Multi-Modal Synchrotron X-Ray Techniques for Visualizing 3D Brittle Fracture

Fracture is difficult to assess in real time. Even when crack propagation can be measured as it is occurring, usually only a 2-dimensional state can be quantified. Assessing 3-dimensional (3D) fracture has traditionally relied on post mortem analysis, but this does not capture the full intricacies of a growing crack front, such as a nonconstant velocity and shape. For brittle fracture, where very little plastic deformation occurs between the accumulation of stress concentration at a crack tip and its extension, these difficulties are compounded. Furthermore, a material's microstructure has a major effect on crack propagation, including its direction, speed, tortuosity, and even whether it arrests at points as a result of unequal stress distribution. Because of these complications, predicting crack paths, speeds, and shapes is a thorny problem which needs more in-situ data from 3D cracks occurring in real materials. Utilizing multi-modal x-ray characterization techniques available at a synchrotron source is a promising method for visualizing a crack's growth and directly relating it to the microstructure and micromechanical state of the studied material.<br/><br/>Our suite of multi-modal characterization techniques include far-field high-energy diffraction microscopy (FF-HEDM), near-field high-energy diffraction microscopy (NF-HEDM), and micro-computed tomography (u-CT). FF-HEDM provides per-grain elastic strain tensors, while NF-HEDM provides a high-quality shape and location for each grain. u-CT identifies the location of a crack in a material. Combining this information allows us to relate the grain structure and mechanical state found by the HEDM techniques with the crack location.<br/><br/>In this work, the material studied was aluminum oxynitride (AlON), a cubic ceramic with randomly oriented anisotropic grains on the order of 100 um. The Rotational and Axial Motion System (RAMS) [1] at the 1-ID beamline of the Advanced Photon Source at Argonne National Laboratory was used to carefully grow the crack and conduct these experiments. The sample being investigated was machined into the double-cleavage drilled compression (DCDC) geometry with parallelipiped dimensions of 18 mm x 1.4 mm x 1.4 mm and a hole diameter of 0.42 mm. The application of a uniaxial compressive load caused a crack to pop in to a length on the order of 100 um, and the crack was grown to over 1 mm by the application of further compressive load.<br/><br/>Every grain in the material had unique stress and strain tensors at each load that were not identical to the applied stress. Crack propagation characteristics such as its speed and path (intragranular or intergranular) were influenced as the crack interacted with these grains and complex micromechanical stress states. Because of the DCDC geometry, the crack could be grown a short distance, and then the loading could be paused along with the crack extension, creating a near in-situ state for analysis. By taking u-CT and HEDM scans in this load-controlled state, the relative extension of the crack with the increase in loading could be monitored, while also providing information on what features and grains in the microstructure caused the crack to slow down, arrest its propagation, or divert its direction of extension. We have found that certain grain orientations and accompanying stress tensors acted as barriers to crack extension and forced the crack either to change directions or to pause and accumulate enough stress concentration energy to advance. These findings can be used to inform computational methods and predictive models for future studies of brittle fracture.<br/><br/>[1] Shade, P. A., Blank, B., Schuren, J. C., Turner, T. J., Kenesei, P., Goetze, K., Suter, R. M., Bernier, J. v., Li, S. F., Lind, J., Lienert, U., & Almer, J. (2015). A rotational and axial motion system load frame insert for in situ high energy x-ray studies. Review of Scientific Instruments, 86(9), 93902. https://doi.org/10.1063/1.4927855