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Rock engineering especially deep rock engineering undergoing long-term effects of external loading and gravity all or most may gradually damage deformation or creep deformation accumulation, leading rock structures to damage, crack, such as severe plastic deformation or even progressive failure. In this paper, based on the nonlinear damage creep characteristics of rock and damage variable, a new nonlinear damage creep constitutive model of high-stress soft rock is defined in series with the improved Burgers model, Hooke model and St. Venant model. This new nonlinear damage creep constitutive model can work out fairly reasonably explanations for the soft rock creep deformation. A series of uniaxial compression creep tests were performed to study the creep damage characteristics of typical soft rock in Jinchuan No.2 Mine in the northwest of China. Using the increment step loading and single-step loading, the results of creep experiments and nonlinear damage creep constitutive model results are very consistent in this study. The new model not only can reflect the whole course of creep deformation, but also can reasonably describe the soft rock under different initial creep stage, steady-state creep stage and accelerated creep stage. Therefore, the new nonlinear creep damage model is a reasonable reference model for the research of soft rock creep.

In 2014–2016, creep tests were performed in a dead-end drift of the Altaussee mine, where temperature and relative humidity experience very small fluctuations. These tests, which were several months long, proved that the creep rate of a natural salt sample is much faster in the 0.2–1 MPa deviatoric stress range than the creep rate extrapolated from standard laboratory creep tests performed in the 5–20 MPa range. In addition, the quasi-steady strain rate is a linear function of stress, and it is faster when grain size is smaller. These findings were consistent with microphysical models of pressure solution creep (rather than dislocation creep, which is the governing creep mechanism at high stresses). A gap in experimental data remained in the 1–5 MPa range, calling for a follow-up experimental program. In 2016–2019, three multi-stage creep tests were performed on salt samples from Hauterives (France), Avery Island (Louisiana, USA), and Gorleben (Germany), which had been tested in the 0.2–1 MPa range during the 2014–2016 campaign. Loads of 1.5, 3, and 4.5 MPa were applied successively on each sample for 8 months. Steady state was not reached at the end of each 8-month stage. However, tests results suggest that, in the 0.2–3 MPa range, the relationship between the strain rate and the applied stress is linear, a characteristic feature of pressure solution. For these three samples, the relationship between strain rate and deviatoric stress departs from linearity when the deviator is larger than approximately 3–4.5 MPa, pointing to a transition to dislocation creep at higher deviatoric levels.HighlightsVery long duration uniaxial creep tests were performed under controlled conditions.The transient phase under low and moderate stress levels is long.Creep rate below 3 MPa is faster than extrapolated from high-stress creep tests.The strain rate-stress dependency is different at low and high deviatoric stresses.The transition between linear and non-linear dependency lies between 3–4.5 MPa.

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.

Triaxial creep tests were performed on diabase specimens from the dam foundation of the Dagangshan hydropower station, and the typical characteristics of creep curves were analyzed. Based on the test results under different stress levels, a new nonlinear visco-elasto-plastic creep model with creep threshold and long-term strength was proposed by connecting an instantaneous elastic Hooke body, a visco-elasto-plastic Schiffman body, and a nonlinear visco-plastic body in series mode. By introducing the nonlinear visco-plastic component, this creep model can describe the typical creep behavior, which includes the primary creep stage, the secondary creep stage, and the tertiary creep stage. Three-dimensional creep equations under constant stress conditions were deduced. The yield approach index (YAI) was used as the criterion for the piecewise creep function to resolve the difficulty in determining the creep threshold value and the long-term strength. The expression of the visco-plastic component was derived in detail and the three-dimensional central difference form was given. An example was used to verify the credibility of the model. The creep parameters were identified, and the calculated curves were in good agreement with the experimental curves, indicating that the model is capable of replicating the physical processes.

This paper presents an experimental study on the mechanical behavior and permeability evolution of basalt, drilled from the dam foundation of the Baihetan Hydropower Station in China. Two groups of triaxial unloading creep tests under hydro-mechanical coupling conditions were conducted. Based on the experimental results, creep deformation and failure characteristics of basalt were investigated. It was found that the relationship between the confining pressure and steady creep rate could be described satisfactorily by an exponential function. A tensile-shear failure mode was observed for basalt under the action of unloading confining pressure and hydraulic-mechanical coupling. The permeability evolution of basalt in the unloading creep process exhibited phased characteristics: permeability decreased in the initial loading phase and remained stable for a certain range of confining pressure in the steady creep phase. Exponential functions were also used to describe the evolution of permeability and confining pressure. The results of this study can provide a comprehensive understanding for the long-term stability assessment of basalt under coupled hydro-mechanical conditions.

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.

