JEC COMPOSITES MAGAZINE - Issue #105 - June 2016 - 54

testing

rotational stage. Everything is enclosed in a
radiation shielding steel/lead/steel cabinet. In addition to the cabinet, a computer
workstation, consisting of a 2D X-ray console
for acquisition, and a 3D CT workstation for
volume reconstruction are required.
A high-end CT scan consists in taking several 2D X-ray images around the object, preferably covering 360° (complete rotation). CT
systems typically acquire between 360 and
3600 images depending on the final desired
resolution. Each image is between 3-10
megapixels and is also averaged and filtered
to reduce noise.
Once the acquisition process of the CT scan
is completed, calibration and reconstruction
algorithms are used to reconstruct the 3D
CT volume. Visualization software allows
the 3D volume to be manipulated in real
time, enabling the viewer to slice through
anywhere inside the object, inspect and look
for defects, and take accurate measurements.
The 3D CT volume non-destructively
reveals a complete model with both external
and internal surfaces. Moreover, CT works
with any surface, shape, colour or material
(up to a certain density and/or thickness
penetrable with X-rays). A modern start-tofinish CT scan can be as fast as a few seconds
or take longer than an hour, depending on
the resolution requirements and size and/or
density of the object.

Combining mechanical testing
with computerised tomography
Combining mechanical testing with DIC
provides much more information on the

behaviour of materials and structures than
is obtained from a traditional test. However,
the full-field strain data is confined to the
surface and the spatial resolution of DIC
is limited (typically 1 mm). CT makes it
possible to look inside the part with a much
higher spatial resolution (typical resolution
20 µm) and offers the exciting possibility of
helping us develop a much deeper understanding of the mechanisms behind the
failure of composite parts.
As an example of the use of CT alongside
mechanical testing, an open-hole woven
composite coupon was tested in an Instron®
5985 250 kN testing machine. The coupon
was scanned before, during and after the test
in a North Start Imaging X5000 series CT
scanner (figure 7).
The scans during the test were conducted
at the points where there were small drops
in the load (accompanied by audible pings)
indicating partial failure of the coupon.
The sectioned 3D images from the scans
are shown in figure 8. Figure 8a is an image
of the coupon before the test; the woven
structure can be clearly seen. Figure 8b is
from a scan taken after the first drop in load
and the zoomed-in view shows both axial
and transverse cracking in the vicinity of the
hole. The final image (8c) shows the coupon
immediately after failure.
The results of this test show the ability of
X-ray CT to produce high-quality 3D images
of composite materials with sufficient resolution to resolve cracks and provide valuable
information on the failure mechanisms of
composite materials.

Conclusion

Fig. 7: North Start Imaging X5000 series CT scanner

54 jec composites magazine / No105 June 2016

Conventional mechanical tests to determine
the static properties of
composite materials
continue to play an essential role in providing
data on materials for use
in models. In addition to static material
properties, fatigue and
high-strain rate data
is important for many
applications. Combining mechanical tests on

Fig. 8: CT images from open-hole test

structures with full-field strain measurement
techniques enhances the understanding of
the behaviour of composite materials and
models.
3D CT is now more accessible than ever
for the composites industry. User-friendly interfaces, increased scan speeds and
decreasing prices have all contributed to
the rapid growth of this technology. Having
very accurate internal dimensions without
destroying the item, along with the ability
to compare to a reference model, is entirely
unique to CT. There are no shaded zones, it
works with all kinds of shapes and surfaces,
there is no post-processing work needed,
and the resolution is excellent. Above all,
the greatest benefit is the ability to non-destructively obtain the 3D internal structure
of the object, and CT is the only technology
capable of achieving such performance. n
References
[1] Mechanical testing of advanced fibre
composites, Woodhead Publishing,
ISBN: 1 85573 312 9
[2] Handbook of Experimental Solid Mechanics, Springer, ISBN: 978-0-387-26883-5
More information:
www.instron.us

http://www.instron.us

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #105 - June 2016