[altaic]

Hey Roy, Creep is the right word. I think you misunderstood when I said it would be nice to know what the rate of creep is-- I mean I would like to know the actual expected value of creep for E/G. Unfortunately, I haven't been able to find much data on the rate of creep of highly packed epoxy-quartz, but I have been looking.

Will

[lgalla]

Creep of epoxy seems to be a concern at high temperatures such as soldering circuit boards or wave soldering.I cannot find any information on creep at room temperatures.Probably not the same as creep,here is a copy...
"Abstract
Adhesive bonding of aircraft primary structures has been in use for over 50 years and is still in use on current aircraft projects as a direct alternative to riveting. Bonding of stringers to skins for both fuselage and wing construction and of metallic honeycomb to skins for elevators, ailerons, tabs and spoilers are the main uses for adhesives. Details of the historical use of adhesive bonding on commercial aircraft and how the adhesive materials developed are given. "
Epoxy bonded aluinium aircraft parts go through extremes of temperature and forces every day.Conclutions:
Should I be afraid to fly?
Should I be afraid to build an E/G machine?
Larry

[lgalla]

With the new,free,three D software,such as Sketchup,one could model the packing density of spheres or cubics.From the pics,cubes look good but actually are very bad.If they all lined up like bricks they would not be pourable.Spherical shapes are micro ball bearings which roll over each other allowing higher packing density.Sorry the threeD files are too large so I am posting various jpeg shots.
Larry

[altaic]

Okay, this is my take on creep in polymers. Bonds in polymers have some probability of randomly breaking and reforming. When some stress is applied to a sample the probability bonds will break in the direction of the stress is increased, and the probability they will reform in the direction of the stress is decreased. IOW, pulling a vertical sample downwards will break more vertical bonds and reform more horizontal bonds. This decreases the tensile strength of the sample in the vertical direction, since the stress is tensile. The sum of the decreased strength is called strain, and the ratio of strain to stress in this case is called creep compliance.

For one pure epoxy system at room temp, the measured curve of creep compliance vs time is logarithmic and tops out at roughly 2E-8 per Pascal (or 1.379E-4 per psi) after 3 years. 20/GPa

This is where it gets a bit fuzzy for me, but I'll take a shot in the dark...

An epoxy column of a mill that is 8"x8"x36" has a volume of 2304 in^3, which at 0.036 lbs/in^3 would weigh about 83 lbs, which equates to about 1.3 psi of stress at the bottom. If you integrate the stress over the length of the column, you get 23.4 lbs/inch. With a pure epoxy creep compliance (remember, strain/stress) of 1.379E-4 inch^2/lb and 23.4 lbs/inch of stress over the length, you get roughly 0.003 inches the column would deform over the 3 years if it was made out of pure epoxy (and since the curve is logarithmic, the rate of strain has fallen off to near zero, meaning that it should practically not deform at all after the 3 years). Actually, that's the bit I'm not too sure about... Intuitively, the more strained something is, the faster it should strain. That logarithmic curve looks to me like the strain rate slows down to a crawl after about 3 years, essentially ceasing to strain any more from the force of it's own weight.

Anyway, that is with neat epoxy, i.e. no additives or fillers. The US patent I linked claimed creep is reduced by a factor of 10 by using 17% epoxy and 83% silica by weight (which I estimate should be 35% epoxy and 65% silica by volume, given 1 g/cc epoxy and 2.65 g/cc silica). I'm once again going to go out on a limb and suppose that 8% epoxy, 27.6% alumina, 42.32% quartz, and 22.08% ceramic by volume reduces creep by a lot more. Cameron indicated that the stiffness of E/G increases drastically if the packing density is near 90% verses 80% (I think he said something like the difference between something half as stiff as steel verses something as stiff as pine). After looking at the exponential curves of cast iron and steel flexural moduli verses packing density from Cameron's post, I extrapolate it to be about a factor of 4 improvement on top of the factor of 10 improvement claimed in the patent. Alumina also has about 4 times the modulus of quartz, so that would be another factor of 1.7 if the quartz was replaced with alumina, which comes out to a factor of 68 improvement: 4E-5 inches of total deformation. Please take these numbers with a grain of salt-- they're really a quick ballpark figure, and I don't even know if they're sensibly generated.

