Abstract
In a naturally saturated state, rocks are likely to be in a stress field simultaneously containing static and dynamic loads. Since rocks are more vulnerable to tensile loads, it is significant to characterize the tensile properties of naturally saturated rocks under coupled static–dynamic loads. In this study, dynamic flattened Brazilian disc (FBD) tensile tests were conducted on naturally saturated sandstone under static pre-tension using a modified split-Hopkinson pressure bar (SHPB) device. Combining high-speed photographs with digital image correlation (DIC) technology, we can observe the variation of strain applied to specimens’ surfaces, including the central crack initiation. The experimental results indicate that the dynamic tensile strength of naturally saturated specimens increases with an increase in loading rate, but with the pre-tension increases, the dynamic strength at a certain loading rate decreases accordingly. Moreover, the dynamic strength of naturally saturated sandstone is found to be lower than that of natural sandstone. The fracture behavior of naturally saturated and natural specimens is similar, and both exhibit obvious tensile cracks. The comprehensive micromechanism of water effects concerning the dynamic tensile behavior of rocks with static preload can be explained by the weakening effects of water on mechanical properties, the water wedging effect, and the Stefan effect.
Highlights
Deep rocks are usually faced with complicated geoenvironments with multifield or multiscale coupling, such as thermomechanical coupling, groundwater disturbance, and static–dynamic load coupling [1,2,3,4,5]
It is difficult to keep the clamping device and rock specimen axis coincident, which induces bending moments in the specimen and causes measurement deviation. To avoid such fatal defects in direct tensile tests, some indirect methods have been proposed, and the Brazilian disc (BD) test is recommended by the International Society for Rock Mechanics (ISRM) to gauge the tensile strength of rocks [19,20,21,22,23]
To compare the dynamic tensile strength at natural and naturally saturated states, the experimental data of coupled static–dynamic flattened Brazilian disc (FBD) tests from Pei et al [32] are cited, and the interpolation is applied to obtain the dynamic tensile strength in the loading rate range of 50–350 GPa/s with various pre-tensions (i.e., 1.1 m3en No Loading14R1a.t8e0(GPa/s) Dynamic Ten1s3il.6e3Strength (MPa), 2.2 MPa, 3.3 MPa, and 4.4 MPa)
Summary
Deep rocks are usually faced with complicated geoenvironments with multifield or multiscale coupling, such as thermomechanical coupling, groundwater disturbance, and static–dynamic load coupling [1,2,3,4,5]. It is difficult to keep the clamping device and rock specimen axis coincident, which induces bending moments in the specimen and causes measurement deviation To avoid such fatal defects in direct tensile tests, some indirect methods have been proposed, and the Brazilian disc (BD) test is recommended by the International Society for Rock Mechanics (ISRM) to gauge the tensile strength of rocks [19,20,21,22,23]. To meet the prerequisite of central crack initiation in BD tests, researchers made some modifications to the loading devices, including using a curved loading jaw and adding cushions to the loading plates [17] These modifications greatly complicate the BD tests. The FBD method was proposed to conduct dynamic tensile tests, and it has proven suitable to gauge the dynamic tensile strength of rocks via this indirect method [28,29]
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