IEC60068-2-27 mentions that the purpose of having shock test is to reveal mechanical weakness and/or degradation in specified performances, or accumulated damage or degradation caused by shocks.
It may be used in some cases to determine the structural integrity of specimens.
Meanwhile, MIL-STD-810H Method 516.8 mentions that shock tests are performed to:
Provide a degree of confidence that materiel can physically and functionally withstand the shocks encountered in handling, transportation, and service environments.
Determine the materiel's fragility level, in order that packaging, stowage, or mounting configurations may be designed to protect the materiel's physical and functional integrity
Test the strength of devices that attach materiel to platforms that may be involved in a crash situation and verify that the material itself does not create a hazard or that parts of the materiel are not ejected during a crash situation
One thing that makes shock test different from vibration test, the duration of the test is not measured based on how many sweeps, cycles, or based on time duration.
The duration of shock test is depending on how many pulses need to be performed.
According to the shape of the pulse, shock test can be classified into:
Classical Shock
Shock Response Spectrum (SRS)
User Defined Shock
The shapes of the pulse which are considered as classical shock:
Half Sine
Trapezoidal
Terminal Peak Sawtooth
Initial Peak Sawtooth
Triangle
Rectangle
Haversine
The first three types are the most common pulse shape used for shock test.
Shock test can be performed by using:
Vibration Test System
The advantage of performing shock test with vibration test machine is easier to achieve the shock demand profile. Since vibration test machine commonly (if not always) uses close loop control system, then we can just set the demand profile to the control software to run the machine. Once we start the test, as long as our test specification is within the machine capacity, the control system will handle the rest of the test. Another huge advantage of running a shock test with vibration test machine is the ability to perform negative shock without the need to modify the UUT mounting setup. Meanwhile, the disadvantages of running shock test with vibration test machine lies in its limited displacement and force. Due to limited displacement, most of vibration test machine is unable to perform shock test with long pulse duration.
Running test with vibration test machine may also be difficult when it involves large and heavy UUT. If the UUT is large, then we may need bigger head expander or slip table. Bigger head expander and slip table will increase total moving mass, which will reduce the maximum acceleration can be achieved by the machine.
2. Shock Test System
Unlike vibration test system that achieves the shock demand profile by using the movement of the moving mass, shock test machine achieves the shock demand profile by using the impact between table to the "shock profile generator".
The advantage of using the shock test machine is less limitation in terms of force and displacement. This means we may be able to perform shock test for shock profile with longer pulse duration than vibration test machine. We may also be able to run very high acceleration test profile for large and heavy UUT compare to vibration test machine.
The disadvantage of shock test machine lies in its open loop control system. Due to this, we may need to perform more trial tests in order to be able to achieve the demand test profile.
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