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Additive manufactured (AM) lattice structures have become very prominent in recent times especially in air and spacecraft industry for their lower weight and specific mechanical properties. Their stiffness and strength can be controlled by their geometrical properties, such as the shape and dimensions of the unit cell. Geometrical and dimensional accuracy of the AM lattices is therefore one of the most important requirements to meet the desired functionality as there could be significant deviations in the as-produced part from the designed model; thus their measurements are of great significance. X-ray computed tomography (CT) has emerged as a promising solution in the field of industrial quality control over the last few years due to its non-destructive approach. However, CT measurement accuracy depends on various parameters (part material, system, operator, environment, data post-processing), among which the resolution or voxel size of the CT data is crucial. In this work the influence of resolution on the measurement of metallic lattice structure is studied by means of simulations and real CT experiments. The optimized CT acquisition settings are obtained with the help of simulated radiographs and design-of-experiment approach. Three different resolutions are achieved by placing the part at different positions from the X-ray source. The computer-aided design (CAD) model comparison reveals that the majority of surface has a deviation of ± 0.2 mm and the results are slightly affected by the resolution. The wall thickness analysis provides a global observation of the strut and node thicknesses. Individual struts are measured with representative regions of interest (ROIs) considering the manufacturing direction. The measurement results are significantly affected by the resolution (or voxel size) of the CT data. Simulated CT scans with different resolutions have been performed for systematic error estimation in relation to the voxel size.

Robotic inspection is one of the acknowledged new trends in X-ray Non Destructive Evaluation (NDE) since it allows more flexibility in the acquisition trajectory and therefore a valued adaptability to object and environment constraints. In this context, we are developing an advanced Computed Tomography (CT) robotic platform (see Figure 1) consisting of two robots equipped, with a micro-focus X-ray tube and a flat panel detector, respectively. In parallel to the equipment installation, we propose to address the new challenges brought by this robotic inspection. In particular we focus on 3D iterative reconstruction algorithms that deal with few and limited-angle data. For this purpose, we consider a regularized algebraic method named Discrete Algebraic Reconstruction Technique (DART) [1] that incorporates prior knowledge about the different materials (attenuation coefficient) of the scanned object. This regularization shows a great improvement in the reconstruction and can be applied for any object consisting of five or less different materials. In this paper, we present an algorithm named DART-TV-FISTA which is based on DART with Total Variation (TV) regularization and Fast Iterative ShrinkageThresholding Algorithm (FISTA) to increase the convergence speed. For performance evaluation, we illustrate a numerical comparison of SART [2], DART and DART-TV-FISTA from both noiseless and noisy data generated by CIVA [3]. We also show reconstruction results using the robotic inspection platform with a view angle limited to 150° and a reduced number of projections. The obtained results show that the proposed DART-TV-FISTA algorithm can improve the image quality and performs better than the original DART algorithm.

The presence of lattice structures is increasing in the manufacturing domain especially in the air/spacecraft and biomedical applications due to their advantages of high strength-to-weight ratios, energy absorption, acoustic and vibrational damping etc. Dimensional accuracy of a lattice structure is one of the most important requirements to meet the desired functionality as there could be significant deviations in the as-produced part from the designed one. Evidently, an approach (non-destructive) to evaluate the dimensional accuracy of all the elements and eventually the lattice quality is of great significance. X-ray computed tomography (CT) has emerged as a promising solution in the field of industrial quality control over the last few years due to its non-destructive approach. In this work, we propose a methodology for geometrical evaluations of a lattice structure by measuring the deviation in shape and size of its strut elements holistically. The acquired CT data of the complete lattice is extracted in the form of a point cloud and then segmented and stored as single strut element with unique identification so that measurements can be performed on strut individually. As demonstrated with a metallic BCCz type lattice structure, the methodology helps in critical evaluation of its quality and the correlation with spatial position of the individual struts; e.g. the lattice exhibits large variations of shape among the inclined struts while the vertical struts possess consistency in their shape.

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