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Tensile Strength Testing

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rsterne:
I recently purchased a Digital Force Gauge from Amazon, so that I could do some testing to better understand the tensile strength of various filament materials, and the effect of changes in printer settings on that strength....



I downloaded some test samples from Thingiverse, both for printing flat to test the X-Y strength, and for printing vertical to test the Z (layer bonding) strength.... The first ones I printed were the flat ones, and I did 3 versions, with 2, 4 and 8 walls, all at 100% fill, to see if having more filament extruded parallel to the applied force was better.... The Infill is always at 100%....



You can see the difference in the above photo.... However, when I measured the parts, the square test section, which was supposed to be 0.200" square was a bit oversize (0.206-0.210"), with an area averaging 0.0436 sq.in. (9% oversize) over 6 samples, and the holes were about 0.360" diameter.... I wanted to use 3/8" bolts/pins to pull them apart, and if the area of the test section is (0.2 x 0.2) = 0.04 sq.in. all I need to do is set the force gauge to read in lbs. and multiply by (1 / 0.04) = 25 to get the tensile strength in psi.... So, I used TinkerCad to redesign them, and design a matching vertically printed test bar, so that I can fine tune the dimensions the way I want.... Here is the first set just printed....



The vertically printed one is 10mm thick instead of 5mm at the ends (as was the original) to make sure the holes don't tear out, but the test section is still 5mm square (as designed on TinkerCad).... The holes were designed at 10mm, on 33mm centers, and the ends of the bars are 22mm wide, and the bars are 55mm long.... The test sections (which are 6mm long) are much closer to what I want, the horizontal one measures 0.194 x 0.204" for an area of 0.396 sq.in, and the vertical one measures 0.197 x 0.201" which is also 0.396 sq.in. (within 1% of desired)... The holes are the correct size, only needing a quick deburr on the corners to fit perfectly on the shank of a 3/8" bolt....



If you haven't got a deburring tool, they are cheap, and if you get the plastic blades are great for deburring holes or corners on 3D prints.... The blade spins in the handle, and it comes with 10 spare blades that store inside the handle....  8)



The test sections are now close enough to the correct size (0.04 sq.in) that they can be fine tuned using the slicer, as they will probably change a bit with different materials anyways.... If you want to work in Metric, to get the tensile strength in MPa (which is 1 Newton/sq.mm), set the force gauge to Newtons, and divide by 25 (assuming the test section is 5x5mm, so correct the print to those dimensions).... Attached are the files for both test bars....

Bob

TorqueMaster:
How high does your force gauge go?  Looking forward to your results!

I did similar, but I was testing the effect of temperature and layer thickness on Z-strength (the weakest direction) for my different filaments, printed at 100% fill.  I used a luggage scale, 50kg max I think, and had to add double pulleys to reduce the force the scale was seeing, even though I used pretty small cross-sections on my specimens.  As I recall I was approaching 300lbs on the specimens, 100lbs on the scale.  PETG layers bond really well!

One factor to consider is -- when printing vertical for Z testing, the area intended to break is very small cross-section, and will print very quickly per layer -- depending on your settings, it may print slower mm/s than usual to maintain "minimum time per layer" settings, or it may print the tiny layers at normal speed, heat soaking the area.  Either way, it may give inaccurate results compared to what strength a normal print, with a decent sized cross-section would have.  The only way I know to avoid that, is to print multiple specimens at the same time, so each one is printed at normal speed, and has a realistic time to cool before the next layer gets added.    Making the cross-section area bigger would also work, but the forces involved would be more than my meager shop tools could handle.


subscriber:
Excellent plan, Bob Sterne

Excellent strategy, Bob TM

rsterne:
Yes, I know that the force is likely more than my 500N (50 kg-110 lb) gauge will read, so I am making a setup with (at least) 2 increases in force, using a lever, at 2X and 4X, so I can handle up to 200 kg (440 lbs), or more if I add another hole in the lever.... I'm working on the setup now, and should be able to share that within a few days.... Hopefully, a nice surprise in store for all of you!.... I will be printing multiple samples at a time, so there will be ample time for cooling between layers in the small vertical test section when doing the layers.... I am very curious about how temperature, cooling, and speed affect the layer bonding....  ;)

Bob

subscriber:
So, unless I screwed up, a 0.2 x 0.2" specimen that fails at 240 lb has an ultimate strength of 6000 PSI.  If so, that would seem close to the strength of the base material.  Very impressive, if it is applied across the layer plane.

Any strength measure is better than none.  It is probably important to apply the load at about the same rate for comparisons between print direction, or fill percentage, for example.  Very slowly applied force allows the material load to even out via creep.  Very fast, and shock becomes a factor.

Anyway, I have been impressed with printed PETG, both in my hands, and indirectly via parts that Bob has printed, holding up better than I expected.  It has taken me awhile to trust this observation and design lighter parts for 3D printing.   

I think PETG is a more durable material than PLA, despite the latter being stiffer and stronger.  PETG is not as easily degraded by environmental factors; and does not creep so badly when a part is left in a vehicle in the sun.

I am sure that you have seen the video below, or ones like it.  Ultimately you only have to satisfy yourselves that your prints are strong enough for purpose, rather than getting into arguments with forum members or potential customers.

Certainly, it sounds like PETG is a friendlier material to print than ABS; stronger even if the cosmetic results are not as good.  But then my own printing experience is limited to a Kodama Trinus using PLA.  And that was a few years back...





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