Creep Stage Research Articles

Compressive creep behavior of selective electron beam melted high Nb containing TiAl alloy

In this study, the compressive creep behavior of Ti–45Al–8Nb alloy fabricated by selective electron beam melting technology at 800 °C and 850 °C was investigated. This alloy is mainly composed of equiaxed γ grains, and minor α2 and B2 grains. Creep results show that compressive creep behavior of this alloy is dependent on temperature, which is manifested in the significant increase of creep strain and creep rate with the increase of temperature. Through a multi-step creep test, the stress exponent n (2.65 at 800 °C and 3.65 at 850 °C) and activation volume V* (36.49 b3 at 800 °C and 53.51 b3 at 850 °C) were measured. Combined with kinetic analysis based on n and V* and TEM observation, compressive creep deformation of this alloy at 800 °C and 850 °C is dominated by dislocation motion, including dislocation slip and climb. Additionally, mechanical twinning also plays an important role in creep deformation. Driven by dislocation pile-ups or mechanical twins, discontinuous dynamic recrystallization (DDRX) occurs during creep. The degree of DDRX at 850 °C is greater. Sufficient DDRX greatly softens the alloy and accelerates creep rate, resulting in no steady-state creep stage at 850 °C.

Study on creep characteristics and component model of saline soil in hexi corridor

Accurate mastery of the creep characteristics of unsaturated saline soil is extremely important for the long-term stability and safe operation of all types of buildings. In this paper, the research object focused on the saline soil of the Zhangye area, Hexi corridor. The indoor triaxial CU creep test was carried out by means of graded loading to study the creep characteristics of saline soil under different salt content and loading stress. The Merchant and Burgers models were used to predict the creep behavior of the saline soils, and the predicted results were compared with the experimental values. The results showed that the triaxial creep curve of saline soil developed in stage III. Namely, transient creep stage, deceleration creep stage and steady-state creep stage. The creep deformation increases with the increase of salt content and loading stress. The stress–strain isochronous curve has non-linear growth, and the cluster of curves develops from dense to sparse after increasing to long-term strength (100∼150 kPa). The parameters of the Merchant and Burgers model vary with salt content and loading stress, and the creep curve predicted by the Burgers model is closer to the test value.

Thermo-mechanical effect on the drained creep behavior of a clay rock

The heat released by high-level nuclear waste can cause a significant increase in temperature within the host rock, thereby affecting the long-term performance of a clay-based nuclear waste repository. This study aims to investigate the effects of stress and temperature on the creep behavior of a representative clay rock through a series of experimental creep tests. The tests were conducted under complex thermo-mechanical conditions using a temperature-controlled triaxial testing system. Drained creep tests were performed under various mechanical conditions, including a heating-cooling cycle (40 °C → 60 °C → 80 °C → 60 °C → 40 °C), with each temperature being maintained for approximately one month. The findings indicate the possible existence of a creep threshold stress even at high temperatures, although the temperature may extend the primary creep stage. Beyond the creep threshold stress, experiments conducted at high temperatures (or low confining pressure) exhibited significantly greater creep deformation and higher creep rates compared to those observed at room temperature (or high confining pressure) during the heating stage. Negligible creep deformation during cooling and the irrecoverable creep deformation resulting from deviatoric stress variation during the creep process demonstrate the path-dependent nature of the creep behavior of clay rock under thermo-mechanical conditions. Finally, the microscopic perspective of the creep mechanism in clay rock under thermo-mechanical effects was discussed, including the rearrangement and combination of clay particles, differential thermoelastic expansion among minerals, and the presence of pore water.