Well, I've been typing for a while now, so I think I'm going to stop.

:cheers:
Will

Edit: Yikes! Somehow my scribbled calculations went horribly wrong. They're fixed now.

Edit #2: One thing I forgot to mention was that since creep in epoxy has a logarithmic curve, one could cast a machine to compensate for creep, and then accelerate the creep by using raised temperature, pre-straining it. This might be along the lines of what RomanLini mentions below.

[lgalla]

Will,you have answered your own questions and my questions on "creeps" Thankyou.
Larry

[veteq]

Epoxy bonded aluinium aircraft parts go through extremes of temperature and forces every day.Conclutions:
Should I be afraid to fly?
Should I be afraid to build an E/G machine?
Larry

...And hold dimensional tolerances of 0,0005 inch al day long,
Dear Larry this aint a comparison.........

Is it wise to build one?
Roy

[RomanLini]

Okay, this is my take on creep in polymers. Bonds in polymers have some probability of randomly breaking and reforming. When some stress is applied to a sample the probability bonds will break in the direction of the stress is increased, and the probability they will reform in the direction of the stress is decreased.
...

That's pretty much summed it up. I've had a little experience with this in one of our previous manufacturing operations. If the epoxy beam members had warpage we would heat them in an incubator to about 65'C (their Thd "temperature of head deformation" spec was rated at 72'C I believe) and press them flat during the process. Then after a few days they would cool, still pressed plat, and after that were straight.

For best strength epoxy should be post cured, usually that means after a week or more initial cure at warm temps (45'C) it is elevated close to the Thd and held there for 10 to 12 hours. The specs vary but the post cure info should be covered in the epoxy datasheet. In general epoxy gets harder logarithmically, so it is still hardening weeks later. Ideally it should be supported for the entire cure of weeks or more. The longer the better if you want it to retain its shape. However the post cure does improve the problem quite a bit.

You can probably build entire structural beams from epoxy gluing together fine grit, but that is not how I would do it. I posted a few pages back that the best way I can think of to make rigid structure is to use a steel outer and fill with larger non-compressible stones physically locked together with compressive forces to lock them hard into the steel structure when it cures. Problems with creep then are greatly minimised as the stones are locked into the steel form as a non-compressible lattice and deflection forces act to try to crush the stones further together. But I'm going to take a tip off Altaic and stop typing now.

[RotarySMP]

Roy, sorry if my post rubbed you the wrong way. I don't want to discourage you, I was trying to encourage you to lay the design tools aside, and start mixing gravel and epoxy.

It is good that you have calculated everything out. What I was trying to say is that the theoretical design variables here are unlikely to be significant vrs the huge process variables.

If creep were a show stopper for E/G, it would no longer be an acceptable material for the production of large expensive machine tooling. The industry has been using it long enough, that it's reputation would be shot if these machines crept significantly over a span of only 15 years.

If you try to have everything perfect before you start, there is a danger that you never start. I would encourage you to buy a litre of epoxy, and obtain filler materials you have designed for and make some test pieces.

You have invested 5 years in the theoretical tasks of designing and calculating your machine, and your design looks really cool, but what has been invested in the practical tasks involved in reaching your goal of a finished machine?

How you mix the epoxy with the filler to get good wetting, how you pour, how you vibrate, how you cure, how you degas etc are critical process steps towards your goal. If you can't control your process variables, your product will likely not conform to the design. Analysing Creep at year 15, is of little value, if your poured and cured E/G is a different material than whatever spec you based your design calculations on.

E/G is a casting process. What does your mold look like? How do you locate your cores? How do you demold the cured part? These are also critical design steps.

Modeling software is seductive, as we can quickly and comfortably "realize" our ideas. In practice, they are only limited tools, which often reflect our own assumptions.

If you don't find solutions to practical issues like these, there will be no creep at the end of 15 years.

