Dual-Energy Computed Tomography for Ceramics and Composite Materials

Abstract

INTRODUCTIONThis work is part of the Brite-Euram project BRE5535 "DUALETO", whose purpose is to set-up a Dual-Energy Computed Tomography (DE-CT) system suited for the examination of fibers-reinforced composites. DE measurements are obtained by a microfocus X-ray source, and a multi-wire proportional chamber which simultaneously collects projection data at two different energies over a section of the object [1].General data processing is sketched in Figure 1. The pre-reconstruction calibration separates the DE projection data into fibers and matrix components, assuming that their photoelectric (low-energy) and Compton (high-energy) attenuation properties differ significantly [2][3]. Then fibers and matrix images are reconstructed using the filteredbackprojection (FBP) algorithm. These images are quantitative in the sense that the pixel value is really a density (glcm 3 ), but their quality is poor for many reasons. The flux emitted by the source is low, the detector has a low counting rate, and matrix and fibers may have similar compositions. Assuming a 40 Ilm spatial resolution, the structures in the images cannot be individual fibers (7-30 J.Lm diameter), but bundles of fibers, matrix and holes. The low photon flux and the DE calibration process imply considerable noise. Further processing is thus needed. A method based on a Bayesian segmentation, and then a non-linear filtering of the density images according to the segmented image, is proposed. This paper first presents the DE calibration method. Then simulated DE images illustrate the need for post-processing. The third part is dedicated to the Bayesian segmentation. Lastly, we describe how the segmented image can be used to enhance the density images and to compute a map of the volumic fraction (VF) of the fibers.Another research [4] is oriented towards a priori calibration (based on a physical model rather than on the experimental determination of the calibration function) and unsupervised segmentation. PRE-RECONSTRUCTION DUAL-ENERGY CALIBRATIONA CT projection over a polychromatic energy band [EI, E2], for a given ray-sum r, and for an object composed oftwo basis materials "fibers" and "matrix", is:

Segmentation and Density-Evaluation of Fiber-Reinforced Materials by Dual-Energy Computerized Tomography

“…Within the Brite project, another segmentation method has been developed for enhancing image quality of DE-measurements by C. Robert et al, [12). That method also uses label images for filtering density images, but the label images are computed by maximizing the a posteriori probability of the solution according to the rule of Bayes.…”

A simple portable low-pressure healant-injection device for repairing damaged composite laminates

“…18,19 Other less conventional methods, such as X-ray computed tomography in single energy and dual-energy modes, are highlighted briefly. 20–23

Figure 2.Types of damage to a CFRC laminate. (a) Point image.

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