To verify the durability of a product or structure, a mechanical shock test is often performed.
An example of a shock time history that a product or structure might be subjected to is shown in Figure 1.
Figure 1: An example of a measured shock pulse.
Sometimes standard test profiles called ‘Classical Shock Pulses’ are used to recreate a mechanical shock environment in a lab. Examples of these classical shock pulses are shown in Figure 2.
Figure 2: Typical classical shock pulse shapes include half-sine, triangle, rectangle, terminal peak, and initial peak.
Classical shock pulses have pre-defined shapes, amplitudes, directions, and durations. They are defined in standard like MIL-STD 810H and often referenced in vibration specifications for specific products.
This article explains classical shock pulses and some of the challenges and considerations of using them on a vibration control shaker.
It has the following sections: 1. Why do we use classical pulses? 2. What is the resulting frequency content from the different shapes? 3. How does pulse duration influence frequency content? 4. What is the purpose of pre-post pulses in mechanical shock testing? 5. How to define and use a pulse in Simcenter Testlab?
1. Why do we use classical pulses?
Typically, ‘Time Waveform Replication’ is considered a superior technique for laboratory shock testing. In a time waveform test, a shock event can be fully reproduced on test structure. This can be done at numerous locations and even along multiple axes as shown in Figure 3.
Figure 3: Multi-axis time waveform replication test.
However, the reality is that obtaining the needed field measurements to perform a proper time waveform test can be difficult. The specific item must be tested, and replicating the shock event can be expensive, also the test preparation/execution is time intensive.
Therefore, classical pre-defined shock pulses are often employed when suitable field measurement information is unavailable. Standards like MIL-STD 810H define classical pulses to be used in lieu of actual measured data. These pulses are well defined and can be run in a very repeatable and consistent manner.
2. What is the resulting frequency content from the different shapes?
The shape of a classical shock pulse determines the frequency range it excites and the degree that the range is excited. A comparison of three different classical shock pulse shapes are shown in Figure 4.
Figure 4: Comparison of the time waveform (left) and shock response spectrum (right) of initial peak (magenta), half sine (blue), and rectangle (green) classical shock pulses. All pulses are 10 g acceleration and 11 milliseconds.
With the same pulse duration and amplitude, varying the pulse shape results in the following in Figure 4:
Half Sine (blue line): Has higher levels at low frequency than at higher frequencies. This is due to its smooth transitions compared to other pulse shapes.
Initial Peak (magenta line): The sharp transition of the initial peak shock pulse generates higher levels at high frequencies than the half sine shock pulse.
Rectangle (green line): Has the highest amount of overall frequency content because of its multiple and very sharp transitions.
Even though a rectangle pulse excites a broad frequency range, it is often harder to run on shaker equipment. The sharp transitions create high frequency and displacement requirements versus a similar duration half sine or terminal peak pulse.
A half sine pulse shock might be used when the low frequency mechanical modes of a structure for potential failures is important. A terminal peak might be more appropriate when inducing failure modes in components sensitive to high frequencies. For example, electronic components that have delicate solder joints are susceptible failures caused by vibration at high frequencies.
It is also possible that a specification might require more than one type of classical shock to cover the entire frequency range well.
3. How does pulse duration influence frequency content?
The pulse duration also influences the frequency content produced by the shock pulse as shown in Figure 5.
Figure 5: Shock Response Spectrum comparison of 10 millisecond (color), 18 millisecond (color), and 22 millisecond (color) classical shock pulses. Pulses are same shape and amplitude.
Increasing the duration of the main pulse (with the same amplitude and shape) generates higher amplitudes at low frequencies.
4. What is the purpose of pre and post pulses in mechanical shock testing on shakers?
A shaker can be a dependable and effective tool to perform vibration shock testing in a laboratory. Nevertheless, shakers can generate only a fixed amount of velocity and displacement. Pre and post pulses are utilized to achieve the required time histories.
Pre and post pulses are required to:
Ensure that the initial and final conditions are satisfied
Allow the shock pulse to be applied with sufficient accuracy
Accommodate the limitations of the shaker
Impose minimal additional damage
Pre and post pulses are illustrated in Figure 6.
Figure 6: Three instances (acceleration, velocity, displacement) of the same classical half sine shock pulse with pre and post pulses.
The following can be observed from the graph in Figure 6:
The pre and post pulses ensure that the initial and final conditions (in acceleration, velocity, and displacement) are all zero.
The pre pulse is also used to position the shaker head as low as possible and simultaneously make the velocity as negative as possible before the pulse is applied. This positioning ensures the largest velocity change during the test that stays within the shaker limits.
The post pulse is used to return the shaker head to the rest position.
Figure 7 shows the shock response spectrum of the same classical half sine pulse with and without pre and post pulses.
Figure 7: Top - Time histories of a the same half sine pulse without (blue) and with (magenta) pre and post pulses. Bottom - Resulting pseudo velocity shock response spectrums have lower displacement and velocity requirements with pre and post pulses but still achieve the same maximum acceleration.
In the pseudo velocity shock response spectrums, there are three distinct parts (bottom graph of Figure 7):
Left Asymptote: Defines the maximum displacement
Center Plateau: Defines maximum velocity
Right Asymptote: Defines maximum acceleration
By introducing the pre and post pulses, the required displacement and velocity to run the pulse are lowered. However, the same maximum acceleration is achieved in both cases.
