[ger21]

Polymers


Some polymers exhibit "creep". Under a continual load, the material will keep on deflecting.

I do not know if polymers exhibit elastic/plastic behaviour up to a point at which they start creeping. Each formulation will certainly behave differently.

It seems to me like you can design out the "creep" factor. As long as there are no continous loads where creep could occur, I don't see it being a concern.

[WilliamD]

I think that's a good point Ger. Where exactly would creep occur? Any examples of creep on something like a bridgeport design knee mill?

[ger21]

I was thinking more in terms of a gantry router. I'd think that any large cantilevered loads might be subject to creep. But, depending on your required degree of precision, it may not be a factor at all anyway. I just have a hard time seeing the thing move.

[Geof]

martinw;
Do a Google search on fatigue failure crack and you will get many hits. The Wikipedia description will be near the top and it gives a good explanation of fatigue.

igalla;
It does not matter what the raw material source is, epoxy is epoxy whether it is made from petroleum feedstock or from feedstock derived from soybean, hemp or whatever. Anyway the ultimate source of petroleum feedstock was plant material way back in the Carboniferous Era.

[martinw]

martinw;
Do a Google search on fatigue failure crack and you will get many hits. The Wikipedia description will be near the top and it gives a good explanation of fatigue.

.

Dear Geof,

I took your advice and had a peek.

Do fatigue failure cracks have any relevence to the creep behaviour of polymers? They might have something to do with why wings fell off Comets in the 1950's, but otherwise, I'm lost.

Best wishes

Martin

[lgalla]

Wikipedia Fatigue is Physiologically"fatigue"describes the inability to function at the level of one's normal abilities.In metals this describes the bending point where there is no plastic return and it finnally cracks.Lucklly for us we can return to our normal abilities unlike materials.
Geof
RU saying I am from the carboniferous era or dinosaur age?Does it show?At least I was not around when granite stress releived.
Larry

[Big S]

I would think that if your E/G frame is flexing enough for the epoxy to fatigue or crack, that you haven't built it strong/thick enough for the task at hand.

FWIW I would also think that if the E/G is made properly that the granite particles (sand or bigger) would touch each other and transfer the loading that way. The epoxy is just there to hold them tight against each other. It should also work out a lot cheaper and easier than carving the machine out of a solid lump of granite .

I have about 20litres of laminating epoxy (thin) sitting in the garage, I am trying to work out where I can get some granite locally and then try a molding.

Shannon.

[Geof]

...Do fatigue failure cracks have any relevence to the creep behaviour of polymers? They might have something to do with why wings fell off Comets in the 1950's, but otherwise, I'm lost.

Best wishes

Martin

I was drawing an analogy between fatigue failure in metals and creep in plastic; sorry if I gave the impression they are the same. What I was trying to say was that in many metals provided the stress is below a certain limit fatigue failure will not happen. With creep in plastics I believe a similar relationship exists; if the stress is below a certain limit then creep does not occur.

[brunog]

IFWIW I would also think that if the E/G is made properly that the granite particles (sand or bigger) would touch each other and transfer the loading that way. The epoxy is just there to hold them tight against each other.

Sandy,
I totally agree with you, :cheers: IMHO aggregate sizing and mixture is the critical point. If the mixture is well proportioned, then the density of the compacted E/G will be higher because there will be less voids and less possible air pockets and the end result will be a strong piece of E/G.

Bruno

By the way Larry, the snow you got this afternoon is heading our way, thanks!

[martinw]

FWIW I would also think that if the E/G is made properly that the granite particles (sand or bigger) would touch each other and transfer the loading that way. The epoxy is just there to hold them tight against each other.

Shannon.

Dear Shannon,

I take the point about the particles all leaning against each other, but there is still going to be quite a lot of exoxy in there.

The mix ratio people were talking about was 10:1 granite to epoxy by weight.

The density figure for epoxy that I have seen is about 1300kg per cubic metre. Granite is about 2700. If I did the calculation right, that means that 17% of the volume of the mix is epoxy.

Best wishes

Martin

[martinw]

I was drawing an analogy between fatigue failure in metals and creep in plastic; sorry if I gave the impression they are the same. What I was trying to say was that in many metals provided the stress is below a certain limit fatigue failure will not happen. With creep in plastics I believe a similar relationship exists; if the stress is below a certain limit then creep does not occur.

