Damping in two bodies - Different shape - Same material 1

[DavidCam]

Hi everyone!

We have experimental data (acceleration – time) in two points of a specimen made of a certain concrete (non-conventional), with a prism shape of 40x10x10 cm. The specimen was vertically oriented, with the lower end fixed and the upper end free. A vertical impact excited the sample. Then, we can obtain de damping factor by means of the acceleration measures.

Now, we are going to make a block, with a prism shape of 50x50x50 cm, made of the same concrete. It is going to be dropped from a height but we can’t take measures during this test.

We are aware that the damping factor is not an intrinsic property of the material. But could we use the same damping factor we got from the previous experimental data to make a finite element model to predict the behavior of the falling block? Could the fact that they are both just a simple body with similar shape justify this assumption? Is there any way to take into account the damping properties from the first test in the last one?

Thanks in advance!

 

[btrueblood]

Yes, you should in theory be able to tune an FEA model to replicate the bulk damping properties of the material, and then use that damping factor to examine other load cases.

Caveat #1 - there is going to be significant damping at what you (and the FEA model) assume is the "fixed" boundary in the first model. In reality, some relative motion across the boundary occurs and contributes frictional damping.

Caveat #2 - I would expect a dropped concrete block to crack/spall at the impact site, which process will create its own damping.

I don't understand why you can't instrument the dropped block - just leave enough length for the accelerometer cables and coil/drape them so they won't get smashed on impact. You might lose an accelerometer or two if the block tumbles and crushes one, but they are fairly cheap these days.

 

[DavidCam]

Thanks for your reply btrueblood.

The point is that the second test (dropped block) consist of a specimen made of the same material but with different shape. The first specimen size was 40x10x10 cm and the second one was 50x50x50 cm. It's not only changing the load case, but also the shape and the size. In that case, is it correct to take the same damping factor for both specimens? What if we have another block of 120x120x120 cm? Could we also use the same damping factor?

 

[btrueblood]

To be perfectly honest, I am not an expert on concrete beyond what I was taugh in school. I understand it is a brittle material, and so (in my admittedly simple view) damping has to occur by non-ductile means, i.e. cracking, and frictional/fluid interaction at the boundaries (ok, there are internal boundaries between the different components, and fluid from the concrete's water of hydration, so maybe there is some internal damping that approximates hystertical damping in metals...but still).

If you can devise your test instrumentation and measurement of damping such that the impact site and any damage/damping occurring there does not influence the damping measured away from that point, then you should be able to back into a bulk material damping constant for the material itself. It would then be reasonable that the measured bulk material damping is similar, independent of the shape/size of the specimen. Those are some big ifs, in my mind. Good luck.

 

[DavidCam]

All right! We'll try to take measures with an accelerometer. Thank you btrueblood, I appreciate your advices!