[mp006ltk]

It looks like there is alot of experimentation going on here. Can anyone recomend a working formula for Epoxy granite with sources for the materials. With 250 almost posts on this subject it's hard to weed out the facts from the experiments. Thanks!

What epoxy works best? US composites seems to be cheap. What products work best? Slow or fast hardeners, Thick or thin resin?

What mix? Can I get everything at my local homecenter, or do I need to special order?

Sorry this may be redundant, but there's alot of posts on this subject.

[sigma relief]

hmm, must not have hit post earlier, so here is a redo.

harryn, an extension to your factorial analysis for data analysis is "Design of Experiments" or DoE http://en.wikipedia.org/wiki/Design_of_experiments (saddly, the wikipedia article is of a lesser quality than most and does not do the process full justice) DoE is a tool to help plan experiments with multiple interconnected variables without doing a full sweep of every possible combination. DoE uses statistics to tease out influencing coeficients between variables and can evaluate how each variable affects other variables, even when multiple things are changed with each sample. As mentioned earlier, the math is a bear, but programs like minitab are realy the only way to tackle this.

Unlike factorial analysis, which is primairly an analysis tool, DoE is used to determine which variables are changed on each sample and statistically reduces the amount of samples needed. The real magic is that DoE can take the influencing coeficients it generates uses them to spit out an optimized formula automatically.

Aside from academic use, I have used this to sort out a year long build/tweak/redesign cycle on a new spring clutch design. The design was broken into 13 variables and I believe we ran 50 or so samples (it may have been a little more, but I'm not sure, either way it was not a lot of samples for 13 varriables). Not surprisingly, none of the samples worked, but that wasn't the point. Once the computer crunched the data, we built up the "optimal" result and it worked perfectly. This was a problem that dozens of engineers could not solve with their best efforts in measuring, tweaking, designing, and guesing, but a 2 day experiment solved everything.

Now the drawbacks, most DoE trials use linear interpolation and assume straight line relationships betweeh the high and low test points used for each varriable. 3 and 4 point per variable tests are possible, but the required samples begin to increase dramatically. Surprisingly, this is rarely a problem, but EG may be a bit unique. The other problem is the requirement to break the test down into individual independant variables. While this may not be possible for some of the agregate tests, the "other" factors like epoxy, and addins are certainly a candidate.

Finally, the most obvious problem is actually creating and testing 15-20 or more unique sample formulations (preferrably with 5 individual specimens each)

If anyone is thinking of an "extensive" test procedure, this is something to look at because you can get a lot more bang for your experimenting buck by correctly choosing which variables to change with each test.

John

[ckelloug]

greybeard John,

Spin casting could indeed preload your castings in compression thus making it the "orbis canis" (My engineering concentration was in mathematical statistics, I didn't take latin. ). It also true that the paper by B.W. Staynes suggests that E/G should be cast under pressure so you have something.

The intent of my comment however was to suggest that the material with the 37-127/37-606 in my test was kind of like a hockey puck: not truly rigid but at the same time pretty hard. It wouldn't be optimal for a precision machine structure but it would make a nice base if the parts placed on top were more rigid.

sigma John,

The idea of careful experiment design is an excellent one. We have an excellent model of the aggregate packing so the variable for aggregate in an actual material is probably Phi, the aggregate packing density. While I would not claim to be a crack statistician, I completely understood the example in the wikipedia article and would be capable of employing such techniques if I were to study some.

There is a huge list of factors that affect E/G. I listed the ones I had identified all the way back at <A href="http://www.cnczone.com/forums/showpost.php?p=306963&postcount=1485"> Post 1485</A>. (I'm not psychic: I used the index thread.)

Briefly speaking, I think the following are the likely model variables:


Epoxy Modulus
Epoxy Strength
Phi (Aggregate Packing Density)
Aggregate Modulus
Aggregate Strength
Void Percentage
Bonding agent concentration
Catalyst concentration
Nano-reinforcement concentration

Epoxy probably breaks down further into:
Epoxide Equivalent Weight of Resin
Amine Hydrogen Equivalent Weight
Mix Ratio
Cure Temperature profile
Post Cure Temperature Profile

The 5 samples requirement won't be too bad for me as the mold I have machined out of UHMW produces a block that is sawn into five samples. In short, I have already tested 4 formulations if you count pure epoxy so the lots and lots of tests problem probably isn't going to be an issue.

