Shock Response Analysis and Shock Response Synthesis

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0.jpgShock control testing is performed to assess whether an object can survive shock loads from an explosive event or a collision (such as in manufacture, shipping, use, or miss-use).

 

There are two main types of shock control testing:

  • Classical Shock: the shock impact is a determined reference time signal such as a half sine, triangular, trapezoidal, etc. pulse.
  • Shock Response Analysis (SRA) with Shock Response Synthesis (SRS): for this type of test, a vibration level is targeted using a Shock Response Spectrum (SRS). This spectrum is calculated from a measured time history (Shock Response Analysis). Then, from the spectrum, a new time waveform is calculated that fits the shaker limitations (Shock Response Synthesis). This new profile is then sent to the shaker.

Note: This article covers the SRA and SRS, not classical shock.

 

Terminology

The following terms are used in the article:

  • Shock Response Spectrum (SRS): a frequency-based function used to indicate the magnitude of vibration due to a shock or transient event.
  • Shock Response Analysis (SRA): the procedure to determine the Shock Response Spectrum from a time trace (such as an impact or explosive event). Shock Response Analysis moves from the time domain to the frequency domain.
  • Shock Response Synthesis (SRS): the procedure to determine a time signal that corresponds to the Shock Response Spectrum. The analysis moves from the frequency domain to the time domain. There are an infinite number of time signals that have the same Shock Response Spectrum. The analysis finds a solution that fits within the limits of the shaker profile.

 

NOTE: It can get confusing! Shock Response Spectrum and Shock Response Synthesis use the same acronym – SRS. Readers of any material should be discerning as to which “SRS” they are reading about. This article attempts to be as clear as possible.

 

Index:

This article has the following sections:

1. Steps to Perform Shock Response Analysis (SRA) and Shock Response Synthesis (SRS)

   1.1 Measure Shock Response

   1.2 Shock Response Analysis

   1.3 Shock Response Synthesis

   1.4 Iterate

2. Shock Response Synthesis in Simcenter Testlab

3. Filtering Options for Shock Response Synthesis in Simcenter Testlab

 

1. Steps to Perform Shock Response Analysis (SRA) and Shock Response Synthesis (SRS)

 

To perform a shock test using the Shock Response Synthesis method, the following steps must be undertaken (Figure 1):

1.pngFigure 1: Defining a reference shock profile for the shock test.

Each step is explained in detail below.

 

1.1 Measure Shock Profile

 

Measure the input into the component that is experiencing a transient event.

 

This is typically done with an accelerometer. For example, accelerometer data from a drop test, driving over a pothole, or from an explosive event might be measured (Figure 2).

 

2.pngFigure 2: An example of a measured time trace from a drop test.

 

It is possible to perform this measurement in applications like Simcenter Signature Testing or Simcenter Spectral Testing in conjunction with Simcenter SCADAS hardware.

 

1.2 Shock Response Analysis

 

Shock Response Analysis (SRA): From the accelerometer data, calculate the Shock Response Spectrum.

 

Shock Response Analysis will derive a Shock Response Spectrum for a time trace. The Shock Response Spectrum quantifies the damage potential of the shock. In this step, the time data is transformed to the frequency domain.

 

The Shock Response Spectrum for the acceleration data in Figure 3 is plotted below.

 

3.pngFigure 3: An example shock response spectrum. This was calculated using the accelerometer time data from Figure 2.

It is possible to combine the shock response spectrum of several environments to get an enveloped damage potential of different shocks. For example, if driving a car on three surfaces (smooth pavement, belgian blocks, and pothole road), the three environments could be combined to create a Shock Response Spectrum that will envelope the damage from the three different environments (Figure 4).

 

4.pngFigure 4: The shock response spectrum can envelope damage from multiple damaging events.

For more details on averaging in frequency domain, check out this article.

 

It is possible to calculate an Shock Response Spectrum in the Navigator workbook of Testlab (Figure 5):

  1. Select a time trace in a plot
  2. Press the SRS calculation button
  3. Press “OK” on the SRS calculation window.

Refer to the Shock Response Spectrum article for information on how to choose the parameters.  

 

5.pngFigure 5: Calculate a Shock Response Spectrum in the Navigator workbook.

The Shock Response Spectrum will automatically be saved to the project.

 

1.3 Shock Response Synthesis

 

Shock Response Synthesis (SRS): The Shock Response Spectrum is then used to synthesize a time signal based on the target Shock Response Spectrum. The calculation is going from the frequency to the time domain (Figure 6).

 6.pngFigure 6: There is only one Shock Response Spectrum for each time signal. However, an infinite number of time signals can be determined from a Shock Response Spectrum.

 

The synthesis problem is an under-determined problem, in that an infinite set of time waveforms can be found that have nearly the same Shock Response Spectrum. This property is what makes the SRS so useful.

