[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

[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.

[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