Regards all,

Cameron

[greybeard]

Now the drawbacks, most DoE trials use linear interpolation and assume straight line relationships between the high and low test points used for each varriable. 3 and 4 point per variable tests are possible, but the required samples begin to increase dramatically. Surprisingly, this is rarely a problem, but EG may be a bit unique. The other problem is the requirement to break the test down into individual independent variables. While this may not be possible for some of the aggregate tests, the "other" factors like epoxy, and addins are certainly a candidate.
John

Hi John.
I think this is the nub of the problem with EG, and from two standpoints.
In your own example, buying springs from Acme Springs Inc. or Zebedee International should not make a difference to anyone trying to reproduce your clutch design, an assumption being that springs will always be used in the linear part of their behaviour.

But in the EG, the ability of a mix to absorb energy is, I would guess, non-linear in the extreme, depending on several unpredictable or non-linear variables.
The general angularity of the aggregates, their relative fracture strengths, and the packing density achieved, the first two being dependant on locally sourced material, and the last being geometrically non-linear.
The ratio of epoxy to aggregate will also have a non-linear effect (akin to the effect of adding water to dry sand), moving from its adhesive properties to becoming the vehicle for fracture propagation.

I think the best we might hope for is that Cameron's testing will give us good pointers to what might be achieved regarding ultimate EG strength in terms of particular recipes, along with a guide to what improvements are possible in terms of additives.

Regards
Greybeard John


The above was written before I read Cameron's posting, but I think you get the layman's drift.
Also thanks for the wiki link. I'm going to add the "milk in tea" reference to my list of important trivia.

[sigma relief]

greybeard

I am concerned with the nonlineatity of the solution as well and am a bit troubled by the what it implies. This came up briefly when I was in school and the professors reply was something along the lines of "You are correct to be concerned about the linearity assumption, but the linearity implied is between each pair of variables, not for the solution as a whole. Almost every 10+ varriable system is "nonlinear", but reasonable approximations can be made between each pair of variables." He went on to explain that unless there is a major inflection point centered between the data points (ie your test points lie on either side of a peak), you are unlikely to ruin the entire solution. This is because every pair is evaluated and effects are summed, if an inflection point is left out, the slope of the line between the two variables will be lower than the real system and this variable pair will have less influence on the final solution.

I did not make a big deal about the 3 and 4 point tests, but those will break up the solution into smaller linear segments and a look at the results showing a V shape in the influencing curve indicates a nonlinearity. That brings me to another point I forgot, the individual influencing curves between each pair of variables can be summed and viewed to give a quick refrence as to if a particular variable has any significance in the results. If the slope is flat or low, it does not affect the results anc can be left out of future trials. It is still a good idea to look at each of the variable pair graphs however as there could still be one pair that has an effect. The results are often all plotted on a single graph for quick comparison of relative effects.

John

[greybeard]

John - thank you for your thoughtful reply.
Reading it I realized that...

....... unless there is a major inflection point centered between the data points (ie your test points lie on either side of a peak), you are unlikely to ruin the entire solution. .....

... perfectly encapsulates what was in my mind re the recipe.

I would think that the effects of most of the additives that are being suggested will be handled by the methodology you are suggesting. It's all the grit in between that worries me

From the beginnings of this thread I've been particularly intrigued by attempting to visualize the packing problem, and how we might achieve the maximum strength by tweaking the aggregate ratios/sizes.

Up to present I've managed to get a static mental picture of a number of different sized particles filling up a 3d space, of smaller and smaller spheres jambed in the ever smaller spaces left by larger ones.
What I'm having a problem with now is a more dynamic picture of how the scene changes as a few too many small particles start to push the larger ones apart. Thus, as this population increases, at some critical point the larger particles are in a sense lubricated, and crack propagation flows around them with little loss of energy.
I know the analogy is a bit sloppy, but I hope the picture conveys my point, because this is what I see as the "major inflection point" mentioned by your prof.
To confound the problem, this situation will be repeated for each of the pairs of different sizes of aggregate in the recipe.

Regards
Greybeard

[ad_bfl]

Question based on this model:

and crack propagation flows around them with little loss of energy.

Regards
Greybeard

If this model is correct then is there a correlation between EG Modulus and the aggregate modulus?

Meaning if we could increase the aggregate modulus by a factor of 10, what would be the effect on the overall EG modulus?

Al

[greybeard]

Hi Al.

My words were a considerable simplification, and the point I was making was aimed at describing how a change in how tightly the aggregate is packed can make a sudden change in overall strength (modulus(?) if you prefer), much like the way damp sand turns into quicksand when you reach a critical percentage of water.