The shape of the pre and post pulses also is also useful in assuring the shock can be reproduced on a shaker system. Four different pre and post schemes are shown in Figure 8:
Figure 8: Illustrations of four different pre and post pulse schemes. Blue arrows indicate the number of sides.
Some of the characteristics of each scheme:
Single Sided: Useful when shaker has asymmetrical stroke capability, typically hydraulic shakers.
Double Sided: More effective use of bipolar shaker armature travel capability.
Optimized: Shocks with high acceleration amplitudes on shakers with only limited stroke but take advantage of full stroke. The pre and post pulse are different.
Minimized: Scheme with least total required displacement of shaker armature.
5. How to define and use a pulse in Simcenter Testlab?
Instructions for using classical shock pulses in Simcenter Testlab Vibration Control:
Launch the Simcenter Testlab Shock Control workbook “Start → All Programs → Simcenter Testlab → Shock Control”
Open a new project by clicking on the ‘New Project’ icon as shown in Figure 9.
Figure 9: After starting “Shock Control: click on the white icon in the upper left to start a new project.
To define the shaker used in a test, go in the top menu to ‘Tools → Options as shown in Figure 10.
Figure 10: After starting “Shock Control: click on the white icon in the upper left to start a new project.
Using the shaker tab press the ‘Configure…’ button, this will launch the shaker configuration wizard to define a new shaker as shown in Figure 11.
Figure 11: The shaker configuration wizard can be used to define the specifications of the shaker that will be used for the test.
Next go to the Channel Setup worksheet as shown in Figure 12.
Activate the input channels that will be used for the test.
Assign at least one control channel and set the ChannelGroupID to ‘Control’.
Assign a transducer direction to each transducer/point
Set the Measured Quantity to Acceleration and Electrical Unit to mV
Set the InputMode: ICP for such transducers and Charge for charge ones
Set the correct sensitivity for all sensors.
Figure 12: The channel setup tab can be used to define which channels will be used for the test.
Go to the ‘Shock Setup’ worksheet to define the test setup
Select a classical shock profile as shown in Figure 13.
Figure 13: The Shock Setup is used to define a reference pulse.
Click on the ‘Profile Editor’ button in the Reference Profile panel as shown in Figure 14.
Figure 14: The Profile Editor is used to define the parameters the reference classical pulse.
After opening Profile Editor, the following menu is opened (Figure 15):
Figure 15: The Profile Editor is used to define the Main Pulse, Pre-and Post-pulses and Alarm/Abort parameters.
The Profile Editor has three main areas:
The left side of the Profile Editor window is meant to define the main pulse, amplitude, pulse duration and polarity.
The middle column of the Profile Editor is intended to define the pre- and post-pulses.
The right side of the Profile Editor is used to define the alarm and abort limits.
After defining the reference profile close the profile editor to get feedback on the maximum acceleration, velocity, displacement, and force levels that will occur during the test. Next to each of these values, a status indicator is shown, that compares these values to the shaker specifications. All status indicators should be green before proceeding with the test as shown in Figure 16.
Figure 16: The Profile Editor is also used to get feedback on the maximum acceleration, velocity, displacement, and force levels that will occur during the test.
Next define the additional test settings using the right side of the Shock Setup worksheet as shown in Figure 17.
Test schedule parameters
Figure 17: Shock Control test settings.
Next click on the ‘Self Check’ worksheet
This step applies a low-level broadband excitation signal to the structure, to get a first estimate of the transfer functions. Based on these transfer functions, an extrapolation will be made to the actual test levels. Also, the software will suggest a range on each channel to be used during the actual test as shown in Figure 18.
Figure 18: Self Check is used to estimate the test transfer function.
Once the self check status is verified, click on the ‘Range/Threshold’ button on the right side of the Self Check worksheet. In the Range/Threshold window, you will find the current range on each channel, and the range that is suggested by Simcenter Testbab, based on the self check [Click on ‘Apply suggested] as shown in Figure 19.
Figure 19: Self Check is used to estimate the test transfer function.
After performing a valid self check, click on to the ‘Shock Control’ worksheet
As soon as you enter the worksheet, a new run name (‘Shock_1’ by default) is proposed in the Run name panel. To change the name of the run, delete the current name and enter a new one as shown in Figure 20.
Figure 20: The run name can be changed with the Shock Control worksheet.
Arm the system by clicking the ‘Arm’ button. The Status field changes to ‘Initializing…’: at this point all defined settings are being sent to the frontend. Wait until the Status field changes to ‘Ready’. Now the reference profile and the abort limits appear in the Control display panel as shown in Figure 21.
Figure 21: Shock Control worksheet test status.
Now, the test is ready to be started by clicking the ‘Start’ action button.
During the run, the actions panel provides various buttons to go back and forth between levels, interrupt/continue the test, open and close the control loop and make a manual measurement as shown in Figure 22.
Figure 22: Shock Control worksheet action panels.
During the run, the actions panel provides various buttons to go back and forth between levels, interrupt/continue the test, open and close the control loop and make a manual measurement.