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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. [ABSTRACT FROM AUTHOR]

In the realm of daily life, ensuring the safety of building structures and civil engineering projects remains a paramount research focus. The creep properties of materials significantly influence their long-term loading process. Specifically, creep load and creep time are pivotal factors that impact material creep damage, thereby playing a crucial role in assessing the safety of engineering endeavors and estimating aspects such as housing construction. This study undertakes creep damage tests on cement-based materials, subjecting them to varying creep loads and creep times, and subsequently conducts uniaxial compression tests on the specimens post-creep damage. The refined Nishihara model is employed for data fitting, facilitating the construction of a creep damage time-stress model. Concurrently, a Neural Network model is utilized to validate the experimental data. The findings indicate that both steady-state creep strain and steady-state creep rate exhibit discernible trends relative to creep load and creep time, effectively mirroring the alterations in creep damage experienced by the specimens. The refined Nishihara model proves adept at predicting and equating creep damage under diverse creep loads and creep times. Similarly, the trained Neural Network model demonstrates capability in measuring and estimating various creep damages. The study successfully explored the correlation between creep time and creep load, enabling the simulation of long-term creep damage within a shorter creep time and facilitating an analysis of its physical and mechanical properties, which is pivotal in predicting the safety of large-scale engineering projects. Concurrently, it advances research on material damage equivalence, offering insights and theoretical groundwork for developing a system to assess material damage equivalence under various damage conditions. [ABSTRACT FROM AUTHOR]

This work mainly focuses on investigating the microstructural and indentation creep behaviour of heat-treated AlSi10Mg alloy processed via the laser-assisted powder bed fusion (PBF) route. The as-built AlSi10Mg samples were initially solutionized at 520 °C for 2 h. Then, the solutionized samples were aged at 180 °C for 2 h. Indentation creep tests were conducted on the aged sample at three different loads 55, 92 and 129 MPa at 100 °C for 2 h. The steady-state creep rate was calculated from the slope of time vs indentation depth graph. In order to supplement the indentation creep results, detailed microstructural analysis has been carried out through optical and scanning electron microscopy. XRD study was carried out for phase analysis. AlSi10Mg in the as-built condition revealed the eutectic mixture at the melt pool boundaries and ultrafine Al cells in the interior. XRD pattern confirmed the presence of Mg2Si in the age-hardened condition. Several defects were seen in the age-hardened condition. As-built alloy showed the highest hardness, whereas age hardening reduced the hardness due to the spheroidization of Si particles. The primary creep region showed an increase in indentation depth increased with respect to time. The depth of indentation remained constant in the secondary creep region. Age-hardened samples subjected to 92 MPa stress showed better creep resistance compared to 55 and 129 MPa. As-built condition showed the highest hardness due to the extremely fine microstructure. [ABSTRACT FROM AUTHOR]

