What "gauge" do I need for 3d filament material strength testing? - Page 1
Forum section: electronics / general

author: pipe2null
author_rank: Regular Contributor
timestamp: February 06, 2020, 10:34:50 pm
content: For various reasons, I want to test the (relative to each other) strength of the different types/brands/colors of the 3D printer filament I currently own or will buy in the future. Plus, I have at least one spool that appears to be inconsistent enough in quality that doing periodic tests on the same spool is merited to confirm it's not just my settings or weird intermittent printer behavior. It's not just the tolerance of the filament diameter, it's also how well the manufacturer mixed the material and how consistent the material is through the full length on the spool. I read an article on (I think) matterhackers where they did a similar test with printed carabiners, but with the amount of time they put into their test rig I think it's bizarre that they did not take an extra 15 minutes in OpenSCAD to design a more appropriate object to print for testing purposes. ANYway...I have a rough idea of the 3D test model I need to make to print out with each filament: a "designated break point" in the middle with known cross sectional area, probably 25ish (?) sq.mm, and the rest of the body having 3x or more times the "break point" cross sectional area, and a solid connection point on both ends. Printed with 100% infill.For a test rig, I have a couple steel 8mmX400mm smooth rods left over from a project I can use, and I think I "should" be able to design and print a plastic good'nuff hinge to use the rods in a kinda lever arrangement. Plus misc M3/M4 nuts/bolts/etc. If there is anything else I need, I'll have to buy it.But I have no idea what kind of gauge I need in order to measure the actual breaking/deformation point. I've done various searches, but the massive variety is not helping me make a final purchase so I can finish designing and building my rig.Thoughts? Recommendations? If you know of a specific (cheapish) gauge, an amazon (US) link would be much appreciated.

author: ve7xen
author_rank: Super Contributor
timestamp: February 06, 2020, 11:11:43 pm
content: What type of strength are you aiming to test? Tension? Strain? Shear? Compression? A more 'realistic' load on your test piece? Generally you'd apply a test force at a fixed rate for deformation, and plot force vs. time or deformation, giving you a stress/strain curve through the elastic and plastic deformation regions, until failure. How you measure the force is going to depend entirely on how your test setup is engineered, but ultimately you probably want a load cell, and most likely a strain gauge type. They're widely available for a pretty affordable price on AliExpress, eBay, etc, just search for strain gauge. You will need a stable voltage source and multimeter to measure the output, and probably some test masses to calibrate your setup with if you want anything more than relative measurements. Increase sophistication from there if you want realtime readout etc.This is the same sort of apparatus in a digital scale, so you could possibly cannibalize the internals of one of those, or just use the scale to measure the force in your setup.A diagram of your test setup might be helpful.

author: MikeLud
author_rank: Regular Contributor
timestamp: February 06, 2020, 11:14:32 pm
content: Pipe2null,Checkout CNC Kitchen YouTube Chanel he does a lot of 3d filament testing. The video below shows how he made his Test Machinehttps://youtu.be/uvn-J8CbtzM

author: pipe2null
author_rank: Regular Contributor
timestamp: February 07, 2020, 02:39:53 am
content: Good feedback, thanks!What I'm trying to accomplish is just to get a very rough idea of how much weight can be supported by a part made of a specific type/brand/color filament, and based on that decide which filament brands I should buy in the future, and which cheapo brands might be crappy but are good enough for stuff I do not care about much. For most of what I do DIY design-wise, I do not need to care much about the mechanical properties of the material beyond the general characteristics of PLA vs PETG vs ABS vs etc, but the mechanical properties are a bit more important when choosing brands. And once in a long while I do actually need a rough idea of what strength characteristics a specific filament has. For example, having a very rough idea of how thick the shell of the plastic VESA monitor hanger I made is good'nuff not to worry when printed with fill-in-the-blank-brand PETG without making it 20mm thick with solid infill. The first hanger I made broke, the second version is good'nuff and is still supporting my monitor, but this was pure trial and error with zero actual calculation. No, it is not 20mm thick with solid infill, that was an exageration, but I still have no idea how near or far the plastic is from it's breaking point.For what I'm doing today, a very simplistic tensile strength test is sufficient. I decided I needed to do some basic (comparative) strength testing after I took identical plastic parts that I printed with 3 different brands, all PETG, and broke them in half with a couple pairs of pliers. 2 broke cleanly at the weak point and a partial break on a thicker part (one brand "felt" a little stronger than the other), and the 3rd brand just turned to taffy all the way down the middle. Doing a second test with the "taffy" one resulted in a complete break even through the thicker region.Thanks for the CNC Kitchen link. From the linked video it looks like the type of "gauge" I am looking for is a crane scale that holds the peak/max value. It would be realllllly nice if the CNC Kitchen guy had data available for all types/brands/colors of filament or if manufacturers would provide that data themselves. As much as I'd like a setup like he has, a simple test rig of a crane scale suspending the printed test object, with some type of manual lever attached to the lower end of the test object to provide enough tensile force to break the plastic should be enough for a quick test...At least that was the idea in my head. I would much rather have the full force vs strain (I think is the term) graph, plus any other measured characteristics that might be useful for one design or another. And the overly simplistic test rig I have in mind would be useless for something like TPU, but a setup like what the CNC Kitchen guy has is way beyond what I have time for.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 07, 2020, 03:01:01 am
content: For a simple test, a spring scale works well:If you want more repeatability and a displacement/force graph, use a geared down stepper motor and a strain gauge. Should be a trivial Arduino project.