Dear Geof,

I misunderstood. Sorry.

I found this article which suggests that you are absolutely right about creep only setting in above a certain load threshold.

http://www.adhesivestoolkit.com/Docs...%20Testing.xtp

Many thanks

Martin

[Geof]

martinw;

That link has a very good explanation of creep, thank you. And possibly my analogy is closer than I thought. This, taken from near the bottom of the page; "Failure through creep tends to follow a linear relationship of log (time to failure) to stress applied to the bond, other than at very low stress levels." could be applied practically word for word to metal fatigue.

[lgalla]

Martin
Your calculations are probably right as a good concrete mix has 7% air.
You have 10% epoxy and 7% air.
Larry

[martinw]

Martin
Your calculations are probably right as a good concrete mix has 7% air.
You have 10% epoxy and 7% air.
Larry

Dear Larry,

At the risk of appearing entirely anal, and reinforcing a probable reputation as being an entire CREEP on this thread, I'm really sorry, but I do not agree.

If you put a 10:1 granite/epoxy ( by weight) mix together, and the density of granite:epoxy is 2700 to 1300, you will end up with a total volume of epoxy (assuming no air voids) of about 17%.

Yes, if you put in a load of air, (probably not a good idea IMVVHO ), you can make the granite:epoxy:air ratio whatever you want as a percentage of the total volume of mix.

The volume ratio of the epoxy:granite within the mix will remain at 17% however.

My best suggestion would be to get rid of all the air you can. It really has little structural value.

There will be those who suggest that air is a further vital ingredient to damp down vibrations. Maybe, but I'm not going to venture into that discussion!

Best wishes,

Creepy Martin

[lgalla]

Sorry Martin math was not a good subject for me.
Here is how I add.Take 10 parts aggregateds to 1part epoxy=10%epoxy.If you endup with 17% great it will lower the cost.I was just trying to account for the 7%I have a reputation for being a little slow.Ask Geof.Maybe you can(chair) pound in the correct figures Sorry for my inad a q c's:withstupi
Spelink was also not a good subject
In any case air entrapment is bad as the heat of exotherm will expand the air.Neverless,air willbe part of the equasion{love my spellink}no matter how much shakin'or vacuum de-gassing you are capable of.If you live in space or a vacuum you will have no air entrapment issues.
Larry

[walter]

Good to have you on board, Martin.

[martinw]

Sorry Martin math was not a good subject for me.

Dear Larry,

Absolutely no need for "sorry". My skills at maths have always been suspect.

They will continue to become more so as time passes and as my calculator has an increasingly more dodgy zero button. Loosing powers of ten can get a bit awkward!

I guess that the point I was trying to suggest is that the weight ratio of aggregates to epoxy is not the same as the volume ratio if the aggregates and epoxy have different densities. Hence the 17%.

Many thanks for your post,

Best wishes

(Creepy)

Martin

[lgalla]

Martin sorry again I missed the reference to volume rather than weight.I am so em bare assed.My X cuse is a couple of 7% German dark beers.:cheers: Over here across the pond,in the colonies it tis 11Oclock :tired: I wonder if the 7% refers to volume or weight?Any thoughts?There is definetaly carbonation in the equation.I will de-gass one more and hopefully record the results.:cheers: again
Larry
addum Ant
I am impressed, you have a calculator.I am in the flintstone era.Having the dodgie zero button experience hath caused mulitple failures.Pencil and paper works for me.

[martinw]

My X cuse is a couple of 7% German dark beers.:cheers: .

Dear Larry,

Gin and red wine are freely available over here in the UK.

I always blame my calculator.

Best wishes

Creepy

Martin

[lgalla]

Hay Martin
Your comments made me look back and see my calculations on mix were wrong.:withstupi I have always stated the 10% epoxy ratio was unobtainable as I read somewhere this was by volume ratio.By weight this ratio is reasonable.I did find it weird by volume ratio as in the business we go by weight.Calculations of final volume are difficult as 1 cu/ft granite is 166 pounds and 1 cu/ft sand or gravel is much less because of air space.
Basically a mix would be 100%,10% resin,90% filler.Is this correct?10lbs resin to 90lbs aggregates.
Larry