Underground energy storage in salt caverns involves operating in high-temperature conditions within deep strata, leading to unloading creep in the surrounding salt rock during cavity construction and natural gas extraction. Therefore, understanding the deformation and damage evolution of salt rock in such conditions is vital for ensuring safe construction and prolonged operation of deep salt cavern gas storage. The present study involves graded unloading creep tests and synchronous acoustic emission tests on the salt rock at 110 °C. The moment tensor inversion theory is employed to analyze focal mechanisms and microscopic crack propagation modes based on the test results. The findings reveal that under constant deviatoric stress, decreasing confining pressure exponentially increases the duration of the deceleration creep stage in salt rock. The creep increment and axial steady-state creep rate exhibited a U-shape trend. Instantaneous strain during stress change mainly involves elastic-plastic strain, where the instantaneous strain decreases first and then increases, exhibiting hardening characteristics in the middle process. The evolution of microcracks in salt rock can be categorized into three stages: compaction stage, stable propagation stage, and accelerated propagation stage. The acoustic emission energy, b-value, and fractal dimension of each stage are different. In unloading creep tests, the peak frequency in acoustic emission signal presents zonal distribution in low-frequency and high-frequency regions. It is found that as the confining pressure decreases, the peak frequency of acoustic emission signal and the proportion of high-frequency signal decrease, and the b-value and fractal dimension exhibit an upward-fluctuation-downward trend. The rupture slip angle of the acoustic emission source can effectively characterize the creep slip behavior in salt rock crystals. The slip gradually shifts from vertical to horizontal, while the strike angle and dip angle of rupture orientation are minimally affected by unloading. The obtained results can provide a reference for the safe construction and the design of the operation pressure for ultra-deep salt cavern gas storage. • The creep behavior of salt rock under high temperature are studied by performing multistage unloading creep tests. • Acoustic emission technology and the moment tensor theory are employed to analyze the microscopic crack propagation modes. • The axial strain increment and axial steady-state creep rate exhibit a U-shape trend as the confining pressure decreases. • The salt rock crystals have a sliding trend from vertical to horizontal with decreasing confining pressure. [ABSTRACT FROM AUTHOR]

Creep resistance and fracture tolerance are considered two essential aspects to look at for material selection and optimization in high-temperature applications. Materials with extensive industrial applications must inherit adequate immune against creep such that operating temperatures can be raised to maximize performance ensuring a long service life of the equipment. The creep study of cupronickel (CuNi) alloy has become crucial because cupronickel alloys are enticing candidates for high-temperature applications. Nevertheless, the fundamental understanding of creep deformation in cupronickel alloys is still obscure. Herein, we executed classical Molecular Dynamics (MD) simulations to reveal the nature of creep characteristics of cupronickel alloy at a molecular level. In particular, we investigated the thermal and mechanical properties of nanocrystalline (NC) Cu, Ni, and Cu0.5Ni0.5 alloy. As the Cu content in the alloy is raised from 0 to 100%, the steady-state creep rate (SSCR) exhibits a ∼12% increment. Based on our MD simulations, we found that the SSCR of Cu0.5Ni0.5 alloy speeds up dramatically under elevated stress, temperature, and decreasing grain size. Our MD results also reveal that at low stress, the governing creep process is the confluence of lattice diffusion, grain boundary diffusion, and grain boundary sliding, while at high stress, the governing creep mechanism turns into dislocation dominated. Admittedly, our computational analysis will help acquire fundamental insights into the creep mechanism of NC cupronickel alloy, which is crucial for the successful usage of this alloy in corrosive, high stress, and high-temperature applications.

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In the present investigation, creep-rupture behavior of P9 steel in two different product forms of 20-mm plate and 300-mm-thick tubeplate forging has been studied at 793 and 873 K. It has been found that steady-state creep rate and rupture life followed power law dependence on applied stress at low and high stress regimes for both forms of products. At high stress regime, P9 steel plate exhibited better creep strength with respect to tubeplate forging in terms of lower creep rate and higher rupture life at 793 and 873 K. On contrary, both product forms exhibited similar creep rate and rupture behavior in the low stress regime at 873 K. Difference in creep ductility has been found to be insignificant in both product forms. Irrespective of test conditions and product forms, fracture appearance remained transgranular ductile characterized by dimples. The comparative evaluation of creep-rupture properties of both product forms has also been described in terms of creep rate-rupture life relationships of Monkman–Grant type, creep damage tolerance and tertiary creep characteristics at 793 and 873 K. [ABSTRACT FROM AUTHOR]

