NDT
59
The demand for neutron imaging is certainly not
diminishing. For turbine blades alone, the casting
community has a demand for around 70,000 images a
year, according to Katie Rittenhouse, marketing director
for Wisconsin, USA-based nuclear technology firm,
Phoenix. “Neutron imaging is used extensively for R&D
in turbine blades,” says Rittenhouse.
Turbine blades are hollow to allow active
cooling when the jet engine is working at
temperatures higher than the blade’s melting
point. The hollow centre is made using a
ceramic core which acts like a mould. The
metal blade is formed around this core.
During the manufacturing process
some trace amounts of ceramic can be
left behind in the cooling channels
and can block or divert airflow,
creating hotspots within the blade.
These would cause that portion of the
blade to get too hot and potentially
catastrophically melt. “We’re seeing
demand as continuing to increase while
turbine blades get more complex and
operate at higher temperatures. We don’t
predict any significant change in the
demand,” MacGillivray says.
Once the metal has cooled and solidified the
ceramic material is washed out chemically. Neutron
imaging is used to find any residual ceramic that would
impair the blade’s performance in the engine. But,
working using neutrons for imaging not easy. “Neutrons
are extremely difficult to direct or coordinate. Focusing
them is much more difficult and a different problem
altogether than x-rays,” Rittenhouse says.
Another use for neutron imaging, whether the
neutron source is a reactor or an accelerator, is imaging
composites such as glass laminate aluminium reinforced
epoxy. “It can see defects in the resin layer, the sandwich
with the aluminum and then between those aluminum
layers, where we have the
glass fibres and epoxy
resin,” says David Mannes, a
scientist at the Swiss Paul
Scherrer Institute’s Laboratory for
Neutron Scattering and Imaging.
“We did tests with this glass laminated
resin and we can see rather well using
neutron imaging,” he says.
The neutron source at the Paul
Scherrer Institute is used to screen
pyrotechnic components such as the fuse
cords and the igniters that are used
inside of the rockets launched by the
European Space Agency (ESA), such as
the Ariane 5. Neutron imaging is the
only technique suitable because testing
them before use is impossible. The
pyrotechnic components are imaged at
PSI and the images evaluated by
engineers at Dassault Aviation to check
the explosives are in place as expected
without defects.
“My dream is to solve
strain mapping in 3D,
which is the holy
grail and is very
hard to do”
AEROSPACETESTINGINTERNATIONAL.COM // MARCH 2020
1 // The quality of
pyrotechnic components for
an Ariane 5 rocket being
checked before launch.
(Photo: Paul Scherrer
Institute/Markus Fischer)
2 // Ariane 5 launch from
the spaceport at French
Guiana (Photo: ESA)
1
2
NEUTRONS FOR IMAGING
Neutrons are found within an atom’s
nucleus, which is surrounded by a
cloud of electrons, negatively charged
particles. Alongside the neutron inside
the nucleus are protons, positively
charged particles.
A neutron has no charge but has
a mass 1,839 times larger than the
electron. While it is normally bound
up inside an atom, as a free particle a
neutron has a lifespan of around
1,000 seconds.
Whether it is a neutron or an x-ray’s
photons, an image is created when
the particles are scattered by the
photographic subject. An x-rays’ photons
are scattered by the cloud of electrons,
while neutrons are scattered when
they interact with the nucleus of an
atom. This allows neutrons to be used
to investigate the interior of materials,
not just the surface layers which x-rays
primarily probe. Another characteristic
of the neutron that allows for deep
interior imaging is that it interacts
weakly with atoms’ nuclei. Because
the interaction is weak the process
is non-destructive even for delicate
and complex polymers or biological
samples. Neutron photography can also
image light atoms, such as hydrogen, in
the presence of heavier atoms.
For the aerospace industry, neutron
imaging is used to analyze pyrotechnics,
such as ejection seats, warheads and jet
engine turbine blades.
/AEROSPACETESTINGINTERNATIONAL.COM
