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Dolomitic limestone is the main surrounding rock material in Yangzong tunnel engineering; the instantaneous mechanical properties and creep behaviors of limestone are significant for stability evaluation during the stages of tunnel excavation and long-term maintenance. Herein, four conventional triaxial compression tests were carried out to explore its instantaneous mechanical behavior and failure characteristics; subsequently, the creep behaviors of limestone subjected to multi-stage incremental axial loading at the confinements of 9 MPa and 15 MPa were studied by employing an advanced rock mechanics testing system (i.e., MTS815.04). The results reveal the following. (1) comparing the curves of axial strain–, radial strain–, and volumetric strain–stress under different confining pressures shows that these curves present a similar trend, whereas the stress drops during the post-peak stage decelerate with the increase in confining pressure, suggesting that the rock transits from brittleness to ductility. The confining pressure also has a certain role in controlling the cracking deformation during the pre-peak stage. Besides, the proportions of compaction- and dilatancy-dominated phases in the volumetric strain–stress curves differ obviously. Moreover, the failure mode of the dolomitic limestone is a shear-dominated fracture but is also affected by the confining pressure. (2) When the loading stress reaches a creep threshold stress, the primary and steady-state creep stages occur successively, and a higher deviatoric stress corresponds to a greater creep strain. When the deviatoric stress surpasses an accelerated creep threshold stress, a tertiary creep appears and then is followed by creep failure. Furthermore, the two threshold stresses at 15 MPa confinement are greater than that at 9 MPa confinement, suggesting that the confining pressure has an obvious impact on the threshold values and a higher confining pressure corresponds to a greater threshold value. Additionally, the specimen’s creep failure mode is one of “abrupt” shear-dominated fracturing and is similar to that under a conventional triaxial compression test at high confining pressure. (3) A multi-element nonlinear creep damage model is developed by bonding a proposed visco-plastic model in series with the Hookean substance and Schiffman body, and can accurately describe the full-stage creep behaviors.

High-temperature titanium alloys are significant materials in the aerospace field, and their service life largely depends on creep aging. However, the creep behavior of the TC25 titanium alloy at high temperatures has not been reported. Here, the creep behavior of TC25 before and after heat treatment at 550 °C under different stresses was investigated. It was found that heat treatment significantly enhanced the creep resistance of the TC25 alloy. An increase in creep stress increased the steady-state creep rate and reduced creep life. The smooth αp/βtrans grain boundaries and refined αs improved creep resistance, and the creep mechanism changed from grain boundary sliding to dislocation climbing after heat treatment. This research provides theoretical data support for the application of the TC25 alloy at high temperatures.

The high-temperature performance of asphalt mastic is a critical factor influencing the resistance of asphalt mixtures to permanent deformation. Despite the importance of this material phase, no standardized test exists for evaluating asphalt mastic behaviour at high temperatures. Therefore, researchers often use the Multiple Stress Creep Recovery Test (MSCRT), originally designed for asphalt binder, although its applicability to asphalt mastic is limited. This study proposes a novel rheological method referred to as Single Shear Creep Test (SSCT) as a more robust alternative for assessing the performance of asphalt mastic at high temperatures. The SSCT applies a constant shear stress over an extended period, allowing for the determination of the steady-state creep rate as a rheological performance indicator. A comprehensive experimental program involving 45 asphalt mastic variants, produced by using 11 asphalt binder types, 15 mineral fillers, and different filler-to-asphalt binder ratios. Each variant was tested using both MSCRT and SSCT in a Dynamic Shear Rheometer (DSR). The results demonstrated that SSCT provides more consistent and rheological meaningful differentiation between materials. The results show that asphalt binder type and the filler-to-bitumen (f/b) ratio strongly influence asphalt mastic behaviour at high temperature. Filler type has a limited influence, except for hydrated lime.

