Explaining SRS Part 1
This is the 1st of a 2 part series explaining the various types of shock testing and SRS (Shock Response Spectrum). Part 2 will cover SRS in defence applications as a replacement for MIL-STD-901D.
Shock Response Spectrum (SRS) Testing – part 1
Shock testing seeks to evaluate the ability of test items to survive high acceleration, short duration impulses. Often the acceleration-time plots of these shock loads are highly complex. Furthermore, the exact acceleration-time waveforms vary considerably from impact to impact even when the cause of the shock pulses is the same.
Thus engineers and scientists sought a way of representing these highly complex and variable waveforms in a simpler and more useful manner. The method of choice in industry is to represent the shock loads not in the time domain but in the Shock Response Spectrum (SRS) domain. SRS plots of a shock give at each frequency plotted, the maximum acceleration of a system subjected to the shock if it was a linear system with a natural frequency equal to that of the frequency plotted. Thus a shock with an SRS plot that has an acceleration reading of 75g at 60Hz means that if the same shock were imparted on a system with a natural frequency of 60Hz, the system mass would experience a peak acceleration of 75g. Each shock recorded in the time domain has a unique representation in the SRS domain. Note that SRS plots are constructed assuming that the system subjected to the shock has a specified damping ratio.
Strictly speaking, the SRS domain applies only to linear, single degree of freedom systems. However, many components – especially on electronic items – can be represented as separate quasi-single degree of freedom systems (each with their own natural frequency) if they are relatively light compared to the relatively heavy base structure that they are attached to.
From the design point of view, the beauty of representing shocks in the SRS domain is then that it enables design engineers to know the peak acceleration that a component or piece of equipment will be subjected to from a given shock, should the designer choose to alter the structure of the equipment or component such that its natural frequency changes to a new amount.
From the testing point of view, complex, real-world shocks on a particular item that look quite dis-similar to each other in the time domain can be found to have quite similar profiles in the SRS domain, which means that they have the same damage causing potential to linear, single degree of freedom (or quasi-single degree of freedom) equipment/components whatever their natural frequency. Thus it makes sense, especially in these cases, to set the shock resistance requirements of an item in terms of an SRS profile that it must survive. Furthermore, real-world shocks on an item that are hard to replicate in the time-domain (and which in any case may be quite inconsistent in the time-domain), can be simulated in the laboratory environment by conducting controlled shocks on the item that have the same SRS as the real-world shock.
An additional advantage of examining shock loads in the SRS domain is that one can estimate whether previous shock testing on an item conducted to particular parameters will meet the requirements of a revised set of shock resistance requirements by comparing the SRS envelopes of the two sets of test specifications. Making such a comparison of course assumes that the components of the item in question can be approximated as a series of linear quasi-independent, single degree of freedom systems, which is often the case.
At Austest Laboratories we are able to produce shocks of specified SRS profiles through a number of methods as detailed below.
Imparting Wavelets on a Large Vibrator
Austest is able to achieve even complex shaped SRS profiles by imparting a superposition of wavelets on the test item using our heavy-duty 120 kN electrodynamic vibrator and SRS control software.
This method of generating SRS profiles can be used for the testing of light and medium-sized items. For example, Austest is able to use this method to achieve the default SRS profile given for Crash Hazard Shocks in Figure 516.7-9 of MIL-STD-810G (with Change 1) for an item with fixture whose effective mass is up to approximately 180 kg
All tests conducted will meet the requirements of MIL-STD-810G (with Change 1) in terms of shock duration and effective shock duration.
Classical Shocks on Free-fall shock Machine
For larger items where the required SRS profile cannot be achieved on a vibrator but can be approximated by classical shocks, Austest can apply classical shocks on our vertical free-fall shock machine. The default SRS profiles given in Figure 516.7-9 of MIL-STD-810G (with Change 1) are examples of SRS that can be well approximated by classical shocks on our shock machine – in this case terminal peak sawtooth pulses.
Complex Shocks With Resilient Fixturing
For more complex SRS requirements which cannot be approximated by simple, classical shocks and which are too heavy to test on our large vibrator, Austest has the capability to design and build customized resilient fixtures which in combination with free-fall shock machine can generate even the very complex shock waveforms. Austest engineers have written our own software to help us design shock and fixture combinations to generate required SRS profiles. We are now in the process of further developing this software to simulate fixtures with asymmetric stiffness and damping to allow us even greater capacity to approximate more complex shaped SRS profiles.
Continued in part 2 – SRS in defence applications and replacement for MIL-STD-901D