Accurate characterization and visualization of the complex inner structure and stress distribution of rocks are of vital significance to solve a variety of underground engineering problems. In this paper, we incorporate several advanced technologies, such as CT scan, three-dimensional (3D) reconstruction, and 3D printing, to produce a physical model representing the natural coal rock that inherently contains complex fractures or joints. We employ 3D frozen stress and photoelastic technologies to characterize and visualize the stress distribution within the fractured rock under uniaxial compression. The 3D printed model presents the fracture structures identical to those of the natural prototype. The mechanical properties of the printed model, including uniaxial compression strength, elastic modulus, and Poissons ratio, are testified to be similar to those of the prototype coal rock. The frozen stress and photoelastic tests show that the location of stress concentration and the stress gradient around the discontinuous fractures are in good agreement with the numerical predictions of the real coal sample. The proposed method appears to be capable of visually quantifying the influences of discontinuous, irregular fractures on the strength, deformation, and stress concentration of coal rock. The method of incorporating 3D printing and frozen stress technologies shows a promising way to quantify and visualize the complex fracture structures and their influences on 3D stress distribution of underground rocks, which can also be used to verify numerical simulations.

Visualization of the complex structure and stress field inside rock by means of 3D printing technology

This paper describes an investigation and comparison of the optical and mechanical properties of the three polymers—PMMA (polymethyl methacrylate), CT200 and MY750 (both based on bisphenol-A epichlorohydrin). The work was undertaken with the specific purpose of assessing the suitability of these polymers for use in contact studies involving three-dimensional photoelasticity and the stress-freezing technique. The optical property investigated was the variation of photoelastic fringe constant with maximum stress-freezing temperature. The mechanical properties sought were the variation of Young's modulus with temperature and the stress-strain behavior at the stress-freezing temperature. The effects of the magnitude of stress and the soak time in the thermal cycle were also investigated for the MY750 resin. The results provide the optimum peak temperature for the thermal cycle to achieve repeatable values of Young's modulus and fringe constant during stress freezing. The nonlinear stress-strain behavior is quantified and should be a useful reference. The dependence of strain on load and soak time is also shown and is useful in specifying the optimum cycle time for stress freezing and the appropriate stress level. It was concluded that the resin MY750 was the most suitable for photoelastic applications which involved high localized stress, such as contact problems or fracture mechanics studies.

Photomechanical properties of some birefringent polymers around their glass transition temperatures

Laminate composites that can show magneto-birefringence effect were suggested and fabricated based on the product effect of magnetostriction and stress-birefringence. Under a magnetic field no stronger than 1900 Oe, a phase difference of about 3.3π was observed for a trilayer composite Tb1−xDyxFe2−y/polycarbonate/Tb1−xDyxFe2−y with the polycarbonate layer in size of 5 × 2.75 × 20 mm3 at room temperature, resulting in a half-wave magnetic field of no greater than 270 Oe. Experiments and theoretical analysis showed that the magneto-birefringence effects in the laminate composites are wavelength-relative and high size-sensitive.

Room temperature magneto-birefringence in composites of stress-birefringence and magnetostriction

In the engineering of underground construction, the discontinuous structures in rock mass have important influences on the mechanical behaviors of the subsurface of rock mass. The acquisition of mechanical parameters is the basis of rock mass engineering design, construction, safety, and stability evaluation. However, the mechanical parameters and failure characteristics of the same rock mass under different mechanical conditions cannot be obtained due to the limitations of specimen preparation techniques. In recent years, with the continuous development of 3D printing (3DP) technology, it has been successfully applied to the repetitive preparation of rock mass samples. The combinations of 3DP and other techniques, such as 3D scanning and CT scanning, provided a new approach to study the mechanical behavior of complex structural rock masses. In this study, through a comprehensive review of the technical progress, equipment situation, application fields, and challenges of the use of 3DP technology, the following conclusions were obtained: (1) 3DP technology has advantages over traditional rock mass specimen preparation techniques, and the verification of test results using 3D printed samples shows that the 3DP has broad application prospects in geotechnical engineering. (2) The combination of 3DP and other advanced techniques can be used to achieve the accurate reconstruction of complex structural rock masses and to obtain the mechanical and failure characteristics of the same rock mass structure under different mechanical boundary conditions. (3) The development of 3DP materials with high strength, high brittleness, and low ductility has become the major bottleneck in the application of 3DP in geotechnical engineering. (4) 3D printers need to meet the high precision and large size requirements while also having high strength and long-term printing ability. The development of 3D printers that can print different types of materials is also an important aspect of the application of 3DP in geotechnical engineering.

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Application of 3D Printing Technology in the Mechanical Testing of Complex Structural Rock Masses

Electro—optical composites based on the product of electro—strictive and elasto—optical effects are developed. Layered composites of PbZr1−xTixO3 and polycarbonate are synthesised. Their electro—optical properties are studied. The nominal transverse electro—optical coefficient of the composite is observed to be about 3.6 times larger than that of LiNbO3. Experiments and theoretical analyses show that the electro—optical effect of the composite has a strong ‘size effect’. With the ratio of thickness/length decreasing or the width of elasto—optical phase increasing, the half-wave electric field intensity increases but the transverse electro—optical coefficient decreases for the layered composite.

https://iopscience.iop.org/article/10.1088/1674-1056/20/3/037802/pdf

Electric field-induced stress–birefringence in layered composites PbZr 1- x Ti x O 3 /Polycarbonate

The possibility of using polycarbonate in the frozen stress photoelastic technique is investigated. The results obtained from strip specimens made of laminated and extruded polycarbonate and from plates confirm this possibility.

Carmine Pappalettre

Papers

Polycarbonate for frozen stress photoelasticity