Severe rheological failure of extremely soft rocks poses a significant threat to the safety and long-term stability of roadways. Herein, four long-term triaxial creep tests were conducted under low confinements and deviatoric stresses. The results show that greater deviatoric stress leads to more obvious creep deformation, while the confining pressure can delay the occurrence of accelerated creep and restrain the creep rate and lateral deformation. The total axial strain reached 4.03% under 0.6-MPa confinement, and the creep strain of the extremely soft coal rock was much larger than that of hard rocks. Additionally, two important features distinguish extremely soft coal rocks from common rocks, namely, the creep rate did not converge in the steady-state creep stage under each applied stress level and a “gradual” squeezing deformation instability occurred in the accelerated creep stage. Furthermore, the steady-state creep rate increased exponentially with an increase in deviatoric stress and decreased following a power function with confining pressure. Then, a modified Burgers model with a nonstationary viscous coefficient was proposed to reflect the dual and nonlinear influence of confining pressure on steady-state creep rate. Moreover, several principles and suggestions for the long-term stability control of extremely soft rock roadway are discussed. Finally, a novel nonlinear creep constitutive model was established by connecting a nonlinear viscoplastic element considering both creep time and applied stress with the modified Burgers model in series. The findings are essential for creep behavior prediction and stability control in extremely soft rock engineering.

The stress and hence strain fields in a cantilever deforming as per power-law creep vary across the length and thickness of the sample, which allow obtaining multiple stress–strain pairs from a single test. Here, a high-throughput method is described to quantify the primary-cum-steady-state creep response of materials by testing a single cantilever sample in bending and mapping strain fields using digital image correlation. The method is based on the existence of stress invariant points in a cantilever, where the value of stress does not change during creep. It is demonstrated that strain evolution throughout primary and steady-state stages at these points is identical to the creep response obtained under uniaxial tests. Furthermore, the gained insights were exploited to obtain various parameters of a power-law type primary-cum-steady-state creep equation by testing only one cantilever sample. The developed method allows obtaining uniaxial creep curves at multiple stresses by testing a single cantilever, thereby reducing the time and number of samples required to understand the creep behavior of a material. The method has been validated by performing bending tests on Al and comparing the results with those of corresponding uniaxial tests.

As the pace of salt rock storage construction in China accelerates, suitable shallow storage sites are becoming scarce, as such, the utilization of deep salt deposits is a foregone conclusion. However, as salt deposits in China all contain a certain amount of impurities, triaxial multistage loading creep tests for deep salt rock with varying impurity contents were carried out to study the influence of impurity content and distribution on the creep properties and failure modes of salt rock. The isochronous stress–strain curves were used to calculate the long-term strength of the salt rocks with different impurity contents. The test results show that the higher the impurity content, the lower the steady-state creep rate, and the greater the long-term strength. Moreover, it was observed that cracks first appeared at the interface between the impurity and the salt rock, subsequently extending to the salt rock. Thereafter, a new nonlinear creep damage model considering impurity content was proposed by introducing the nonlinear viscoplastic damage body and using the fractional-order derivative theory. The proposed three-dimensional creep model was verified based on creep test data and compared with the traditional creep models. The proposed model has fewer parameters and increased fitting accuracy. Thus, this study contends that the proposed model can accurately simulate the entire creep process for impurity salt rock, which provides a theoretical basis for the safe operations in salt caverns containing impurities. Highlights The triaxial multistage loading creep tests for deep salt rock with varying impurity contents were carried out. The influence of impurity content on steady-creep rate, creep deformation and long-term strength of salt rock were analyzed. A nonlinear fractional-order creep damage model for deep impurity salt rock was proposed. The proposed three-dimensional creep model is in good agreement with the test data and can accurately simulate the whole creep process of impurity salt rock.

Shear creep is one of the most important mechanical behaviors of rock discontinuities. The creep mechanism and prediction of starting point of the accelerating creep stage are vital for establishing the creep model and predicting creep failure. In this study, a series of multi-step creep tests are conducted. The three creep stages of shear creep tests are investigated in detail, and a method for predicting the accelerating creep stage is proposed. Distinct nonlinear and local fluctuations caused by cracking are observed in the creep curve. To describe the transition creep stage and steady creep stage, an empirical creep model is established, and the creep characteristics related to the joint roughness coefficient (JRC) and the normal stress are explored in detail using the model’s parameters. The creep process can be described as involving the JRC resistance weakening and frictional resistance compensation, and a model also established to describe this process. The frictional resistance cannot compensate for the loss of JRC resistance; consequently, creep failure occurs. The starting point of the accelerating creep stage can be predicted by combining the JRC weakening and frictional mobilization model and the empirical creep model. A new method for determining long-term strength is also proposed based on the relationships between the starting point creep deformation and the shear creep stress.

