When do I use a classical shock test?
Similar to sine and random testing, shock tests are often listed in vibration test specifications. An example of a classical shock test definition might appear as 3 – 10mS, 20Gpk, half-sine pulses in all six orthogonal axes for a total of 18 shocks.
Outside test specifications, a shock test may be employed to test a system’s capability to survive a drop, hit, impact, fall, explosion, or any other source of transient vibration that may occur in the real world.
Many vibration test standards define classical shock pulses. However, more advanced shock testing may require a complex transient pulse that cannot be replicated by a classical shock.
What is the difference between a classical and complex shock test?
Classical shock pulses are a simple method of generating a shock pulse. The library of basic pulse shapes derives from archived data, so they have historical precedence. Although simple, classical shock tests generate a reliable and understandable response that can be used for product evaluation and durability testing.
However, a vibration test may require a user-defined transient or shock-response spectrum (SRS) to generate more complex shock pulses than possible with classical pulses. There are specialized software packages designed to re-create these complex pulses, such as Shock Response Spectrum.
Complex shock pulses better represent real-world conditions. Many synthetic pulses can represent a complex transient waveform with a frequency response comparable to the original environment. It is also possible to replicate a recorded signal from the real world and process the signal using an iterative shock control loop in order to effectively generate and control the complex waveform.
Which classical shock pulse should I use?
Each classical shock pulse has a defined purpose. Generally, every unipolar pulse excites a wide range of frequencies in a short period, but in different ways.
If a test standard requires a classical shock test, it will also provide the pulse shape. Otherwise, the engineer should select the pulse shape based on the transient event being replicated. The half-sine pulse has a smooth transition and generates low amplitude, high-frequency energy; the terminal-peak pulse has a smooth ramp and a sharp transition back to zero, which generates notably higher amounts of high-frequency energy; and so on. An understanding of the classical pulse shapes will help to guide the engineer’s decision.
What machinery does a shock testing system require?
A shock test can be performed with a drop shock machine or an electrodynamic shaker. Several factors determine the type of drop shock machine used to generate classical shock pulses. Similarly, a shaker can be a cost- and time-efficient option for a routine shock test but presents a few caveats.
View Vibration Research’s selection of shakers here.
Complex Shock Testing
A vibration test may require a user-defined transient or shock-response spectrum (SRS) for more complex shock pulses. There are software packages available for generating more complex shock testing, including Shock Response Spectrum control – SRS VR9302 and Transient Waveforms control VR9301.
Additionally, there are software packages in VibrationVIEW designed for IEEE-344 standards, chatter monitoring, and other advanced analyzer functions. These transient events are difficult to characterize and analyze with basic tools. Additional analysis tools like waterfall plots, energy spectral density, coherence plots, and transfer functions are commonly used for shock testing depending on the environment and application.
Other Shock Options
Standard Pulse Shapes
Access the most used classical shock pulses in the industry: half-sine, haversine, initial-peak sawtooth, terminal-peak sawtooth, triangle, trapezoid, and square. Fulfill requirements from test standards such as MIL-STD-810, DO-160, ISTA, ASTM, SAE, and IEC 60068. Alternatively, select the user-defined transient option to upload a recorded waveform.
- Test and level scheduling: Repeat a pulse from 1 to more than 2 billion times, with a configurable repetition rate. Tests can be configured to run pulses at different amplitude levels.
- Frequency range: Standard frequency range is DC-4,990Hz. High-Frequency Shock extends sample rates to 108,000Hz (VR9303 High-Frequency).
Vibration System Control
The control signal can be a single input channel or an average of 2 to 4 different input channels.
To protect the test article and shaker system, the user can set configurable acceleration and drive limits. The control input is also verified against shaker force, velocity, and displacement ratings.
The controller automatically equalizes the response of the shaker/fixture/product prior to running the test. This equalization can be memorized and stored with the test to quickly start a test at a full equalized level.
Shock Graph Display Options
Available graph display options include acceleration, velocity, displacement, output voltage, acceleration, and drive spectra. Graphs can be easily auto-scaled, and the cursor display can be adjusted. Data and text annotations can be easily placed on the graphs, and data values update live with changes.
Shock Testing FAQ’s
- Why shouldn’t I use multiple channel control for shock?
- What counts as a pulse?
- When do I need to add pre-/post-pulse compensation?
- What is the difference between using a drop shock machine and an electrodynamic shaker?
- Find all your answers here!
Shock Fundamentals Webinar