Design of Custom Testing Programs - Part I
Testing is crucial to the successful development of any type of new product, whether it’s meant for military, commercial, or
consumer use. While testing is often done based on widely acknowledged specifications, it doesn’t mean that the process is
similar for every item. As a matter of fact, testing programs are unique to the unit under test, even if the same type of test is
performed on countless other items. While personalizing a test for a specific unit often only consists of designing a fixture
which attaches it to a testing apparatus like a shaker table or shock machine, other times a complete test program needs to
be developed around the specific test parameters for that unit.
Testing revolves around subjecting equipment to worst-case scenario conditions that could be expected in its service life.
Clearly, depending on the type of item being tested, any of a multitude of tests may be performed on it. It is up to design
engineers and acceptance personnel to determine the appropriate testing to be performed on an item. If equipment is
designed for military shipboard use, it is expected that it will successfully pass such standard tests as shock, vibration, and
salt spray. However, certain equipment will experience unique conditions that will dictate the type of testing it must undergo.
For example, when a train enters a tunnel opening or passes alongside another train at high speed, there is a momentary
positive and negative pressure pulse that acts upon the outer surface of the train. These aerodynamic loads and pressure
waves occur due to the flow behavior of fluids (one must remember that although invisible, air is certainly a fluid). Not only do
train windows have to be designed robustly enough to be able to handle this pressure pulse, they must be able to withstand
the fatigue induced by the thousands of these waves encountered throughout their lifetime.
Pressure pulse values are measured experimentally by setting up a false window with pressure transducers, and subsequently
running the train through tunnels and beside other trains at specific velocities. This image, and experimental results were
obtained from the paper "Aerodynamic Loading of Trains Passing Through Tunnels" by Tarada, Himbergen, and Stieltjes.
Determining the appropriate test setup for pressure cycling of train windows (as well as for any test) starts by taking
measurements in the field to determine the magnitude of the conditions that may be encountered. A lot of times these
values are specified in published test specs, but when dealing with such a specific condition as pressure waves on the
outer surface of a train, it may be necessary to obtain data experimentally (see image above). Once the test condition is
known, it is up to the test engineer to determine the best way to simulate these conditions in a laboratory setting.
Check back soon for Part II !