Right now I’m sitting in the lab testing these composite specimens so I decided to make a video about my project. As I finish I’ll post more pictures and details and when I’m finished I’ll put the full report up here.
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What I’m doing is an undergraduate research project. I’m given a small budget and about 6 months and do basic level research in some topic of my choosing. I do this on top of a full academic load of classes. Because the work done is by an undergraduate, the researcher, me in this case, is relatively uneducated and unfocused compared masters and PhD students. Due to this the objective of these grants is to give us basic level research experience, essentially practice for the real thing. I wish I could do a lot more but as always I have to prove myself with small projects before anyone will let me loose in a big lab and big money!
My range was limited by time and money, I do this on top of being a full time student.
As for the higher vacuum range, I’ve already completed research at 27 in Hg (Max of my pump) for strength tests, and 27, 25 and 22.5 for density.I want to be able to make a more comprehensive and complete test of results but right now I don’t have a place to work or money for more CF, Epoxy and waterjet cutting.
This is all undergrad research though, when I move onto grad school I should be much less limited!
Thats alot of specimens haha, i personally take every chance i get to use a tensile strength machine, but i have not gotten a chance to do a test of carbon yet. Do you mind maybe posting a video of the entire test?
From the samples you showed in the video it seems as though the test samples did not fully fracture, but you said you ran the test to failure point. Im not familiar with the failure mechanism of carbon but since its considered a plastic does it stretch and yield as opposed to fracturing?
Hi Canyon
1)Do you believe is it really worth to investigate different vacuum levels ? Maybe do you expect better performance for lower vacuum ?
2) Did you follow some material test standard ?
The epoxy readily yields but carbon tends to strain linearly until break, as brittle materials tend to do. For this research the failure load is the peak load of the composite. Once its load carrying capability has reduced it has failed. The reduction in load carrying capacity is always coupled with large fiber bundles snapping, I can hear this and it’s visible when inspecting the specimens, as well as evident on the stress strain graphs. I’ve posted some that show the initial break then regions of what I suspect are fiber pullout.
Here’s a video of my previous round of test. This one fails at it’s peak load without question.
And yes this is an Instron, series I’m not sure but 3300 sounds right. I’ll tell you later today when I’m back in the lab
I’m not sure if it’s worth it or not. The main driver for different vacuum levels is that for student composites our vacuum levels are all over the place, especially true of student club composites. I decided to also test different vacuum levels just to see what happens and also because it was an easy parameter to vary. Because of this I was interested to see what would happen and decided to pursue it.
As for the standards I wanted to follow ASTM D4762 but it turned out to be prohibitively expensive. Same for ASTM D3171, It was only possible through an outside lab. With the student discount it ended up being $300 for ONE sample :eek:.
For these projects I get a materials budget but I’ve blown through it everytime since my school lacks pretty much everything needed to make composites. This time around I had to buy things like vacuum oil, which wasn’t cheap at all, and other supplies and this eats into my budget. In addition to this I don’t get paid for my time. If I ever do future research it won’t be as an undergrad though so hopefully I get more equipment and financial support!
I’m going to weigh them next. I’ll give you results. I did do specific gravity measurements for 2x2 in squares infused at 27, 25 and 22.5 in hg and the vacuum level showed no correlation to volume fraction. However take this with a huge grain of salt, my sample size was two specimens wer vacuum level.
I forgot to mention the above stress strain graphs are deceiving. At the start there’s two distinct slopes, the first steeper one is the true strain measured by an extensometer. The second strain is an approximated strain measured by the grip position. This is why you see two different moduli, if I left the extensometer on until failure it would more or less be a single line. However the extensometer would be destroyed.
I have a question canyon… Did you mix all your resin at once and record room temp and humidity?
Might sound just a bit anal but… Across the board if you were to mix freeze then thaw in degas chamber there would be no variable in mixing. In aerospace most adhesives are mixed then freezed. Or are used in conjunction with a mixing nozzle. This keeps end user quality at a very high standard. Then you have the click bonds and quick sets in packets.
The days where you can just go mix up a hot batch are numbered as the selling point is astm91oo, iso, and bisi in most reputable shops.