But the underlying idea is that if there is sufficient space around the aggregate particles, cracks will more easily travel through a uniform material, the epoxy, rather than crossing the interface between the epoxy and the aggregate.

If additives/surface treatments are also incorporated to improve the adhesion between the epoxy and the aggregate, this will help prevent the cracks from finding a weak path round the particles. So if the packing is very tight, then the crack has no alternative but to try to shear the aggregate itself, and lose a lot of energy in the process. The more energy it takes for a crack to propagate, the stronger the material is.

It's only at this stage that improving the modulus of the aggregate will start to have an effect.

Regards
John Greybeard

[Gizmot]

personnal opinion here:
Since I have spend the last months trying to design an highly accurate machine, I must say that in order my priorities for the perfect material are:

1. Dimentional stability (no creeping)
2. ability to do not need vibration
3. Low cost
4. Low thermal expansion
5. good vibration damping
6. then mechanical strenght

[ad_bfl]

Hello John

My reason for asking is a far fetched, but "possibly" relevant. Notice I am hedging this idea BIG time , so please do not laugh.

Lets assume we have a achieved a high packing density with little to no segregation, and any dislocation has a high chance of being pinned by an aggregate or zerosphere.

In that case overall EG modulus could be highly dependant on aggregate modulus and shear strength.

So here is where it gets weird, I was watching a DVD by Nova last night on the Venus probes and the geology of venus.

What they discovered on Venus was that existing compressive and shear strength values for "earth" rocks would never support such drastic geologic formations, the mountains would literally crumble under their own weight.

In order to explain this discovery, they did a simple experiment, take some earth "rocks" and cook them at 900F, which is the surface temp of Venus, what they found was a 10x increase in modulus of the "rock".

Given the new strength, the formations were possible, but only in a environment totally devoid if water and with the "rock" cooked at kiln level temperatures.

Hence this weird thread.

Imagine taking our mix, cooking it in a oven at 900F to drive off all moisture in the granite components, mix it up and measure the modulus.

I have not had a lot of luck tracking down the relevant papers describing the experiments, this could be a total waste of time if the rock samples they used were incompatible with our mix. When/If I find any papers that shed some light on this I will post the relevant links.

and again, please no heckling, I just like the idea of being able to increase the modulus of our mixes.

Thanks for humoring this...
Al

[greybeard]

.........
In order to explain this discovery, they did a simple experiment, take some earth "rocks" and cook them at 900F, which is the surface temp of Venus, what they found was a 10x increase in modulus of the "rock"..........

Given the new strength, the formations were possible, but only in a environment totally devoid of water and with the "rock" cooked at kiln level temperatures. .....
Imagine taking our mix, cooking it in a oven at 900F to drive off all moisture in the granite components, mix it up and measure the modulus. .....

Very interesting. But note the line I've made bold. I fear you might have to keep the rock moisture free to avoid the transition reversing.

I can go with the idea that the rock may well convert its crystaline form by having it cycle through this sort of temperature range.

In doing so it might also reduce the degree of micro cracking that there would normally be.
These would be the starting points for major cracks to start under stress, so that too would be an improvement.

Driving off surface water has already been mentioned for an improvement in adhesion, so it adds up to an interesting idea, taking the starting point that you do, of all other parameters being optimised.

John

[ad_bfl]

Hi John

Thanks for not laughing

It appears when Nasa did their testing, it was a compressive strength test.

The researchers took a "column" of the original and cooked samples and loaded then from the top in compression, the uncooked sample deformed to 1/2 the size of the cooked sample before it fractured.

So far I have found that the formations are basaltic in nature, and the reported increases in modulus ranged from minimum of 2x to 7x. Highly dependent on the sample as one would expect.

Assuming cooking works, then once the aggregate is bound in the epoxy, would we expect moisture to penetrate the epoxy?

Thanks
Al

[roach]

sorry ive been gaming got a new mouse got to get used to it
ill get some data on one of our samples
we're making a 4.5 m x 1m x 1m if this deflects the figures your saying it will be scrap
think the customers gonna set the machine to 2 micron for this one as its gonna rough the item first the next 10 machines have gotta be better to finish the item
what size test pieces do u make?

[ckelloug]

Al,

Ignoring the effects of fracture mechanics, there are several uniformly increasing bounds that can be drawn around the modulus of the composite as the modulus of the components goes up. These are the rule of mixtures and the Hashin Shtrikman relation. The relation ship is uniformly increasing but not linear. Googling the rule of mixtures should produce some good results.