Direct shear creep tests have scarcely been used for long-term creep behavior studies of landslides in the Three Gorges reservoir area. In this study, based on field investigations and monitoring of the Huangtupo Landslide, direct shear creep tests were performed on the sliding zone soil of Riverside Slump #1, and the creep characteristics of sliding zone soil after varying cycles of reservoir water level fluctuation were studied. Using the creep results, the Mohr–Coulomb parameters were obtained by numerical simulation, and the deformation pattern of the reservoir landslide was analyzed. The results show that the direct shear creep of sliding zone soil can mainly be divided into stages of attenuation creep and steady-state creep. Under the same shear stress, with the increase of loading–unloading cycles N , the soil's strain and shear strain rate in the sliding zone decreased accordingly, and the long-term strength gradually improved. As the shear stress increases, the shear strain rate increases and the creep of the soil in the sliding zone has an obvious time effect. Our numerical simulation results showed good agreement with both the landslide deformation monitoring data and direct shear testing data. The Burgers model is suitable for describing creep deformation of landslides under fluctuating reservoir water levels. Under high shear stress, the fitted curve showcased both attenuation and constant velocity characteristics. Numerical simulation and burger model can reflect the direct shear creep test characteristics well. These research findings can provide an important reference on the creep characteristics of landslides, potentially aiding geotechnical engineering applications.

The layered surrounding rocks of tunnels undergo large creep deformation due to the presence of planes of weakness, thereby the deformation severely endangers the safety of tunnels. This study conducts uniaxial compression creep tests to experimentally investigate the transversely isotropic creep characteristics and the damage mechanism of layered phyllite samples having bedding angles of 0, 22.5, 45, 67.5, and 90°. The results indicate that the creep deformation of the specimens takes place in four stages: the instantaneous elastic deformation stage, the deceleration creep stage, the steady-state creep stage, and the accelerated creep stage. The cumulative creep deformation and the creep time during the steady-state creep stage of the specimens initially decrease and then increase as the bedding angle changes from 0 to 90°, thereby, corresponding to the initial increase and subsequent decrease in creep rate during the deceleration creep stage. Based on the existing viscoelastic-plastic damage creep model, the creep parameters E1, E2, η2, and η3 are observed to initially decrease and then increase with the increase in bedding angle, hence demonstrating that the creep characteristics and damage mechanism of the layered rock mass are controlled by the effect of the natural weakness planes and show significant transversely isotropic characteristics. Then, the damage creep model was embedded into FLAC3D numerical analysis software to analyze the displacement change of surrounding rock and its supporting structures of the tunnel with different bedding angles. The results of the calculation accurately reflected creep deformation characteristics of the layered surrounding rocks of tunnels induced by excavation.

Understanding the long-term creep mechanical behavior of granite is critical to evaluating the safety of the nuclear waste repository. It is unreasonable to extrapolate the creep test results of hard rock samples at high stress to lower stress. In this work, the uniaxial creep test of granite lasting for 1117 days is carried out at lower axial stress levels (60 and 87 MPa) to reveal its long-term time-dependent deformation characteristics. The test results show that under 60 MPa and 87 MPa, the creep deformation of granite accounts for only 21.7% and 36% of the total deformation, while that of salt rock is about 80%, making the creep of hard rock difficult to be detected. The steady creep rate of granite is 9.14 × 10−12 s−1, which is 1–2 orders of magnitude slower than that of salt rock under low stress, and 3–6 orders of magnitude slower than that under short-term granite creep test. At the same stress ratio of about 0.45, it takes 47.6 days for granite samples to enter the steady creep stage, which is much shorter than 213 days for salt rock samples. In the short-term (several hours or days) creep test under low stress level, the hard rock sample does not actually enter the steady creep stage. Based on the fractional derivative theory and damage mechanics, a novel nonlinear creep-damage constitutive model that can well describe the long-term brittle creep characteristics of granite is proposed. This research reveals the very long-term time-dependent deformation behavior of hard rock, which is conducive to the evaluation of long-term stability of nuclear waste repositories.