The purpose of this work is to explore the impact of 0.5, 1.5, 2.5 and 3.5 wt.% Bi additions on the microstructure and mechanical performance of Sn-1Ag-0.5Cu solder alloy. Scanning electron microscope (SEM) and X-ray diffraction (XRD) were utilized to examine the microstructure of the present solders. Creep measurements have been used for the preliminary assessment of mechanical properties. The steady-state creep rate, έst, diminished as the Bi’s concentration increased and reached 2.5 wt.%, with this trend altering above this point. Furthermore, increasing the aging or testing temperature caused the έst values to increment for all the investigated solders. έst variations with different Bi content and aging temperature were observed by examining the Sn-Ag-Cu solders’ structural evolutions. The mean value of the activation energy of all investigated solder alloys was found to be ∼52 kJ/mol. This value is appropriate to that quoted for the dislocation climb through the core diffusion as the dominant operating mechanism. The XRD findings supported the microstructure and lattice parameters variations with both aging temperatures and bismuth concentrations.

The objective of this paper is to model the effect of direct current upon the steady-state creep of metals. The modeling of this phenomenon has been accomplished in terms of the synthetic theory of permanent deformation. The constitutive equation of the theory is supplemented by a term taking into account the passing of DC. Results obtained in terms of this theory show good agreement with experimental data. [ABSTRACT FROM AUTHOR]

The paper addresses the issue of the plastic and creep deformation of tin, a material with low melting point; main attention is focused on its plastic deformation and steady-state creep. The modelling of these deformations has been accomplished in terms of the synthetic theory of permanent deformation whose main peculiarity is only one constitutive equation governs both plastic and creep deformation. The stress–strain as well as strain-rate vs. stress diagrams constructed on the base of this theory show good agreement with experimental data. [ABSTRACT FROM AUTHOR]

Short-term and creep tests for saturated red sandstone under different pore pressures were carried out by a rock servo-controlled triaxial equipment. Based on the experimental results, the influences of confining pressure and pore pressure on short-term mechanical behavior of red sandstone are firstly analyzed. The results show that the peak strength of dry red sandstone increases with the confining pressure, which can be better expressed by the nonlinear Hoek-Brown criterion than the linear Mohr-Coulomb criterion. But with the increase of pore pressure, the peak strength and elastic modulus of saturated red sandstone all decrease step by step. The short-term failure mode of red sandstone is dependent of the confining pressure, but independent of the pore pressure. And then, the influence of pore pressure and axial deviatoric stress on the creep mechanical behavior of saturated red sandstone is analyzed quantitatively. Creep contribution to rock deformation increases with the pore pressure, and the specimens show significant time-dependent effect at higher deviatoric stresses. However, the steady-state creep rate of saturated red sandstone increases nonlinearly but the viscosity coefficient decreases gradually with increasing axial deviatoric stress and pore pressure. An exponential function is suggested to characterize the relationship between the creep parameters (including creep strain, steady-state creep rate and viscosity coefficient) and the axial deviatoric stress, and pore pressure. The calculated curves using the proposed function are compared with the experimental results, which is in a good agreement with the experimental data. In the end, the mechanism that the pore pressure and deviatoric stress affect the creep deformation of saturated red sandstone is discussed in detail. The presented experimental results on short-term and creep mechanical behavior of red sandstone under different pore pressures are very significant for evaluating the long-term stability and safety of deep underground rock mass engineering. [ABSTRACT FROM AUTHOR]

Highlights: [•] We model the steady-state creep of metal as a function of preliminary ultrasonic irradiation. [•] We model how the stacking fault energy of metal affects the above phenomenon. [•] We use the synthetic theory of irrecoverable deformation to model the phenomena above. [ABSTRACT FROM AUTHOR]

Abstract: Biaxial load was applied on the tube specimen by the inert gases medium, the inner pressure creep property of domestic Mo-3Nb alloy single crystal were studied. Up to now, the creep experiment has been done at 1600°C for 3000hours under three kinds of hoop stress. The three kinds of hoop strees is 5MPa,10MPa and 15MPa. Ultimately the creep curves under different conditions were obtained. The steady-state creep rate and the stress exponent in the Sherby-Dorn function have been deduced out. The stress exponent of Mo-3Nb is 5.4. Compare with the Russian single crystal, the creep property is close to Russian''s. [Copyright &y& Elsevier]

