[ckelloug]

getting my mech E degree in 4 days

Gizmot,

I think you just signed up to handle all the beam calculations people come up with around here Congratulations on finishing your degree. Does it come with a free Iron Ring?

--Cameron

[Gizmot]

Gizmot,

I think you just signed up to handle all the beam calculations people come up with around here Congratulations on finishing your degree. Does it come with a free Iron Ring?

--Cameron

its all about the ring man

[sigma relief]

Its great to see some real user generated data on this stuff. I keep thinking about how to process some samples in my appt, but SWMBO just caught me using the bathroom as a wood shop (forgot to get the router dust of the top of the towl bars... doh) so I will watch the first part of the race from the side.

Best of luck to all.

[lgalla]

Race update.
Jack on BYK996 is just bubbling along,although confused of degrees C.The track conditions are good:no NANO mud or surfactants.Wait a minute,here comes Cameron riding Carbon Black Beauty.The ratios are correct.Still laging is Roach riding Red Lead.Please hang on to your tickets for the photo finish.

[ckelloug]

Well,

I've learned a bit this week. I have discovered that UHMW makes great molds and the epoxy doesn't stick to it at all unless there's a place where it can do the monkey with the fist stuck in the jar routine. UHMW is quite temperature sensitive however and has low thermal conductivity so I warped the sample mold earlier this week by putting a heat lamp on one side of it.

On the bright side however, my big surplus 36x36x48 industrial oven works except for the off part of the on/off switch so I was able to cook my sample of pure epoxy for 2 hours at 100C. I guess I am stuck with the breaker until I figure out which relay is cooky and why. I also cooked the UHMW mold which seems to have normalized out the stress caused by the heat lamp incident and gotten it to flatten back out.

I also noticed based on a lab accident that it appears that a bit of extra hardener gets the material to set up harder. Being that it was a measurement accident, I of course don't know how much. . .

I hope to have flexural modulus and strength numbers for the reichhold 36-127/37-606 epoxy mixture in the next few days.

Regards all,

Cameron

[jhudler]

I also noticed based on a lab accident that it appears that a bit of extra hardener gets the material to set up harder. Being that it was a measurement accident, I of course don't know how much. . .

which begs the question; where does the extra hardener go? As I understand it, there's no place for it to link in.

[ckelloug]

Jack,

In answer to your question on off ratio epoxy, I found this very interesting paper from Brazil. I haven't read all of it yet but it looks like there are several curing reactions possible which lead to different mollecular linkages:
<A href="http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1516-14392003000300017">Mat. Res. vol.6 no.3 São Carlos Apr./June 2003</A> I know I have seen at least one other paper on the topic.

greybeard John,

You're the polymer chemist. Comments?

--Cameron

[lgalla]

I was taught to have slightly more hardener to ensure full cure if mixing small batches.Small batches are easier to be off ratio.With large mixes you should not have ratio problems.
Larry

[harryn]

Hi Cameron

Nice work. So, now that we are starting to get real data again, a real experimentalist would do what is called a "factorial analysis.". It is based on the work of Box, Hunter and (I think Hunter ?).

The basic idea is that you run an experiment similar to the idea of a body centered cubic crystal. (corners and center of the box) for 3 variables. All results lie within this "box" of collected data, and a little math can extract it out. This method works remarkably well for many variables / dimensions, although sometimes I don't understand how.

Any good materials guy uses it. :stickpoke

In any event, when I had to do it in 1982, the math was all performed manually, and was a PITA (about of week of analysis for 1 day of testing). I think it is now built into mathmatica and similar pc programs and is much easier to use.

Why use it ? - because it takes a lot fewer samples to get the same quality of results - often a factor of 5 - 10 less. It does require you to let the math tell you what your samples should be, not the other way around.

[harryn]

Hi, just to add to the post cure question - 100 C is not going to make it for an epoxy that calls for 120 C cure. Time does not always make up for a lack of activation energy, but even if it did, a good approximation is that for every 3 C lower, you need to double the cure time. This would make it about 128 hrs at 100 C if I did the math right.

[ckelloug]

harryn,

Good Call and nice method description.

For the record, I'm still in the process of testing equipment and test procedures so I'm not focused yet. The data I'm producing is still almost incidental. I've already learned I need to get a mixer instead of my sticks and a balance big enough to measure all the components of my test batches simultaneously. Almost mixed epoxy that is several percent off the desired ratio is not going to cut it with these kinds of sample sizes.

It's amazing how much work it is to produce and test samples. And if anyone thinks watching paint dry is fun, try babysitting an industrial oven for two hours on the off chance it might try to burn down the building. It might have tried had I not heard buzzing and realized the off switch was not working on my way out the door

Thanks for the suggestion about three factor testing. I haven't though about it yet as I am still trying to isolate and prevent boneheaded mistakes.

Regards all,

Cameron

[greybeard]

My first read through prompted several thoughts.
Firstly, the structural chemistry certainly bears out observations of increase in strength with slight excess of hardener.
Secondly, this work, though mentioning composites in passing, is testing variables within a bulk material. Even in simplistic terms, one could guess that a degree of elasticity is far better than having a brittle matrix.
Thirdly, the mention and photos of the fracture structure with the 100 micron size shown suggests the inclusion of aggregates, especially including ones down to and below this size is going to have a major impact on the behaviour, if the fracture structure is such a good indicator of tensile strength.
This is pure supposition on my part, but there you go.
Finally, there is a mention several times of water absorption having an effect, but unfortunately no hint of the order of magnitude it has. So we don't know if other effects are greater or less than the one discussed here.
In fact what really worries me is the number of extra dimensions that are being introduced to the problem of maximizing the strength of our EG formulation.