Role of microstructural stability and superdislocation shearing on creep behavior of two low-cost Ni-based single crystal superalloys at 1100 °C/130 MPa

In order to satisfy the pursuit of advanced aeroengines with low cost and high temperature capacity, the novel Re-free and Re-low single crystal superalloys were designed based on the strategy of Re-reduction. The creep lives of the two experimental alloys at 1100 °C/130 MPa both exceed that of René N5, and the Re-low alloy displays better creep performance and microstructural stability than the Re-free alloy. At the end of steady creep stage, due to the strong hindrance of interfacial dislocation networks, only a few superdislocations with Burgers vectors a<110> and a<001> enter into γ′ phase in the Re-low alloy. In the Re-free alloy, non-uniform plastic deformation leads to the severe γ/γ′ topological inversion, resulting in high-density a<110> superdislocation arrays in γ′ rafts, which can further react to form superdislocation networks. Moreover, the increasing μ phase precipitated with the prolonging creep time also promotes earlier failure of the Re-free alloy. In summary, the Re addition can improve creep resistance through reducing rafting rate of γ′ particles, stabilizing interfacial dislocation networks and increasing the APB energy of ordered γ′ phase. The above results provide favorable experimental basis for the development of low-cost single crystal superalloys.

Experimental and Numerical Study of Water–Rock Coupling Creep under Uniaxial Compression

In order to study the influence of the long-term strength of the rock surrounding deep roadways under the action of groundwater on surrounding rock stability, taking the rock surrounding the deep roadway of the Wanfu Coal Mine as the main research object, uniaxial compression and uniaxial creep tests were carried out on sandstone samples under different water-content states. It was found that the water content had an obvious softening effect on short-term and long-term strength, and both strengths showed a negative exponentially declining relationship. The viscosity modulus (E¯v) was put forward to describe viscoelastic creep deformation. And damage variables corresponding to E (the instantaneous elastic modulus) and E¯v were proposed. A sticky element that can describe the accelerated creep behavior was also established to improve the Nishihara model, based on the experimental results and damage theory. A comparison of the identified parameters and the experimental curves showed that the model can describe the mechanical behavior of various creep stages well. The model was developed using the ABAQUS user subroutine function, and the uniaxial compression creep experiment was simulated. The simulation results were basically consistent with the experimental results, which provide a basis for the further long-term stable use of roadway and creep failure simulation and have important practical and guiding significance.

Creep mechanical properties and creep damage model of sandstone considering temperature effect based on acoustic emission

AbstractThis work studies the creep mechanical properties and creep damage model of high‐temperature heat treated sandstone. Acoustic emission (AE) technology is used to monitor the crack evolution of sandstone in uniaxial compression and creep experiments in real time. The influence of temperature on the mechanical properties of sandstone is analyzed through AE count, cumulative AE count, and AE absolute energy. The relationship between cumulative AE count and axial strain is discussed, and the cumulative AE count increases exponentially with increasing axial strain. Moreover, an appropriate damage evolution equation is established based on the cumulative AE count, and a nonlinear viscoelastic‐plastic model capable of describing the decay creep, steady‐stage creep, and accelerated creep stages of high‐temperature heat treated sandstone is constructed. Through the model validation and parameter inversion, the new creep damage model can be employed to study the creep instability of geotechnical engineering under high temperature environment.

Microstructural Evolution of a Re-Containing 10% Cr-3Co-3W Steel during Creep at Elevated Temperature

Ten percent Cr steels are considered to be prospective materials for the production of pipes, tubes, and blades in coal-fired power plants, which are able to operate within ultra-supercritical steam parameters. The microstructural evolution of a Re-containing 10% Cr-3Co-3W steel with low N and high B content during creep was investigated at different strains at 923 K and under an applied stress of 120 MPa using TEM and EBSD analyses. The studied steel had been previously normalized at 1323 K and tempered at 1043 K for 3 h. In the initial state, the tempered martensite lath structure with high dislocation density was stabilized by M23C6 carbides, NbX carbonitrides, and M6C carbides. At the end of the primary creep stage, the main microstructural change was found to be the precipitation of the fine Laves phase particles along the boundaries of the prior austenite grains, packets, blocks, and martensitic laths. The remarkable microstructural degradation processes, such as the significant growth of martensitic laths, the reduction in dislocation density within the lath interiors, and the growth of the grain boundary Laves phase particles, occurred during the steady-state and tertiary creep stages. Moreover, during the steady-state creep stage, the precipitation of the V-rich phase was revealed. Softening was in accordance with the dramatic reduction in hardness during the transition from the primary creep stage to the steady-state creep stage. The reasons for the softening were considered to be due to the change in the strengthening mechanisms and the interactions of the grain boundary M23C6 carbides and Laves phase with the low-angle boundaries of the martensitic laths and free dislocations.