New methods of coupled mathematical modeling of steady-state creep rate and creep rupture strength of metals in tension have been devised. Two nonlinear fractional power functions with four material constants are used as basic dependences of the steady-state creep and creep rupture life on stress. Computation of the functions is based on optimally solving two nonlinear and inconsistent — in the conventional meaning — equations sets by the method of minimization of quadratic residuals. The authors outline the methods for calculating material constants, which were used to derive analytical expressions that optimally approximate the test results for 10Kh15N27T3MR steel at 600°C under various stresses. A method of piecewise-linear approximation of creep rupture strength test results, which involves the use of a two-segment broken line, is put forward. It implies that the locations of kinks as well as other numerical characteristics of the broken line are determined from the condition of the line’s optimal arrangement relative to the experimental data points. The method takes a more comprehensive account of various damage accumulation mechanisms in steel under various stress levels. [ABSTRACT FROM AUTHOR]

The Monkman-Grant (M-G) and its modified parameters were evaluated for type 316LN and modified 9Cr-Mo stainless steels prepared with minor element variations. Several sets of creep data for the two alloy systems were obtained by constant-load creep tests in 550-650°C temperature range. The M-G parameters, m, m’, C, and C’ were proposed and discussed for the two alloy systems. The m value of the M-G relation was 0.90 in type 316LN steel and 0.84 in modified 9Cr-Mo steel. The m’, value of the modified relation was 0.94 in type 316LN steel and 0.89 in 9Cr-Mo steel. Although creep fracture modes and creep properties between type 316LN and modified 9Cr-Mo steels showed a basic difference, the M-G and its modified relations demonstrated linearity quite well. The m’ of modified relation almost overlapped regardless of the creep testing conditions and chemical variations in the two alloy systems, and the parameter m’ was closer to unity than that of the M-G relation. [ABSTRACT FROM AUTHOR]

The purpose of this work is to explore the impact of 0.5, 1.5, 2.5 and 3.5 wt.% Bi additions on the microstructure and mechanical performance of Sn-1Ag-0.5Cu solder alloy. Scanning electron microscope (SEM) and X-ray diffraction (XRD) were utilized to examine the microstructure of the present solders. Creep measurements have been used for the preliminary assessment of mechanical properties. The steady-state creep rate, έst, diminished as the Bi's concentration increased and reached 2.5 wt.%, with this trend altering above this point. Furthermore, increasing the aging or testing temperature caused the έst values to increment for all the investigated solders. έst variations with different Bi content and aging temperature were observed by examining the Sn-Ag-Cu solders' structural evolutions. The mean value of the activation energy of all investigated solder alloys was found to be ∼52 kJ/mol. This value is appropriate to that quoted for the dislocation climb through the core diffusion as the dominant operating mechanism. The XRD findings supported the microstructure and lattice parameters variations with both aging temperatures and bismuth concentrations. [ABSTRACT FROM AUTHOR]

Since the δ phase is stable at temperatures higher than the normal service conditions but the γ" phase looses its strengthening effects, the formation of large amount of δ phase might be advantageous. In this work, it was revealed that such a delta processed microstructure can be obtained via overaging at 810 °C in Inconel 718 superalloy and its modified version known at Allvac 718Plus. While the peak aged samples (720 °C for 8 h) with the strengthening γ" phase lost their creep resistance at temperatures higher than 649 °C, the delta processed samples showed lower steady state creep rates and higher creep lives (rupture lifetime). Therefore, this study might shed light on the applicability of well-designed delta processing for creep resistance at higher temperatures in similar alloys. • Suitable δ-processed microstructure can be obtained via overaging in Inconel 718. • The peak aged sample with γ" phase loses its creep resistance at T > 649 °C. • δ-processed sample retains good creep resistance due to already available δ phase. [ABSTRACT FROM AUTHOR]