Characterization of a Model Composite Material
with X-Ray Cone-Beam Microtomography
A. Shih1, S. J. Pan1, W. S. Liou1, M. S. Park1, W. Chang1, G. Wang2,
S. P. Newberry3, H. Kim4, D. M. Shinozaki5, P. C. Cheng1
1
Advanced Microscopy and Imaging Laboratory,
Department of Electrical and Computer Engineering,
State University of New York, Buffalo, NY 14260, USA
2
Mallinckrodt Institute of Radiology,
Washington University, School of Medicine, St. Louis, MO 63110, USA
3
CBI Labs, Box 11, S. Wescott Rd., Schenectady, NY 12306, USA
4
Department of Material Sciences and Engineering,
Kwangju Institute of Science and Technology,
Kwangju, Republic of Korea
5
Department of Mechanical and Materials Engineering,
University of Western Ontario, London, Ontario, Canada, N6A 5B7
Abstract. X-ray cone-beam microtomography was used in the evaluation of
encapsulated spherical and cylindrical structures in an epoxy matrix. The
volume and surface area of the encapsulated structures were obtained by using
multi-dimensional image analysis system developed at SUNY.
1 Introduction
The examination of the three dimensional structure of optically opaque composite
solids is of great interest in a wide variety of applications in engineering. Assembled,
cast or fabricated parts often must be examined after manufacturing to understand
failure processes, and for quality control of final product. The spatial resolution
requirements for this kind of application are not demanding, with part dimensions of
the order of a fraction of a millimeter. However for practical implementation, the
tomographic imaging must be relatively rapid, and real time reconstructions would be
a valuable engineering tool. A much more demanding application is in the study, for
quality control or for failure analysis, of typical composite materials such as fiber
reinforced polymer systems. The spatial resolution required in these cases would be
some fraction of the reinforcing phase dimension, or if possible, sizes of the order of 1
micron or less, which would be useful in detecting flaws introduced during
fabrication, or developed in service. In composites, in many cases, microcracks or
discontinuities on this scale can contribute to premature failure of the component.
This kind of available resolution would also be useful in examining reinforcement
sizes and distributions for quality control or failure analysis.
Conventional scanning electron microscopy (SEM) can be used to examine only
the surface of the sample. The distribution and sizes of second phases are typically
inferred from the two dimensional image taken from fractured or etched surfaces,
often prepared by random cutting through the opaque solid. In complicated
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manufactured components, the exact distribution of second phases and flaws with
respect to the component geometry is normally important, and true three dimensional
reconstructions of internal microstructures is critical.
Recent developments in confocal and two-photon microscopy provide the
capability of 3D imaging of transparent materials as deep as 250µm from the surface.
However, for bulky specimens or opaque materials, X-ray imaging must be used.
With an appropriate selection of the wavelength, which depends on the elemental
composition of the various phases being examined, X-ray microtomography can
provide good image contrast and high resolution. This paper reports our attempt to
produce accurate three dimensional images of sample model composites using a
relatively rapid procedure, which involves unique instrumentation and specialized
reconstruction algorithms. These initial stages of the project will address low
resolution, rapid examination of manufactured structures. The high resolution
techniques are for future developments.
2 Material and Methods
In order to test the cone-beam tomographic reconstruction algorithm [1, 2, 3] and the
multi-dimensional image analysis system, a test specimen was constructed by
embedding glass beads (G-9393, Sigma Chemical Company, MO) and a glass
capillary tube (with broken ends) contained in an Epoxy matrix (Mid-Cure Epoxy,
Bob Smith Industry, Atascadero, CA) in the inner halve of a #0 gelatin capsule. The
average thickness of the capsule wall is approximately 120µm. A drop of Elmer’s glue
(Borden Inc., Columbus, OH) was placed on the outside surface of the capsule tip.
Then the specimen was placed into a creaked outer halve of a gelatin capsule. Fig. 1
shows a diagram of this specimen. Fifty cone-beam projections (at 7.2 degree spacing)
were obtained with a conventional X-ray source (Aztech 65, Boulder, Colorado) at the
source-specimen distance of 58mm and specimen-detector distance of 6.8mm. The
integration time was 0.1sec for each projection.
Fig. 1. Diagrammatic representation of a “model” sample used in this study. (1): inner
gelatin capsule, (2): outer gelatin capsule with broken edge, (3) air bubbles, (4): glass
beads, (5) glass capillary, (6) drops of Elmer’s glue and (7): Epoxy matrix.
Characterization of a Model Composite Material
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3 Results
Figure 2 shows a stereo-pair of original X-ray cone-beam projections (transmission
intensity) of glass beads and capillary tube embedded in Epoxy. In contrast, Fig. 3
shows the integrals (Radon transform) of linear absorption coefficients of a stereo
pair. After tomographic reconstruction of 50 equal-angular projections, a volumetric
data set was generated. Fig. 4 includes representative cross-sections of the
reconstructed image. Note the high absorbing glass beads (4) and capillary tube (5).
Volumetric projection and isosurfaces rendering can be used to visualize the threedimensional data set. Fig. 5 shows isosurfaces rendering of eight glass beads (4), a
capillary tube (5), air bubbles (3) Elmer’s glue drop (6) and gelatin capsules (1 and 2).
Once the volumetric data is reconstructed by tomographic software, the size and
orientation of the encapsulated object can be obtained using a multidimensional image
analysis method [4]. Fig. 6 outlines the surface contours of the encapsulated
structures. With the knowledge of the surface contours, both the radius and volume of
each encapsulated structure can be computed as listed in Table 1.
Fig. 2. Stereo-pair of the model specimen showing in X-ray transmission.
Fig. 3. Stereo-pair of the model specimen showing in X-ray absorbance
(brighter the image, the higher the absorbance).
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Fig. 4. Representative cross-sections through the reconstructed data set. The outer circle
defines the reliable reconstruction region of cone-beam tomography. Various structures of
interest are clearly defined: (2) gelatin capsules, (3) air bubbles, (4) glass beads, (5) a glass
capillary and epoxy matrix (7).
Fig. 5. Isosurfaces rendering of the reconstructed specimen, showing various components.
Fig. 6. Surface contour of the four glass beads in the reconstructed image.
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Table 1. Volume of various components in the model sample.
Bead 1
Bead 2
Bead 3
Bead 4
Bead 5
Bead 6
Bead 7
Bead 8
Capillary wall
Radius (mm)
0.4875
0.4983
0.4846
0.5097
0.5176
0.4851
0.5048
0.4648
-
Volume (mm3)
0.4851
0.5180
0.4765
0.5543
0.5804
0.4779
0.5385
0.4204
5.0402
4 Discussion and Conclusion
The tomographic technique described above is intended for industrial QC. Hence, the
major consideration is placed on turn-around speed instead of image resolution. We
have demonstrated that 50 equal-angular projections are adequate for our purpose. We
plan to perform further tests with precisely made model specimen to calibrate the
system, and evaluate the performance.
In conclusion, a composite material specimen can be tomographically reconstructed using X-ray cone-beam microtomographic system, and subsequently segmented and quantitatively described. This technique has the capability of characterizing
encapsulated structures for QC, and holds the potential for other applications.
Acknowledgment
This project was supported by the University Academic Development Fund to PCC.
This work is part of the PhD thesis of AS.
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