In situ experimental techniques to study the mechanical behavior of materials using X-ray synchrotron tomography
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In situ X-ray synchrotron tomography is an excellent technique for understanding deformation behavior of materials in 4D (the fourth dimension here is time). However, performing in situ experiments in synchrotron is challenging, particularly in regard to the design of the mechanical testing stage. Here, we report on several in situ testing methods developed by our group in collaboration with Advanced Photon source at Argonne National Laboratory used to study the mechanical behavior of materials. The issues associated with alignment during mechanical testing along with the improvements made to the in situ mechanical testing devices, over time, are described. In situ experiments involving corrosion-fatigue and stress corrosion cracking in various environments are presented and discussed. These include fatigue loading of metal matrix composites (MMCs), corrosion-fatigue, and stress corrosion cracking of Al 7075 alloys.
KeywordsX-ray tomography In situ Fatigue Corrosion Crack AA7075
A sound understanding of mechanical behavior requires a thorough understanding of the evolution of deformation with time. Traditionally, the study of material structure has been limited by two dimensional (2D) analyses. This approach is often inaccurate or inadequate for solving many cutting-edge problems. In addition, it is often laborious and time-consuming. We can now use three dimensional (3D) tools and analyses to resolve time-dependent (4D) evolution of a variety of important phenomena [1, 2, 3, 4].
X-ray tomography is an extremely promising, non-destructive technique for characterizing microstructures in 3D and 4D. The use of high brilliance and partially coherent synchrotron light allows one to image multi-component materials from the sub-micrometer to nanometer range.
Over the years, many ex situ X-ray tomography experiments have been performed to study the behavior of materials under mechanical loading [5, 6, 7]. These experiments have consisted of post-mortem characterization after testing. More recently, in situ mechanical testing has become more attractive, as a means of visualizing and quantifying microstructural changes as a function of time. Experiments under tension [8, 9, 10], cyclic loading [11, 12, 13, 14], corrosion-fatigue , and creep [16, 17] have been conducted.
Several challenges exist when designing an in situ mechanical testing stage for synchrotron tomography beamlines. The stage will be on top of several motion stages which have weight limits for precise and reproducible movement. This constrains the size and weight of the motorized linear actuator. Also, since precise rotational movement is necessary for tomography, so the load frame’s weight must be distributed as symmetrically as possible about the rotational axis. In addition, the stage requires an X-ray transparent material in the scanning area which should also be able to bear the applied load without excessive deformation. Furthermore, the load train has to be precisely aligned for the smaller samples used in tomography. This becomes even more important for in situ experiments on fatigue crack growth, for example. Finally, the distance between the fixture and the detector should be sufficient enough such that there is no contact between the load bearing sleeve and the scintillator during scanning. This can potentially affect the amount of absorption (density-based) contrast, with smaller distances yielding more absorption contrast.
In this paper, we report on several in situ testing methods developed in our group at ASU in collaboration with scientists at the Advanced Photon Source at the Argonne National Laboratory. We begin by describing our initial attempts at in situ loading in tension. This is followed by a discussion of the important issues associated with alignment, both in tension and in fatigue loading. Finally, we conclude with a section on in situ experiments in corrosion and moisture environments, which carry with them a unique set of challenges.
Experimental details on X-ray synchrotron tomography
X-ray tomography was performed at the 2-BM beamline of the Advanced Photon Source (APS) at Argonne National Laboratory. The details of the tomography system at 2-BM have been described elsewhere [18, 19, 20]. In all the experiments, a monochromatic beam with energy of ~ 24 keV (except in the case of corrosion-fatigue experiment, where pink beam was used and will be described later) was focused on the specimen. A scintillator screen (CdWO4 or LuAG:Ce) was used to convert the transmitted X-rays to visible light. This was coupled with an objective lens and a camera (CoolSnap K4 CCD or PCO Dimax CMOS camera) to obtain images. 2D projections were acquired at angular increments of 0.12° over a range of 180°. The tomography at one time step can be completed in about 15–20 minutes when a monochromatic beam is used. These 2D projections were then input into a filtered back-projection reconstruction algorithm (Shepp-Logan filter) to obtain 3D information.
Tensile and fatigue behavior
A few limitations of the loading stage include the maximum cyclic frequency of 2 Hz and the maximum load of only 500 N. Also, given that the fixture must rotate 180 degrees, and that it has a load bearing sleeve, 50 mm was the closest distance our samples could safely be from the scintillator without hitting it - a distance that yields significant phase contrast when using 24 keV X-rays (produced by a bending magnet).
Using the second loading stage shown in Figure 3, in situ fatigue crack growth tomography was performed on AA7075-T651 samples [12, 13]. The fatigue crack growth rates obtained from these experiments [12, 15] were comparable to published data by others . The errors in the ∆K (stress intensity factor range) measurements were up to a maximum of only 7% due to error in measurement of force. The crack lengths can be measured accurately with an error of 1–2 pixels, the effect of which was very small in the crack growth rates (da/dN) and ∆K measurements. To achieve higher resolutions in tomographic reconstructions, the width of our samples was chosen to be about 2.8 mm to achieve a desired resolution of 2 μm. A single edge-notched (SEN) geometry was chosen for these experiments because a CT specimen would be too wide to fit into a 3 mm field of view (necessary to achieve a 2 μm resolution), and a 3 mm wide MT specimen could not be machined precise enough to achieve symmetric crack nucleation on each side of the central hole. The experiments were performed in the Paris law regime because of the limited amount of time available at the beamline (typically three days), and because the highest cycling frequency achieved by our linear actuator was only 2 Hz.
We present some preliminary results on the fatigue tests at high R-ratio (R = 0.6) in Al-SiC metal matrix composites (MMCs) using the third loading stage. The effect of high R-ratio on the crack propagation in MMCs has not been well understood. A 2080 aluminum alloy (3.6% Cu, 1.9% Mg, 0.25% Zr) reinforced with 20 volume percent SiC particles (average particle size of 25 μm) was used. The composite was processed by blending SiC and Al powders, compacting the powder mixture, hot pressing, and hot extrusion (Alcoa Inc., Alcoa, PA). Details of the powder metallurgy process for fabrication of these composite materials can be found elsewhere . SEN specimens were machined by EDM for the fatigue tests.
Corrosion-fatigue and stress corrosion cracking (SCC)
“Pink beam”, which is a polychromatic beam with low and high energies removed from the white beam spectrum, was used during corrosion experiments, because the higher photon flux compared to a monochromatic beam allowed for significantly faster data acquisition. Thus the progression of corrosion could be documented with a higher frequency. While a typical scan with a double-multi-layer monochromatic (DMM) beam takes twenty minutes to complete, a pink beam scan can take only 0.5–1 second. Although much faster scans are technically possible using a pink beam, significant X-ray attenuation by the corrosive fluid required higher exposure times (2–3 minutes) to achieve a suitable signal-to-noise ratio.
In situ techniques using X-ray synchrotron tomography to understand the mechanical behavior of materials under variety of conditions have been explored. We have described several in situ loading methodologies, challenges, and solutions. The provisions for alignment led to symmetrical crack front in the second generation in situ mechanical testing stage. The third generation in situ loading stage provided load-control capability and additional stiffness to perform high R-ratio fatigue crack growth experiments. These loading stages were used for mechanical testing (monotonic and cyclic loading) in ambient air as well as in corrosive environments such as EXCO solution or moisture.
We acknowledge financial support from the Office of Naval Research, under contract number N000141010350 (Dr. Asuri Vasudevan, Program Manager). Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. We also thank Carl Mayer at Arizona State University for useful discussions.
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