The Hashin Shtrikman equations produce better bounds but until I've finished reading their dusty paper, I can't explain it and there are no usable web references I could find. Not even a wikipedia entry. . .

greybeard John's comment however involved fractures flowing around the aggregate. This is an interesting point and one I haven't thought about. I'd hope to load the E/G in a region that we aren't getting fractures but it is a concern.

Regards all,

Cameron

[jhudler]

1. Dimentional stability (no creeping)
2. ability to do not need vibration
3. Low cost
4. Low thermal expansion
5. good vibration damping
6. then mechanical strenght

Your best chance of achieving this with EG or any other material, is to have a separate Metrology frame made of Invar that is coupled kinematically at the machine at the base. This allows the machine components to expand as needed and all measurements reference to the frame.
Read Slocum's book Precision Machine Design.

Also, number 3 is mutually exclusive.

[sigma relief]

I am a little worried that some of the investigations into modulus are interchanging modulus which we desperately need with ultimate strength which is largely irrelevant (The required stiffness will result in more than enough strength). Crack propagation energy or fracture toughness is an indicator of brittle failure which we expect to see, but is not directly linked to modulus. Visualizing packing and crack propagation require bit of mental dexterity, but modulus is difficult to impossible to accurately visualize. The quick cross sectional area approach to visualizing modulus is easy to think about, but it does not correlate directly with the data.

I believe what we are looking for lies somewhere between visualizing how cracks will form, flow, and be pinned and the basic cross sectional area approximation of modulus. We know there are no mobile cracks when the structures are well designed, so that theory is not applicable, but raw data tells us that averaging does not work either.

I apologize for not having answers, but we need to make sure we use the appropriate models because one wrong assumption can waste a lot of time and effort in testing.

The other John (K.)

[lgalla]

Recent observations.
I prefer a composite that has some flexibility.Picture an extremely stiff E/G gantry and a servo runaway or spindle crash.The stiffer epoxy will have catastrophic failure.Carbon fiber is similar.No "give"it shatters like glass.
From producing epoxy coatings in the 1970's,epoxy's were extremely strong,but brittle.Adding ,I remember glycidyl esters gave flex strength and lowered viscosity to boot.
To answer a question:Yes,fast hardeners generally produce brittle composites.
Larry

[harryn]

Hi Sigma Relief

Thank you for posting that enhancement / correction to the design of experiments concept. As you noted, the concept can be used to work through many variable problems with surprisingly (at least to me) valuable results. I was a little hesitant to propose a complex experiment to start with since there is a bit of "leap of faith" involved the first time you use it.

In a test set like this, I tend to use the approach to first test some basic variable space, then refine it - let it take me into a direction for more testing. This is certainly less efficient, but perhaps more intuitive for the normal user (like me)

A good place to start might be a 3 variable test
- Constant aggregate loading / sizing mix
- Vary epoxy %
- Vary Hardner content
- Vary one other parameter such as replacing one aggregate item

I think that ad_bfl has made a very interesting observation. While annealing epoxy at 900 C is not possible, there is a well recognized (for many years) technique used in the semiconductor industry we call "annealing glass". The materials are technically SiOx films with various dopants, and often contain large amounts of hydroxyl bonds.

These films are commonly heated to 900 - 1000 C for an hour, which dramatically improves their structural and chemical resistance properties. (at least 5 X ). I did not really think about it, but especially the fumed silica in our mix is probably not post annealed to this tougher state.

What I am not so sure about, is if the silica will start to fuse together during the anneal.

[rowbare]

Cameron,

Some posts back, you talked about mixing problems etc... when you were creating your samples. That reminded me of a video I had watched on Freeman Supply's site:

http://www.freemansupply.com/video/preparing/liquid.htm

There is also a video on vacuum degassing which some might find interesting. Of particular note is where he mentions that not pulling enough vacuum can actually create problems in some cases.

http://www.freemansupply.com/video/p...g/vacdegas.htm

Now I know that they are dealing with very different materials and the scale is different but the basic principles are similar. They have quite a few other video tutorials about casting and mould making that I think are relevant to readers of this thread especially to those unfamiliar with casting processes.

BTW, I love this thread. I stumbled upon it when it was a mere 200 pages or so and spent several evenings reading through it all. It will be interesting to see what comes out of it all.

bob

[roach]

no when it breaks it should shear thru the granite not follow the resin its a pity my phones camras no good the pics are terrible you'd see what i mean
you should'nt see any stratas of resin if you can then its wrong