Synchrotron-Based X-ray Computed Tomography During Compression Loading of Cellular Materials
Nikolaus L. Cordes , 1 Kevin Henderson , 1 Tyler Stannard , 2 Jason J. Williams , 2 Xianghui Xiao , 3
Mathew W. C. Robinson , 4 Tobias A. Schaedler , 5 Nikhilesh Chawla , 2 and Brian M. Patterson 1 * 1 Polymers and Coatings Group , Materials Science and Technology Division , Los Alamos National Laboratory , P.O. Box 1663 , Los
Alamos , NM 87545 2 Department of Materials Science and Engineering , School for Engineering of Matter , Transport, and Energy , Arizona State
University , Tempe , AZ 85287-6106 3 Advanced Photon Source , Argonne National Laboratory , 9700 South Cass Ave. , Argonne , IL 60439-4837 4 Atomic Weapons Establishment , Aldermaston , Reading , Berkshire , RG7 4PR , UK 5 HRL Laboratories LLC, 3011 Malibu Canyon Rd., Malibu, CA 90265
*
bpatterson@lanl.gov
Introduction T ree-dimensional (3D) X-ray computed tomography
(CT) of in situ dynamic processes provides internal snapshot images as a function of time. Tomograms are mathemati- cally reconstructed from a series of radiographs taken in rapid succession as the specimen is rotated in small angular increments. In addition to spatial resolution, temporal resolution is important. Temporal resolution indicates how close together in time two distinct tomograms can be acquired. Tomograms taken in rapid succession allow detailed analyses of internal processes that cannot be obtained by other means. T is article describes the state-of-the-art for such measurements acquired using synchrotron radiation as the X-ray source. Lab-based CT . Laboratory-based micro-scale X-ray CT,
fi rst developed in 1982 by Elliot and Dover [ 1 ], is oſt en used to image (in 3D) samples in a static state. With temporal resolutions of ~1 h to 36 h, this technique may be used for the imaging of dynamic processes that occur over long periods of time, similar to time-lapse photography. A recent example of time-lapse CT was published by Lowe et al., in which the authors imaged a living butterfl y chrysalis at diff erent stages of metamor- phosis [ 2 ]. For fast dynamic processes that are not easily controlled, a “post mortem” imaging approach is typically taken, in which X-ray images (that is, tomograms) are acquired before and aſt er the dynamic process [ 3 , 4 ]. For processes that can be easily controlled, a “quasi-static” (also referred to as “interrupted in situ ”) approach is taken [ 5 , 6 ]. With this approach, a sample is incrementally exposed to the dynamic process, such as tensile or compressive strains, with tomograms acquired in between each interrupted step; however, some information is lost when the dynamic process is paused (for example, crack propagation) while the tomogram is acquired. Oſt en, either the dynamic process may not be paused, or some information is lost during the pause and therefore the interrupted in situ modality is not possible. For example, the interrupted in situ compression of soſt materials (for example, polymer foams) has been investigated with laboratory-based micro-scale CT, by taking a full tomogram at incremental compression steps [ 5 ]. A simple observation of the stress-strain curve (see Figure 7A in reference 5 ) of a silicone foam shows that stress relaxation of the polymer foam occurs as a result of the applied uniaxial compression. T erefore, in between compression steps, a pause
12
of typically 5 to 15 minutes is needed before the tomogram is acquired. Without this pause, residual motion within the sample leads to blurring of the tomogram and a complete loss of stress relaxation information. Synchrotron-based CT . For adequate imaging of systems undergoing fast dynamic processes, faster imaging is needed. T is requires a high photon fl ux of the X-ray source, high quantum effi ciency of the scintillator (that is, material that converts X-ray photons to visible light photons), and a fast-frame-rate imaging detector. Synchrotron-based X-ray CT, fi rst hypothesized by Grodzins in 1983 [ 7 , 8 ] and fi rst realized by T ompson et al. in 1984 [ 9 ], off ers the ideal solution for this problem. T ird-generation synchrotron sources [ 10 ], such as Argonne National Laboratory’s Advanced Photon Source, can off er X-ray photon fl uxes ~10 12 to 10 14 photons s -1 , which is 3 to 5 orders of magnitude brighter than what is available in the laboratory. With the availability of high-speed optical cameras, 4D (the fourth dimension being time) synchrotron-based CT is readily achievable.
Many experiments involving synchrotron-based CT are similar to the lab-based post mortem [ 11 ] or interrupted in situ [ 12 , 13 ] experiments. However, recent studies have focused on fast imaging, with temporal resolutions on the order of sub-second to tens of minutes. Recent examples include the investigation of insect mechanics [ 14 , 15 ], simultaneous heating and continuous compression of an aluminum alloy [ 16 ], continuous tensile testing of matrix composites coupled with acoustic emission characterization [ 17 ], investigation of creep damage in a brass alloy [ 18 ], the solidifi cation of Al-Cu alloys [ 19 ], and investigations of fatigue crack growth in SiC particle- reinforced aluminum alloy matrix composites [ 20 ]. A current “hot topic” in engineering is additive manufacturing (AM) (also referred to as 3D printing or rapid prototyp- ing), which is the building of 3D structures layer-by-layer with minimal material waste, as opposed to traditional casting or machining methods. For a recent review on the history and current developments of AM, see reference [ 21 ]. With AM, unique structures are possible, especially with foams and lattice- structured materials that do not rely on stochastic foaming or pore-templating agents in order to produce voids [ 22 ]. Foam structures using AM may be created through direct engineering. Herein, we describe synchrotron-based CT imaging of two advanced cellular materials undergoing dynamic, continuous
doi: 10.1017/S155192951500019X
www.microscopy-today.com • 2015 May
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