Damping Test of Concrete & Steel Tube

I posted all this data in another thread on my 'Fast Cartisian Printer Build' but summerised it in one posts for another forum.

I thought I would post it here as a single thread so it can be read and searched more easily.

I was discussing a build that will use a steel tube frame and I said I was planning to fill it with concrete / SBR mixture. SBR is basically a liquid latex that adds flexibility and waterproofing to concrete. Styrene Butadiene Rubber.


Another user mentioned that he did some calculations and thought that: A) The steel would carry nearly all the stress/strain and so the concrete core would not offer much lossy damping, only increase mass. B) Therefore, might as well buy thickest steel tube and not bother filling.

As I have the acoustics tools to measure damping I thought it would be an interesting experiment.

I took two 90cm lengths of 40x40 steel tube. (I didn't have 1m scraps). One had a 2mm wall, the other had a 3mm wall. I hung them so they were free to resonate and attached an exciter transducer and an accelerometer.

Attachment 445140

This is the result for the hollow tube, before filling.

Thin Tube Hollow.
Attachment 445142

Thick Tube Hollow.
Attachment 445144

We can see the first strong resonance frequency in each tube is pretty much the same frequency 138Hz (thin) vs. 128Hz (thicker). However we can see a significant difference in amplitude. The thin walled tube has higher amplitude and the width 'Q' of the resonance is more narrow. The thicker walled tube resonance is -5dB in comparison, which is a lot. 3dB is a halving of amplitude. It has a broader 'Q' which usually indicates higher damping factor, but we can look at that more closely in a moment.

Below we look at the same data, but in a different way, this a waterfall plot and shows the decay of energy over time.

Thin Tube Hollow.
Attachment 445146

Thick Tube Hollow.
Attachment 445148

Now we start to see the difference between 'lossy damping' and simply increasing mass. Yes, in terms of amplitude adding mass damps the resonances. However when we look at energy decay in the system we can see that the thicker higher mass tube actually has slower energy decay.

This is not surprising really. A higher mass object will not move as far as a lighter one given the same energy input, but it has greater inertia so it will keep moving for longer once in motion.

We do see a cleaner decay in the range below the big resonance on the thicker tube. I think this is probably a reflection of simple increased rigidity but I'm not entirely sure.

Okay lets fill the beams! I used a pre-mix concrete bag, but instead of water I used only SBR mixture.

I gave them about 10 days to cure during the hot weather at the time.

Thin Tube Filled.
Attachment 445150

Thick Tube Filled.
Attachment 445152

Thin Tube Filled.
Attachment 445154

Thick Tube Filled.
Attachment 443944
https://www.cnczone.com/forums/attac...936909&thumb=1

Wow there is a difference there! I'm surpised just going 2mm to 3mm wall thickness did that.

The filling gave a significant increase in mass and has reduced the initial amplitude of the resonances in both tubes by over -10dB (that's HUGE!). They are now equal in terms of initial amplitude, probably because the filling equalises the difference in mass.

Looking at the decay of energy we see now the thin walled tube decays significantly faster than the thicker filled tube. Both are improved over the decay rate of the hollow tubes.

So in conclusion, Does the SBR concrete mixture contribute lossy damping or only mass? It clearly is able to contribute lossy damping. Great!

Looking at the results we can also conclude that the thinner walled tube is passing more of the stress/strain to the concrete core and this benefits from greater lossy damping. There is of course a catch, that you still want a steel tube with enough outright stiffness to be suitable for your machine design, concrete is good in compression but weak in tension so you can not rely only on the concrete to give the structure strength.

In my own design I was wondering if I should go for 2mm walled tube and fill, or go for 5mm tube with/without filling. The choice for me is clearly in favour of the thin tube filled, as that still offers easily enough stiffness for my planned design with improved lossy damping.

At frequencies below the resonances, the stiffness and mass still dominate over lossy damping. So if you know a machine will not excite those higher frequencies, stiffness is all that needs to be considered. However, also remeber that longer beams and larger structures will have lower frequency resonances than these ~1 meter samples.

I'd very much like to try a thin walled tube with concrete filling under compressive pre-load. In buildings they sometimes cure concrete around steel bars stretched under tons of load. When the concrete is set, the steel is released from it's load and it contracts on the concrete putting it under a massive compressive pre-load. I imagine this would get the best of the visco-elastic damping at higher frequencies, while also gaining great stiffness and at the lowest frequencies. Trouble is, that isn't so easy in a complex shape.

https://www.designingbuildings.co.uk...edconcrete.jpg

Great findings. Thanks for sharing!

Thanks Bouyen - well done - Peter

Thanks for positing the test results.

Hi Sash - To clarify my thoughts, if we look at the thin tube as an example. The unfilled tube resonance was at about 350htz (eyeballed from image) and took 2400ms to stop vibrating. The filled tube resonance was about the same 350htz yet took 600ms to stop. So the filled tube is damper. Can you calculate the decrement of the vibration? so it can be compared to published other material dampness?

To extend this a little can you calculate the modulus of a material using your rig? I make composite bits and its hard to get their modulus....See attached...Peter

Did a quick modal on a hollow steel 900mm long 40x40x2 and its first mode is 331htz so all's good on that...

Pete, I'm not sure if the loss factor or modulus would be any help because my test set-up is not industry standard.

I did try to take a measurement with one end of the beam clamped to my drill press and the other end free. I saw much the same modes but they were all more suppressed, I assume by the clamping interface. This is probably why they use quite thin samples in the industry standard procedure.

Hey ssashton,

Nice work!

One thing I'm curious about is the effect of screwing something to help with dampening on the outside of a tube or plate. For example, If using a box type Z axis, the inside of that structure is where the spindle goes, you can't really fill that.

Also, have you taken any measurements from either 3d printers or mills in operation to see what frequencies they tend to generate?

Nic,

You wouldn't get much low frequency damping effect with a simple damping layer on the outside of a plate. It would need another stiff shell to constrain it.

I haven't taken measurements of my machine in action.Any kind of impulse like a cutter hitting material or a sudden direction change will generate broad frequency components.

Lossy damping is not going to be a substitute for a rigid, high mass and well braced machine design, but if you can add increased loss just by including some SBR admixture or choosing synthetic cement rather than standard or something, then that's worth knowing I think.

The point is, adding damping is not pointless and if you want to add more mass to a machine it looks to me like using thin steel and filling with something like synthetic cement will have advantages over just using thicker tubing.

If anyone wants to play with the data, I uploaded the files to a google drive folder. There is an install file for the (free) software to view and analyse the files. The only limitation of the free version is that you can't save.

Once you open the file it will directly display the impulse response. You can zoom vertical and horizontal axis and scroll with the buttons at the side.

Attachment 445358

https://drive.google.com/drive/folde...j9?usp=sharing

Awesome work, thanks for sharing.

What ratio you use Sbr with concrete for filling tubes