Combining Digital Image Correlation and Pulsed Ultrasonic Signals for Stress and Strain Analysis in Synchrotron-Based High-Pressure, High-Temperature Oscillatory Deformation Studies

Deformation of minerals plays a vital role in material transpor within the earth. The D-DIA apparatus coupled with synchrotron radiation allows for controlled deformation of minerals under high pressure and temperature while permitting in situ measurements of stress and strain. Conventionally, lattice strain (stress) is determined by fitting diffraction peaks converted from 2D powder diffraction images into 1D intensity profiles, whereas macroscopic strain is calculated from pixel positions of strain markers in radiographs. Both techniques are limited in resolution. In the case of stress, this arises from the need to integrate the 1D profiles over a finite azimuthal range, whereas (macro)strain resolution is limited by pixel size. Digital image correlation (DIC) provides a method of accurately tracking changes between images to subpixel accuracy. In this study, experiments were run on CaTiO3 samples at ~2GPa and 700C under oscillatory deformation. Using DIC to track intensity displacements in the powder XRD images, we were able to resolve lattice strains of 0.003 angstroms with the expectation of improved resolution through averaging and careful positioning of raster points. Applying DIC to the radiographs we were able to resolve oscillatory (macro)strains to 1/3 of a pixel. In addition to DIC, we also measured (macro)strain by detecting sample length changes based on travel times required for pulsed ultrasonic signals to traverse the sample. The strain measured using this technique is in good agreement with that computed using DIC. This suggests that in future experiments we may be able to dispense with collecting radiographs, which require ~1 min. of dead time in XRD collection since it involves moving the camera/scintillator into and out of beam path. This technique will also be discussed in the context of how a cyclic stress field can be used to generate stress/strain hysteresis loops. These can be used to identify the amount of elastic energy dissipated during an oscillatory cycle, which is relevant to the process of seismic wave attenuation in minerals in the deep mantle.