Given a Shock Test? Here's How to Know if Your Vibration System Can Do It.

Shock testing is a critical aspect of product validation, simulating the sudden and intense accelerations and decelerations that products might experience during handling, transportation, or operation. Determining whether your existing vibration test system can adequately perform a given shock test is crucial for obtaining reliable results.

At Dongguan Precision Test Equipment Co., Ltd., we understand the importance of matching your testing needs with the capabilities of your equipment. This guide will walk you through the key parameters to consider when evaluating if your vibration test system can meet specific shock test requirements.

1. Understanding Shock Pulse Types:

According to common testing standards, shock pulses are generally categorized into three primary waveforms:

• Half-Sine Pulse: This waveform is well-suited for simulating the shock effects caused by impacts in linear systems or the deceleration of linear systems, such as the collision of elastic structures. It's the most commonly used waveform, particularly for component-level testing.
• Trapezoidal Pulse: The trapezoidal pulse generates a higher response across a broader frequency spectrum compared to the half-sine pulse. It's often employed to simulate the effects of shock environments caused by events like explosive bolt firing during the launch phase of Space detector or satellites.
• Terminal-Peak Sawtooth Pulse (A type of trapezoidal pulse with a rapid decay): Compared to the trapezoidal pulse, the terminal-peak sawtooth pulse offers a more uniform response spectrum in certain applications.
• 

Note: While the standards outline these three waveforms, half-sine pulses are the most prevalent, with trapezoidal and sawtooth pulses being less frequently used for component-type samples.

2. Defining the Severity Level of Mechanical Shock:

The severity level of a mechanical shock test is defined by three key parameters:

• (1) Pulse Waveform Type: As described above (Half-Sine, Trapezoidal, Terminal-Peak Sawtooth).
• (2) Peak Acceleration: The maximum instantaneous acceleration reached during the shock pulse, typically expressed in 'g' (acceleration due to gravity).
• (3) Nominal Pulse Duration: The approximate time the shock pulse lasts, usually measured in milliseconds (ms).

Testing standards often provide quick reference tables correlating these parameters for various applications and severity levels, allowing for a preliminary assessment of test requirements.

3. Key Control Parameters of Electrodynamic Vibration Test Systems (Typical Values):

The ability of your vibration test system to perform a specific shock test is limited by its inherent performance specifications. Common control parameters to consider include (refer to your specific equipment's datasheet for accurate values):

• (1) Maximum Displacement (Peak-to-Peak): Typically ranges from 25 mm to 100 mm (or more) depending on the model. This limits the low-frequency, high-amplitude capability for longer duration shocks.
• (2) Maximum Shock Force: Often related to the sine force rating of the shaker. A common rule of thumb is that the maximum shock force (for durations less than 6 ms) can be up to twice the sine force rating. For longer durations (e.g., around 11 ms), the maximum shock force might be closer to the sine force rating. Always consult your system's specifications.
• (3) Maximum Velocity: The maximum speed the vibration table can achieve, typically around 2 m/s, with some advanced systems reaching 2.5 m/s or higher. This parameter is critical for achieving the required velocity change during the shock pulse.
• (4) Maximum Acceleration: The highest acceleration the system can generate, often around 100g, but can vary. This directly limits the peak acceleration achievable in the shock test.
• (5) Supported Shock Waveform Types: Most modern electrodynamic vibration test systems are capable of reproducing the three common shock pulse types (Half-Sine, Trapezoidal, Terminal-Peak Sawtooth) through appropriate controller programming.

4. Evaluating Your Vibration Test System Against Given Shock Test Conditions:

Typically, a shock test specification will provide the following information:

• Pulse Waveform Type (e.g., Half-Sine)
• Peak Acceleration (e.g., 50g)
• Pulse Duration (e.g., 11 ms)

You can perform a preliminary evaluation of your existing vibration test system's suitability by comparing these required conditions against the system's control parameters:

• Maximum Acceleration: Ensure your system's maximum acceleration rating meets or exceeds the specified peak acceleration. You might need some margin to account for fixture dynamics and potential overshoot.

• Maximum Displacement: For longer duration shocks, the required displacement can be significant. A rough estimate of the required displacement for a half-sine shock can be calculated using the following approximation:

  $Displacemen{t}^{peak}\approx PeakAcceleration\times (PulseDuration/1000{)}^2$

  Where acceleration is in m/s2 and duration is in seconds. Compare this calculated peak displacement with half of your system's maximum peak-to-peak displacement.

• Maximum Velocity: The maximum velocity reached during a shock pulse can be estimated as:

  $Velocit{y}^{peak}\approx PeakAcceleration\times (PulseDuration/1000)\times 2$ /π(for half-sine)

  Ensure this estimated peak velocity is within your system's maximum velocity rating.

• Maximum Shock Force: Estimate the required shock force using Newton's second law (F=MA), where M is the moving mass (specimen + fixture + armature) and A is the peak acceleration. Compare this with your system's maximum shock force capability for the given pulse duration (remember the relationship to the sine force rating).

• Supported Waveform Type: Verify that your vibration controller and system software support the generation of the specified shock pulse waveform.

Example using the quick reference table :

If the standard provides a table correlating peak acceleration and pulse duration for different severity levels, you can directly compare your required values to the system's maximum capabilities. For instance, if the table indicates that a 50g, 11ms half-sine shock is within a certain severity level, you would check if your system can achieve at least 50g peak acceleration and has sufficient displacement and velocity for an 11ms pulse.

Important Considerations:

• Fixture Mass and Dynamics: The mass and resonant frequencies of your test fixture will significantly influence the system's ability to achieve the desired shock profile at the specimen. Heavier fixtures require more force.
• Head Expanders and Slip Tables: Using these accessories can further impact the system's effective performance in shock testing.
• Controller Capabilities: The sophistication of your vibration controller is crucial for accurate shock pulse generation and control.
• System Calibration: Ensure your vibration test system is properly calibrated to guarantee accurate and reliable shock test results.

Conclusion:

Evaluating whether your vibration test system can meet given shock test conditions requires a careful comparison of the required peak acceleration, pulse duration, and waveform type against your system's maximum acceleration, displacement, velocity, shock force, and controller capabilities. While quick reference tables in standards can provide a preliminary assessment, a more thorough evaluation involving calculations and consideration of fixture dynamics is recommended.

At Dongguan Precision Test Equipment Co., Ltd., our team of experts can assist you in determining the suitability of your existing vibration test system for specific shock test requirements or help you select a new system tailored to your needs. Contact us today for a comprehensive evaluation and guidance on achieving accurate and reliable shock testing for your products.