3D printing has recently been combined with frozen stress techniques to create a new way of directly characterizing the 3D interior discontinuities and the full-field stress caused by man-made disturbances in deeply buried rock masses.
However, the method has also recently raised concerns over the similitude of the mechanical behaviors in the printed model and its prototype rock mass.
A team of Researchers from China have developed a transparent, polymeric and photoelastic printable material to study its physical and mechanical properties in an attempt to improve the similarity between its properties and those of natural rocks.
Man-made activities in deeply buried rock masses, such as construction and mining projects, have the possibility to cause large deformations in the surrounding rocks and disturbances in the geo-stress field, resulting in the potential for geological hazards to occur in the surrounding area(s).
Accurate characterizations of the surrounding rock strata (and its stress field) in these locations are essential to ensuring geological safety of the region and to ensure the exploitation optimization of the underground resources.
However, the complex network of rock structures and geological processes under the ground have made it extremely difficult for Researchers to assess and characterize the stress fields and 3D discontinuities in such rock systems.
3D printing methods coupled with frozen stress techniques now afford a new and promising way to physically display complex underground structures and to probe rock strata stress fields. But whilst these methods have been found to be useful for solving many rock mechanics problems, they have not been able to directly measure the stress field and its evolution inside media.
The team of Researchers have now used an epoxy-based polymer (VeroClearRGD 810) to create a transparent, light and photoelastic printable material for use as a modification material to help improve the similarity between its strength and stiffness and those of natural rocks.
The study aimed at providing greater knowledge to aid in better simulations of real rock masses in terms of both mechanical and geological performance.
The Researchers used Objet Connex 3D printers to create a series of cylindrical and dog-bone-shaped specimens for mechanical tests. The Researchers first created the specimens using AutoCAD, which were then exported in Stereo Lithographic (STL) format and imported into the Objet Studio program ready for printing.
Ensuring that the properties of the 3D printed material were as close as possible to naturally occurring rocks was of utmost importance to the Researchers, and as such they characterized the materials through infrared spectroscopy (IR, Spectrum 400) X-ray diffraction (XRD, Bruker D8 Advance), pyrolysis gas chromatography and mass spectrometry (PY-GC/MS, model EGA/PY-3030D pyrolyser and QP2010-Ultra GC/MS) and scanning electron microscopy (SEM, S4800). Mechanical properties were tested through a digital servo-control universal testing machine.
The Researchers first determined the chemical constituents in the specimens, and followed this by analyzing the impacts of building orientation and heat-treatment temperatures on the mechanical properties of the samples.
The Researchers imposed many stress-related environments on the samples in an effort to evaluate and improve the mechanical properties of the printable materials against their prototype rock. Such measures included printing orientation, post-processing, temperature control optical stress sensitivity, stress-visualized properties, stress-frozen performance of the material, uniaxial compressive strength, direct tensile strength and triaxial compressive strength tests.
By adjusting the build-up direction, and post-processing the materials to above their glass transition temperatures, the Researchers were able to increase the strength and stiffness of the printed material(s).
The printed polymeric material was also found to possess a good birefringence and photoelasticity at room temperature, showed stress-freezing at temperatures above its critical freezing temperature and exhibited a prominent optical stress sensitivity.
The stress sensitivity of the material also allowed for the fringes of force-bearing points and discontinuities to be clearly displayed and captured.
The process employed by the Researchers has helped to improve the similitude between the printed material and natural rock formations. However, natural rock strata are much more unpredictable due to random imperfections, pores, fractures and joints. This could mean that the physical characteristics differ and simulating these environments with printed materials remains a challenge.
However, despite the challenges ahead, the Researchers have made great progress in this field and the printed material shows a great potential for solving various engineering problems associated with 3D full-field stress in natural environments, such as in unconventional oil and gas extraction processes, reinjection of hydraulic fracturing waste, CO2 geological sequestration, disposal of nuclear waste, seismic prediction, geothermal energy utilization, civil and building construction and deep underground coal mining.
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“The mechanical and photoelastic properties of 3D printable stressvisualized materials”- Wang L., et al, Scientific Reports, 2017, DOI:10.1038/s41598-017-11433-4