to the materials in question:
POM (aka Delrin):

http://plastics.ulprospector.com/datasheet/E94861/Delrin-500P-BK602

PBT 30GF:

http://plastics.ulprospector.com/datasheet/E4726/VALOX-420SE0-Resin

Stress Strain for Delrin 100, a similar material

The difference in modulus between the two materials was evident pre failure. Under light and moderate loads the difference in deflection amount was very noticeable (the PBT deflected much less under approximately the same load).

In addition, it was not surprising to me that the delrin sample required much more deflection before failure than the PBT.

What I was expecting was the following:

I would need to apply a given load to the Delrin which would bend and bend and bend until it finally snapped. Then with the PBT, I would apply the same load and experience only moderate deflection. Then I would increase the load approximately by 33% and then the PBT would snap suddenly.

What I actually experienced:

I loaded the PBT one handed with the intent to break it, and it broke on the first try. I then attempted to repeat this on the Delrin, and despite using all of my one handed strength, could not cause the delrin to break. I then tried two handed as hard as I could and still could not get it to fail. It was not until I figured out a method to increase my leverage (holding the sample close to my chest) that I was able to get it to fail.

From this qualitative analysis, I was required to apply a larger force to fail the delrin than the PBT. My understanding is that given these conditions force should correlate well to stress experienced by the material. (samples same size and cross section, same loading conditions). Therefore break stress of the delrin appeared higher than the yield/break stress of the PBT, which is contrary to the material specs.

I attached some images of the failed samples.

Could the PBT failure have been premature (sub 120MPa in the bulk of the loaded tensile surface) have been the result of a stress concentration at the root of the lettering?

 

[rb1957]

remember that spec values are the minimum strength of a material (not the average or actual)

another day in paradise, or is paradise one day closer ?

 

[3DDave]

The samples have stress risers built in; they are not uniform thickness. The stress riser is the ridge around the edge.

Stress = MC/I, where C is the distance from the neutral axis. A ridge makes C larger without making I larger, so the stress increases in the outer fibers.

When a crack forms in the raised edge it can propagate rapidly - perhaps a factor of 1000 stress concentration at the tip of the crack.

The site you linked to requires an account, free though it is, to see the values.

In any case, unless the values show a great deal more than 2% elongation to failure, then it is no surprise that a brittle material with a stress-raising configuration will fail more easily than a ductile material in the same configuration.

 

[rb1957]

these are all valid points, but i think the OP is saying given that the same part (shape) was made from two materials that have essentially the same E, why did the nominally weaker material support a higher load ? The unscientific result can be understood with more data, most useful would be a pull test on both specimens, to get Ftu of the actual parts.

i realise that this probably will never happen, as the query was made based on the results of an unscientific test ... clearly there's no research project underlyingthis, just someone with too many plastic labels and too much time on their hands !

another day in paradise, or is paradise one day closer ?

 

[dgallup]

How certain are you that they have the same dimensions? These materials have different shrinkage so if shot in the same mold the dimensions will be different.

How certain are you that the parts were processed correctly to obtain the specified properties? Molding parameters can make huge differences.

How certain are you that the part that failed easily did not already have a crack or other defect that led to the rapid failure? Were there knit lines or other molding defects?

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[Marmight]

Thanks for all the replies everyone.

3DDave

That actually makes a lot of sense, using some rough approximations, I predict that the uneven geometry of the card would result in ~45% stress riser due to an increase in the distance from the neutral axis.

Assuming I is of a rectangle 60mm x 2.2mm (taken from measurements) with 0.5mm ridges that do not contribute to I (as an approximation), but act to increase c from 1.1mm to 1.6mm (1.6 / 1.1 = 1.45 = 45% stress riser).

rb1957

Just a clarification, the E of the two different materials is quite different, as was experienced. However the failure stress of the PBT was supposed to be 33% greater than that of the delrin, and instead it felt 50% weaker or more.

dgallup

They are approximately 2% different in size, the PBT is larger as expected because the glass fibers are acting to reduce shrinkage.
I do not know how they were processed.
No visible defects, certainly no knit lines given the gate position.

 

[Dougt115]

From the geometry of the samples I would expect that the Delrin's elongation allowed it to deform and decrease the stress concentrations. This would allow it to come closer to the yield rating of the material. The other material being brittle would not deform and the stress concentration would cause failure below the rated yield strength.

Truly these are poor samples to do any real comparison on.

 

[racookpe1978]

OK, so the two cards were bent until one broke.

Almost crudely put, but so what?

What is your actual design usage of the two plastic laminates? What is the real world failure criteria (it MUST be able to do this so many times) and what is - as noted above - the "bending a plate of glass" problem? "Yes, the glass breaks when bent, but I will not design glass to go in a place where it will bend."

A credit card, for example, is a different life cycle application than a computer baseplate.

 

[LittleInch]

The reason you're getting "strange" results is that the test is not a yield strength test, but a bending test. However the bending forces and stresses the sample experiences are quite different. The much stiffer material will have a much higher bending stress a the point of your hand than the more flexible material which allowed a bigger bending radius to develop which greatly reduces bending stress.

If instead you had clamped your sample to the edge of the desk then clamped a much stronger item to the overhanging portion such that the bending force was concentrated in a small length then applied different loads to the over hanging section then I think you'll find that the stiffer stronger material will hold more load than the other.

