There’s an old saying “If the only tool you have is a hammer, then all of your problems start to look like nails.”
In the old days of shock testing (writer spits into spittoon) all we had was drop table test machines. 1/2 sine shock pulses were easy to perform (just use rubber under the table), they looked good in the report (when you filtered the heck out of them), and they were absolutely repeatable.
If there is anything an engineer likes better then accuracy in Half Sine Shock Pulse Testing, it’s repeatability.
But what is our excuse today? If we are really interested in finding out how our designs will stand up to real world shocks, why would we use a shock pulse for our testing which never appears in the real world?
Passing a 1/2 sine shock test will NOT indicate that your product will survive the shocks encountered in use and in transportation, nor does failing the test tell you very much useful either, yet these tests are still performed routinely around the world.
It looks like another case of “We’ve always done it this way.”
We live in the era of “Test Tailoring”. It is NOT a big issue to instrument a package, and (gasp) take measurements! Use those measurements to determine a realistic reliability test requirement. Then you are performing useful testing.
I have a client who is asking me what the difference between a half-sine 20 g’s 11 mS shock and a terminal peak sawtooth 20 g’s 11 mS shock? Is the TPS more representative of real world shocks?
No it is not, however if my ONLY two choices for test are 1/2 sine or TPS, I’ll take TPS every time. The reason is that if you were to perform an analysis of the energy distribution of those two pulses (such as Shock Response Spectrum) you would find that the energy of the shock is much more evenly distributed across the frequency band with the TPS. Neither shock represents the shocks encountered in service usage.
I cover this in much more detail in my seminars. See http://www.equipment-reliability.com/open_courses.html
Thanks Steve! Your help is greatly appreciated.