Real life seemingly not matching material properties
Real life seemingly not matching material properties
(OP)
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?
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?





RE: Real life seemingly not matching material properties
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RE: Real life seemingly not matching material properties
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?matgui... for SABIC Innovative Plastics Valox® 420 PBT, 30% glass fill.
RE: Real life seemingly not matching material properties
another day in paradise, or is paradise one day closer ?
RE: Real life seemingly not matching material properties
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.
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Also: There's usually a good reason why everyone does it that way
RE: Real life seemingly not matching material properties
Link to the materials in question:
POM (aka Delrin): http://plastics.ulprospector.com/datasheet/E94861/...
PBT 30GF: http://plastics.ulprospector.com/datasheet/E4726/V...
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?
RE: Real life seemingly not matching material properties
another day in paradise, or is paradise one day closer ?
RE: Real life seemingly not matching material properties
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.
RE: Real life seemingly not matching material properties
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 ?
RE: Real life seemingly not matching material properties
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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RE: Real life seemingly not matching material properties
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.
RE: Real life seemingly not matching material properties
Truly these are poor samples to do any real comparison on.
RE: Real life seemingly not matching material properties
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.
RE: Real life seemingly not matching material properties
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
RE: Real life seemingly not matching material properties
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.
RE: Real life seemingly not matching material properties
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RE: Real life seemingly not matching material properties
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.
RE: Real life seemingly not matching material properties
RE: Real life seemingly not matching material properties
Material Properties from Matweb:
http://www.matweb.com/search/DataSheet.aspx?MatGUI...
http://www.matweb.com/search/DataSheet.aspx?MatGUI...
A source that lists bend strength for Delrin:
https://www.plasticsintl.com/datasheets/Delrin_150...
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.
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.
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?MatGUI...
“Delrin”, Flexural Strength, ASTM D790: 11,500 psi
https://www.plasticsintl.com/datasheets/Delrin_150...
This would imply a relationship between Young’s Modulus and bending strength when loaded in bending. I will do more reading on this.
Just trying to rectify my understanding of theory with (crude) experimental observations.
Valox 420, Flexural Strength, ASTM D790: 186 Mpa (27,000 psi)
http://www.matweb.com/search/DataSheet.aspx?MatGUI...
“Delrin”, Flexural Strength, ASTM D790: 11,500 psi
https://www.plasticsintl.com/datasheets/Delrin_150...
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?
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.
RE: Real life seemingly not matching material properties
RE: Real life seemingly not matching material properties
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."
i'm guessing your expectation was that the delrin should've failed first (as it is weaker); a not unreasonable expectation.
in the light of the results, i suggest that you need to refine your expectation to investigate the stresses in the test samples. I'd hope that the PBT is failing at it's spec stress, and that the delrin was over-strength. It is of course possible that the stress concentration of the lettering and the ribs is creating a hot spot in the PBT, particularly if it is more brittle than the delrin.
i was looking at your post of stress/strain curves and assumed that you were showing both materials (to have similar E).
another day in paradise, or is paradise one day closer ?