Initial Creep Stage Research Articles

Investigation on Creep Deformation and Age Strengthening Behavior of 304 Stainless Steel under High Stress Levels.

The creep deformation behavior and age strengthening behavior of 304 stainless steel under high stress levels were systematically studied by uniaxial creep test, tensile test, XRD diffraction test and transmission electron microscopy. The results show that the total creep strain and the initial creep strain rate increase with the increase in stress level, and the creep strain in the whole aging process is mainly produced in the initial creep stage. The calculated stress exponent shows that the main mechanism of creep deformation of 304 stainless steel at 453 K is dislocation slip. The strength and plasticity of 304 stainless steel after creep aging are improved simultaneously. Microstructural observations indicate an increase in dislocation density and martensite content, as well as austenite and twins, leading to an improvement in strength and plasticity, respectively. In addition, considering the influence of dislocation density on creep behavior, the relative dislocation density increase is introduced into the hyperbolic sine creep model, and a simple mechanism-based creep aging constitutive model is established. The creep strain predicted by the model is in good agreement with the experimental data of 304 stainless steel. The findings can provide theoretical support for the application of creep age forming in 304 stainless steel parts.

Initial Vacancy-Dependent High-Temperature Creep Behavior of Nanocrystalline Ni by Molecular Dynamics Simulation

Nanocrystalline metals possessing excellent mechanical strength have great potential to replace traditional metal materials as structural materials, but their poor resistance to creep deformation seriously restricts their engineering applications at high temperatures. The high-temperature creep behavior of nanocrystalline Ni with different volume fractions of initial vacancies ranging from 0% to 10% was studied systematically by molecular dynamics simulation in this study. The results showed that the steady-state creep displacement first increased and then decreased with increasing initial vacancy concentration, reaching the maximum when the initial vacancy concentration was 6%. The microstructural characteristics, such as quantity increment and distribution of the vacancies, the number and types of dislocations, and shear strain distribution during creeping, were analyzed in detail. The deformation-induced vacancies formed at the grain boundary (GB) in the initial creep stage, and their variation trend with the initial vacancy concentration was consistent with that of the creep displacement, indicating that the initial vacancy-dependent high-temperature creep behavior of nanocrystalline Ni was mainly determined by the rapidly increasing number of vacancies at the GB in the initial creep stage. Afterwards, the deformation-induced, vacancy-assisted 1/6{112} Shockley partial dislocation activities dominated the creep deformation of nanocrystalline Ni in the steady-state creep stage. The results can provide theoretical support for expanding the application of nanocrystalline metals from the perspective of crystal defect engineering.

Effect of applied stress level on anisotropy in creep-aging behavior of Al–Cu–Li alloy

In the present work, the anisotropic creep aging (CA) behavior and microstructural evolution of Al–Cu–Li alloy are investigated under different stress levels. The results indicate that as the stress level increases, the anisotropic creep behavior of Al–Cu–Li alloy changes significantly. As the applied stress level increases, the in-plane anisotropy (IPA) value of creep strain decreases and then increases. In addition, the strength also exhibits significant anisotropy, and as the applied stress level increases, there are differences in the strength changes of samples with different orientations at different creep aging times. Electron backscatter diffraction (EBSD), Energy-dispersive x-ray spectroscopy (EDS) and transmission electron microscopy (TEM) methods are used to study the evolution of texture and microstructure, as well as their effects on the anisotropy evolution of materials. Microscopic observation reveals that the anisotropy changes in creep and strength can be attributed to the difference in dislocation density and grain boundary slip phenomenon during the initial creep stage in samples with different orientations, as well as the stress orientation effect of precipitated phases, especially T1 phase. The study reveals a new mechanism of anisotropic CA mechanism in Al–Cu–Li alloy under different applied stress levels.

Microstructure Evolution and Dislocation Mechanism of a Third-Generation Single-Crystal Ni-Based Superalloy during Creep at 1170 °C.

