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

[StathPol]

From the high speed videos, it is clearly visible that the bullet vibrates a lot. Maybe is it beause we use a threaded steel rod instead of welding the bullet on it? Is there any way to attach videos here ?

[johnsold]

Can you mount accelerometers on the concrete or on the supports which hold the concrete and record the data simultaneously with the signal from the bullet?

Is the bullet suspended or falling freely or connected to something at the time of impact?

Lynn

[StathPol]

I have mounted accelerometers on the concrete (please see below). The accelerometers are recorded simultaneously as they were implemented in the same task.

The bullet is falling in the vertical direction. The bullet consists of three parts: the front bullet, a steel rod that is used to add incremental weights and another steel part at the top (magnet). Then the bullet is released and hits the concrete.

 When the beam experiences downward movement,  accelerometers read positive.

[johnsold]

Thanks.

Here is my wild guess.

The relatively smooth part of the bullet acceleration - the first ~300 us - is when the bullet hits the concrete and compresses. The later parts with much higher frequency components occur after the bullet has bounced back and is no longer touching the concrete. The video shows the bounce although the timing is not easy to determine from what you posted.

The smooth ~20 g concrete accelerations around the 2 ms point occur during the time the bullet is in contact with the concrete. At around 2.5 ms the bullet and concrete separate and the crack forms.  The much higher accelerations are the result of crack propagation and the release of the energy of the bent concrete.

The high frequency components in the bullet acceleration may be due to resonances in the steel rods.  Or it may be that contact with the concrete is sufficiently damping to mask the higher frequency vibrations during the short contact duration.

I suspect that waht you are seeing is what the accelerometers are actually experiencing within the limitations of the sensors' frequency responses and the DAQ system.

Lynn

[StathPol]

Thanks Lynn for your wild guess.

What I cannot explain is the bullet acceleration. Note that the high speed camera gives the same predictions with the accelerometer mounted on the bullet.

When the bullet hits the concrete, it is expected to form a gradual increase in the amplitude and then form lower oscillations. From the camera, I suspect that the bullet might be the problem, due to the resonances in the steel rods as Lynn suggested. If this is the reason, I need to fabricate another bullet or weld the steel rod with the front bullet?

[johnsold]

That is way outside my area of expertise. I have worked with people doing accelerometer-based measurements but I was mostly the electronics and data acquisition guy.  Metals are highly elastic and will oscillate enthusiastically if given the proper excitation. Concrete has good compressive strngth and weak tensile strength as well as being a composite of several materials with different sizes, making its behavior highly non-linear and quite complicated.  Unless you are or have access to a highly skilled mechanical engineer/strength of materials expert, it may be difficult to develop an accurate model for the detailed behavior at impact.

One thing that occurs to me: Try wrapping some of that rubber you showed in one image around the steel rod. Secure it as tightly as possible with nylon cable ties or duct tape. My thought is to see if you can damp the oscillations in the rod enough to detect a difference. That might help you decide if changes in the suspension might be helpful without having to make a new device.

Lynn

[Ben]

I do work wiht a Mech-E that thinks that Lynn is onto it. He sisupected the assembly to which the bullet is fastened is compressing prior to the crack and the bullet springs back and does oscilate.

Ben

[StathPol]

Guys, thank you . I am grateful to you.

I just got a steel cylinder and welded an L-bracket to fit the accelerometer and do a simple test. I will post the results soon. Lets see what we will get.