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1. Effect of the State of Stress in the Fracturing and Micro-Seismic Activity of Hydraulically-Fractured Granite

   Gunarathna, G ; Goncalves da Silva, B ( January 2019 , US Rock Mechanics/Geomechanics Symposium)

   Hydraulic fracturing can be recognized as an emerging method used in the mining of heat in Enhanced Geothermal Systems as well as in the extraction of oil and gas entrapped within shale formations. While there are several experimental studies focusing on the initiation and propagation of hydraulically-induced fractures under uniaxial and biaxial loading conditions, a very limited number of experimental studies investigate the effect of triaxial loading conditions on fracture initiation and propagation. This study describes an experimental setup, which was designed to allow one to independently apply and control three orthogonal stresses in prismatic granite specimens while simultaneously applying a hydraulic pressure inside pre-fabricated flaws. Moreover, the test setup allows one to observe and subsequently interpret the fracturing processes through visual and acoustic emission (AE) monitoring. The observations obtained in the current study using a triaxial state of stress were interpreted and compared with existing experimental studies that used other states of stress. It was observed that whitening of some grains and high-amplitude AE events occurred where visible cracks eventually developed for the triaxial state of stress investigated. Comparison with previous studies, in which only vertical loads (uniaxial) were applied, shows that the aperture of the hydraulically-induced fractures for the triaxial condition is significantly smaller than for the uniaxial loadings and that the coalescence patterns are, in general, stress-state-dependent. In terms of AE data, the total number of AE events in the specimens subject to triaxial stresses were significantly higher than in the tests using uniaxial stresses, even though most of the events (65%) had a relatively low-amplitude (<50dB) in contrast to the uniaxial tests, in which low-amplitude events were typically less than 50%. 

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2. Effects of fluid diffusivity on hydraulic fracturing processes using visual analysis

   Baptista-Pereira, C ; Goncalves da Silva, B ( January 2019 , US Rock Mechanics/Geomechanics Symposium)

   Hydraulic fracturing arises as a method to enhance oil and gas production, and also as a way to recover geothermal energy. It is, therefore, essential to understand how injecting a fluid inside a rock reservoir will affect its surroundings. Hydraulic fracturing processes can be strongly affected by the interaction between two mechanisms: the elastic effects caused by the hydraulic pressure applied inside fractures and the poro-mechanical effects caused by the fluid infiltration inside the porous media (i.e. fluid diffusivity); this, in turn, is affected by the injection rate used. The interaction between poro-elastic mechanisms, particularly the effect of the fluid diffusivity, in the hydraulic fracturing processes is not well-understood and is investigated in this paper. This study aims to experimentally and theoretically comprehend the effects of the injection rate on crack propagation and on pore pressures, when flaws pre-fabricated in prismatic gypsum specimens are hydraulically pressurized. In order to accomplish this, laboratory experiments were performed using two injection rates (2 and 20 ml/min), applied by an apparatus consisting of a pressure enclosure with an impermeable membrane in both faces of the specimen, which allowed one to observe the growth of a fluid front from the pre-fabricated flaws to the unsaturated porous media (i.e. rock), before fracturing took place. It was observed that the fracturing pressures and patterns are injection-rate-dependent. This was interpreted to be caused by the different pore pressures that developed in the rock matrix, which resulted from the significantly distinct fluid fronts observed for the two injection rates tested. 

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3. The Role of Background Stress State in Fluid‐Induced Aseismic Slip and Dynamic Rupture on a 3‐m Laboratory Fault

   https://doi.org/10.1029/2022JB024371  

   Cebry, S. B. L. ; Ke, C. ‐Y. ; McLaskey, G. C. ( August 2022 , Journal of Geophysical Research: Solid Earth)

   Fluid injection stimulates seismicity far from active tectonic regions. However, the details of how fluids modify on‐fault stresses and initiate seismic events remain poorly understood. We conducted laboratory experiments using a biaxial loading apparatus with a 3 m saw‐cut granite fault and compared events induced at different levels of background shear stress. Water was injected at 10 mL/min and normal stress was constant at 4 MPa. In all experiments, aseismic slip initiated on the fault near the location of fluid injection and dynamic rupture eventually initiated from within the aseismic slipping patch. When the fault was near critically stressed, seismic slip initiated only seconds after MPa‐level injection pressures were reached and the dynamic rupture propagated beyond the fluid pressure perturbed region. At lower stress levels, dynamic rupture initiated hundreds of seconds later and was limited to regions where aseismic slip had significantly redistributed stress from within the pressurized region to neighboring locked patches. We found that the initiation of slow slip was broadly consistent with a Coulomb failure stress, but that initiation of dynamic rupture required additional criteria to be met. Even high background stress levels required aseismic slip to modify on‐fault stress to meet initiation criteria. We also observed slow slip events prior to dynamic rupture. Overall, our experiments suggest that initial fault stress, relative to fault strength, is a critical factor in determining whether a fluid‐induced rupture will “runaway” or whether a fluid‐induced rupture will remain localized to the fluid pressurized region.

