Damping Test of Concrete & Steel Tube

[Tenson]

Hi Everyone!

Over at CNCZone 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.



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

Thin Tube Hollow.


Thick Tube Hollow.


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.


Thick Tube Hollow.


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.


Thick Tube Filled.


Thin Tube Filled.


Thick Tube Filled.


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.