Larry, Bonded aircraft structure is normally Al to Al, so there is limited differential expansion. The pioneers like the Shorts SC-7 and Beech Sundowner only used it in secondary structures like skins to ribs. Anyone have any examples of bonded Aluminium primary structure on aircraft?

[ckelloug]

Will,

I ran the model with the inhance TiC9100 particles. You can replace the g200 zeeospheres in the last recipe with no material change in packing density. Predicting 88.3% packing density plus or minus 3%.

Creep is worth thinking about but I agree that E/G in high end machine tools means that it isn't too terrible. Since the aggregate portion doesn't creep, keeping the epoxy down definitely helps. I remember reading in a paper somewhere that tertiary amine hardeners increase creep but I can't find the reference. Creep requires a critical loading so if you are building a desktop machine, I suspect it might be hard to load the E/G enough to cause creep.

Take a look at post 351 by martinw. It has an article attached that describes creep.

Regards all,
Cameron

[altaic]

Hey guys, I made a series of serious errors in my calculations. Sorry about that; I think they're fixed now.

Will

[altaic]

Hi Cameron,

Thanks for the reference to the article on creep. Very interesting.

One of the papers I've found very enlightening is the dissertation of Chih-Wei Feng at Texas A&M in 2004, entitled Prediction of Long-Term Creep Behavior of Epoxy Adhesives for Structural Applications (pdf here). It's long but thorough.

While it makes sense that creep is much greater at high stress, it appears that an epoxy structure still "settles" to some degree over the three years. It's certainly possible the degree of settling is so minute that commercial manufacturers of E/G machines can ignore it for the most part. OTOH, it's also possible that commercial E/G machine manufacturers may pre-strain their machines in some way or otherwise take some kind of preventative measure.

I find it interesting because it may indicate that we may need to do something special to make an E/G machine that will say in spec for a long time. Roy mentioned that there were reports on cnczone.nl of large commercial machines going out of spec due to creep, so it seems worthwhile to investigate.

Will

[veteq]

Hey Guys,

Mark, i ended the replay wrong, sorry for it (the machine is my baby..) On the froum it is sometimes difficult to speak without seeing the other in person. Still like the discussion and idea`s that you brought to this forum in the years.

Will,
That is what i mean, if the big ones have problems with it (maybe not that often do) its a point to mention, personally i dont want to take the risk for a full base.
The big companies have the ability to test and measure in time and labs to analyse the EG, they at extra dopes etc. (were we dont know of, they dont use regular epoxy i think, from the market).
We are on the right path here, but my opinion is that we arent near Zanite or Epucret or something like it.

Kind regards,

Roy

[ckelloug]

Will,

You have a good point. I have ignored creep because it is difficult to quantify. I've made the tacit assumption that with low epoxy content that it's negligible. It may in fact not be negligible but it isn't something I have researched. I look forward to what you find. I'll look into the paper you just posted when I have a bit more time.

--Cameron

[veteq]

Will,

You have a good point. I have ignored creep because it is difficult to quantify. I've made the tacit assumption that with low epoxy content that it's negligible. It may in fact not be negligible but it isn't something I have researched. I look forward to what you find. I'll look into the paper you just posted when I have a bit more time.

--Cameron

Hey Cameron,
Next week i will go to the proffessor on my university and explain to him the excact subject. He is a smart guy and appreciate in the field by a lot companies. He specializes in material properties and fly`s all over the world as trouble shooter. (its a funny strange chinese guy with 7 long hears on his chin, LOL, a cool smart dude)
I will make notes so i dont forget it and hope to translate right here on the forum.

[jonpry]

Actually, that's the bit I'm not too sure about... Intuitively, the more strained something is, the faster it should strain. That logarithmic curve looks to me like the strain rate slows down to a crawl after about 3 years, essentially ceasing to strain any more from the force of it's own weight.