author: ve7xen
author_rank: Super Contributor
timestamp: February 07, 2020, 03:02:18 am
content: Those crane scales are just strain gauges packaged up neatly with electronics to measure the bridge, calculate and display the weight. It's fundamentally the same thing, but doesn't do the datalogging for you to generate the stress/strain curve. You could DIY something similar with a strain gauge and a datalogging multimeter which with some calibration and Excel could generate stress/strain curves for you. Or just use a crane scale, point a video camera at it, and collect the data afterwards manually. Just make sure to size the scale properly so you have decent resolution. I wouldn't expect most 3D prints to hold up to more than a few KG when you're pulling the layers apart. A cheap luggage scale might be better than a crane scale in that respect.I am not sure the simplest way to rig it so you can pull it consistently. Some pulleys and rope is the 'obvious' solution but kind of takes up a lot of space. Maybe use the 3D printer gantry itself?

author: tautech
author_rank: Super Contributor
timestamp: February 07, 2020, 08:06:41 am
content: Quote from: ve7xen on February 07, 2020, 03:02:18 amLike that will produce results.....not !Some feet for a HP scope beanflying made me:https://youtu.be/H0vTK5Li-NQ

author: pipe2null
author_rank: Regular Contributor
timestamp: February 07, 2020, 09:14:55 am
content: Arg. I had hoped that I could keep it simple for my little tensile strength tester, but the more I think about it (and review the info you guys have graciously provided), I'm thinking that I should probably figure out how to make a mini version of what the CNC Kitchen guy has and just size the cross section of the printed test object accordingly. Doing my lever idea with manually provided force will only work for a subset of materials and only get me a small amount of usable info, much better than the pliers version I already did but if I'm going to put time into something, might as well do it right. Or in this case, a mini version of the right way, and since CNC Kitchen is an open source project, shouldn't be too hard to adapt for my purposes.I have a bad tendency to try really hard to avoid adding new projects to my massive list of series of projects that I already have. But then I fail miserably and add another project to the list anyway. Eh, oh well. It's all fun in the end. Speaking of my bad tendency to add projects to my ridiculous list:@tautech: Oh no, the "TEA Enabler" has arrived! Hehe. Thanks to you I've started budgeting for a VNA, or maybe a decent SA with tracking, but more likely a siglent vna... haven't finalized that bit yet, but that's topics from other boards... But I don't have a problem, so no need for a TEA intervention. Hehe.

author: tautech
author_rank: Super Contributor
timestamp: February 07, 2020, 10:06:37 am
content: Quote from: pipe2null on February 07, 2020, 09:14:55 amI'm sure you can achieve the tensile strength you seek with the advice of others with real 3dp experience.Ask here for guidance and tips:https://www.eevblog.com/forum/chat/3d-printer-yet/QuoteYou might have some trouble deciding with what's about to be released......a 7.5 GHz SVA and a 7.5 GHz RT SA.But seriously we have never been so spoilt for choice of well priced and capable gear.I'm always around if you wanna bounce thoughts or ideas at me.