Impact force measured using an accelerometer mounted on the bullet - urgent please

[StathPol]

The results with the cylinder look to be reasonable. The bullet seems to be the problem. Below, you will find attached on how the bullet arrangement is. I was thinking of welding the bullet or increasing the diameter of the steel rod.

[StathPol]

GUys,

I have done a test without the steel rod, just the hemispherical bullet to see the difference. For this purpose, the distance should be increased to cause damage in concrete. Here are the results. This is how the impact force should look like. You think the steel rod is the problem right?

[Ben]

I would believe that. Cany ou "hear" the ringing?

Ben

[StathPol]

I cannot 'hear' the ringing. the event is too fast. if i understood your question 😕

[johnsold]

It certainly looks like the rod is ringing. Try to calculate the resonant frequency of the rod and then run a spectral analysis of the signals you get when the rod is attached to the bullet. Is ther a reasonable correlations?

Lynn

[StathPol]

Lynn

Fundamental frequency of the 0.650 m long steel rod

Steel bulk modulus = 163 GPa = 163 x 109 N/m2   (1 pa = 1 N/m2)

Density of mild steel = 7850 kg/m3

Speed of sound, velocity of plane longitudinal wave in mild steel.

Fundamental wave frequency of the rod, f = v/ 2L

f = [4556 m/s] / [2 x 0.650m] = 3504.6 Hz.

Below is the FFT of the accelerometer on the bullet, for three identical tests, it shows a frequency of around 3.5 kHz, but is this for the ringing? does the first peak corresponds to the impact?

[johnsold]

It is very clear from that calculation and measurement that the steel rod (or something) is vibrating at its fundamental frequency. The correlation between the calculated frequency and the FFT is much closer than I expected. Most of the high frequency components in the smoothed data shown in your earlier images appear to be about 3.5 kHz. However, the response of the bullet only graph shows a similar frequency.

I am not sure how to calculate the resonance of the bullet but I wonder if it might be in the same range?

Try tapping the bullet with a small hammer while it is resting on that rubber pad. Try it with and without the steel rod attached. Look at the accelerometer outputs and FFTs in both cases.  The accelerations will be much smaller but the frequencies related to the bullet and rod will not change much.

Lynn

[StathPol]

Lynn

Actually, there is a mistake in the calculations above. I used the bulk modulus instead of the young modulus. The speed of sound in a solid depends on the young modulus. The bulk modulus is for fluids.

Fundamental wave frequency of the rod = 3978 Hz, not 3504.6 Hz.

I will do the test tmr.

[StathPol]

Lynn,

Please see image below. i used an impact hammer to apply an impulse to the bullet with and without the steel rod. The acceleration in the first case shows a first negative peak because i hit the bullet in the opposite direction so it exhibited upward movement which is negative.

The frequencies do not seem to change much with and without the steel rod.

[billko]

I just want to comment that this is all FASCINATING stuff.  I hope you don't mind me being a passive oberver.  🙂