AEROSPACETESTINGINTERNATIONAL.COMAPRIL 2015 |^79
Environmental trials z
S
trict environmental regulations
and a highly competitive market
call for affordable and efficient
aircraft that are durable and easy to
maintain. Fuel consumption and
engine emissions are directly related
to aircraft weight, and in the design
phase, much time and effort is spent
on optimizing the structural
components and mechanical
subsystems that make up the airframe.
Testing starts in the early design
stage, when materials are established.
Later, it needs to be done to validate
structural design solutions and analysis
methods. Finally, full-scale tests on the
complete aircraft or on major structural
components are needed to generate
input for the certification process.
In all these tests, attention must be
given to the effects of parameters
other than loads. Of these, temperature
is very important. Material properties
are known to change with
temperature, and predicting the
temperature dependence of specific
structural properties, such as buckling
behavior and subsequent failure, is
not straightforward.
This is why the test house of the
National Research Laboratory (NLR)The Dutch National Research Laboratory has
developed a new method for environmental testing at
low temperatures, which improves the handling and
delivery of liquid nitrogen to save time and lower costBY HOTZE JONGSTRACold is
LEFT: Overview of
full-scale flap track
test setup at -55°Cin the Netherlands has developed both
modular and specific cooling solutions
for use in material and structures
testing, based on the use of liquid
nitrogen. Attention has been paid to
affordability, maintainability, safety,
scalability, accuracy and operating
temperature range. The developed
systems feature a high degree of
autonomy, which makes them suitable
for 24/7 applications.
It all started some decades ago
with the development of a simple
system for testing material coupons
at low temperatures, typically -55°C.
Liquid nitrogen was used to cool a
climate chamber that was built around
the specimen. A controlled amount of
liquid nitrogen was simply poured into
the chamber until the temperature of
the specimen had stabilized. Manual
adjustment in a slow feedback loop
was subsequently applied to the flow
of liquid nitrogen until the desired
temperature level was reached.
At that time, far more advanced
cooling methods were in use by NLR’s
Space Department. For standard
mechanical coupon testing in a
commercial environment, however,
the cost was (and still is!) of crucial