The present study investigates the creep behavior and deformation mechanism of a third-generation single-crystal Ni-based superalloy at 1170 °C under a range of stress levels. Scanning electron microscopes (SEM) and transmission electron microscopes (TEM) were employed to observe the formation of a rafted γ' phase, which exhibits a topologically close-packed (TCP) structure. The orientation relationship and elemental composition of the TCP phase and matrix were analyzed to discern their impact on the creep properties of the alloy. The primary deformation mechanism of the examined alloy was identified as dislocation slipping within the γ matrix, accompanied by the climbing of dislocations over the rafted γ' phase during the initial stage of creep. In the later stages of creep, super-dislocations with Burgers vectors of a<010> and a/2<110> were observed to shear into the γ' phase, originating from interfacial dislocation networks. Up to the fracture, the sequential activation of dislocation shearing in the primary and secondary slipping systems of the γ' phase occurs. As a consequence of this alternating dislocation shearing, a twist deformation of the rafted γ' phase ensued, ultimately contributing to the fracture mechanism observed in the alloy during creep.

Повзучість малопластичних жароміцних матеріалів при вигині…

Express method of testing the initial creep stagiess of low-plastic heat-resistant materials that work in extreme conditions using the bending scheme proposed and developed. The features of stress and deformation calculation are analyzed. The limitations of using the elastic approximation are outlined: the degree of plastic deformation of the sample should not exceed 1—1,5%. The deflection should not exceed 10% of the distance between the supports, the height should not be greater than 1/10 of the distance between the supports. Under these conditions, the first and second stages are well distinguished on the creep curves. This makes it possible to analyze the influence of phase and structural changes on the mechanisms of creep at each stage, and the conditions for the transition to stationary creep. The results of model experiments performed on TNM titanium aluminide alloys and Fe3Al powder alloy samples indicate the expediency of using the three-point bending scheme for researching the initial stagiess of creep of heat-resistant materials. Experimentally determined values of the deformation rate vary in the range έ ~ 10-5—10-8, which is the most characteristic for the creep of heat-resistant materials. For low-plastic intermetallics, the influence of temperature and loading force on creep curves was studied. The dependences deformation rate vs time on the first and second stages of creep were obtained from bending tests. Thermal activation parameters are defined for the stage of permanent creep. The proposed method allows to study the speed sensitivity and to determine the thermal activation parameters of creep. In extreme conditions of operation of low-plastic heat-resistant materials, the proposed method allows to take into account and analyze the contribution to the creep of cracking and slow destruction processes. Keywords: three-point bending test, rate of deformation, initial stages of creep,high temperature materials, thermal activation parameters.

Characterization of geometrically necessary dislocation evolution during creep of P91 steel using electron backscatter diffraction

In this study, the change of geometrically necessary dislocation (GND) has been investigated during the elevated temperature creep of P91 steel. Firstly, the crept microstructures of P91 steel at 873 K and 165 MPa were characterized by electron backscatter diffraction (EBSD). Then, the resultant GND distributions were analyzed in detail. Meanwhile, the GND density was quantitatively estimated by two methods of kernel average misorientation (KAM) and Nye's dislocation density tensor, respectively. The obtained results indicated that during creep of P91 steel, the values of GND density calculated by the KAM method are highly consistent with those of Nye's dislocation density tensor method. Besides, the GND density increases significantly during the initial creep stage and attains the maximum approximately at the transition from initial creep to steady creep. During the subsequent creep, it gradually decreases. In addition, it suggested that the internal stress predicted by the obtained GND density is consistent with previous experimental data. Accordingly, it further demonstrated that the present characterization of GND evolution offers a convenient tool for the microstructure degradation analysis of P91 steel during creep. • The microstructure of P91 steel during creep has been investigated by EBSD method. • The change of GND density was quantitatively characterized by two methods of KAM and Ney's dislocation density tensor, respectively. • The GND densities obtained by the above methods are highly consistent. • The GND densities vary closely with the creep stage, which mainly results from the interaction between dislocation and precipitates.

