Introduction to the Shock Response Spectrum (SRS)

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In the research and development stages of manufacturing, shock testing helps define the performance of a test article during and after a transient event. A classical shock test employs regular shock pulses with a set acceleration over time. However, unidirectional classical pulses cannot replicate many transient events that occur in the real world.

The shock response spectrum (SRS) is a tool for evaluating the effect of complex shock. It is a mathematical model that allows engineers to examine the theoretical response of their device under test (DUT) in the event of a shock. Examples of “complex” events—i.e., high-frequency and multidirectional—include earthquakes, rocket separation, and shipboard shock.

Classical shock tests apply user-defined pulses that excite DUT responses, including resonances. More advanced methods, such as the SRS, evaluate how the DUT would respond to a transient event by calculating the peak response of a series of single-degree-of-freedom (SDOF) systems. The SRS is a measure of the maximum response across a range of frequencies.

Live SRS data in VibrationVIEW.

Single Degree-of-Freedom (SDOF) Systems

A single mass connected to a spring represents the most simple vibratory system. The mass can only travel along the direction of the spring’s elongation. Therefore, the system has a single degree of freedom (SDOF).

The SRS uses a set of theoretical SDOF mass-damper-spring oscillators that are mathematically excited by a time-history base input. It displays the peak response of the series on a graph (Figure 2). It is important to note that the SRS is not the actual response of the DUT but a calculated model of the response.

Figure 2. A basic model of the SRS featuring a graphical representation of a series of single-degree-of-freedom oscillators.

The horizontal axis of the SRS plot is the natural, or resonant, frequency in hertz (Hz). The resonant frequencies of the SDOF oscillators are typically logarithmically spaced. The vertical axis is the absolute maximum acceleration of the SDOF oscillator in response to the pulse, a value also known as the maximax (Gpk).

The oscillators are also characterized by either the critical damping factor (ζ) or resonant gain (Q), where Q = 1/2ζ. The same value applies to all the oscillators (normally ζ = 5%, Q = 10).

Validity of the SRS

Acceleration pulses that differ in amplitude, frequency content, and duration can produce equivalent shock spectra, resulting in an extensive process of waveform selection. Despite this, SRS remains a standard tool for complex shock evaluation. The SRS method characterizes a shock environment in a straightforward manner, and engineers can define a test with data from multiple shock events.

Engineers use the SRS to generalize a transient event, estimate the event’s damage potential, or perform data analysis. It also functions to identify resonances, define test specifications (particularly for seismic tests), and replicate failure modes.