Synchrotron-Based X-ray Computed Tomography
anticlockwise. T is procedure was then repeated 18 more times to acquire a total of 20 tomograms per sample. Tomograms were recorded every fi ſt h second. T e com- pression stage was held stationary during the fi rst tomogram then continuously compressed during the remainder of the tomographic acquisition at a strain rate of 10 -2 s -1 . T is pro- cedure was repeated for a total of 20 tomograms, aſt er which a fl at-fi eld background and dark-fi eld background, each consisting of 50 radiographs, was acquired. T e total duration of the 20-tomogram acquisition took place during a 100 s time span.
Compression tests . T e custom loading stage ( Figure 1 ) used for these experiments is described in reference [ 27 ]. Compressive loading of the sample was initiated aſt er the fi rst tomogram was acquired, at a strain rate of 10 -2 s -1 . T e displacement distance and displacement rate of the loading stage was set to -2.5 mm and 18 μ m s -1 , respectively, for the 3D printed foam. T is resulted in a total compression time of 138.8 s. For the NiP microlattice, the displacement distance and displacement rate was -1 mm and 18 μ m s -1 respectively, resulting in a total compression time of 55.5 s. T erefore, compression of the NiP microlattice was halted aſt er the 11 th tomogram. T e drive platen moved approximately three vertical pixels per tomogram and did not lead to image blurring. During compression, samples were not constrained by the X-ray transparent poly(methyl methacrylate) (PMMA) sleeve and were not physically attached to the compression platens. Although total data acquisition time was 100 s, a period of 15–20 minutes was required before the next sample could be imaged, to allow for data transfer from the PCO Dimax camera to the data acquisition PC. Upgrades to the camera controller will increase write and read-out speeds making it possible to increase experimental compression rates by a factor of two. Data processing . Tomogram reconstruction was performed using TomoPy [ 28 ], a Python-based reconstruction program developed at APS. T e data were reconstructed and saved as 16-bit slices. Filters used during reconstruction include stripe removal (for example, coif16); phase retrieval (alpha set to 1 or 5 × 10 -5 for our samples), which helps to set the best contrast; normalization to maximize the grayscale; and fi nally a zinger (despeckle) removal [ 29 ]. Stripe removal, phase retrieval, and normalization fi lters are not independent of each other and require several iterations to balance for the best reconstruction. Each data set of 20 tomograms was 96 GB in size. Reconstructed tomogram visualization and analysis was performed using Avizo Fire version 8.1 (FEI Visualization Sciences Group, Burlington, MA). Smoothing of the reconstructed tomograms of the 3D printed foam was performed using the Edge-Preserving Smoothing fi lter found in Avizo Fire. No post- reconstruction processing was required for the NiP microlattice tomograms because of the high signal-to-noise ratio of those tomograms. Segmenta- tion of the tomograms was performed using the Interactive T reshold module found in Avizo Fire, resulting in binary images suitable for volume rendering.
14
Post-reconstruction image processing and visualization was performed using a Hewlett-Packard Z820 workstation equipped with 96 GB of RAM, 2 Intel Xeon quad-core processors, and a Nvidia Quadro 6000 graphics card.
Results
3D Printed Foam . Figure 2A presents slices in the XY (that is, top-down view) direction of the reconstructed tomograms of the 3D printed foam at 0%, 15%, 29%, and 45% compressive strain. As can be seen in Figures 2 A and 2 B, four tubular voids are present, which obviously decrease in height and width during uniaxial loading. T is decrease is best visualized in the XZ (side view) direction presented in Figure 2B . While the two inner tubular voids completely collapse at the last stage of compression (45% strain), there is still void structure present in the two outer tubular voids. T is is due to the lack of stress in the lateral direction caused by the limited amount of ligament material in the outer region of the voids. Figure 3 is a stress-strain curve of the 3D printed foam acquired during dynamic uniaxial compression. T e curve mostly follows that of a typical elasto- meric foam as described by Gibson and Ashby (see Figure 5.1a in reference [ 30 ]). T e stress-strain curve exhibits a plateau
Figure 3 : Stress-strain curve of the 3D printed foam acquired during CT imaging. Square symbols indicate times at which a tomogram was acquired. Black squares indicate the tomograms presented in Figures 2 and 4 .
Figure 4 : Volume rendering of the 3D printed foam at different stages of compression. The scale bar is 3,000 micrometers.
www.microscopy-today.com • 2015 May
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