A new forecasting method for failure time of creep landslide based on nonlinear creep behavior and new pre-warning criterion

The forecast of failure time of unstable slope and the definition of early warning threshold are very important for preventing landslide disaster and reducing its losses. Based on the monitoring curve of unstable slope deformation varying with time, the mathematical models are used to accurately describe the nonlinear creep behavior in the initial creep stage and the unstable creep stage of unstable slope and a forecasting method for creep landslide is proposed. In addition, this study examines an improved tangential angle criterion obtained by a new forecasting method. The results show that the initial creep stage of unstable slope can not be neglected for forecasting the failure time. The initial creep velocity v0 and the viscoelastic hysteresis coefficientξof the slope are determined at initial creep stage, which together control the creep process of the unstable slope. Moreover, in the secondary creep stage, there is an inverse relationship between the velocity and the critical tangential angle. According to the average velocity of the secondary creep stage, the early warning criterion of the landslide tangential angle is proposed. Combined with the forecast parameters of the unstable slope, comparative analysis of the critical tangential angle warning criterion and an improved tangential angle criterion, the early warning and forecasting system of landslide with creep characteristic is established. This system is applied to the early warning and forecasting analysis of reported soil and rock landslides cases with creep characteristics. The forecasting effect and applicability of the new method are studied in order to make it a better supplement to the early warning strategy of landslide.

Study on very long-term creep tests and nonlinear creep-damage constitutive model of salt rock

To better understand the creep mechanical behavior of salt rock under very long-term tests and more accurately predict the long-term deformation of the salt caverns , a series of long-term uniaxial creep tests lasting for 21000 h (875 days) were carried out. The results show that the creep strain and creep rate of salt rock increase with the increase of axial stress . The initial creep stage under lower stresses lasted for 4.3–8.3 months, which was much higher than that under high stresses. Predicting the true creep rate of the salt cavern based on the test results of the creep rate in this paper is more accurate than the existing research results. A new nonlinear creep-damage constitutive model is proposed by introducing the nonlinear damage body and using the fractional derivative theory. Using the presented model and the classic creep constitutive models to fit the experimental data, it is found that the presented model best characterizes the creep evolution characteristics of salt rock. The fitting function obtained from the experimental data at different times predicts the final creep results, revealing that the longer the test duration, the more accurate the predicted results. The research in this paper provides the necessary basis for intensive investigation and prediction of the long-term creep mechanical behavior of salt storage caverns.

Crystal plasticity phase-field simulation of rafting and creep properties with a continuous change of the mismatch strain in a Co-Al-W superalloy

The lattice mismatch strain between the precipitates and matrix has an essential effect on the creep morphology and properties of Co-based superalloys. The rafting morphology with a continuous change of the mismatch strain is clearly presented in a Co-10Al-8W at.% alloy by utilizing crystal plasticity phase-field simulation. The large mismatch strain accelerates the rafting of γ′-Co3(Al, W) phases, the hardening effect caused by eigenstrain elevates the creep properties in the initial creep stage, while the accumulation of damage variable induced by large eigenstrain reduces the creep life in tertiary creep stage. These results suggest that the suitable lattice mismatch is beneficial for the creep properties of superalloys.

Triaxial creep tests of glacitectonically disturbed stiff clay – structural, strength, and slope stability aspects

Abstract This study concerns the creep impact on strength parameters of the selected very cohesive soils (PI = 30–70%). The analysis refers to Neogene clays characterized by a complex structure, resulting directly from a complicated load history in the geological time scale and identified glacitectonic deformations. In the process of samples’ preparation for strength tests as well as during the interpretation of the post-failure state, particular attention was paid to the soil structure. The imaging methods (X-ray densitometry and computer microtomography) enabled the comparison of the soil structure and the selection of samples with similar characteristics. The completed program of strength tests consisted of two series of tests in the triaxial stress state, differentiated by the occurrence of the initial creep stage, preceding the typical strength test scheme under undrained conditions. This study allowed to obtain a quantitative assessment of the influence of the creep process on the strength parameters of tested soils. Constant stress lower than 60% of the shear stress deviator leads to the deceleration creep course (m parameter 0.64–0.89). As a result, higher values of internal friction angle (20% increase comparing to triaxial tests without creep stage) and cohesion reduction are obtained from triaxial creep tests. Creep parameter m is found to be a valuable indicator for differentiation of landslide activity trend. The tests proved low values of axial strains (1–5%) at failure, which was associated with lithogenesis. By the implementation of obtained strength parameters into the 3D finite element model of the slope, the potential influence of the creep process on the stability of an exemplary cross section of the Warsaw slope could be determined.