Rheology Dependent on the Distance to the Propagating Thrust Tip—(Ultra‐)Mylonites and Pseudotachylytes of the Silvretta Basal Thrust

AbstractTo evaluate how the presence of pseudotachylytes affects the strength of crustal rocks, deformed pseudotachylytes and their relationship with pristine pseudotachylytes at the base of the Silvretta nappe are analyzed. Pseudotachylytes formed associated with high‐stress crystal plasticity (σd &gt; 400 MPa), as indicated by twinned amphiboles in gneisses. Mylonitic quartz clasts enclosed within deformed pseudotachylytes and mylonitic vein‐quartz, hosting folded pseudotachylyte injection veins, reflect creep at lower stresses (ca. 100 MPa) after seismic rupturing. Deformed pseudotachylytes can be crosscut by pristine pseudotachylytes, indicating a second, independent stage of coseismic rupturing after creep. The evidence of dynamic dislocation creep of quartz and the presence of stilpnomelane and epidote associated with all fault rocks indicate similar ambient greenschist facies conditions during all deformation stages. Whereas the intermediate stage of creep is interpreted to represent deformation at large distance to the propagating thrust tip, the pristine pseudotachylytes represent the last stage of rupturing eventually leading to nappe decoupling from its basement. Gneiss clasts in an ultramylonitic matrix (i.e., deformed pseudotachylyte) reveal that pseudotachylytes have a lower strength during creep in relation to the hosting gneisses. In contrast, during coseismic high‐stress crystal plasticity, the coarse gneisses accumulate a higher amount of strain. This strength‐relationship explains that only those rocks rupture, which have not been previously deformed before. The study demonstrates the importance of different strengths of crustal rocks at specific stress‐ and strain‐rate conditions in dependence on the distance to the propagating fault tip.

Creep Performance and Life Prediction of Bamboo Scrimber under Long-Term Tension in Parallel-to-Grain

Creep performance is a crucial factor that must be considered in structural design. This paper aims to investigate the creep failure mode, creep strain, creep compliance, and other creep properties of bamboo scrimber under long-term tension in parallel-to-grain. To establish a general creep life prediction method for the full stress level of the bamboo scrimber, a multi-branch Kelvin–Voigt model, a generalized Maxwell model, and a creep finite element simulation were employed. The results showed that the creep strain curve of bamboo scrimber included the unsteady creep stage and the stable creep stage, but not the accelerated creep stage. When the stress ratio was less than 0.3, the residual strength decreased gradually. Below 70% of the ultimate load capacity, the creep characteristics of the bamboo scrimber were linear viscoelastic, and the creep compliance was generally independent of the load level. The creep finite element model of bamboo scrimber could accurately calculate the creep deformation of specimens. Based on this creep finite element model and creep failure rules, a life prediction model for the full stress level of bamboo scrimber was established, which could accurately predict the creep life. This paper provides theoretical guidance for the creep design of bamboo scrimber in engineering structures.

Creep deformation and constitutive modelling of 316LN SS with varying nitrogen content

In this investigation, two different constitutive models were employed to examine the creep deformation and damage behaviour of nuclear grade austenitic 316LN SS with four different nitrogen contents (0.07–0.22 wt%) at 923 K. Creep deformation resistance of the alloys investigated by “internal-stress based kinetic creep law” for primary and secondary creep stages revealed an increase in internal stress and decrease in obstacle spacing accompanied with lower dynamic recovery with increasing nitrogen. The aforementioned outcomes substantiated the experimentally observed beneficial role of nitrogen on lowering of creep strain rate with increasing nitrogen level. In contrast to the above observations, the analysis of coupled creep deformation and damage using the Kachanov-Rabotnov model and the subsequent development of iso-damage contours indicated evolution of higher creep damage at lower strains with increasing nitrogen content. With increasing % nitrogen, damage rate dominated over the strain rate thus justifying the observed significant intergranular damage at and above 0.14% N. Based on the above results and in-house studies on low cycle fatigue and creep–fatigue interaction, optimum nitrogen content is suggested between 0.07 and 0.14% N in 316LN SS that combines the benefits of low nitrogen content and strengthening effects of high nitrogen content.