Or you could tape / fix a sample to a open file or book and try and close it, holding the sample to the face of the file forcing the same bending radius on both samples.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[dhengr]

Marmight:
I think the problem with your test was that you used the wrong kind of desk. It has to be an obtangular desk without bendy edges. And, the desk top color is all wrong. Furthermore, the desk should always be oriented with its long axis running north and east when you run your test, but it shouldn’t be running too fast or it’ll get away from you. And finally, is this really the true state of professional engineering and technical activity and inquiry these days? Talk about scientific inquiry with a few uncertainties and ambiguities, I hope your product design is better founded than your material research.

 

[IRstuff]

Isn't a yield test supposed to stretch the material in question by pulling on it?

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[cowski]

It sounds like you performed a flexural strength test on the parts. Compare the flexural strength characteristics on the data sheets to see if they correspond better to your informal test results.

Also bear in mind that the injection molding process will tend to orient some filler materials (such as glass fibers) and the end product may have a "grain". It may be that you bent the glass filled plaque along the 'weak' axis.

 

[Compositepro]

If you had one sample made of rubber, you would not be able to break it using all of your strength. That does not mean that rubber is a stronger material.

 

[Marmight]

Thanks again everyone for taking the time to reply. Really appreciate all the activity in the thread.

Material Properties from Matweb:

http://www.matweb.com/search/DataSheet.aspx?MatGUID=eba8ff43155542e0af2d8e731863c589&ckck=1

http://www.matweb.com/search/DataSheet.aspx?MatGUID=f3e120b399214a5f9b15e7ab0399ffe7

A source that lists bend strength for Delrin:

https://www.plasticsintl.com/datasheets/Delrin_150.pdf

From the geometry of the samples I would expect that the Delrin's elongation allowed it to deform and decrease the stress concentrations. This would allow it to come closer to the yield rating of the material. The other material being brittle would not deform and the stress concentration would cause failure below the rated yield strength.

Truly these are poor samples to do any real comparison on.

Dougt115, I suspect you are correct in saying that stress concentrations are the root cause for brittle PBT failing before its rated yield strength. It could either be stress concentrations caused by ridges increasing the distance from the neutral axis, or it could be sharp internal corners at the base of some lettering on the plaques.

The failure crack follows the contours of some of the letters in a very suspicious manner, as can be seen on page 3 of the attached pdf.

I agree that these are poor samples to do any real comparison on. Luckily I am not doing a real comparison between them; rather I am attempting to correlate some observations from a simple test to my understanding of the underlying theories of material mechanics.

OK, so the two cards were bent until one broke.
Almost crudely put, but so what?

I had a certain expectation based on my understanding of the theory.
I observed a different outcome than expected in a rough, poorly controlled test.
I am seeking to rectify the discrepancy in my understanding of the theory.

The reason you're getting "strange" results is that the test is not a yield strength test, but a bending test. However the bending forces and stresses the sample experiences are quite different. The much stiffer material will have a much higher bending stress at the point of your hand than the more flexible material which allowed a bigger bending radius to develop which greatly reduces bending stress.

If instead you had clamped your sample to the edge of the desk then clamped a much stronger item to the overhanging portion such that the bending force was concentrated in a small length then applied different loads to the over hanging section then I think you'll find that the stiffer stronger material will hold more load than the other.

Or you could tape / fix a sample to a open file or book and try and close it, holding the sample to the face of the file forcing the same bending radius on both samples.

Interesting, does ASTM D790 control the bend radius of the test subjects? Based on this video, it appears to me they do not:

https://www.youtube.com/watch?v=veVwA3-Vpmg

Valox 420, Flexural Strength, ASTM D790: 186 Mpa (27,000 psi)

http://www.matweb.com/search/DataSheet.aspx?MatGUID=eba8ff43155542e0af2d8e731863c589&ckck=1

“Delrin”, Flexural Strength, ASTM D790: 11,500 psi

https://www.plasticsintl.com/datasheets/Delrin_150.pdf

This would imply a relationship between Young’s Modulus and bending strength when loaded in bending. I will do more reading on this.

And finally, is this really the true state of professional engineering and technical activity and inquiry these days? Talk about scientific inquiry with a few uncertainties and ambiguities, I hope your product design is better founded than your material research.

Just trying to rectify my understanding of theory with (crude) experimental observations.

It sounds like you performed a flexural strength test on the parts. Compare the flexural strength characteristics on the data sheets to see if they correspond better to your informal test results.

Also bear in mind that the injection molding process will tend to orient some filler materials (such as glass fibers) and the end product may have a "grain". It may be that you bent the glass filled plaque along the 'weak' axis.

Valox 420, Flexural Strength, ASTM D790: 186 Mpa (27,000 psi)

http://www.matweb.com/search/DataSheet.aspx?MatGUID=eba8ff43155542e0af2d8e731863c589&ckck=1

“Delrin”, Flexural Strength, ASTM D790: 11,500 psi

https://www.plasticsintl.com/datasheets/Delrin_150.pdf

No correlation.
Based on gate location, I absolutely was bending it along the “weak” axis, good catch! In your experience will material properties for a filled material be at least as good as the base material (PBT unfilled) or could it actually be compromised by the presence of filler?

If you had one sample made of rubber, you would not be able to break it using all of your strength. That does not mean that rubber is a stronger material.

Umm, actually I think that the vernacular “stronger” would be totally applicable to a rubber material whose break (engineering) stress of the bulk material was higher than Delrin or 30GF PBT. Of course it’s Young’s Modulus would be much less than either of the named plastics. Perhaps you misread my question Compositepro.

 

[chicopee]

The graph tensile stress vs strain is developed from satistical results per some ASTM procedure and in all likelihood a tensile strength tester was used to stretch dogbone specimen. This graph is not intended for flexure calculations unless some form of correlation has been made between flexure and tensile stress.