Long-term mechanical and acoustic emission characteristics of creep in deeply buried jinping marble considering excavation disturbance

To obtain a complete understanding of the stability of deep caverns after excavation disturbance, a laboratory simulation method for testing surrounding rock under excavation disturbance was systematically proposed. The rock creep mechanical behavior considering excavation disturbance was compared with that of rock without excavation disturbance, and the results show that the long-term rock strength considering excavation disturbance is significantly lower than that without excavation disturbance at a 2400 m depth. Based on this method, a step-loading triaxial creep and associated acoustic emission (AE) test was carried out with a single-stage loading time of 5 days and a total loading time of approximately 50 days. The results show that with an increase in stress level, the transient strain of the deep rock increases nearly linearly, while the axial creep strain rate increases exponentially. The AE monitoring results demonstrate that when the rock creep transforms from the initial creep stage to the steady-state creep stage, and from the steady-state creep stage to the accelerating stage, the AE amplitude will clearly exhibit a band-shaped distribution, and the AE count rate and AE energy rate will show a single peak or multiple peaks. Multistage AE data can be used to determine the long-term strength of rock. In addition, with an increase in the stress level, the microcracks of the deep rock under excavation disturbance transform from closed primary microcracks to initiating, propagating and coalescence secondary microcracks, which ultimately leads to macroscopic shear failure. The above research results can provide a reference for deep resource exploitation and deep cavern stability evaluation.

Molecular dynamics simulation-based study of creep–ratcheting behavior of nanocrystalline aluminum

In the present study, molecular dynamics simulations have been performed to investigate the creep–ratcheting deformation behavior of nanocrystalline aluminum (NC Al) having an average grain size of ~ 8 nm. The influence of deformation temperature on creep–ratcheting behavior has been studied and associated with underlying mechanisms based on the structural evolution of the material identified. The vacancy concentrations, strains and dislocation densities have been evaluated at the end of each stage of creep–ratcheting process for two ratcheting stress ratios and three different temperatures. In the mean time, the microstructural and defect evolution has been investigated. Accumulation of creep–ratcheting strain is found to increase with the deformation temperature in the range of temperature investigated: 10–467 K. Cyclic hardening dominates in the initial stages of creep–ratcheting, whereas cyclic softening dominates in the final stages at a higher temperature. The creep–ratcheting plots exhibit a primary and steady state regions at room temperature (300 K). In addition, a tertiary region is also perceived at high temperature (467 K). The NC Al specimen is also found to be damaged earlier at a higher temperature (i.e., 467 K) than at 10 K and 300 K. The highest dislocation density is attained for room temperature creep–ratcheting deformation. Finally, it is seen from the dislocation analysis that the Shockley partial and full dislocations are the driving dislocations for the creep–ratcheting deformation process.

STUDY ON CREEP CHARACTERISTICS OF SALT ROCK UNDER UNLOADING CONDITIONS

The underground salt karst caverns are considered as an excellent oil and gas reserve site due to the dense low permeability and excellent creep. The creep characteristics of salt rocks under frequent gas injection and gas production activities are the focus of salt cavern gas storage. The 700h loading and unloading creep tests were performed on salt samples taken from Jintan, Jiangsu Province, China, and the creep behaviors of rock salt under different loading and unloading stress paths were compared and analyzed. The results showed that the steady-state creep strain rate increases with the increase of deviatoric stress during multi-stage creep test, The deviatoric stress has a great influence on the steady-state creep strain rate, and the steady-state creep strain rate increases obviously with the increase of the deviator stress in the loading stage, but in the unloading stage, steadystate creep strain rate increases with deviatoric stress increase, and the increase range is small. The Fractional Derivative model, the Nishihara model and the Burgers model are used to fit the experimental data of the initial creep stage and the steady-state creep stage during loading stage, the results show that the Fractional Derivative model can better describe the creep curve. The creep deformation curve is almost straight and the creep curve is fitted by a linear function in unloading stage, the results show that the fitting curve is consistent with the experimental results.