The rheological phenomenon of rock mass affects the long-term safety of rock mass engineering. In this study, gneiss samples with different 3D morphologies are prepared by splitting tests and are tested through multi-step creep tests. The long-term strength of rock discontinuities is determined by using several methods. The test results show that as the 3D morphological parameter increases, the creep deformation, creep rate, and the duration of failure all decrease. The long-term strength of rock discontinuities is linearly related to the 3D morphological parameter. Based on the principle of damage mechanics for rock mass, a damage variable is introduced in the creep model, and an improved non-linear Burgers model is established. Research results are of great theoretical significance and practical value for the design, construction, and long-term safety of rock mass engineering.

Different temperatures and continuous loads have significant effects on the long-term performance of asphalt concrete facings. The effects of temperature and stress on creep strain and creep rate were analyzed by designing a bending creep test of impermeable asphalt concrete under different temperatures and stresses. Based on the test data, a time–temperature–stress-dependent creep constitutive model was constructed to predict the long-term creep behavior of asphalt concrete at low temperature. The results showed that the creep behavior of asphalt concrete showed significant temperature and stress dependence. The creep behavior accelerated as the temperature or stress increased, especially under high-stress conditions, indicating obvious nonlinear characteristics. Under the condition of 0.2376 MPa, when the temperature increased from 0 °C to 20 °C, the strain at the creep time of 9330 s nearly increased by 24 times. Under 0 °C, the loading stress increased from 0.2376 MPa to 1.3176 MPa, and the strain nearly increased by six times at a creep time of 880 s. The creep strain is expected to increase to 8% after 8 years at −15 °C and 0.2376 MPa. The results can provide a scientific basis for engineering practice and significant implications for designing and maintaining asphalt concrete facings.

The design of underground facilities in rock salt relies on constitutive models whose parameters are derived from laboratory tests and (when possible) recalibrated on the basis of field data. In this sense, laboratory tests should cover the load paths expected in real-scale applications. However, the literature review proves that most tests have been conducted at stress deviators greater than about 5 MPa, which is not representative of the whole spatial and temporal spans of the facilities. In this paper, results of confined multi-stage creep tests performed on salt samples from two different origins are shown. Stress deviators between 0.5 and 20 MPa have been applied to different samples under a constant temperature of 50 ∘C. The experimental procedure allows to reduce uncertainty associated with sample-to-sample variability, sample preparation and preconditioning. The analysis of the experimental data shows that the stress dependency of the creep rate is different at low and high differential stresses. In particular, the creep rate is underestimated by several orders of magnitude when the creep tests are performed at high differential stress levels and the trends are extrapolated towards low and moderate levels. On the basis of these results, the Lemaitre and RTL constitutive models for rock salt have been modified, allowing for more accurate predictions of underground facilities. Properly designed laboratory tests are necessary to limit extrapolation; however, the duration of such experiments is negligible compared to the lifespan of the underground facilities, and long-term aspects should be further investigated.HighlightsConfined multi-stage creep tests were performed covering stress deviators from 0.5 to 20 MPa.Rock salt specimens from two different origins were tested.The deviatoric stress dependency of the creep rate is different at low and high deviatoric stresses.Two phenomenological viscoplastic models for rock salt are extended to better reproduce creep behavior.Although stress extrapolation is addressed, time extrapolation remains a major challenge.

Nanoporous membranes based on the single crystalline nickel-based superalloy CMSX-4 are a promising class of materials for membranes, especially for use in premix membrane emulsification. In addition to the pore size, the strength and stability of the membrane structure are key factors for subsequent use. The production of the membranes is based on the directional coarsening of the γ/γ′-microstructure by creep deformation, in which the material is subjected to a tensile load at high temperatures so that a bicontinuous network of the γ- and γ′-phase is formed. The subsequent dissolution of the γ-phase leaves a network of γ′-phase, which can be used as a membrane structure; the former γ-matrix channels now serve as pores. Previous investigations focusing on the evolution of the microstructure during membrane fabrication found that a particularly small pore size can be achieved when the creep deformation temperature is lowered from 1000 °C to 950 °C while increasing the stress from 170 MPa to 250 MPa. This study will now investigate the strength and fracture behaviour of membranes produced by these improved parameters. For this purpose, four creep states with creep strains between 1.3% and 5.7% are investigated in tensile tests at room temperature, with the load being applied perpendicular and parallel to the raft structure. The results show that the strength of nanomembranes during perpendicular loading essentially depends on the cross-linking between γ′-rafts. Generally, an increase in creep strain leads to an increase of the cross-linking resulting in higher tensile strength. During parallel loading, γ′-inhomogeneities play an important role resulting in a loss of strength. The analysis of the fracture surfaces and evaluation of EBSD measurements reveal an insufficient cross-linking between dendrites and around γ′-inhomogeneities, leading to preferred crack paths. Therefore, the differences in orientation within the single crystal play a key role in the strength of the nanomembranes.