Somewhere way back there was a brief discussion justifying the development of the thread along the lines that, by following the experimental method that de Larrard produced for high strength concrete, we could achieve a greater strength than shown previously by casual mixes made along the lines of traditional concrete formulations.
I firmly believe that this will be borne out by the current testing of samples.
However, unless we can get a handle on what order of magnitude a small adjustment in the hardener ratio will make to the strength, we may all drown in an ever deepening sea of variables.

[Zumba]

Cameron, those are some interesting numbers in post #3002.

Anocast's PDF shows that their material has a flexural strength of 2500 psi (much weaker than jhudler's sample) but a young's modulus of 5.25 x 10^6 ksi (much more rigid). Hexion numbers are very similar.

Having slightly less than ideal aggregate packing could account for a lot of this added strength and elasticity, since that would mean there's more epoxy between the rocks and epoxy is quite elastic by nature.

I'm curious as to how different resins and hardeners compare. What kind of resin is used for laying carbon fiber?

[ckelloug]

I have performed the D790ish tests on the 37-127/37-606 mixture at about 65 degress F.

The flexural modulus at .035 deflection is 366ksi plus or minus 25 ksi. n=10

The deflection at failure was around 1.05 inches for pure epoxy when I was able to induce failure whereas for Jack's E/G samples the maximum deflection was around .05 inches at failure. Only 2 of 10 samples failed. I had to stop the test machine manually on the remainder to avoid the sample and test head bottoming out on the test fixture. 10% is a large deflection in a D-790 test. These samples had 400%.

I have not corrected the following data for deflection (which may be impossible due to there being several hundred percent deflection) but uncorrected, the epoxy had ultimate flexural strength of at least 12.2 ksi plus or minus 0.6 ksi. n=2 as only 2 samples went to failure and these were way outside of normal D-790 conditions.

In summary, the epoxy we have is very rubbery and tough after a cure at 100C. It is this rubberyness combined with the rigidity of the aggregate that make the E/G a good damping material. We still do not know however how damping is effected by aggregate percentages.

Also, it looks like harryn is right that I bungled my cure as it should have been at 121C according to the data sheet.

In conclusion, the epoxy holds it together and the aggregate makes it rigid.

Regards all,

Cameron

[ckelloug]

Zumba,

Thanks for the tip. I just looked at Hexion's numbers again. We're only off by a factor of a bit more than 2 on the modulus numbers. There's some effect on the aggregate packing from the size of the molds Jack's samples were poured in but in general, from the packing models, our aggregate mix is near optimal for that grading span.

My current round of pure epoxy samples were poured into a larger mould than Jack's and then sawn to size with a diamond saw which may turn out to be a better procedure for future samples.

The hexion mineral casting epoxy is a bit more active than the reichhold stuff we've got as it has 30g/equivalent lower EEW. I'm thinking that this means more cross linkage and thus higher modulus.

Because of all the aggregate modeling that we have done, I hypothesize right now that the aggregate is about as good as can be achieved and that our modulus problems lie in technique, epoxy and additives probably in that order.

Regards all,

Cameron

[lgalla]

I'm curious as to how different resins and hardeners compare. What kind of resin is used for laying carbon fiber?

Apparently Northrop Grumman and the US Air Force doesn't have that answer.
[http://query.nytimes.com/gst/fullpag...5BC0A961958260
Larry

[ckelloug]

To really have fun however, you need Northrup working with the Army, not the Air Force. If genius can be described as poetry in motion then Army plus Northrup equals poseurs in motion.

That being said, it was observed by Zumba that our current epoxy, due to the huge deflection it will support, is stronger than the hexion stuff but less rigid. This begs the question of whether there is some other hardener or mixture of hardeners and additives that will produce closer to the desired properties. My assumption based on the Brazilian paper and greybeard's comments is that faster hardeners probably produce stiffer epoxy. It also leads me to start wondering about the cobalt acetyl acetonate again.

The formula we have right now with the high compliance is probably quite good at damping because of the seemingly viscoelastic properties of the epoxy. It's possible that a base of this stuff with more solid components in the actual mechanical portion would be advantageous.

Roach of course pointed out that the fact that our epoxy is similar to their secret formula might not be good enough.

So ladies and gentlemen, get out your thinking caps and lets figure out whether we need to change resins, hardeners, additives, or all three and what should they be? This is not a rhetorical question: longtime readers, newbies, and anybody else with an opinion is encouraged to put forward his/her theories.

I'm going to write to the reichhold apps engineer and send him the hexion datasheet.

Regards all,

Cameron

[Zumba]

I started using epoxy when I was in elementary school, building model airplanes. Got the stuff in little 4-8 oz bottles from the hobby shop. The 5-minute epoxy definitely cured harder than the 20-30 minute stuff.

I wonder if BYK has some answers. They make additives specifically for epoxy mineral castings... surely they'll know how to obtain a stiffer (and more brittle) product.

[greybeard]

.........
It's possible that a base of this stuff with more solid components in the actual mechanical portion would be advantageous.
..........
Cameron

Cameron - Is this to suggest that my spinning idea( desperate to get other things out of the way so I can get up and spinning you samples) might, by loading the outer surface of my castings, be the dog's spheres ?

John

[brunog]

Epoxitech makes an epoxy resin formulated for marble repair, please refer to the following attachment: http://www.cnczone.com/forums/attach...0&d=1180708464

Epoxitech is also known as East system epoxy. The product has a 45-50 minute pot life and cures at room temperature.

Best regards

Bruno