Stress Distribution During Cold Compression of Rocks and Mineral Aggregates Using Synchrotron-based X-Ray Diffraction

This method can help answer key questions in the geomechanics field about compaction processes in rocks. The main advantage of this technique is that it allows quantitative measurement of the local elastic field within rock and mineral aggregates. Though this method can provide insight into stress distribution within samples during cold compression, it can also be applied to other techniques such as high temperature compression.

Generally, individuals new to this method struggle because of challenges in preparing the sample and the cell assembly. Sample and cell assembly preparations are difficult to learn without visual demonstration because of how small and delicate the individual samples and parts of the cell assembly are. To begin preparing a mineral aggregate sample grind about 15 grams of a rock specimen or pre-existing powder, to approximately 4 micron diameter grains using a mortar and pestle, or a rotary tool with a grinding head.

Pour the mineral grains into a 20 centimeter decantation column containing ethanol. Allow the grains to settle for an appropriate duration for their density. Then partition the suspension by height into three beakers.

Allow the contents of the beakers to dry in air overnight. Measure the average grain diameter of each batch, and select the batch in which the average grain diameter is closest to four micrometers. Next, grind the ends of an alumina rod to be flat in parallel within 0.5 degrees.

Clean the rod in an alumina ring by sonication in ethanol for 10 seconds. Allow the components to dry on a delicate task wipe. Working on a clean surface, cover one end of the hole in a D-DIA cell assembly cube with adhesive tape.

Insert a boron nitrate sleeve into the assembly cube. Then use tweezers to slide a graphite ring in the alumina ring onto the alumina rod. Place the alumina rod and rings in the assembly cube with the graphite ring on the bottom.

Ensure that the rod and rings are settled at the bottom of the hole with the graphite ring in contact with the tape. Mark the corner of the cube that will eventually be aligned with the incoming x-ray beam. Then cut a 1.5 millimeter by 17 millimeter piece of tantalum foil and fold it into a U shape.

Insert the foil into the cube with the foil aligned so that the 2D projection of the foil is minimized with respect to the x-ray beam direction. Use a pin to gently press the foil against the edges of the space. Insert the previously prepared rock core or mineral aggregate reference sample into the the cell assembly cube.

Lay a 1.7 millimeter by 1 millimeter piece of tantalum foil flat on top of the reference sample with the long side of the foil perpendicular to the x-ray beam. Then, carefully pack the mineral aggregate sample into the cell assembly cube using a small spatula. Leave 1.4 millimeters of space in the cube above the packed sample.

Gently remove excess grains adhering to the walls of the cylindrical space with air. Use a pin and calipers to confirm that the final sample height has been reached. Place another 1.7 millimeter by 1 millimeter piece of tantalum foil on the packed sample.

Then grind the ends of another alumina rod to be flat in parallel and clean the rod in an alumina ring. Use tweezers to fit the alumina ring and a graphite ring onto the rod. Place the rod on the samples so that the graphite ring is on top.

Finally, seal the exposed alumina rod at each end of the cube with a zirconia powder based cement, being careful not to use excess cement. Trim the exposed tantalum foil once the cement has dried. Collect and analyze a diffraction pattern from an alumina standard.

Remove the alumina standard and collect an open press x-ray spectrum with an exposure time of 500 seconds to measure the background without a sample assembly. After cleaning the anvils, insert the prepared sample assembly into the center of the experiment set up, ensuring that it is properly aligned with the x-ray beam. Slowly lower the opposing pairs of lateral anvils simultaneously.

Gently push the anvils into alignment so that the anvils are level and the bottom and lateral anvils are in contact with the sample assembly. Then release the safety latch and insert the spacer into the hydraulic press. Close the hutch and enable the shutter to allow the x-ray beam to enter the hutch.

Turn on the low pressure pump. Then move the top ram up until it rests against the spacer. Guided by real-time x-radiographic imaging, slowly and carefully move up the bottom ram until the anvils appear in the radiograph.

Leave a fine gap so that the sample is not overloaded prior to the experiment. Then, turn off all controls on the low-pressure pump controller and close the pressurized valve. In the high pressure hydraulic pump controlled software, move the sample assembly parallel to the beam so that the center of the mineral aggregate sample is aligned with the diffraction focus mark.

Collect a diffraction spectrum of the sample with an exposure time of 500 seconds. Capture a radiograph with an exposure time of six milliseconds. Then, move the assembly to center the rock core reference sample in the diffraction focus mark.

Acquire diffraction spectrum in a radiograph under the same conditions. Next, start the hydraulic pump motor to drive the anvils inward. Set the target load to 50 tons and enable feedback with a rate of one second and a gain of 20.

Set the upper limit of the speed to seven to achieve the slowest possible compression. They hydraulic pump motors drive the anvils inwards to provide pressure for the compression of the experimental cell. In the data collection window, define the locations of the rock core reference in the mineral aggregate sample in terms of the x and y press locations.

Ensure that the exposure time is set to 500 seconds. Set the number of cycles required to zero, so that the data collection will continuously repeat. Then start the data collection.

As compression progresses, update the sample and reference locations in the software as needed. Upon reaching the target load, stop the data collection. Set the lower limit of the speed control to minus 10 and the target load to zero tons to decompress the sample.

Manually collect diffraction spectra in radiographs for the core and aggregate following unloading. Then, open the pressurized valve on the low-pressure pump panel. Turn on the low-pressure pump, and lower the top and bottom rams until the down indicator lamp is illuminated.

Move the spacer arm to the out position and drive the top ram up until the safety lock engages. Then, turn off the controls in the pump motor control unit. Slowly move the lateral anvils outward manually and remove the sample assembly.

At ambien pressure, the diffraction spectra of a quartz aggregate with the grain size of approximately 4 micrometers and novaculite reference sample with a grain size of six to nine micrometers were similar in intensity with end position. The quartz aggregate spectrum began to broaden with increasing pressure. The quartz peak continued broadening on the higher energy side as the pressure increased, whereas the novaculite peak shape was essentially unchanged.

Both the quartz and novaculite peak positions shifted in the higher energy direction which corresponds to lower D spacing. The quartz aggregate showed a significant amount of differential stress in both axial and transverse directions, with nearly twice the amount of stress in the transverse direction than in the axial direction. This indicated that the transverse direction supported a significantly greater load than the axial direction.

The modest broadening on the low energy sides of the quartz axial and transverse peaks indicated that a considerable amount of the grains remained stress free in both directions which can only occur if a significant number of grains had at least part of their surface area bounded by voids supporting zero pressure throughout the experiment. Once mastered, this experiment can be done in 16 hours if it is performed properly. After its development, this technique paved the way for researchers in the field of rock mechanics, mineral physics, geotechnic engineering, and material science to explore stress distribution in materials of interest.

While attempting this procedure, remember to have the end surface contacts flat so that the applied load can be evenly distributed across the entire surface area. Following this procedure, this method can be used for high temperature applications and ultrasonic sound velocity measurements to determine other information about the rheological and elastic properties of the interior of the earth. After watching this video, you should have a good understanding of how to characterize local stress distribution within mineral aggregates using a multi-anvil deformation apparatus with synchrotron x-radiation.