The Shock Response Spectrum (SRS)

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November 23, 2021

While we feature a complete blog series on electrical transients, the technical team at Elite is also continuing our companion series on mechanical shock transients. Follow along as we discuss the basics of the Mechanical Shock environment and see how testing is performed. Read Part 1 here.

Engineers use the Shock Response Spectrum (SRS) to understand the dynamic response of systems to mechanical shock impulses. Its applications include engineering and validating electronics mounted on vehicles, evaluating the severity of pyrotechnic shock in stage-separating rockets, and earthquake evaluation of building structures, to name just a few.  

SRS is an analytical calculation of a field-recorded waveform showing mechanical shock acceleration versus time. The output of the SRS calculation is the maximum response acceleration (in Gs) for a single-degree-of-freedom spring mass that is resonant at a particular frequency. 

In the illustration (right) a shock pulse excites a spring mass that is resonant at 20Hz. The maximum acceleration response for the spring mass is then plotted for 20Hz and repeated for an entire range of spring masses generally spaced apart fractionally per octave and each having a unique resonant frequency.

The maximum acceleration for each resonant frequency spring mass is plotted to make a continuous curve. The analogy is that of a range of individual tuning forks resonant at discrete frequencies mounted to a common base. The base is struck by the shock pulse, which sets each fork into resonance to the extent that the shock pulse contains frequencies that excite some or all the forks. J. Jang provides an excellent illustration of the SRS concept in 60 seconds.

Using SRS information, design engineers can determine the maximum acceleration at specific frequencies for their product’s shock environment. Based on their findings, PCBs can be stiffened to increase natural resonances of the board and shift away from high-G conditions; shock isolators can be applied to dampen modal velocities, or components can be moved to sections of a PCB that experience less displacement.  

Once an SRS pulse is characterized, it can be re-applied to a product to evaluate the product’s response. 

How to Apply SRS Pulses in the Lab

SRS shock can be performed using several different types of test equipment. A common method is to accelerate a mass to strike a tuned resonant beam or resonant plate where the Equipment Under Test (EUT) is fixtured. The impact mass transfers its kinetic energy to the plate, fixture, and the EUT. The shock pulse includes the energy of the initial impact as well as all harmonics that puts the surface into resonance.    

Other SRS test systems use pneumatic or hydraulic-driven impactors that are accelerated to a resonant surface. Pyro-technic shock test methods such as MIL-STD-810H Method 517 in some cases use (as the name implies) explosive pyrotechnic charges that create the SRS input wavefront. 

Electro-Dynamic (E-D) vibration systems are also capable of generating SRS shock pulses. They provide the best solution for quickly and repeatably producing SRS tests with results captured as shown in the image on the right. However, E-D vibration systems generally are limited in their ability to test heavy EUTs or run tests that include high-frequency shock energy.   

Elite offers SRS testing primarily using our E-D shakers, so all inquiries are reviewed in advance by our technical experts to confirm capabilities against the application requirements. We also rely on our high-G pneumatic shock machines to produce classical shock pulses that approximate SRS requirements.

Want to learn more about SRS and Mechanical Shock Testing?  Contact Elite for a review of your application and see how to apply SRS to design and validate your products in their mechanical shock environment.

Learn more in Part 1 of our Mechanical Shock Testing series.