[fyffe555]

I will ask a simple engineering question.
Take a 4"X4"/1/4" A36 steel tube.What would the deflection be on a 4'length,8'length with a 200lb centre load?Would filling withE/Q make it stiffer?
Thankyou
Larry

Hey Larry - we did this ages ago You can use the spread sheet from post #5 http://www.cnczone.com/forums/showthread.php?t=14227.

Ckelloug - Please write something to calculate this properly. The spreadsheet is basic at best and ignores a lot of stuff although its close enough for home built ..

Anyhow, ignoring the deflection of the beam due to its own weight (as it's constant);

A 4' 4x4x1/4 steel tube with centre 200lb load and *fixed* ends would deflect 0.0008 or less than a thou'.

The same beam at 8' and same load centred would deflect 0.0064 or about 6 1/2 times more than the 4' beam.

Fill the same beam with E/G and it gets a touch more complicated. Do we have a concencus on the modulus for E/G? accurescasting.com web site has a modulus of 4.5x10^5 which seems a touch low, hardwood is usually taken as 4.9x10^5, steel used in the beam is 29x10^6, so E/G is 64.4 times weaker than Steel.

You can simplify the beam and E/G filling problem as its doubly symmetric in cross section and the neutral point will be located at the centre of the beam with or without E/G. You can modify the E/G section to replace the E/G with an equivalent steel section at the centre to use the same modulus for the calculation and then simply add the two Area moments of inertia from the tube beam and E/G equivalent and calculate.

Before the engineers amongst us scream, yes this does ignore any and all sheer caused by the E/G glued to the inside of the tube. I've no idea what the sheer strength for that would be and it's not going to be a major contributor under these loads anyway. The variables of a good verses bad E/G cast, resin ratios, resin elasticity and agregate sizes will will affect it more.

So, if the modulus of E/G at 4.5x10^5 is right(?) then steel will have the same strength at ~ 1/64th the cross sectional area. The 3 3/4 sq section = 14.06sqin. 14.06/64 = ~0.22sqin or ~0.47" square section in steel. In otherwords a 3 3/4 square cross section E/G beam is equivalent to a steel 0.47" square cross section beam.

Adding the E/G equivalent MOI to the tubes MOI you/re adding 0.004 to 4.854. Not much at all and it will have no significant affect on stiffening the beam in the conditions given.

That is, an 8' composite beam with 200lb centre load will deflect 0.00643" . the same beam without E/G to the same five figures will deflect 0.00644'.

If the E/G had any tensile strength the calc's would be different but since it doesn't this method is a decent approximation and a measure of the effect of composite beams in this scale.

It gets more interesting since the E/G will weigh enough to cause a static deflection of the composite beam about 20 times more than the E/G will reduce the deflection under load.

So Why Do It? the E/G and beam will deflect under gravity alone - but that is naturally accounted for during construction and machine alignment. The E/G will not contribute to strength in bending untill you get to very large sections and 64x steel seems to be the measure - until someone shows me I've used the wrong modulus but the ratio is still large.

The biggest benefit is Larry's original ideas - E/G is castable and it's a relatively cheap means to massively dampen vibration by adding easily formed mass where you want it.

Andrew

[Geof]

....The biggest benefit is Larry's original ideas - E/G is castable and it's a relatively cheap means to massively dampen vibration by adding easily formed mass where you want it. Andrew

But if you are building a steel structure which you then want to dampen the easiest and least expensive is the concrete mix with added aluminum powder to prevent shrinkage as described in the Bamberg thesis.

And if you want the ultimate vibration damping you use the rubber bags inside the steel members and then filled with the concrete. This casts in place a constrained layer damping system.

[fyffe555]

And if you want the ultimate vibration damping you use the rubber bags inside the steel members and then filled with the concrete. This casts in place a constrained layer damping system.

Now there's a good idea.... I'll have to go find this paper..

[ckelloug]

Fyffe,

Are we sure that the compressive and tensile moduli are the same for E/G. I'm not experienced enough with composites to not want to double check this. At any rate, I think that the accures number for Young's Modulus is probably wrong: For thinking numbers, I'm thinking in terms of the numbers for Zanite available from http://www.zanite.com/Zanite_brochure.pdf. 4.5e6 is the value they give for E at the Zanite site while 4.5 e5 is the number they give over for E at accures. It engineering: What's an order of magnitude right?

[fyffe555]

Fyffe,

Are we sure that the compressive and tensile moduli are the same for E/G.

Heck no! , or rather there's definetly a question of what the numbers actually are. They are an order or more less than steel certainly and, No, comp and tensile are not the same for E/G, but they're not the modulus for the material either. The modulus sounds wrong as I tried to suggest but I've since found reference to the same 4.5e5 so who knows...

I'm not experienced enough with composites to not want to double check this. At any rate, I think that the accures number for Young's Modulus is probably wrong:

Agreed, 4.5e6 is more likely but there's such a wide range possible with the material, just look at the experiments in this thread. A carbon or glass laminate composite which I was once more comfortable with 15 years ago can increase by an order if the laminate is heat cured after layup for example.

If it IS 4.5e6 the same calcs I tried to show still apply with E/G at 1/6.4 that of steel. It still will not make much difference in Larry's examples. Not to say E/G isn't an excellent material in larger cross sections.

I do have a question about the Zanite pdf as it seems to say E/G is 45 times better than steel in suppressing vibration, because E/G has higher 'dynamic stiffness' whatever that's supposed to mean. Someone's playing loose with the dictionary I think, generally stiffer materials transfer vibrations better....

[Geof]

....I do have a question about the Zanite pdf as it seems to say E/G is 45 times better than steel in suppressing vibration, because E/G has higher 'dynamic stiffness' whatever that's supposed to mean. Someone's playing loose with the dictionary I think, generally stiffer materials transfer vibrations better....

I think Zanite plays loose with a lot of things;I think they are the ones giving numbers like +/-0.0001" precision off the mold in a page I found on their site.

I think the claim for 45 times better damping is more or less believable but I think the dynamic stiffness, whatever that means, has nothing to do with it. I think the E/G, because it is composed of two materials with radically different individual moduli, simply acts as a constrained layer system.

[brunog]

Fyffe,

Are we sure that the compressive and tensile moduli are the same for E/G. ?

Based Report #6361 from the NIST the specs (page 3)are the following:

Density: 1950-2400 kg/m3

Linear shrinkage: .003-.05%

Compresive strength: 50-150 Mpa

Flexural Strength: 15-50 MPa

Tensile Strength: 8-25 Mpa

Modulus of elasticity in compression: 20-40GPa

Modulus of elasticity in tension: 12-15 GPa

Brinell hardness: 240-400 MPa

Grindability: .10-.30 cm

Poisson's ratio: .30

Linear thermal expansion 10-35 (1/K)*10^6

Adhesion to steel: 5-14 MPa

Adhesion to concrete: 4-6 MPa

Water absorption: .02-1 %

Best regards

Bruno