Shock testing with long durations can be a challenging endeavor. DES recently had to perform a 35G peak, half sine shock with a 50 millisecond duration. The video below shows this shock test being performed.

This sounds like an easy shock to carry out because a peak of 35G is low compared to many shocks. However, this is a difficult shock to perform because 50 milliseconds is a long duration. Most typical shock durations are less than 20 milliseconds.

A half sine shock impulse has the shape of a half sine wave. More details can be found elsewhere on our blog, in an article titled “Classical Shock Testing“.

A half sine shock impulse is created when the shock machine table accelerates downward, then impacts a rubber material and changes direction abruptly. This abrupt change in direction causes a rapid velocity change which creates the shock impulse. Different rubber or foam materials are used to create half sine shocks with different magnitude and durations.

Half sine shocks can be performed on Electro Dynamic (ED) Shakers, Drop Tables or Drop Towers or Pneumatic Shock Machines. For this particular shock, DES used its Drop Tower Shock Machine. This machine has large bungee cords to increase its capability.

The required velocity change for a 35G, 50 millisecond duration half sine shock is 430 inches per second. A typical free fall drop table shock machine is only capable of a maximum velocity change of approximately 260 inches per second. A free fall drop machine could not attain a 35G peak, 50 millisecond duration half sine shock.

The addition of large bungee cords is used to create higher shock table velocities. The bungees act like large rubber bands or springs pulling the table downward. Higher shock table velocities can also be used to create higher peak G levels.

The impact material (sometimes called the shock programmer) for the 35G, 50 millisecond duration half sine shock was various density foam rubber pieces that were a total of 10 inches thick. It is a dramatic shock, as seen in the video above. As our customer described it, we are moving the shock table at a very high velocity to hit a bunch of pillows!

A typical plot for this shock impulse is shown below. DES was able to easily obtain both the 35G peak and 50 millisecond duration.

What sets DES apart from other labs is our in-depth experience and technical capability to understand and reproduce the most complicated shock profiles. DES has tested to the most complex shock requirements on products that are used in outer space, rockets, missiles, military environments, etc.

In your example you noted that the required velocity change for a 35G, 50 millisecond duration half sine shock is 430 inches per second. How did you determine this?

The velocity change for a half sine shock is dV = (2/3.14) x 35G x (386 (in/s^2)/G) x (.050 second duration) = 430 in/sec

Hi DES, thanks a lot for the quick response. I see how you got that

Hello DES, Thank you for this. I am not understanding the part of (2/3.14). I know the dV=a*dt. The velocity change is from 0 to max and max to 0 so it is 2. But how did you get the 3.14 to divide? I am sorry for my slow understanding.

Hello, I am sorry. I figured it out. the equation is actually a=sin2Pifdt. So then the integral of that is the velocity change. Thank you very much.

Which is more difficult on the product being tested – A 6 ms duration shock or a 10 ms shock. If the duration is longer does that make it easier on the unit being tested?

Not necessarily. For a half-sine pulse,the relation of the time duration of the pulse to the frequency is 1/2T = f, where

T = time duration, seconds

f = frequency, Hz

The frequency of a 6ms shock is different than for a 10ms and so on. So it depends how your product responds to the fundamental shock frequency.