Pendulum Hammer Impact Force Calculation

[JBA]

If your review the above referenced document http://www.sandv.com/downloads/1611alex.pdf starting on page 10 you will see the the response wave(s) form are sinusoidal. This same result is also repeated in other papers I reviewed on testing of this machine. It is used for shock testing for shipboard mounted equipment in accordance with MIL-S-901D.

For more specifics go to:
https://en.wikipedia.org/wiki/MIL-S-901

 

[JBA]

Additionally, you learn more than you probably want to know by reviewing a copy of the 80 page reference Mil spec by going to: https://cp-techusa.com/PDF/MIL-S-901D.pdf

 

[hutchphd]

How low is “low enough” ?
How sharp is “any sharp step” ?
How do you define “effectively instantaneous” ?

I was trying to understand your point.
If you are interested in the high frequency components of the transfer function then the exact shape of the velocity step is important.

 

[Dr.D]

I thought we put this one to bed over at the ME Forum. Is there something new required?

 

[JBA]

This is the ME (Mechanical Engineering) Forum and hopefully about all that can be covered given the available information now has been.

 

[Baluncore]

I was trying to understand your point.
If you are interested in the high frequency components of the transfer function then the exact shape of the velocity step is important.

That is a false assumption.

If the hammer and anvil is being used for destructive failure testing, then the step profile is a standard defined by the construction of the machine, as specified in the standard. The exact shape is irrelevant as it is standard.

The acceleration can be read from accelerometers mounted on the anvil. Indeed, I would instrument both the anvil and the device being tested to get acceleration and compute displacement. I would then compute the FFT of both those synchronous records, and divide the device spectrum by the anvil spectrum to get the transfer function of the system being tested.
That eliminates any assumptions about the exact shape and spectrum of the stimulus from the analysis.

 

[hutchphd]

If the hammer and anvil is being used for destructive failure testing, then the step profile is a standard defined by the construction of the machine, as specified in the standard. The exact shape is irrelevant as it is standard.

The acceleration can be read from accelerometers mounted on the anvil. Indeed, I would instrument both the anvil and the device being tested to get acceleration and compute displacement. I would then compute the FFT of both those synchronous records, and divide the device spectrum by the anvil spectrum to get the transfer function of the system being tested.
That eliminates any assumptions about the exact shape and spectrum of the stimulus from the analysis.

All correct and on point. But I disagree with

That is a false assumption.

...
That eliminates any assumptions about the exact shape and spectrum of the stimulus from the analysis.

Practically, if the step function is not sharp, the high frequency components of the anvil spectrum will be effectively zero (smaller than the noise) and it does not test the high frequency response. Further, when you deconvolute by dividing the FFT you will get junk for the high frequency parts of the transfer function because you are dividing by ~(zero +noise). This is a fact, not an assumption.

 

[Baluncore]

Practically, if the step function is not sharp, the high frequency components of the anvil spectrum will be effectively zero (smaller than the noise) and it does not test the high frequency response.
Further, when you deconvolute by dividing the FFT you will get junk for the high frequency parts of the transfer function because you are dividing by ~(zero +noise). This is a fact, not an assumption.

There is a fine line between presenting a ridiculous argument, and appearing ridiculous. Your reductio ad absurdum argument is clearly ridiculous. If the required high frequency components were not present, then the mechanism could not be used for testing.

Since the mechanism is used for testing, the required HF components must be present, and so they cannot be completely obliterated by noise when computing the transfer function.

 

[hutchphd]

That step is being used to test the high frequency components of the transfer function

What makes you think this? I believe they may be more interested in the low frequencies where indeed the step shape is less relevant. But your circular argument relies upon this "fact":

Since the mechanism is used for testing, the required HF components must be present, and so they cannot be completely obliterated by noise when computing the transfer function

I do not find this in the test protocol.

 

[JBA]

In the referenced MIL spec there no criteria as to the magnitude or frequency etc. of the applied shock or any other elements related to the applied shock profiles or levels. All references to the test results criteria are based strictly upon the condition of the item being tested after testing.
The only statements related to the actual performance of the test is that "the machine approved the Class of Testing required shall be used", "the hammer height used shall be determined by the machine indicator marking", "the minimum travel height for the anvil table for that class shall be as per table no. ...", the maximum allowed equipment weight for each of the three classes of testing i.e. Light, Medium or Heavy and the mounting of the test equipment, etc.

For more details on all of these issues, see the referenced MIL spec.

 

[hutchphd]

In the referenced MIL spec there no criteria as to the magnitude or frequency etc. of the applied shock or any other elements related to the applied shock profiles or levels. All references to the test results criteria are based strictly upon the condition of the item being tested after testing.
The only statements related to the actual performance of the test is that "the machine approved the Class of Testing required shall be used", "the hammer height used shall be determined by the machine indicator marking", "the minimum travel height for the anvil table for that class shall be as per table no. ...", the maximum allowed equipment weight for each of the three classes of testing i.e. Light, Medium or Heavy and the mounting of the test equipment, etc.

For more details on all of these issues, see the referenced MIL spec.

Very good. I'm sure the test does what they want. I just wanted to be sure our analysis was coherent.