I think documented and controlled mixing would be keen in your research. Granted digital scales are great but a single mix can be issued a batch number and testing against batches for difference would greatly improve the resulting data.
Great work man! I really like your curiosity! Might want to also look into studying the compaction certain vacuum levels produce. Micrography is expensive but no doubt an ndi shop would help you out. most people that see an opportunity to learn or advance their understanding jump on it, at least that’s been my experience.
Just a 5 minute chat and showing them some of your hard work would no doubt be the ticket for some after hours testing.
You’re completely right that I should be controlling more variables. I wasn’t able to control temp and humidity, it just happened to be whatever it was that day. Other variables that I wish I could control better are lack of consistent curing temps and resin flowrate which varies slightly with each layup. Lack of controls over these variables does hurt the quality of the findings. I also lack an industrial freezer to store epoxies, I’d have to stick it next to my dad’s Lean Cuisines :rolleyes:. I have tried using residential appliances for composites manufacture. A few people were not too happy.
I called places for stuff like NDI and fiber volume fraction but I wasn’t able to get affordable prices for it. Material suppliers like Airtech and Fiber Textile Inc were more then happy to help out but testing shops weren’t as much. A single fiber volume fraction test costed 275 and blew my budget.
I have many gripes with the research, in terms of money and time cost and its effects on the final results. My grades have taken a large hit because of these projects. I know these won’t end up being rock solid PhD results but in the end I’m very happy with what I’ve done. When you consider that when I started two years ago I knew nothing about composites and my position now I sincerely hope I can keep up this pace of learning and practice. Like my signature says hopefully I can end up being a pro like you guys someday. And by then I should be done with these baby research projects and instead hanging out with the big boys making cars, planes and rockets At least that’s the dream.
Could you elaborate on this further? I’ve tried cooking composites for curing purposes but the epoxy smelled up the room and there were immediate concerns about the health and safety of everyone around.
The same concerns were raised when I asked to use the school’s foundry for volume fraction measurements. In this case I wanted to use the resin burnoff method detailed in ASTM D3171 but again wasn’t allowed to due to health/environmental concerns.
He is talking about burnoff. You weight the sample in air and water to get density, weight crucibles, burn off at 565c, and then weigh it all after, to find out the weight lsot in the matrix.
HOWEVER (there is always a however), when burning off carbon, you will oxidize the fiber in air, and loose mass!!! You have to burn off in inert atm, like N2. Most furnaces are not meant for this, but if they are sealed well, you can add a flow of N2 in. Still can loose 1-2% in mass.
But yeah, burn off causes a LOT of smoke…but it’s better than dealing with acids!!!
There are several softwares that I dont know the names of, which can calculate the Vf from a polished micrograph.
Ah yes I already looked into that and it wasn’t possible at all. Same for matrix digestion procedures. The ASTM procedure lists several but the methods are somewhat dangerous. One involved microwaving an acid, another involved filtering nitric acid through a partial vacuum. It requires equipment I don’t have.
I did also try polishing specimens and managed to get an area fraction from there. Saying it takes a long time though is an understatement. I must have spent at least 6 hours just polishing two specimens, another 4 hours taking high zoom photographs and then another 4 identifying the voids, filling them in and then finding the area fraction. Limitations I ran into here are that a plane image doesn’t say much about volume, I could be cutting into a section with a lot of voids, or a section with a little, and on top of that it doesn’t say anything about the 3D shape. I found two papers that calculated void fraction using microscopy. One researcher used an automated microscope. The other said his results were iffy at best, and he had ten micrographs per sample.
I have somewhat reliably calculated the resin fraction using two methods, one using the areal weight of the fabric, and standard bouyancy tests. Using both in combination I can estimate the volume fraction but again the results are shaky.
All this research is great though even if I can’t get perfect results. It really makes me think about all the ways to do even the most simple things, and really exposes me to how complex composites are. Hell it really got me focused on composites in the first place, I’m really happy about that.
In all seriousness I’m looking to get an advanced degree. I’ve been researching schools but it’s been very hard to find specific information about the quality of the equipment in their facilities.
Do you guys also have a machine shop? Your school looks amazing because it’s obvious that they actually care about composites, moreso than just acting it through some BS marketing text.
