Synchrotron-Based X-ray Computed Tomography
Table 1 : Physical dimensions of NiP microlattices reported in reference [ 23 ] and in this work. Sample
Description Low density a Mid density b High density c Current work d
1 mg cm -3 14 mg cm -3 43 mg cm -3 unknown
a See Figure 3C in reference 23 . b See Figure 3A in reference 23 . b See Figure 3A in reference 23 . d Physical dimensions were measured from CT data.
at 0% compressive strain, after applying an edge-preserving smoothing filter [ 31 ] to the data set (compare to Figure 2A , top left). For this data set, the filter performs an adequate smoothing of the noise in three dimensions while keeping desired edges intact, allowing for sufficient grayscale-based segmentation of the tomogram ( Figure 5 , middle). The histograms of the unprocessed and filtered tomograms ( Figure 5 , bottom) are radically different, with an increased separation of the two grayscale distributions. By applying this filter to the tomogram, noise is greatly reduced and ring artifacts are virtually eliminated.
From the volume renderings at four stages of compressive
strain ( Figure 4 ), the surface roughness of the foam can be visualized. Additionally, the volume rendering allows for a better visualization of the two outer void structures at the 45% compressive strain, when compared to the reconstructed slices presented in Figure 2A . Volume renderings of the two inner tubular voids, at 0% compressive strain and 29% compressive strain, are presented in Figure 6 . Segmenting the tomograms for the void structures allows for not only visualization of the void volume, but also quantification of the void height and width as a function of compression. NiP Microlattice . As stated in the introduction, a NiP microlattice was also imaged during uniaxial loading. This material is similar to those reported by Schaedler et al. [ 23 ]. In their report, NiP microlattices exhibited different stress responses during cyclical compressive strains, up to ~50%,
Figure 7 : Volume renderings of the NiP microlattice at different periods of compression. The volume renderings correspond to the 1 st tomogram (top left), the 6 th tomogram (top right), the 8 th tomogram (bottom left), and the 13 th tomogram (bottom right). The scale bar is in micrometers.
16
which were attributed to different physical dimensions of the lattice. These physical dimensions include L , the lattice member length; D , the lattice member diameter; and t , the lattice member wall thickness ( Table 1 ). NiP microlat- tices with a small wall thickness-to-diameter ratio ( t / D ) exhibited reversible compressive behavior and excellent recovery of their initial shape and height, whereas those with a large t / D ratio exhibited deformations typical of metallic cellular structures. For the sample examined in this work, the t / D ratio is much larger (by 2 to 3 orders of magnitude) than those that exhibit reversible compressive behavior ( Table 1 ) because the large wall thickness was 40 µm. This resulted in a loss in plastic deformation, moving directly to brittle fracture of the microlattice, evident in the volume renderings presented in Figure 7 , where a broken lattice ligament can be seen lying on the bottom of the loading stage (bottom two images of Figure 7 ). The stress-time curve of the NiP microlattice sample ( Figure 8 ) exhibits high stress (~1300 kPa) at a time of 25 s, corresponding to the sixth tomogram acquired ( Figure 7 , top right). Figure 9 presents a series of radiographs, acquired at different time intervals of the dynamic compression of a separate NiP microlattice sample. These radiographs, acquired during the 6 th tomogram of the series, highlight the brittle fracturing of the NiP microlattice during compress- ion. The radiograph acquired at 7 ms (top right) exhibits significant motion blur of the NiP ligaments, due to mechanical failure of the microlattice at the ligament nodes. This failure occurs at a shorter time scale than the radiograph acquisition time, causing the blurring of the image. Also, the direction of the ligaments’ movement is perpendicular to the camera. At 396 ms, the sample has rotated 70° from the previous image, thus giving the appearance that ligament bending has not occurred, when in fact the bent ligaments are now parallel to the camera. The black arrow in this radiograph indicates a ligament that broke off of the main body of the sample at two nodes, as can be seen in the 398 ms radiograph (only 2 ms later). In the next four radiographs, the ligament
www.microscopy-today.com • 2015 May
L (µm) 4,000 1,050 1,050 1,600
D (µm) 500 150 150 260
t (µm) 0.12 0.50 1.4 40
t / D
0.24 × 10 -3 3.33 × 10 -3 9.33 × 10 -3 154 × 10 -3
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