Study on the Creep Characteristics of Gentle Dip Red Bed Mudstone of the Surrounding Rock Tunnel

Red bed soft rock exhibits poor physical and mechanical properties and certain creep characteristics, which cause continuous deformation of tunnels and underground engineering structures during operation. The rock mass at the bottom of a railway tunnel in Sichuan Province is mudstone of the Penglaizhen Formation of the Upper Jurassic. It has a thin to medium-thick layered structure. Moreover, the rock formation is nearly horizontal. In this paper, the shear creep test and deep learning are employed to study the creep characteristics of gentle dip red bed mudstone.The results indicate that the red bed mudstone in the tunnel site exhibits medium-low creep characteristics. When the stress level is relatively low, the rate of the creep deformation gradually reduces with time, and when it reaches a certain time, the deformation no longer increases, and the final deformation tends to a stable value. Conversely, when the stress level is relatively high, although the rate of the creep deformation gradually reduces as the time increases, it remains unchanged when it reduces to a certain value. At the initial stage of creep, the deformation of each grade is evident; at the middle stage, the deformation is slow; and at the later stage, the deformation remains unchanged, that is, it enters the stable creep stage after a rapid decay in creep rate. The long-term strength of red bed mudstone in the tunnel site is low, which easily causes continuous deformation of the surrounding rock of tunnels under the action of high ground stress. According to the long-term deformation monitoring data at the bottom of the tunnel, combined with 20 groups of red bed mudstone creep parameter samples and the upper arch deformation data of the tunnel numerical model, the deep learning inverse analysis model of the creep parameter of rock mass is established based on the deep learning algorithm. Finally, we obtain the creep parameter of the red bed mudstone via inverse analysis. The research results provide a basis for engineering structure design and long-term deformation prediction of red bed mudstone strata in this area.

Localised creep cavitation on boron nitride in the heat affected zone of 9% Cr tempered martensitic steel welds

An excessive concentration of boron in the 9% Cr tempered martensitic steels alloyed with nitrogen has a detrimental effect on creep performance, as relatively large boron nitride phases are formed and can act as the preferred sites for cavity nucleation. In the current research, a systematic investigation has been performed documenting details of creep damage in the heat affected zone of a weld joint constructed by using a 9% Cr Grade 92 steel. A combination of correlative characterisation techniques has been applied to monitor and quantify the evolution of creep damage within the heat affected zone through the entire creep life. The utilisation of cryo-fractography, two-dimensional electron-based microscopic characterisation and three-dimensional tomography based on focused ion beam microscopy have confirmed the close association between creep cavities and boron nitride phases from the initial stage of creep. It is revealed that the thermal transients imposed during welding result in the decohesion at the interface of boron nitrides with the surrounding matrix. This decohesion is a result of significant thermo-mechanical mismatch between boron nitrides and the tempered martensitic matrix. The voids initiated at these locations grow and eventually link-up during creep exposure, leading to cracking and fracture.

The slope creep law for a soft rock in an open-pit mine in the Gobi region of Xinjiang, China

The lithology of the strata in the Gobi region of the Xinjiang autonomous region of China is mainly composed of mudstone, silty mudstone, and other soft rocks. Because of the low strength of the rock mass and the serious effects of physical weathering in this area, the slope stability in open-pit mines is poor, and creep deformation and instability can readily occur. Taking the Dananhu No. 2 open-pit mine as a typical example, the creep test of a mudstone sample under different stress levels was studied. Then, based on a bottom friction experiment and a FLAC3D numerical simulation, the deformation and failure processes of the slope were analyzed. The stress–displacement curve and the displacement–time curve for the monitoring points were plotted to obtain the relationship between the stress and displacement for the slope of the soft rock. The results showed that the long-term strength of the mudstone was between 8.0 and 8.8 MPa, and that stable creep occurred when the slope was under low stress. The potential failure mode for this type of slope is that the front edge creeps along the weak layer and then a crack is formed at the trailing edge of the slope. When the crack penetrates the weak layer, cutting bedding and bedding sliding occur. The deformation process of the stable creep slope includes an initial deformation stage, an initial creep stage, a constant velocity creep stage and a deceleration creep stage.