    

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4. Experimental Investigation on Forming Limit Curve at Elevated Temperature Through Dome and Biaxial Test

   https://doi.org/10.1115/IMECE2020-23085  

   Nikhare, Chetan P. ; Teculver, Evan ; Aqlan, Faisal ( November 2020 , Proceedings of the ASME 2020 International Mechanical Engineering Congress and Exposition)

   null (Ed.)

   The characteristics of metal and materials are very important to design any component so that it should not fail in the life of the service. The properties of the materials are also an important consideration while setting the manufacturing parameters which deforms the raw material to give the design shape without providing any defect or fracture. For centuries the commonly used method to characterize the material is the traditional uniaxial tension test. The standard has been created for this test by American Standard for Testing Materials (ASTM) – E8. This specimen is traditionally been used to test the materials and extract the properties needed for designing and manufacturing. It should be noted that the uniaxial tension test uses one axis to test the material i.e., the material is pulled in one direction to extract the properties. The data acquired from this test found enough for manufacturing operations of simple forming where one axis stretching is dominant. Recently a sudden increase in the usage of automotive vehicles results in sudden increases in fuel consumption which results in an increase in air pollution. To cope up with this challenge federal government is implying the stricter environmental regulation to decrease air pollution. To save from the environmental regulation penalty vehicle industry is researching innovation which would reduce vehicle weight and decrease fuel consumption. Thus, the innovation related to light-weighting is not only an option anymore but became a mandatory necessity to decrease fuel consumption. To achieve this target, the industry has been looking at fabricating components from high strength to ultra-high strength steels or lightweight materials. This need is driven by the requirement of 54 miles per gallon by 2025. In addition, the complexity in design increased where multiple individual parts are eliminated. This integrated complex part needs the complex manufacturing forming operation as well as the process like warm or hot forming for maximum formability. The complex forming process will induce the multi-axial stress states in the part, which is found difficult to predict using conventional tools like tension test material characterization. In many pieces of literature limiting dome height and bulge tests were suggested analyzing these multi-axial stress states. However, these tests limit the possibilities of applying multi-axial loading and resulting stress patterns due to contact surfaces. Thus, a test machine called biaxial test is devised which would provide the capability to test the specimen in multi-axial stress states with varying load. In this paper, two processes, limiting dome test and biaxial test were experimented to plot the forming limit curve. The forming limit curve serves the tool for the design of die for manufacturing operation. For experiments, the cruciform test specimens were used in both limiting dome test and biaxial test and tested at elevated temperatures. The forming limit curve from both tests was plotted and compared. In addition, the strain path, forming, and formability was investigated and the difference between the tests was provided. 

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5. Cracking behavior in lead zirconate titanate films with different Zr/Ti ratios

   https://doi.org/10.1063/5.0106340  

   Cheng, Christopher ; Peters, Travis ; Trolier-McKinstry, Susan ( October 2022 , Applied Physics Letters)

   Crack initiation stresses for different lead zirconate titanate (PZT) film compositions were investigated. PZT/Pt/TiO2/SiO2/Si stacks with 2.0 μm thick {100} oriented PZT films at the morphotropic phase boundary (MPB) showed a characteristic strength of 1137 MPa, and the film thickness served as the limiting flaw size for failure of the film/substrate stack. In contrast, for Zr/Ti ratios of 40/60 and 30/70, the characteristic stack strength increased while the Weibull modulus decreased to values typical for that of Si. This difference is believed to be due to toughening from ferroelasticity or phase switching. X-ray diffraction showed that the volume fraction of c-domains increased in Ti-rich compositions. This would allow for more switching from c to a-domains under biaxial tensile stress. Zr/Ti concentration gradients were present for all compositions, which contributed to the observation of a rhombohedral phase off the MPB. Due to the reduced tendency toward cracking, off-MPB compositions are potentially of interest in actuators, albeit with the trade-off of needing a high actuation voltage.

    

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