Creep and Hardening Characteristics of Anthracite under Graded Static–Dynamic Coupled Loading

Remaining coal pillars in remining areas exhibit clear creep characteristics, and dynamic pressure accelerates their instability and failure. The creep and hardening characteristics of coal under dynamic pressure are of great engineering significance for the stability of remaining coal pillars in remining areas and their control. To investigate the creep and hardening characteristics of anthracite under static–dynamic coupled loading, graded loading creep tests with different loading rates were conducted. In this research, the creep strain, instantaneous elastic modulus, and creep rate of anthracite were studied under different graded loading rates. The results showed that the hardening effect of the samples manifested as an increasing instantaneous elastic modulus at the loading stage and a decreasing strain rate at the creep stage. When the graded loading rate increases from 0.01 to 0.1 mm/s, the instantaneous elastic modulus increases by 0.16–2.32 times. The sudden increase in the instantaneous elastic modulus at the failure stress level explains the instantaneous failure of the samples well. The actual yield levels corresponding to the peak instantaneous elastic modulus of the samples linearly decreased with increasing graded loading rate. The functional relationship between the graded loading rate and the elastic modulus hardening coefficient, the actual yield stress, and the strain rate decay coefficient were established, which could quantitatively describe the influence of different graded loading rates on the creep and hardening characteristics of anthracite and predict its creep damage.

Compressive creep behavior of high Nb containing TiAl alloy: Dynamic recrystallization and phase transformation

In this work, compressive creep behavior of Ti-43Al-6Nb-1Cr-1.5V alloy fabricated by vacuum induction skull melting technology was investigated at 800-950 °C. The results show that compressive creep behavior of this alloy strongly depends on temperature. At 800 °C and 850 °C, this alloy exhibits conventional three-stage creep characteristics with a well-defined steady-state creep stage, and creep strain is small. However, at 900 °C and 950 °C, this alloy exhibits unconventional creep characteristics, and creep strain increases sharply. Microstructure observation proves that this is mainly due to the occurrence of dynamic recrystallization (DRX). During creep, continuous dynamic recrystallization and discontinuous dynamic recrystallization appear simultaneously, but the latter is more significant. At 800 °C and 850 °C, creep deformation mainly depends on dislocation motion, while at 900 °C and 950 °C, DRX also greatly contributes to creep deformation. During creep, recrystallized grains preferentially form at β-segregation and α-segregation, and rising temperature from 850 °C to 900 °C facilitates the sharp increase of DRX fraction. The occurrence of DRX reduces dislocation density, which greatly softens the alloy. Besides, more α2 grains generate at γ grain boundary and grain boundary/twin intersection during creep, and maintain Blackburn orientation relationship with γ grains. This is the result of stress-induced γ→α2 phase transformation.

Estimation the Remaining Life for Reformer Unit Tubes Furnace Operated Beyond the Design Life

The aim of this work is to study and model creep damage accumulation for remaining safe working life estimation of tube material used in petroleum industry plants operated at a high temperature for long time beyond the design life. Material constants and other data required for life estimation were extracted from the output results of real accelerated creep rupture tests conducted at three test temperatures (700, 725, and 750(oC and at three constant stress applications (120, 130, and 140 MPa). Also, a time-temperature parameter method (Larson-Miller Method) was modified to be applied based on the results of the accelerated creep rupture tests to build life predicted master curve for austenitic stainless steel type 321H. It is found that creep rupture time for stainless steel alloy decreases as temperature or stress increase. Also, it distinguished from the creep curves for the serviced stainless steel alloy, that the onset of tertiary creep stage starts early (about 50% of creep curve), this reflects the reduction in creep strength of the alloy as a result of the degradation in alloy properties that take place during the service life. It is also found that the estimated remaining working life for the serviced tube samples made from austenitic stainless steel type 321H is about (54703 hr) based on the calculated stress level (40 MPa) and service temperature 570 oC. The difference between the estimated remaining life and the experimental creep test life is due to the differences between the applied stresses, where higher level of stresses in the experimental creep testes are used to accelerate failure of specimens. The application for the result of this work can be the petroleum industries that where heaters tubes serviced at such industry need to be estimated for the remaining life after the design life expire.

