An In-Depth Look into Classical Shock Pulses
April 13, 2020
The various classical shock pulses have unique purposes. Generally, each of these unipolar pulses excites a wide range of frequencies in a short period of time but in different ways.
Classical pulses are primarily defined in acceleration; the shape of the acceleration waveform determines the frequencies and the degree to which they are excited. However, delta velocity, or the change in velocity, determines the severity of the shock.
If we look at classical shock pulses on a graph, we can easily note the differences between the pulse shapes.
Half-Sine and Terminal-Peak Pulses
Half-sine and terminal-peak are two of the more common pulses. They are defined in a variety of standards for different applications. In Figure 1.2., the acceleration waveforms show the clear difference between the half-sine and terminal-peak shapes.
The half-sine pulse has a smooth transition; all the change is uniform. The half-sine generates low amplitude, high-frequency energy. Much of the wavelength energy is at lower frequencies and is primarily used to test for responses and resonances in mechanical systems.
The terminal-peak pulse has a smooth ramp and a sharp transition back to zero. The sharp transition generates notably higher amounts of high-frequency energy. This energy generates minimal displacement and has less of an effect on mechanical resonances. It is effective for testing electronic components, solder joints, and electronic circuitry. The high-frequency energy excites potential failures in these components and is commonly used when testing components with embedded electronics.
Visually, the differences between the half-sine and terminal-peak pulse shapes are easy to see. When running on a shaker, the overall velocity and displacement of the terminal peak are lower, but the amount of high-frequency energy generated by the terminal peak is much greater. These characteristics must be evaluated when selecting a classical shock pulse.
Square and Trapezoid Pulses
Square and trapezoid pulses appear as extreme outliers in the displacement and velocity waveform plots. Neither pulse shape is common.
The square pulse will never be perfectly square on any mechanical system. A mechanical system cannot change positions so sharply and the square wave will always have some trapezoidal shape.
The trapezoidal pulse, with constant acceleration at the peak, is also difficult to create with a mechanical system. There are shock machines designed to create trapezoidal pulses and typically use pneumatics to propel the table up and down. These pulses are exceptionally difficult on electrodynamic, hydraulic, and other shaker systems that are limited in velocity and displacement.
Test Standards and Multiple Shock Pulses
Test standards often require multiple shock pulses with different amplitudes and durations. For example, EN-60068-2-27 specifies two pulses: a 30G, 11mS half-sine pulse and a 50G, 3mS half-sine pulse.
Although lower in actual peak amplitude, the first pulse will generate a larger amount of displacement and velocity and a lower amount of overall high-frequency excitation. The second pulse will have less displacement and velocity but a much higher energy spectral density at the higher frequencies.
The combination of shock pulses is very common in test specifications. The first, longer-duration pulse is designed to excite lower frequency mechanical resonances. The second pulse drives the higher frequencies and can cause more damage if the product’s resonances are in bands of higher frequency.
Moving Toward Complex Shock Pulses
More and more often, vibration tests are recommending real-world data to generate test profiles. Requirements are moving away from classical shock pulses and are instead using a complex time history, the shock response spectrum (SRS), or a combination of both. Complex shock pulses are the more appropriate and realistic method of evaluating a product’s response to real-world environments.
All classical shock pulses excite a range of frequencies in a short period of time, but each pulse shape had a unique purpose. In the end, no classical shock pulse occurs in the real world; rather, they are historical references that have been used for many years and are still used to evaluate products today.