Tensile Strength Testing

[rsterne]

The coupons are acting as they should now, so unless I see a problem, I won't bother with more photos.... I will just give the results graphs....

I have read that reducing the cooling can increase the strength on PLA, so I tested that out.... I picked a temperature, and then starting with the data from the coupon I had already done (at 100% fan), I reduced the fan speed to 80%, 60%, 40%, 20% and zero, and plotted out the results.... When I used 230*C nozzle temperature, where the Overture PLA was the strongest, the strength dropped slowly as the cooling decreased (instead of increased!).... When I thought about this, it made sense, as increasing the temperature above 230 reduced the strength.... So, I dropped the temperature back to 210*C, where the strength was the lowest, as shown in Reply #31 above.... Now, I got an initial increase in strength, as shown below....



The strength peaked at 40% cooling, although I did not try 50%.... The Yield Strength was in between what it was when this PLA was printed at 100% cooling at 220 and 230 deg.... so yes, you can increase the strength of PLA by reducing the cooling, but only if you are printing at a lower temperature than what gives the strongest print at 100% cooling.... This makes perfect sense, and may have some applications, where your appearance (particularly stringing) is suffering if you print at too high a (cosmetic) temperature to get the highest strength.... I realize that this is a data set of "one", but I won't be testing different cooling settings on a regular basis....

The first PETG tests are on the printer, so stand by!....

Bob

[TorqueMaster]

Good stuff, Bob.  I never tried this test, but your conclusion makes sense.  Printing "too hot" for best strength can cause more oozing and dripping, too, so great to know less temp, with less fan can give similar strength.

On my Ender 3, the cooling fan is showing its age -- it will not spinup by itself with any command under 30-35% -- I don't know if yours has or will suffer the same fate, but the 0% and 20% being almost identical made me remember that tidbit.

[subscriber]

Bob,

Surely the shallower the helix angle (finer pitch), the more mechanical advantage a thread has?  Therefore clamping force should go up with a finer pitch at a given diameter.  Yes, friction eats up a lot of clamping force, but if you think of the thread as a spiral cam, the helix angle seems to matter; a lot.

The calculator seems to use "half angle", rather than pitch.

https://www.engineersedge.com/calculators/bolt_torque_from_thread_pitch_calculator_15830.htm
https://securitylocknut.com/how-is-clamp-force-for-a-bolt-and-nut-assembly-calculated/

[rsterne]

I thought exactly the same, but just as the finer pitch moves less per turn, stretching the bolt less per revolution, it is also easier to turn.... A coarser pitch will reach a given "torque" sooner, which balances out the increased stretch of the bolt....

I looked at those calculators you linked, and the first doesn't have pitch as an input (a 60 deg. thread has a 30 deg. "half angle").... and the second one is for calculating the maximum clamping force, based on the grade of the bolt and the pitch (which affects the shear area).... If you can find an axial force calculator where the pitch makes a difference, I'm sure "Engineers Edge", which is where I linked, would be most interested.... Here is a link to the equations that calculator is based on....

https://www.engineersedge.com/torque.htm

Bob

[subscriber]

I will take another look, Bob

[rsterne]

Really, REALLY Counterintuitive.... 

Bob

[rsterne]

sub, here is a possibility for a test block....



It is a 1" square, 1.75" long, with 1/2"-20 threads in one end, enlarged to 13mm diameter (but length not changed) which should be pretty close to a thread in for a 1//2"-20 bolt or rod.... The threads have a cylinder behind (above) them that is 14mm diameter, and limits the thread engagement to 12.7mm (1/2"), and gives room for the end of a tap if one is needed to clearance the threads.... Above that is a transverse 10mm hole (fits a 3/8" bolt shank), with enough material around it to be (several times) stronger than what I can pull with my setup.... If you use 100% fill, it takes just over 2 hrs. to print, so quite acceptable.... I would mill an aluminum adapter that would fit over the block, and the other end fit inside my moving slot.... If you want to try less thread engagement, just modify the print to shorten the threads by subtracting material from the outside, easy peasy....

Bob

[subscriber]

Brilliant, Bob!

With that much wall thickness around the thread the fact that the wall thickness varies is academic.  But one might round the outside of the section that has the cylindrical hole to make the wall even.

Rather talk about it, I went ahead and designed three test piece, based on your template: 
1. One intended to be ready for use, straight off the printer (might benefit from de-fuzzing with a tap).
2. One intended to be chased with a tap before it can be used.
3. One that is blank hole for tapping, with its ID equal to the minor diameter of the for chasing version.

I designed the parts to be self supporting when printed upright, so the threads are facing the platen.
I took the opportunity to reduce print time, by removing material from the rectangular part.

To reduce thread engagement, either don't screw the bolt all the way through the exactly 1/2" deep threaded section; or trim the STL in the slicer, by "sinking" it below the platen level so that a specific number of threads are left.  The latter option would have a different thread termination than the 45 degree chamfer at both ends of the full threaded section.