Those curves are designed more or less for springs. So they compress the spring a known amount, which causes a certain amount of stress and strain. Then over time the spring constant changes. With a fixed load like gravity, the creep rate is at least at its maximum. It actually increases eventually. The reason for this is as the beam bends more and more under constant creep. It gets closer and closer to the point of maximum bending, at which it fails. It is important to note that creep is not like molding. Say you have a 1 foot beam that when setup cantilever, will break when it deflects 1". Now say you put a weight on it that only bends the beam .5", and then wait for creep to kick in. When the beam gets to 1" of deflection, it will still break, even though the beam at/near this time would appear to be naturally bent .5", which an induced bend of .5".

That being said, it's doubtful your beam fails at .003. And I think the aggregate will help in untold amounts. This is going to be the same kind of thing as carbon fiber, which has zero creep.

[oxford]

Here is the Epoxy Mike recommended to me

http://www.precisionepoxy.com/SlurryCoat.htm

He told me that the aggregate would help release gas from the epoxy. This mix would have enough epoxy in it that there would be a little settling of the aggregate which would create a glass like top. He said one could blue flame the top to get rid of any bubbles.

Mike likes a sand sized aggregate because it gets in all parts of the molds well, packs well, and traps less air.

He said that their company's colored epoxies would have a chemical in them to help the mix release mixed in air, making vibration (if using the right sized aggregate) superfluous.

Specs for linked epoxy:
tensile elongation 15%
tensile strength 9000psi
adhesion to steel 2700- 3000 psi
compression strength 12-17.5kpsi
flexural strength 14000psi
235-290F temp for deformation
shord hardness 80-85
creep not a factor

[harryn]

Hi, Just checking in. I like to read about 500 - 1000 posts in this thread at a time and gain an overview every once in a while.

The posts on creep are very interesting, although I am not at all sure how to approach this aspect. Lucky for me, I just play with wood routers, so all of these materials have less "creep " than my workmanship.

[jonpry]

I had a couple of random ideas about the creep situation. I only managed to read the first ~150 pages of this thread, so all of this might be old news. First of all, using hollow beam profiles, or other profiles, like I-beams would reduce the static weight while maintaining an appropriate level of stiffness. I see this reducing the creep by as much as a factor of 10. If i were to build one of these machines, I would definitely go hollow if for nothing more than to reduce the cost.

That all might be obvious/lame. The other idea is to put a piece of pre stressed steel under the beam. So say you have a beam that weighs 1,000 lbs. You bend a piece of steel flat bar, or whatever, into a radius such that it takes 1,000 lbs of force to make it flat again. Then bolt it to the bottom of the beam. This totally negates any sagging going on in the beam. Reducing any creep to be nothing more than the beam becoming fatter. It would also be orders of magnitude slower. You don't even need to get the prestressing very precise. Somewhere between 900 and 1100 pounds would still get you a factor of 10 improvement in stress. Not to mention that the beam would once again have a surface that is as flat as the earth.

[brunog]

Oxford,
You might just be going in a direction we all have overlooked!

We have all considered that the EG mix would start with "raw" components: aggregates, fine particules and epoxy resin.

Maybe, by starting with a product like SlurryCoat and adding aggregates could result in a very good product!

The question is: Can we add aggregate to the Slurry Coat without reducing it's mechanical properties, and to what limit??

Maybe EpoxyMike can give you an answer on this!!??!

Nice work Oxford!

Best regards,

Bruno

[brunog]

I really believe that in reality creep is NOT an issue!

I don't believe that we can get to an EG mix that will not creep!

However machine bases that I have seen, professional or DIY, are made of EG and threaded inserts alone! Why? Maybe that's because epoxy creeps.

Structural inserts are and will be necessary, like on any concrete structure!

To reduce the impact of creeping among other things.

EG is a COMPONENT of total machine structure, we must all consider and well utilize it's strengths and use our imagination to overcome it's weaknesses!

No machine is eternal, sometimes we need to overhaul once in a blue moon.

Conclusion: Epoxy creeps if and we can calculate it's impact.

Can we cure the problem in a DIY environment? Maybe not!

Can we get around the creepy problem in a DIY enviroment? Sure we can!

So,what's next??

Guys, keep on the good work!!

Best regards to all

Bruno