Study on creep behaviors and nonlinear creep constitutive model for sandy marine hydrate-bearing sediments

Marine natural gas hydrate (NGH) production is critically affected by reservoir damage, which is usually presented as creep deformation. Clarifying the creep behaviors of hydrate-bearing sediments (HBS) is essential for predicting potential geoengineering risks during long-term NGH mining. In this study, a series of multi-step loading creep tests on HBS samples are conducted and a modified creep model specialized for sandy HBS is proposed. The creep strain, creep rate, and long-term strength are experimentally evaluated under various hydrate saturation (Sh) and/or effective confining pressure (Pc). Three typical creep stages of deceleration creep, stable creep, and accelerated creep are observed in all experiments. The results show that the creep deformation of HBS is enhanced with the decrease in Sh under the same axial load. The long-term strength decays linearly with decreasing Sh and Pc. The creep model is established by modification of both the Kelvin element and the ideal viscoplastic element in the conventional Nishihara model. A nonlinear viscous coefficient and a time-damaged elastomer considering are introduced to describe the nonlinear creep and damage characteristics of HBS. The model is able to describe the whole failure process, especially the accelerated creep stage. The results might have some significance on the prediction of nonlinear failure behaviors of the reservoir during long-term NGH mining.

Effect of temperature on the creep properties of polycrystalline Cu-Ni alloy: insight from molecular dynamics simulation

Creep is a phenomenon where a particular material tends to deform due to highly subjected parameters, such as temperature and pressure. This deformation is promoted by the diffusion of the material across the grain boundary, as critically assessed by Coble in 1963. This phenomenon is primarily crucial since the most critical application of materials is subjected to high temperature and pressure. This study intends to assess the effect of temperature of polycrystalline Cu-Ni alloy by utilizing molecular dynamics simulation. The simulation is run with different temperatures, thus allowing studying the critical differences between the applied parameters. The results show that the temperature affects the creep rate at every creep stage, primary and secondary, by a significant value. This is due to the deformation promoted by the diffusion of atoms across the grain boundary, decreasing the crystallinity of polycrystalline Cu-Ni alloy. Although the effect of temperature on the creep mechanism of polycrystalline Cu-Ni alloy seems straightforward, it is necessary to assess whether it increases the creep rate at a low temperature in further detail.

Measurement of Creep Stress Exponent of TC17 Titanium Alloy by Nanoindentation Method at Room Temperature.

The creep stress exponent is commonly employed to characterize the deformation mechanism during the steady-state creep stage, serving as an indicator of creep behavior. The creep phenomenon of high melting point metallic materials is not obvious at room temperature. However, the nanoindentation method proves suitable for investigating the creep properties of metallic materials under such conditions. Consequently, this paper places emphasis on measuring the creep stress exponent of TC17 titanium alloy at room temperature using the load preservation stage of the nanoindentation method with a constant loading rate. In order to investigate the effects of loading rate and maximum load on the experimental results, different loading rates were applied to the diamond Berkovich indenter to reach different maximum loads. The indenter was held under the maximum load for a duration of 360 s, and the relationship between the indentation strain rate and indentation stress during the holding process was used to obtain the creep stress exponent of the material at room temperature. The findings indicate that within the loading rate range of 1.25 to 15 mN/s and maximum load range of 50 to 300 mN, the influence on the experimental results is insignificant. Ultimately, the distribution range of the creep stress exponent for TC17 titanium alloy at room temperature was measured to be 8.524-8.687.

