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.
A straight and parallel alignment specimen was used for initial alignment. It was machined such that it would only fit in the upper and lower grips if they were closely aligned. The benefit of X-ray tomography is the fact that 3D data sets are available, so we used actual images from X-ray tomography for finer alignment, without having to resort to strain gauges. By applying increasing loads to the pre-cracked specimen and measuring crack opening displacements, concentric alignments could be made to reduce the contributions from cracking in modes II and III. After alignment, the x-axis misalignment was about 1.8 μm, close to zero in the y-axis, and the tilt misalignment about 0.21, as shown in Figure 4. A symmetric crack growth front was observed when alignment was performed compared to misaligned condition, as shown in Figure 5. It can be seen that with misalignment, the crack grows much further on the right side of the specimen whereas the crack is quite symmetric after alignment.
Adem 9.0 Crack
Two dimensional (2D) X-ray tomography slices showing types of misalignment (a) crack growing into the plane, (b) side surface of the specimen and (c) 3D rendering showing axial and angular misalignment.
Third generationin situloading stage to perform mechanical testing with load-control capability and additional stiffness for higher R-ratio fatigue crack growth experiments.
2D X-ray synchrotron tomography images showing the progression of fatigue cracking have been shown in Figure 7. The fracture of a particle ahead of the crack tip during high load ratio fatigue can be clearly seen. Figure 7a shows that the particle (circled) was not fractured when the experiment was paused for tomography after 7,000 fatigue cycles. However the particle fractured at 8000 cycles (Figure 7b) and then the crack passed through the same fractured particle as shown in Figure 7c. This provides insight into the fundamental understanding on how the fatigue crack interacts with SiC particles, and the role of particle fracture, both ahead and right at the crack tip, in controlling fatigue crack propagation at high R-ratios.
In order to study the effects of corrosion-fatigue as well as stress corrosion cracking, new designs were developed. Corrosion-fatigue experiments were performed in a liquid environment, as shown in Figure 8. The PMMA sleeve has not been shown here in order to clearly show the arrangements inside the stage. Corrosion-fatigue experiments were performed on AA7075-T651 in EXCO solution (4 M NaCl, 0.5 M KNO3 and 0.1 M HNO3). EXCO was chosen to ensure a significant amount of corrosion in the limited time available at the synchrotron beamline. SEN specimens were machined along LT orientation by EDM for the corrosion-fatigue tests. The biggest challenge in the experiment was to avoid the reaction of corrosive fluid with any part of the loading stage, especially the grips. Therefore, the material used for the bottom grip must be chemically inert to the solution while also sustaining the applied load. A polymeric PEEK (Polyether ether ketone) cylindrical grip was chosen to replace the bottom steel grip since it is chemically resistant to the solution and has good mechanical strength [23]. The bottom grip was fitted with the load cell. A rectangular hole was made at the top part of the PEEK grip to accommodate the sample. A hole was made on the side wall of the cylindrical grip to accommodate the stainless steel pin which passes through the hole made in the sample. Epoxy was added to make this a permanent and strong connection. The top part of the sample was clamped as previously described. Kapton tubing, which is also chemically inert to EXCO solution, was attached to the PEEK cylinder with wax to hold the solution around the sample during the test. The height of the Kapton tube was chosen such that the notch of the specimen was immersed in the corrosive fluid.
Using the above design, we showed that the fatigue crack growth rate of AA7075-T651 was much higher in EXCO solution than in ambient air [15]. Figure 9(a) shows a 2D X-ray tomography image of the fatigue crack along with corrosion product and the hydrogen bubbles inside the crack formed due to reaction between AA7075 and the EXCO solution. As shown in Figure 9(b), the 3D reconstruction of the crack, bubbles, corrosive fluid and corrosion products was performed using commercially available software (MIMICS, Ann Arbor, MI). Corrosion products were segmented on a few slices (from the part of crack), as shown in Figure 9(b). Figure 10 shows the changes in the shape of the bubbles and the formation of a new bubble in a fatigue cycle. Formation of a new hydrogen bubble can be clearly seen at position 3 which was not present in position 2. All bubbles were squeezed as the crack closes during unloading (position 4). These results provided insights to the fundamentals of evolution of hydrogen bubbles inside a growing fatigue crack. In particular, local variations in pH cause inhomogeneous formation of reaction products and will affect crack growth.
In situX-ray tomography of corrosion in aluminum alloy (a) 2D X-ray tomography slice showing hydrogen bubbles, corrosive fluid, and corrosion products, (b) 3D reconstruction of the fatigue crack (bubble + fluid) and corrosion products from selected area of the segmented crack.
3D in situ stress corrosion cracking experiments in moisture. (a) Schematic of in situ stage for experiments in moisture. The PMMA sleeve is not shown to show the arrangements clearly and (b) relative humidity as a function of time. The relative humidity remains constant throughout the test.
The in situ stress corrosion cracking experiment was performed on AA7075 in under-aged (UA) condition under constant load. The under-aged AA7075 was used due to the limited time available at the synchrotron beamline as it has already been established that the under-aged alloy is most susceptible to stress corrosion cracking in moisture [24]. Small SEN specimens were machined along the ST orientation by EDM for the SCC tests. Preliminary results obtained from these arrangements are shown in Figure 12, which contains 2-D X-ray tomography images of a SCC crack over time at constant load at the center of the specimen. These results show that the SCC tests in moisture can be performed by these arrangements. It should be noted that the same arrangements can be used to perform fatigue test in moisture by changing the constant load to cyclic load.
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.
SS carried out experiments, analysis of data, and wrote the manuscript. JJW designed the testing jigs, carried out experiments, and analysis. PH helped with experimental analysis and image segmentation. XX and FDC provided experimental support at APS. NC helped design the experiments, ideas for crack growth and corrosion, and helped in writing the manuscript. All authors read and approved the final manuscript.
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Due to their layered architectures, most properties in laminated composites are inherently anisotropic. In terms of mechanical properties associated with crack growth, the most favorable properties are obtained when layers and interfaces are orientated perpendicular to the crack growth direction (crack arrester orientation) for quasistatic [19,20,21,22] as well as cyclic [23,24,25,26] loading. This can be attributed to the presence of extrinsic toughening mechanisms, resulting in a reduction of local stress intensity at the crack tip and thereby reducing the local crack driving force [27,28,29]. A comprehensive overview of different toughening mechanisms observed in laminated metal composites is provided by Lesuer et al. [27]. In crack arrester orientation, crack deflection [30,31,32], crack blunting [33], crack bridging [33], and stress redistribution mechanisms were observed in different LMC systems. 2ff7e9595c
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