Design of Custom Testing Programs - Part II
Changes in pneumatic pressure can be obtained in a variety of ways and with a number of devices, but determining a way to
apply these pressures to one side of a train window, for example, may be challenging. A chamber or enclosure that the
window will be mounted on must be fabricated so that pressures on one side may be varied according to the required
conditions. In effect, the difference of the pressures acting on the two sides of the window is what causes a load on the glass
pane. The test engineer must determine the means by which the pressure and vacuum cycles are to be applied to the side of
the window within the enclosure. An air compressor, blower, or fan may be utilized, but other less conventional methods like a
bellows system could also be considered.
Just like there are many possible means of applying pressure to one side of the window, there are also many different ways to
complete the other aspects crucial to the test. Other important processes to consider are venting the pressurized air, applying
a vacuum, measuring the pressure changes, timing the cycles, ensuring minimal deviation of pressure values between
cycles, and accurately repeating these pressure cycles thousands, if not millions of times. Since it is the fatigue life of these
train windows due to pressure fluctuations that is being tested, and it may take millions of cycles for fatigue cracks to develop,
the most important thing to consider when designing the test setup is its reliability. The test engineer needs to diligently
forecast the anticipated lifespan of each and every piece of equipment used during the test so that appropriate quantities could
be ordered, and costly surprises don’t occur.
Reliability of equipment used in a test program must be closely considered, especially when testing is expected to last for an
extended period of time, or if it simulates a large number of fatigue cycles. Source: en.wikipedia.org
The test setup needs to be designed so that manufacturability of any custom parts is not unnecessarily difficult.
Furthermore, when a test program is expected to last for weeks or months such as this one, the setup needs to be
designed so that minimal labor needs to go into maintaining the desired test conditions. Automation is always a desirable
option when it is cost-effective. Naturally, pricing is one of the most important aspects of designing a testing program. The
setup must always be optimized so that material, equipment, labor, and power costs are minimized. A lot of the times when
multiple units need to have the same tests performed on them, it is more cost-effective to test them simultaneously. Both
the customer and the testing laboratory benefit from a streamlined, efficient, and cost-effective test program.
While attempts must always be made to minimize the cost of testing, concessions cannot be made which may affect the
validity of the test program. The specifications to which an item needs to be tested must never be compromised; otherwise
the test is inconclusive. The most important aspect of designing a custom test program is the ability to foresee any issues
that may come up once the test is in progress. Issues relating to durability, reliability, strength, stiffness, stress
concentrations, temperature, data measurement and recording, safety, and many others need to be considered. While
testing may be the last step in the product design process, designing the test program may be as complex as designing
the item itself.