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micro Stress vs Strain Machine

Dedicated uSSM

Here I'm using some

img

End of the line here is (1) to do small-scale material testing, OK. Next step is (2) to automate material search, doing a kind of real-time simplex where function evaluation is testing and DOF are process and recipe parameters. This is a cohesive, realizeable story.

I would like to know:

  • evaluation: pick: Stiffness and Tensile Strength? some weighted score from both?
  • ... no hard yesses, want to be able to pick & make
  • so far doing tensile strength and elastic modulus
  • to add: poisson ratio with vision and actuation
  • inputs: if we are to pick, say, 3 process parameters (to have 3D search space), what are they?
  • water, glycerin, agar
  • humidity and temperature / during drying time
  • heating profile ... how long the cook is

FEA

I ran a quick simulation to see that this flexure was OK. Here it is loaded in the direction it is meant to be stiff in, with 2.5kN applied vertically. There are two of these members being tested so this would be equivalent to 5kN of force, about where I expect this thing to top out. Displacing about 40uM (I'm looking at the face-to-face distance between the loaded zone and the fixed zone, not that rotation that appears from my non-rigorous fea constraints) with 70MPa stress maximum.

fea fea

And out of plane, with 250N applied, about 20uM displacement. So 1/10th the load and 1/2 of the displacement, it's at least 5x stiffer in one DOF than the other. Is that grounds for a decent flexure?

fea fea

The Cutting

0.1mm offset, 0.5mm fillet for test.

Next Steps

design tasks remaining

  • fixturing

  • dbner ... in gh

  • minor offsets for fits

  • measure material carefully

  • confirm sdp-si parts fitments

  • control box ? psu home ?

  • fab this thing

  • finish waking up new network, new stepper boards / code

  • finish hello-world CV on markers

  • plot a stress/strain chart

  • show high resolution stress / strain measurement w/o high stiffness machine

  • show machine reference via known material?

Generalist does uSSM

To test requisite parts for a small stress and strain machine, I attached some jaws and a load cell to this 'generalist' machine.

machine

I used automatakit network controllers and atkapi to program the beginnings of a materials testing system: here I'm just stepping the axis along 25 steps at a time and capturing a photo on each step. No data yet.

cam bone neck

gif

We're going to try to track the endpoints with CV, rather than fancy encoders etc. Sam has done some prior work on this, here.

Microphone Stiffness Testing

Alisya did this in FabAcademy and it would probably be a fun piece of kit / example for the NIST Project. Looks pretty simple... and compelling.

Design Spec / Notes

The goal here is to design and build a machine, which can be fabricated in $250k size fab labs, that can generate stress-strain curves for a wide set of materials as well as perform hardness testing.

img img

To ballpark, I'd like to see 100mm diameter plates having a total travel from 0mm separation -> 500mm, this leaves enough room for fixturing etc.

Generating kN

We want lots of force, with very fine control of position. This means a nice linear transmission. To estimate the forces we might want to see, I wrote a quick table of forces required to rip apart ~ 3mm square (0.001mm^2) samples of a few materials.

Material Yield (MPA) F at Break (N)
ABS 40 360
Nylon 6 70 630
HDPE 15 135
6061 ALU 310 2790
4140 Stainless 655 5895
6AL-4V Ti 950 8550

Brinell hardness tests range from 10N through to 30kN (for steel and cast iron) but non-ferrous materials normally see 5kN only.

So, a ballpark of ~ 10kN would be ideal - this is a big number - off the bat I'm going to estimate that 5kN will be a more reasonable target. 1kN is enough for a complete set of plastics, but that's only allowing for a realtively small sample.

To generate the force required, we're going to need some motor / transmission oomph. Here's a list of typical NEMA size motors, and the torques they can generate. The atkstepper can supply enough current to power any of these.

Motor kg Nm
N23 56mm 1.2 1.3
N23 100mm 1.8 3.0
N34 68mm 1.8 3.4
N34 156mm 5.4 13.0
N42 150mm 10.9 22.0

Racks and Pinions

We'll be using two linear stages (one on either side of the platform), so, from our N34 156mm motor with 13Nm of torque we'll need 26, 5.2 and 2.6mm lever arms respectively for 1, 5 and 10kN total force.

Considering practical limits on pinion diameter (with a shaft of 14mm, and 19 3mm-pitch teeth, we'll have an 18mm diameter pinion - 9mm lever arm) we will only realistically achieve 1.44kN of linear force per motor with direct-drive rack and pinion on a Nema 34 size motor. This makes a 3kN machine, but to add some safety factor we're at at 2kN goal with this approach.

The next step would be to check against tooth shear stress for 3mm pitch.

From reasonable WEDM (time) and Waterjet (washout) limits, I expect the thickness of any fabricated pinion to have a limit around 12mm. To make this all simple, I'll say 10mm. For the waterjet, this is a bit of a stretch (get it?)...

To guess at resolutions, we'll take our sample above in aluminium having a length of 100mm - with elongation at break being 12%, we're interested in a 12mm 'long' stress / strain curve. For 1000 pts on this curve, we're interested in a step size of 12 microns.

From our 18mm diameter pinion having a circumference of 56mm, this means 4667 counts per revolution. In a 200-step motor, we would need 32 microsteps per revolution - most drivers will go to 256 - but microstepping isn't exactly linear. To do this really well, we will want to finish work on closed loop stepper driving, where we can use a 14-bit encoder to control around ~ 4096 counts reliably.

All in, a direct rack and pinion drive can land us at a 2kN machine with some desired resolution, but we're at or near most of the limits here.

Belts

img

Since I was spinning this up to test on a belt-driven gantry system, I was able to confirm that this is a bad idea. We defeat the elastic stretch of the belt by measuring at the specimen (not through the machine's structural loop) but a meagre GT2 belt profile skips teeth pretty quickly as we approach any tens of kilos of load.

Ball or Leadscrews

Racks aside, a ballscrew is the obvious way to do this. Ballscrews can be had for less than hundereds of dollars. Besides transmitting motion smoothly, their ratios are favourable. For example, with a 5mm pitch ballscrew having an efficiency of 85%, we can drive 14kN with our 13Nm motor - so 28kN for the machine.

For the same ballscrew, to achieve 12 micron resolution we'll only need 416 steps in each rotation - this is easy to get.

Step Two: Force Measurement

For load measurement, Sam has worked through a great loadcell design.

cell

Ok, doing this with a wheatstone-bridge type load cell now.

I believe I want my ADC to have the green line 'on top' and white on the bottom, of a differential channel.

Fixturing

img

! important