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The Boeing 787 Dreamliner made
headlines for being the first major airliner
to use composite materials to construct the
majority of its airframe. Long before any
aircraft like the Dreamliner took off, scientists
and engineers around the world used the
3D inspection capabilities of computed
tomography to evaluate the structure,
processing and properties of the composite
materials. Industrial computed tomography
(CT) provides digital 3D x-ray models of test
subjects, thereby allowing the analysis of
interior structures of composites.
Industrial CT uses a series of 2D images
taken at specific intervals throughout a
complete (360°) rotation of the test object.
A typical North Star Imaging CT system
consisting of three principal components: an
x-ray tube, an x-ray detector, and a rotational
stage. The x-ray tube shoots high energy x-rays
through the test object to form images on the
x-ray detector. Once anything between 360 and
3,600 2D images are collected, CT calibration
and CT reconstruction algorithms reconstruct
the images into a 3D volume. The use of
visualization software of
the 3D volume allows one to slice through
anywhere in the object, inspect and look for
defects, take accurate measurements, and
reconstruct a surface model.
Computed tomography is uniquely suited for
composites because they often consist of
complex structures of multiple materials and
varying densities. Traditional 2D x-ray fails to
differentiate between layers in composite
structure with intricate geometries. For a given
part geometry, the amount of signal received by
the x-ray detector decreases as density
increases, thereby allowing different materials to
be highlighted. The image bottom right shows a
composite of lower density plastic surrounding
higher density fibers. CT enables one to see the
orientation of the fibers, flow lines in the plastic
and voids in the material structure.
The high level of contrast that CT provides
allows for clear identification of delamination,
wrinkles, fiber orientation, misweaves, fiber
pull-out, cracks and voids. 3D rendering
capabilities of CT software, such as efX-CT
produced by North Star Imaging, allow for
multiple virtual cross-sections through the part
in multiple axes. The resolution of industrial CT
often allows seeing individual composite fibers
that are only a few micrometers in size. High-
resolution CT is used to detect delamination,
to detect fiber distribution patterns, and to
quantify and measure porosity.
Unlike traditional mechanical testing or
sectioning, industrial CT is a non-destructive
technique that preserves the test object for
future use. This non-destructive testing
technique is not only valuable throughout
a product’s lifetime, it also saves money.
During research and development, industrial
CT may be used to periodically check a partundergoing accelerated testing. In a multi-year
test, hundreds of parts are placed in an
environmental chamber so that a single part
can be pulled out periodically for destructive
inspection. CT allows the parts to be inspected
and then returned to the test chamber. During
production, industrial CT is used to inspect
parts before they are shipped. High-value
casting houses, including turbine blade
manufacturers, have been using industrial CT
to measure wall thickness and rib thickness for
years. If material that failed incoming inspection
accidentally makes its way into a production
run, parts are quickly CT scanned to screen for
defects. In the event of a field failure, industrial
CT provides a 3D model without sacrificing a
unique specimen of a failed component.
Industrial computed tomography systems,
like the X5000 manufactured by North Star
Imaging, are powerful tools that provide
precise and automatic material analysis when
combined with powerful software. As
consumers demand larger and safer aircraft
with more amenities, aircraft manufacturers
continue their search for low-density materials
that are strong, stiff, and abrasion and impact
resistant. More and more composites are
meeting these needs and industrial computed
tomography provides a unique and non-
destructive inside view of composite structures
and properties.
North Star Imaging designs systems,
develops software, and manufactures x-ray
and CT equipment, near Minneapolis, USA.
The company built its first digital radiography
system in 1991. In 2006, North Star Imaging
founded the Inspection Services Group
offering need-based consulting to anyone
needing x-ray and/or computed tomography
scanning. In 2010, North Star Imaging was
acquired by ITW, a global company withscanning for problems
Computed tomography is the most effective way
to evaluate the properties of composite materials
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ABOVE RIGHT: Internal voids are
highlighted by creating a
transparent shell
RIGHT: Fiber orientation analysis of
a composite part. The colors in the
middle picture show the angle of
the fibers and indicate the
orientation strength of the part