A novel nonlinear fractional viscoelastic–viscoplastic damage creep model for rock-like geomaterials

In this study, a novel nonlinear fractional viscoelastic–viscoplastic damage creep model is developed for rock-like geomaterials in an attempt to describe the 3-stage creep behaviour. The proposed model consists of viscoelastic and viscoplastic parts. The former considers the viscoelastic response at different time scales, and incorporates the improved Kelvin and Maxwell components modified by the Abel dashpot. The two Abel dashpots represent the effects of the time dependence on the decaying and steady creep stages. The latter part is described by using a plastic slider parallel to a damaged Abel dashpot, and characterises the viscoplastic response adjusted by the overstress and non-orthogonal viscoplastic flow theory in the accelerated creep stage. The proposed model was written in finite difference form and transcribed into object-oriented C++ for implementation in FLAC3D. By calibrating the available experimental data, the numerical simulation with the proposed model shows a successful description of the full 3-stage behaviour. In addition, the proposed model adequately predicts the delayed deformation caused by the accelerated creep in the tunnel simulation, which provides a valid reference for the subsequent support.

Effect of Re/Ru on constituents distribution and creep performance of nickel-based single crystal alloys

ABSTRACT The effects of Re/Re on the element distribution and creep performance of nickel-based single crystal alloy by using atom probe tomography (APT) and creeping property testing has been studied. Results show that the elements Re, Ru, W, Mo, Cr, and Co are the former of γ phase; the elements are distributed in γ and γ‘ phases of two alloys according to various ratios. Some of the Ru atoms in Re-containing alloy make more Al, Ta atoms are dissolved in γ matrix and make more Mo, W, and Re atoms dissolving in γ‘ phase, which increases the alloying degree of γ“, γ phases to enhance the strength and creep resistance of alloy. Particularly, the Re and W atoms dissolved in γ” phase are excluded, during creep, for the enrichmrnt in γ phase near the interface to form their peak content; the lattice distortion coming from the enriched Re and W atoms restrains dislocations gliding to delay the γ’ phase from being sheared. The deformed mechanisms of alloy in the later stage of creep are the dislocations gliding in γ phase and shearing γ’ phase. Wherein, the dislocations of shearing γ’ phase are both glided on {111} planes and cross-glided from {111} to {001} planes to form the KW locks, the ones that restrain the gliding and cross-gliding of dislocations for improving the creep resistance of alloy. While the interaction of the Ru with Re and W atoms make some Reand W atoms reserved in the γ′ phase to delay the diffusion of other elements, which prevents the KW locks from being released, to keep the good resistance of alloy.

Modelling the creep behaviour and induced failure of clay rock from microscale viscosity to large-scale time-dependant gallery convergences using a multiscale numerical approach

The large-scale creep behaviour of Callovo-Oxfordian (COx) clay rock is modelled from small-scale viscous mechanisms of the rock using a multiscale numerical approach in the context of radioactive waste repositories. At the mesoscale, the saturated non-homogeneous rock is represented in digital 2D Representative Elementary Areas (REAs) as a cracked composite material consisting of rigid elastic mineral inclusions (quartz, calcite, and pyrite) embedded in a clay matrix. The modelling of these materials is considered within double-scale finite element framework, in which the homogenised responses of the REA to the enforced global kinematics serve as a numerical constitutive relation at the macroscale. To reproduce the rock creeping, two viscous mechanisms have been introduced to consider the creep of the clay matrix: either the viscoplasticity of the clay aggregates or the viscoelasticity of their contacts, or both. Firstly, laboratory biaxial creep tests of clay rock samples are simulated. A three-stage creep stage is reproduced and the creep failure process is discussed. Then, large-scale underground engineering structures are modelled to reproduce its time-dependent behaviour. It is found that the developed multiscale model is able to provide some valuable insights into the large-scale creep behaviour of clay rocks through the morphological and material small-scale characterization of REA.

Design of a constitutive model based on the evolution of creep cavities damage for 9Cr-1Mo steel

Considering the impact creep cavity evolution damage of 9Cr-1Mo steel, a new creep constitutive model was established based on the damage-coupled Norton equation and the strain hardening term in the theta projection method model, and the parameters were solved by optimization technique. The creep behavior was compared with that of the Kachanov-Robotnov model. According to the simulation results, the proposed constitutive model can represent the strengthening phenomenon during the first creep stage of 9Cr-1Mo steel. It provides a more accurate description of the behavior during the three creep periods.