Acceleration signal initial peak under impact load - LabVIEW

[StathPol]

Hi all,

A hemispherical bullet weight of 10.6 kg hits a concrete sample dropped from distance of 200 mm.

The recorded acceleration using labview is shown below. Does anybody knows why I am getting this initial peak (3 to 5.7 ms) at the start since the acceleration should go down during the impact?

Thanks all.

[Bob_Schor]

There are several possibilities.  One is a Code/LabVIEW problem -- without seeing the code, it is almost impossible to say much more.  Another has to do with the (possibly interesting non-linear) properties of concrete -- maybe this is actually real!

When faced with a puzzle such as this, my first attempt at getting a better understanding (especially if I've shown the code to colleagues and we've all agreed that the code seems to be correct, and have reason to believe that the measuring instrument is correct) is to do a "proof of concept" experiment.  I would replace the concrete slab with a piece of metal (maybe a metal ruler), mount the accelerometer to the metal, and drop a penny from 10 cm onto it.  Now you know it has to go down, so if you see it go up first, you know you have your acceleration "sign" wrong, for instance.

Bob Schor

[StathPol]

Bob - You are absolutely right in terms of the material properties. It has an interesting non linear strain hardening and strain softening behaviour. It is 10 times stronger than normal concrete as I use fibres..woohoo 🙂

I did a test as u suggested. (By the way, the accelerometer reads -1g under free fall. Another one accelerometer is mounted on the impactor). Some people say 1g.

I also measure the impact force as well. An accelerometer is attached on the free fall bullet. This is on concrete slab.

[Bob_Schor · Accepted solution]

The Coin test shows an Acceleration at rest of about 0, going to +20 when you drop the coin.  I'm assuming that the -1g of gravity has been zeroed out, and that when the beam starts to move down, this is recorded as a positive acceleration, right?  [Getting the signs right in something like this can be a real challenge!].

I'm glad my guess that "the data are correct" was right.

Bob Schor 

[StathPol]

Again, you are right. The -1g of gravitational acceleration has been zeroed out.

What I understand is that when the beam moves down, the accelerometer is compressed and therefore it reads positive acceleration. However, the accelerometer – time in the first post seems a bit weird. Need to put the high speed camera to check. I assume that the accelerometer is compressed and then expands as it reads negative?

[StathPol]

Bob - just to let you know.

I used a high speed camera and indeed, there is uplift force upwards which tends to be negative. The vertical acceleration indicates positive and it is less than the rebound force as the concrete is very strong with high elastic modulus.

Thank you for your responses.