The Effect of Acid Mine Drainage Corrosion on the Creep Deformation of Cemented Gangue Backfill

Acid mine drainage (AMD) is a hazardous by‐product of coal mining, which may intensify the creep deformation of cemented gangue backfill (CGB) in solid backfill mining. In this study, the long‐term stability and creep property of CGB soaked in AMD were quantitatively measured by experimental and theoretical analysis. The uniaxial compressive strength, elastic modulus, and mass of CGB soaked in AMD, with pH of 5.56 and soaking time of 30 to 360 days, were tested. The change trend of compressive strength, elasticity modulus, and mass first increased, then decreased, and finally ended at 94.37%, 96.68%, and 98.00%, respectively. The respective creep processes of CGB soaked in AMD and untreated CGB were similar, initial creep stage lasted only for a short time, and no creep deformation occurred in the steady creep stage. Moreover, the long‐term strength of CGB soaked in AMD was 98.6% of the untreated CGB. The creep law of the CGB soaked in AMD was accurately fitted by using the Kelvin model. The theoretical analysis revealed that the final creep deformation of the CGB column soaked in AMD was only 4.4 mm. The results of this paper show the possibility of using CGB in AMD‐accumulating goaf.

Regulating effect of pre-stretching degree on the creep aging process of Al-Cu-Li alloy

Regulating the deformation and property synchronously is a key point of creep aging process. And pre-deformation has been considered to be a good regulating method. In this paper, the laws of the influence of pre-stretching degree on creep strain, strength property, precipitate during creep aging process of Al–Cu–Li alloy were studied, and the pre-stretching design methods suitable for creep aging process were explored. The results indicate that the introduction of different pre-stretching degrees can have various influences on the creep strain of Al–Cu–Li alloy and the nucleation, growth and composition of the precipitates, so as to possess the capacity to regulate deformation and property. The introduction of proper pre-stretching can significantly prolong the duration of the initial creep stage, therefore, the total creep strain in 24 h are 2 times greater than that of the un-pre-stretched specimen. Besides, appropriate pre-stretching can effectively inhibit the orientation precipitation effect of T1 phase and promote a large deal of nucleation and dense distribution. However, when the pre-stretching degree is too large, the composition of the precipitates will be changed, so that the number of θ′ precipitates increases and T1 precipitates decreases, and the alloy strengthening effect is obviously reduced.

Creep and solvent squeeze behavior of κ-carrageenan gels under compression

Creep by solvent squeeze of cylindrical κ-carrageenan gels is investigated. A phenomenological model to describe the creep and the solvent squeeze of polymer gels is also presented. Under compression, the creep of gels proceeds at a constant diameter except for the initial stage of creep. The change in height of the gels is well reproduced by the sum of three exponential terms, each of which is specified by the combination of displacement and retardation time. The displacement for the second mode, which corresponds to the second longest retardation time, becomes largest for all gel specimens. Three retardation times are commonly proportional to the square of initial diameter of gel specimens. Based on the proposed model, the spring constant in the second mode becomes smallest and the permeability corresponding to the third mode becomes larger than those in the other two modes.

Effect of Asphalt Binder Characteristics on Filler–Asphalt Interactions and Asphalt Mastic Creep Properties

Creep properties of asphalt binder and mastic are critically important to asphalt concrete’s high-temperature antirutting resistance. The Burgers and three element models were applied to determine the creep parameters of four asphalt binders and 24 mastics with varying filler volume fractions. The influence of asphalt binders and filler volume fraction on the creep compliance, J, instantaneous elastic deformation compliance, Je, delayed elastic deformation compliance, Jde, viscous flow deformation compliance, Jv, and the proportion of Je, Jde, and Jv to total creep compliance were analyzed according to the model results. A simple two-point reciprocal relative creep compliance was applied to determine the critical filler volume fraction of asphalt mastics with different asphalt binders. The physicochemical interaction between asphalt binder and filler was evaluated with the Einstein coefficient, KE. Based on Fourier infrared spectroscopy, four kinds of functional group indexes were calculated, and then associations of KE and the functional group indexes were determined by grey relational analysis (GRA). The results showed that filler volume fraction had a similar degree of influence on Je of the two kinds of modified asphalt mastics, but an obviously different influence on Jde and Jv of mastics with various asphalt binders. Nonrecoverable viscous flow characterized the deformation of matrix asphalt mastics. However, in modified asphalt mastics, elastic deformation played an important role in the initial stage of creep, and then the delayed elastic deformation constituted the principal component. Adding fillers caused an increase in the proportion of instantaneous elastic deformation and delayed elastic deformation, while the viscous flow deformation decreased significantly. The critical filler volume fraction was 0.273–0.426 for the four asphalt binders. Grey relational analysis results showed that the influential priority of the four functional groups of asphalt on KE were as follows: sulfoxide>carbonyl>aliphatic>aromatic.