A_T_I_2015_04_

AEROSPACETESTINGINTERNATIONAL.COMAPRIL 2015 |^81

Environmental trials z

importance, which precluded the
use of these advanced methods.

DISADVANTAGES
The simple ‘pouring method’ worked
all right, at least for relatively small
coupons, but there were some
disadvantages, the main one being
spillage. The amount of nitrogen
escaping from the setup was
considerable and the area around the
test setup – and the operators – was
exposed to low temperatures.
A sufficient supply of nitrogen for
a static test on a smaller specimen was
not a problem, but for larger specimens
or longer lasting tests, the supply was
problematic. Moreover, the spillage of
nitrogen was undesirable and led to
unacceptable safety issues. Nitrogen
is not poisonous; at sea level the
atmosphere contains about 78%
nitrogen. However, if sufficient liquid
nitrogen is vaporized and the oxygen
percentage is reduced from its normal
level of 21% to below 19.5%, the risk of
asphyxiation greatly increases. Since
nitrogen is colorless and odorless, this
happens without much warning.
In the meantime, a number of
customers and prospective customers
had expressed a requirement for low-
temperature testing of larger structural
items, such as stiffened panels and
even full-scale structural components
and mechanical systems. The sustained
endurance testing of large components
at -55°C for the 24 hours a day and
seven days a week that some customers
desired required the development of
a radically new approach.
This new approach still involves the
use of liquid nitrogen, but the control
system is much more elaborate. Instead
of pouring the liquid nitrogen in bulk
into the climate chamber that encloses
the test article, nozzles are used to
evaporate and spray controlled amounts
of liquid nitrogen around the test
article at specific locations. In this way
the test article is cooled very efficiently
because of the swirling evaporation near

the specimen. This is quicker and more

accurate, and by optimizing the number/

placement of nozzles, the temperature

gradients in the test article can be

minimized. This is done empirically,

prior to the start of a structural test.

The system is totally modernized

for different materials. Client names

cannot be discussed, but after a

successful series of low-temperature

static specimen tests using the new

approach, a fatigue crack growth test

was performed at -55°C on a specimen

made of Glare (aluminum layers

interspersed with glass fiber layers, as

used in the Airbus A380). This test had

to run continuously for several days.

The amount of nitrogen used was

relatively small and the multipoint

LIGHTNING STRIKES

Wichita State University’s

National Institute for Aviation

Research (NIAR) recently

added lightning transient

analysis, high-intensity

radio frequency (HIRF) and

direct effects of lightning

testing capability to the wide

range of services offered

by its specialist

Environmental Test and

Electromagnetic Effects Labs.

In February 2015, NIAR

completed two full-vehicle

lightning transient tests. The

first was for Garmin’s

supplemental type certificate

on the Beechjet 400, followed

by full-vehicle tests for the

Cirrus SF50.

Lightning transient tests are

intended to measure the actual

transient levels induced into

aircraft electrical wiring as a

result of lightning attachment

to an aircraft. This is done to

ensure that the transient level

does not exceed the wiring’s

transient control levels. To

measure actual transient

levels, a simulated lightning

strike at a reduced level is

injected onto the aircraft; the

field generated by the lightning

strike travels the length of the

aircraft, inducing voltages and

currents on wiring inside the

aircraft. These voltages and

currents are measured and

analyzed to make sure that an

actual lightning strike will not

damage any critical equipment

during flight.

NIAR teamed with QinetiQ

to perform HIRF testing for

the Cirrus Vision SF50

personal jet, a newly

designed composite five-

plus-two-seat, single-engine

turbofan-powered aircraft.

The aircraft was tested using

low-level swept coupling to

measure the level of induced

currents and voltages on

system components as a

result of radiated fields below

400MHz. It also underwent

low-level swept fields testing

above 400MHz to determine

the transfer function relating

the external field to the

internal bay fields at specific

locations in the airframe.

NIAR will offer direct

effects of lightning testing in

the second quarter of 2015.

With a 200,000A generator,

the lab is able to test all

common lightning strike

levels. Direct effects lightning

testing evaluates the aircraft

components’ ability to resist

the possible damaging effects

associated with lightning

attachment to the aircraft.

Effects can include melting;

the development of holes at

points of contact; temperature

rises in metal conductors;

magnetic force effects;

acoustic shock effects; arcing

at structural bonds, hinges,

and joints; and the ignition of

flammable vapor in fuel tanks.

FAR LEFT: Starting

practice of the

cooling of material

test specimens by

pouring liquid

nitrogen in the

climate chamber

LEFT: Thermal

blocks, or heat

sinks, are mounted

on the LIDs to

thermally insulate

the test article from

the test rig

controllability of the temperature over

the test article was excellent. Minor

problems such as frost on the test

article and frozen window panes were

easy to solve. Frost on the test article

and the inner side of the window pane

was prevented by pre-filling the

climate chamber with dry nitrogen:

no humid air, no water, no problem.

The heat from a paint stripper solved

the problem of a misted or frozen

window pane.

INCREASED SCALE

The next step in the development was

increasing the scale: doing panel tests

at low temperatures. For this purpose,

a stiffened panel was tested at -55°C in

an MTS 500 testing machine under