Synchrotron-Based X-ray Computed Tomography
compression (10 -2 s -1 strain rate), with a tomographic temporal resolution of 1 s and spatial resolution of ~15 μ m.
Materials and Methods Foam and microlattice . T e fi rst sample described is a 3D printed foam, which was fabricated using an Objet500 Connex 3D printer (Stratasys Ltd., Eden Prairie, MN). T e printed material was a rubber-like elastomer (TangoBlack-FullCure 970). T is foam exhibits parallel tubular pores in the horizontal direction. T e pores exhibit a diameter of 0.75 mm and 1 mm pore spacing. T e second sample is a NiP microlattice. T e microlattice was fabricated by self-propogating photopolymer waveguide prototyping, which forms a periodic polymer lattice. T e metal microlattice is then fabricated from this template by coating the polymer lattice template using electroless deposition. Etching the polymer template leaves a periodic hollow tube metal microlattice. Details of microlattice fabrication can be found in references [ 23 ] and [ 24 ]. Tomogram acquisition . Micro-scale X-ray CT was per- formed at the 2-BM beam line at Argonne National Laboratory’s Advanced Photon Source. Details of the tomography setup have been described elsewhere [ 25 , 26 ]. A polychromatic (pink) beam (centered at ~27 keV) was used. T e high photon fl ux of the pink beam, as compared to a monochromatic white beam, was required for suffi cient signal-to-noise in the 1 ms-radiographs. Radiographs were collected using a PCO Dimax high-speed CMOS camera equipped with a 2×Mitutoyo long working distance objective lens, which resulted in 5.8 μ m isotropic voxel size in the reconstructed tomograms. Each tomogram consisted of 900 radiographs acquired over 180° sample rotation in 1 s, resulting in a radiograph every 0.2° of sample rotation at ~1 ms intervals. T e camera can operate much faster than 900 frames-per-second used here. T e rectan- gular beam geometry produced a fi eld-of-view of 2,016 pixels horizontally (11.09 mm) by 600 pixels vertically (3.30 mm). T e memory on the camera was suffi cient for ~18,000 radiographs. Laboratory-based CT operates so that the sample is rotated in discrete angular increments. Rotation motion is halted to allow for radiograph acquisition, typically on the order of seconds to minutes. However, a requirement for tomogram acquisition of fast dynamic processes is that the rotation stage must be syn- chronized with the radiograph acquisi- tion time. T e short 1 ms radiograph acquisition time used in this work required the sample rotation stage to be accelerated to a constant angular velocity during data acquisition in order to be synchronized. Any varia- tion in this velocity would result in a less than ideal tomogram. Ideally, the rotation stage would rotate at a con- stant angular velocity in one direc- tion for the total duration of the data acquisition (for this study, ~100 s). However, this was not possible due to the location and length of the load
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Figure 1 : Photo of the loading stage and X-ray CT setup at the APS 2-BM beam line.
stage’s power supply cord, which was located at the bottom base of the load stage. T erefore, an oscillating “washing machine” rotation was used. T e details of the rotation are as follows: for each tomogram, the sample rotation stage was accelerated clockwise from 0 o to 180° in 2 s to a constant angular velocity. T e stage was then rotated another 180° clockwise for 1 s at this constant angular velocity, during which 900 radiographs were acquired (constituting the fi rst tomogram). T e rotation stage was then decelerated over 180° clockwise for 2 seconds, for a total of 540° rotation per data acquisition cycle. T e rotation stage then stopped and reversed direction for the next tomogram, accelerating for 180° anticlockwise, maintaining a constant angular velocity for the next 180° anticlockwise during data acquisition, then decelerating for the next 180°
Figure 2 : Reconstructed XY (A) and XZ (B) slices of a 3D printed foam at different stages of compression. The scale bars are in micrometers.
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