Clément Jailin

Measurement of 3D displacement fields from few tomographic projections

The present paper aims at providing 3D volume images of a deformed specimen based on i) a full 3D image describing the reference state as obtained e.g., from conventional computed tomography and ii) the 3D displacement field accounting for its motion. The displacement field, which is described by much fewer degrees of freedom than the specimen volume itself, is here proposed to be determined from very few projections. The reduction in number of needed projections may be larger than two orders of magnitude. In the proposed approach, the displacement field is described over an unstructured mesh composed of tetrahedra with linear shape functions. The mesh is based on the reconstructed reference volume so that it provides a faithful and accurate description of the specimen, including its boundary. Nodal displacements are determined from the minimization of the quadratic difference between the computed projections of the deformed configuration and the acquired projections (radiographs) for the selected orientations. Well-posedness of the problem requires the number of kinematic unknowns to be small. However, in cases where the geometry is complex, the displacement field may call for many parameters. To deal with such conflicting demands it is proposed to use a regularization based on the mechanical modeling of the displacement field using a linear elastic description.

On the use of flat-fields for tomographic reconstruction

Seeking for quantitative tomographic images, it is of utmost importance to limit reconstruction artifacts. Detector imperfections, inhomogeneity of the incident beam, as classically observed in synchrotron beamlines, and their variations in time are a major cause of reconstruction bias such as `ring artifacts. The present study aims at proposing a faithful estimate of the incident beam local intensity for each acquired projection during a scan, without revisiting the process of data acquisition itself. Actual flat-fields (acquired without specimen in the beam) and sinogram borders (when the specimen is present), which are not masked during the scan, are exploited to construct a suited instantaneous detector-wide flat-field. The proposed treatment is fast and simple. Its performance is assessed on a real scan acquired at ESRF ID19 beamline. Different criteria are used including residuals, i.e. difference between projections of reconstruction and actual projections. All confirm the benefit of the proposed procedure.

Fast Tracking of Fluid Invasion Using Time-Resolved Neutron Tomography

Water flow in a sandstone sample is studied during an experiment in situ in a neutron tomography setup. In this paper, a projection-based methodology for fast tracking of the imbibition front in 3D is presented. The procedure exploits each individual neutron 2D radiograph, instead of the tomographic-reconstructed images, to identify the 4D (space and time) saturation field, offering a much higher time resolution than more standard reconstruction-based methods. Based on strong space and time regularizations of the fluid flow, with an a priori defined space and time shape functions, the front shape is identified at each projection time step. This procedure aiming at a fast tracking the fluid advance is explored through two examples. The first one shows that the fluid motion that occurs during one single 180

Separation of superimposed images with subpixel shift

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Experimental database with full-field measurements for mixed-mode crack propagation in concrete: comparison between experimental and numerical results

∘ scan can be resolved at 5xa0Hz with a sub-pixel accuracy whereas it cannot be unraveled with plain tomographic reconstruction. The second example is composed of 42 radiographs acquired all along a complete fluid invasion in the sample. This experiment uses the very same approach with the additional difficulty of large fluid displacement in between two projections. As compared to the classical approach based on full reconstructions at each invasion stage, the proposed methodology in the studied examples is roughly 300 times faster offering an enhanced time resolution.

In situ μ CT-scan Mechanical Tests: Fast 4D Mechanical Identification

The motion of a sample while being scanned in a tomograph prevents its proper volume reconstruction. In the present study, a procedure is proposed that aims at estimating both the kinematics of the sample and its standard 3D imaging from a standard acquisition protocol (no more projection than for a rigid specimen). The proposed procedure is a staggered two-step algorithm where the volume is first reconstructed using a “Dynamic Reconstruction” technique, a variant of Algebraic Reconstruction Technique (ART) compensating for a “frozen” determination of the motion, followed by a Projection-based Digital Volume Correlation (P-DVC) algorithm that estimates the space/time displacement field, with a “frozen” microstructure and shape of the sample. Additionally, this procedure is combined with a multi-scale approach that is essential for a proper separation between motion and microstructure. A proof-of-concept of the validity and performance of this approach is proposed based on two virtual examples. The studied cases involve a small number of projections, large strains, up to 25%, and noise.

Self-calibration for lab-μCT using space-time regularized projection-based DVC and model reduction

The problem of the separation of superimposed images is considered in the particular case of a steady background and a foreground that is composed of different patterns separated in space, each with a compact support. Each pattern of the foreground may move in time independently. A single pair of these superimposed images is assumed to be available, and the displacement amplitude is typically smaller than the pixel size. Further, assuming that the background is smoothly varying in space, an original algorithm is proposed. To illustrate the performance of the method, a real test case of X-ray tomographic radiographs with moving patterns due to dust particles or surface scratches of optical elements along the beam is considered. Finally an automatic and simple treatment is proposed to erase the effects of such features.

Digital Volume Correlation: Review of Progress and Challenges

The measurement of four-dimensional (i.e. three-dimensional space and time) displacement fields of in situ tests within X-ray computed tomography scanners (i.e. lab-scale X-ray computed tomography) is considered herein using projection-based digital volume correlation. With a single projection per loading (i.e. time) step, the developed method allows the loading not to be interrupted and to vary continuously during the scan rotation. As a result, huge gains in acquisition time (i.e. more than two orders of magnitude) need to be reached. The kinematic analysis is carried out using predefined space and time bases combined with model reduction techniques (i.e. proper generalized decomposition with space–time decomposition). The accuracy of the measured kinematic basis is assessed via gray-level residual fields. An application to an in situ tensile test composed of 127 time steps is performed. Because of the slender geometry of the sample, a specific beam space regularization is used, which is composed of a stack of rigid sections. Large improvements on the residual, the signal-to-noise ratio of which evolves from 9.9 to 26.7u2009dB, validate the procedure.