Screwing in the bolt by 10 threads (1/2") and then backing it out by so many turns should be easy enough.  Else, I can make shorter test print files with the same thread termination as these ones.

All three files are attached in one ZIP file.  See Images below comparing the three test files.

[rsterne]

Yep, those should work fine.... My model doesn't have the 45 deg. taper at the outside of the threaded hole, as I downloaded the McMaster-Carr file for a 1" long threaded rod, and "chopped" off both ends, then increased the diameter to 13mm (but left the length alone, of course).... Here is the file for mine.... Printing with smaller layers will reproduce the thread profile better, because of better resolution, of course.... maybe the strength too?....

BTW, on my "Z" coupons, when I rounded off the end, they broke through the sides of the eye, when I went back to square (with the same margin of material), they no longer broke.... It must be something to do with the loading from where the bolt pulls on the coupon being spread out by the layers over the entire end of the coupon when squared off.... That is why I left mine square, and didn't worry about the extra material....

Bob

[WhatUPSbox?]

I'm curious how you are going to test these.

[rsterne]

I will make aluminum adapters to connect the printed test block to my existing setup....

Yesterday and today I ran the tests on Overture PETG Gray.... I did both the Horizontal (X) Coupons and the Vertical (Z) coupons.... I used both the thin and thick ended horizontal coupons, and found so little variation in the results that in future I will only use the ones that at 4mm thick throughout, as they print a bit faster.... Here are the results....



The vertical coupon showed a greater response to temperature change than the horizontal one, which varied only a small amount with nozzle temperature.... However, both were the strongest at a nozzle temperature of 240*C.... The bed was at 80*C, and the cooling at 60% for all tests.... All the vertical coupons broke suddenly near one end of the 5.6mm x 4.8mm test section, where the 45 deg. taper started.... All the horizontal coupons yielded at the psi's given in the graph above, but all of them (both thick and thin) continued to stretch in the area of failure for 30 sec. more, at which point I stopped the test.... They all yielded and stretched somewhere in the middle of the 5.6mm x 4mm test section.... The PETG is about 78% of the Yield Strength of the Overture PLA, but once it yields, it resists breaking immediately.... I now have a standardized test which I will use in the future, unless some filaments (such as Carbon Fibre filled) exceed the capabilities of my setup with the horizontal coupons, in which case I will thin up the test section....

Next up will be Wood PLA, and I have to go back and test the vertical coupons with the Overture PLA… It takes about 2 hrs. to print one each horizontal and vertical coupons, and another half hour to break them and record the data, times the number of temperatures I test (about 6)… Realistically, that works out to 2 days for each material, and uses about 100 grams of material (about 33 metres) to run one set of tests (assuming no failures)…

Bob

[subscriber]

Thanks, Bob

Interesting that the rounded test samples broke.  I will take the round top features off the ones I designed.

[subscriber]

Same files as before, but V2 indicates square tops.

3 STLs in zip file.

[rsterne]

The rounded tops probably have more strength than my test setup can stand.... just though I would mention it....

When I get a chance, I will machine up an adapter for a 1" square bar, and make an adapter for the other end to a 1/2"-20 male thread....

Bob

[subscriber]

Thanks, Bob

As always, you are free to use whichever arrangement makes most sense to you.

I generated the CAD designs for fun.  Using any of them is optional.

[rsterne]

I wonder if I should put the axis of the second set of 3/8" pins at 90 deg. to my current setup (ie horizontal)?.... It would make kind of a universal joint, free to rotate in both planes?.... Any advantages/disadvantages?.... For a disadvantage, I guess they could fall out, currently gravity is my friend.... 

Bob

[subscriber]

If there is any lack of alignment in the cross axis, the a universal joint seems to make sense.  Using a bolt that is not fully threaded would allow one to place a nut on the other end, so nothing drops off.

A 3/8" pin with a cotter pin hole near the other end could do the same thing; if it is a problem.  If you testbed is horizontal, then the test sample and support hardware might be able to lay on the testbed, rather than coming apart.  If the axis lifts the parts slightly under load, there should be be any added friction.

If the test pull axis is vertical, I think the universal joint idea might sometimes move to a position where one of the pins is tempted to drop out.  A partial wrap of masking tape might be enough to prevent that, if cotter pins are not a good idea.

[rsterne]

Pic of my setup is in Reply #7.... Pins are made from shortened 3/8" shouldered bolts, and drop in from the top.... Sure saves time over putting on nuts!....

Bob

[subscriber]

It helps tp look at post#7 again.  From that, I don't think cross bolts to form a U-joint will fall out.

[WhatUPSbox?]

Bob, I think if you want to limit the loads on the leadscrew, relocating the pivot point off of the cross slide to either the fixed side or in between, will limit the load onto the leadscrew to what comes through your force gage. You will need to make an extra bar though.