[S12P-01] A simple direct shear friction apparatus using a uniaxial compression tester
Introduction
Measurements of rock friction is crucial to evaluate the fault stability around the area conducted unconventional resources extraction such as shale oil and gas and enhanced geothermal system. The initiation of the slip on a fault does not always lead to an earthquake. Whether a fault slips seismically or aseismically depends on the characteristic of fault friction. That means that measuring fault friction parameters to evaluate fault stability could be widely demanded in the engineering field. Many types of friction machines have been proposed to conduct friction experiments, such as the double direct shear apparatus and rotary shear apparatus. However, the design of these apparatus is complex and not standardized so that it is not popular in engineering applications. Therefore, here, we would like to propose a simple, cost-effective and accurate friction apparatus to perform friction experiments with a commonly available uniaxial compression testing machine.
Experiment system
We used a uniaxial compression tester installed at Kyoto University (MTS 50t press with FlexiTest SE) as a part of loading system of the direct shear friction apparatus. The loading process of the rock is controlled by the function generator through the servo-controlled system of uniaxial tester. The shear stiffness of the apparatus is 7.89×107N/m. The center rock specimen (A) is a cuboid with a height of 8cm and a width of 2 cm. And the two outer rock specimens (B) are also cuboids whose height is 6 cm. The normal and shear stress on the sliding surface is measured by two loadcell (The measurement range is 50kN for normal load and 100kN for shear load). The outer specimens B are supported by the u-shaped metal pad to ensure the sufficient displacement to slide for the center rock A. During the experiments, the normal stress ranged from 2.4 MPa to 7.2 MPa, provided by four bolts that tighten a pair of fixed plates. At this moment, we used metagabbro rock from Tamil Nadu, south India for the experiments for the validation of the apparatus.
Some results and conclusion
We conducted velocity step change tests and slide hold slide tests to estimate the rate- and state- dependent friction parameters a, b and Dc of the metagabbro specimen. We used the Levenberg-Marquardt method to fit a nonlinear least-squares to experimental data(Skarbek and Savage, 2019, Geosphere).The a-b values are estimated between 1.3×10-3 and 12.7×10-3. The characteristic slip distance Dc is estimated between 9.9μm and 20.8μm. The estimated a-b value is more or less consistent with that by previous studies using different apparatus but using the same rocks (e.g., Urata et al., 2017, PAGEOPH), suggesting that this simple apparatus provides reasonable friction values in spite of its simplicities. In summary, we constructed a compact friction apparatus using uniaxial compression tester and conducted two conventional friction experiments. We confirmed that our friction apparatus could be available for engineering purpose due to its efficiency in cost and accuracy.
Measurements of rock friction is crucial to evaluate the fault stability around the area conducted unconventional resources extraction such as shale oil and gas and enhanced geothermal system. The initiation of the slip on a fault does not always lead to an earthquake. Whether a fault slips seismically or aseismically depends on the characteristic of fault friction. That means that measuring fault friction parameters to evaluate fault stability could be widely demanded in the engineering field. Many types of friction machines have been proposed to conduct friction experiments, such as the double direct shear apparatus and rotary shear apparatus. However, the design of these apparatus is complex and not standardized so that it is not popular in engineering applications. Therefore, here, we would like to propose a simple, cost-effective and accurate friction apparatus to perform friction experiments with a commonly available uniaxial compression testing machine.
Experiment system
We used a uniaxial compression tester installed at Kyoto University (MTS 50t press with FlexiTest SE) as a part of loading system of the direct shear friction apparatus. The loading process of the rock is controlled by the function generator through the servo-controlled system of uniaxial tester. The shear stiffness of the apparatus is 7.89×107N/m. The center rock specimen (A) is a cuboid with a height of 8cm and a width of 2 cm. And the two outer rock specimens (B) are also cuboids whose height is 6 cm. The normal and shear stress on the sliding surface is measured by two loadcell (The measurement range is 50kN for normal load and 100kN for shear load). The outer specimens B are supported by the u-shaped metal pad to ensure the sufficient displacement to slide for the center rock A. During the experiments, the normal stress ranged from 2.4 MPa to 7.2 MPa, provided by four bolts that tighten a pair of fixed plates. At this moment, we used metagabbro rock from Tamil Nadu, south India for the experiments for the validation of the apparatus.
Some results and conclusion
We conducted velocity step change tests and slide hold slide tests to estimate the rate- and state- dependent friction parameters a, b and Dc of the metagabbro specimen. We used the Levenberg-Marquardt method to fit a nonlinear least-squares to experimental data(Skarbek and Savage, 2019, Geosphere).The a-b values are estimated between 1.3×10-3 and 12.7×10-3. The characteristic slip distance Dc is estimated between 9.9μm and 20.8μm. The estimated a-b value is more or less consistent with that by previous studies using different apparatus but using the same rocks (e.g., Urata et al., 2017, PAGEOPH), suggesting that this simple apparatus provides reasonable friction values in spite of its simplicities. In summary, we constructed a compact friction apparatus using uniaxial compression tester and conducted two conventional friction experiments. We confirmed that our friction apparatus could be available for engineering purpose due to its efficiency in cost and accuracy.