 

If a measured shock pulse cannot be directly applied due to shaker limitations, the Shock Response Synthesis allows to find an alternative pulse with the same damage potential.

 

1.4 Iterate

 

Iterate to ensure the shock pulse fits within the limits of the shaker. To arrive at a usable time pulse, some constraints are applied. For example, the resulting pulse must meet the shaker limitations for acceleration, velocity, and displacement of the shaker head. It is also desired to have velocity and displacement equal to zero at the start and end of the time pulse. This is possible with DC removal or with high-pass filtering of the time-pulse event. It is possible in Simcenter Testlab Shock Control to iterate between several time pulses until a desirable one is determined in the software.

 

A detailed section on how to perform Shock Response Synthesis in Testlab is included in the next section.

 

2. Shock response synthesis in Simcenter Testlab:

 

To perform Shock Response Synthesis in Simcenter Testlab, open the Simcenter Testlab Shock Control Workbook (Figure 7):

  1. Go to the “Shock Setup” workbook.
  2. Ensure the Reference Pulse drop-down is set to “SRS” .
  3. Then, enter the “Profile Editor”.

 

7.pngFigure 7: Access the SRS Profile Editor from the Shock Setup workbook of the Shock Control workbook.

In the Profile Editor window, it is possible to manually type in the frequency and amplitude points for the Shock Response Spectrum. However, it is more common to import an existing Shock Response Spectrum.

 

To compute a Shock Response Spectrum, reference Figure 5, above.

 

To import an existing SRS (Figure 8):

  1. Press “Select SRS Block…”
  2. Browse to the block on your PC
  3. Press "OK"

 

8.pngFigure 8: Import the SRS block using the “Select SRS Block” option.

The SRS frequency and amplitude will import into the table (Figure 9): 

  1. A preview of the spectrum will appear in the lower left of the window.
  2. Ensure that the min and max frequency on the left side of the window match that of the imported spectrum.
  3. Move to the “Time Synthesis” tab.

9.pngFigure 9: The Shock Response Spectrum information will populate and a preview will appear in the lower left window.

Next, a time waveform created by component waveforms will be synthesized. These component waveforms are characterized by narrowband frequency content in the frequency domain. The three component waveforms available in Testlab are wavelet, damped sine, and chirp.

 

In the Time Synthesis tab (Figure 10): 

  1. Press the Default button to generate the first time-pulse synthesis. This will serve as the starting point for the iterative process. The initial (default) amplitudes are set equal to the specified shock response spectrum.
  2. The three colored boxes indicate whether the time pulse is within the shaker limits. The red box indicates the pulse exceeds the acceleration limits.

 

10.pngFigure 10: The results from the default calculation.

The graph on the lower left shows the acceleration profile vs the target. The center graph shows the SRS error. Clearly, the profile is off target, so it is necessary to iterate. 

 

Typically, only one or two iterations are necessary to bring the profile very close to the target and to bring all levels within the capacity of the shaker.

 

To bring the levels within target (Figure 11):

  1. Check if the target acceleration profile is close to the target.
  2. If not close to the target, press the “Iterate” button a few times. 

 

11.pngFigure 11: Results after iterating. Note that the target Shock Response Spectrum is now within the testing limits and the error is close to zero dB.

Now notice that the profile is very near the target, the error is close to zero, and the profile fits within the shaker limitations, causing all boxes to turn green.

 

Press “Done” to accept this profile for the test. 

 

3. Filtering Options for Shock Response Synthesis in Simcenter Testlab

 

When performing Shock Response Synthesis, depending on the input settings, it is possible that the resulting generated time waveform will have discontinuities (Figure 12, right).

 

12.pngFigure 12: It is possible to get discontinuities in the simulated time pulse.

The time histories shown are estimates.  If desired, it is possible to use a filtered integration when executing the synthesis which might reduce the discontinuity shown.

 

To turn on this option:

  1. Go to C:\Program Files (x86)\Simcenter\Testlab 18\central\Configuration (Figure 13)
  2. Open the TestLabEnvironmental.ini file
  3. Scroll to the bottom of the file and uncomment the [SRSsynthesis] field (delete the pound sign)

13.pngFigure 13: Remove the comment tag in the TestLabEnvironmental.ini file.

This enables band pass filtering when integrating the synthesized pulse to better estimate the velocity and displacement and therefore effecting the quality of the estimated time histories. Save the project file and then re-open the Shock Control software and re-perform the Shock Response Synthesis.

 

The discontinuity can be reduced (Figure 14, right).

 

14.pngFigure 14: By turning on the filtered integration, the discontinuity is removed.

Depending on the time history, this setting may be changed to see what is appropriate for your application.

 

Questions? Email jacklyn.kinsler@siemens.com or contact Siemens PLM GTAC support.

 

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