Variable Zoom technique for X-Ray Computed Tomography
Introduction
X-ray Computed Tomography (CT) has evolved into an indispensable imaging method [1,2]. The objectivity of X-ray CT reconstructions made it a unique method for industrial diagnostics applications [3,4]. Modern cone-beam industrial CT systems capable of high-resolution imaging at micron scale are becoming essential in materials research and development [5,6], as well as structural diagnostics [[7], [8], [9]]. The fidelity of the nondestructive inspection (NDI) needed to quantify the smallest defects that impact structural performance is key to structural diagnostics [10]. For example, small individual flaws present at critical locations can dramatically affect strength and durability of composite structures [11]. Assessment of structural integrity of composite elements depends on accurate detection of structural defect locations and sizes leading to estimation of residual strength after fatigue or impact damage [12,13]. Due to three-dimensional (3D) nature of the critical defects and damage modes, high-resolution nondestructive methods able to capture the location and geometry/shape of the individual flaws impacting structural performance are required.
Despite the advances in CT, high-resolution 3D reconstruction of small defects in objects with large in-plane dimensions remains a fundamental challenge for the X-ray CT-based NDI. Current microfocus CT technology is based on full scanning (360° around the object, or at least 180° plus cone beam angle [14]), which limits the applicability of the technology to small cross sections. Furthermore, even the objects that can be scanned in the existing micro-CT facilities may not allow for sufficient magnification of the composite structure if the resolution requirements place the inspected objects too close to the X-ray tube such that a full scan cannot be completed. Reconstructions using the limited ranges of projection angles below 180° are available in the industrial CT systems, but they often result in inconsistent 3D reconstructions associated with missing projections due to partial access [14,15].
Extensive research has been undertaken to overcome the size limitations in X-ray CT. Pioneering works emerged in the medical field attempting to reconstruct small-scale regions of interest (ROI) in human bodies, with later expansion to industrial CT. Penβel et al. investigated modified 360° trajectory for ROI inspection of the partially accessible object [16]. The unconventional scanning trajectory was driven by the size and shape of ROI continuously shifting a specimen as a function of a rotation angle. The findings showed that the proposed trajectory could achieve a high-fidelity scan in the area of interest on simulated 2D phantoms. Dabravolski et al. used the acquisition trajectory following the convex hull of a specimen [17]. The proposed Adaptive Zooming technique showed a superior reconstruction quality on the artificial data. Maaβ et al. [18] tested several novel approaches combining data from low- and high-resolution scans to improve quality of a reconstruction. Kyrieleis et al. [19] have shown that extension of projections is suitable for reasonable approximation of the area of interest where high resolution is not required.
Computed Laminography (CL) is an alternative technique for large objects that cannot be accommodated in an X-ray CT system [20,21]. The technique allows partial access to a test specimen by irradiating an object at an inclined angle. CL has been shown to result in smaller artifacts as compared to limited-angle tomography [22]. Despite the improvement in overcoming geometric constraints related to large in-plane dimensions, CL is not capable to provide the same quality of the out-of-plane defect detection as the full-angle (360°) CT [[23], [24], [25]].
This work presents a new Variable Zoom X-ray CT technique that is targeted to overcome the limitations of large width-to-thickness aspect ratio in cabinet X-ray CT systems and to allow for additional flexibility in achieving high resolution for structures with large in-plane dimensions. The method eliminates the need to destroy the inspection article by cutting out a small section enabling accurate inspection of the composite structure at sufficient magnification. The proposed technique includes two essential aspects: nonconventional radiograph acquisition trajectory and a modification to the industry-standard Feldkamp–Davis–Kress (FDK) reconstruction method that includes weighting of radiographs based on the distance from the panel to the X-ray source and enables higher quality of 3D reconstruction. The analysis of reconstruction quality of CT images produced by the Variable Zoom technique, including dimensional and unsharpness measurements, is carried out on an artificial 3D phantom and on the CT scans of articles with the features of known dimensions. The capability of the method, with the special radiograph acquisition implemented into Shimadzu inspeXio SMX-225CT FPD HR industrial microfocus X-ray CT system, is also demonstrated on the detection of complex structural damage due to low-velocity impact, including accurate detection of through-the-thickness features, in large and thin composite laminate panels, which is among the worst-case scenario for the conventional CT methods.
This paper is organized as follows. Section 2 describes the scanning trajectory and the reconstruction method. Section 3 provides the assessment of Variable Zoom technique's accuracy and sharpness based on the artificial phantom. Section 4 provides similar assessment for CT scans of articles with known features. Section 5 demonstrates the technique's capability for impact damage detection in CT scans of large and thin laminate composite panels.
Section snippets
Scanning trajectory
Fig. 1 illustrates a cone-beam CT scan setup of a 401-mm Carbon/Epoxy composite laminate panel accommodated inside the Shimadzu X-ray CT system. The panel contains impact damage, which is localized in the central part of the laminate. For conventional CT scan, distance from the X-ray source to the axis of specimen's rotation, known as the source-to-object distance (SOD), remains stationary during the scan. The specimen rotates for a full 360° angular range while being imaged from an X-ray
Phantom definition
In this section the performance of the Variable Zoom technique is assessed using an artificial 3D phantom. The phantom volume represents a rectangular 400 mm-wide and 3.5 mm-thick block with a cylindrical defect (0.5 mm in diameter and height) located in the center of the phantom. Dimensions of the phantom mimic the composite panels presented in Section 5. The cone-beam projections of a phantom are generated using the projector function [32] and using the geometric parameters and acquisition
Dimensional measurements in a composite panel
Measurement-based validation was carried out on a large aspect ratio Carbon/Epoxy composite panel shown in Fig. 1 manufactured by Boeing using the Hexcel prepreg [38]. A 0.5-mm hole was drilled in the middle of the panel, and the hole diameter was confirmed by the Keyence Digital Microscope VHX-950F. The depth of the defect was 2.5 mm, which was estimated by a dial test indicator with 0.001 in (25.4 μm) precision. The dimensional measurements as well as the unsharpness measurements were
Test specimens
The Variable Zoom technique is demonstrated on X-ray CT scans of pre-impregnated continuous fiber-reinforced polymer composite panels which have been subjected to low-velocity impact damage. These specimens represent a challenge for the conventional X-ray CT due to large width-to-thickness aspect ratio. Large size of the panels prevents conventional CT scanning techniques from obtaining desired spatial resolution in the area susceptible to damage, which is typically of the size comparable to
Conclusions
This work demonstrated a novel X-ray Computed Tomography technique that is able to increase the spatial resolution for non-destructive inspection of plate-like objects with large in-plane dimensions relative to thickness. For instance, detection of damage in large composite plates can be identified as an important application of the proposed technique. The method incorporates two essential parts: a nonconventional trajectory of radiograph acquisition and a novel reconstruction weighting.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work is partially sponsored by the U.S. Army Research Laboratory under Cooperative Agreement Award W911NF-17-2-0195, technical monitor Dr. Robert Haynes. Such support is gratefully acknowledged. The views and conclusions contained in this manuscript should not be interpreted as representing the official policies, either expressed or implied, of the U.S. Government.
The work would not be possible without the support from Shimadzu Corporation that provided the necessary acquisition software
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