Creep Deformation Rate Research Articles

Simulation of crystal plasticity of P92 heat resistant steel considering martensitic lath structure

Based on the theory of crystal plasticity, coupled with dislocation and lath hardening models, this paper establishes a crystal plasticity finite element model describing the high temperature creep of P92 steel. Open source software was used to generate lath models with an average size of 350nm, 650nm and 950nm to explore the effect of lath coarsening on the high-temperature creep behavior of P92 steel. The results show that the roughening of the slats increases the rate of creep deformation, resulting in a decrease in the service life of the material. Observing the slat model, it can be seen that the roughening of the slats enlarges the numerical gradient of stress and strain, and aggravates the overall plastic strain of the model. The coarsening of the slats accelerates the movement of dislocations, causing the density of movable dislocations to increase, and at the same time the shear strain amplitude of the slip system increases, thereby reducing the hardening behavior of the material.

Creep performance of carburized 316H stainless steel at 550 °C

The creep behaviour of two ex-service type 316H austenitic stainless steel casts with different Mn contents and austenite grain size, that were pre-carburized for either 8000 h at 550 °C or 3000 h at 600 °C in simulated CO2-rich gas environment of UK Advanced Gas-cooled Reactors, was evaluated by uniaxial creep testing at 550 °C in air. The low Mn (0.98 wt.%) steel grade with an average austenite grain size of 142 ± 110 µm presents a higher creep deformation rate and creep ductility compared to the high Mn (1.52 wt.%) steel cast with an austenite grain size of 81 ± 67 µm. Below an applied stress of 318 MPa, the presence of the sub-surface carburized layer reduces the minimum creep rate, likely due to the significantly modified material properties of the carburized layer and the presence of local compressive stresses. The stress exponent determined for the high Mn steel carburized at 600 °C (n = 9.6), and its reference non-carburized microstructure (n= 9.8), identifies the dominant creep mechanism to be dislocation climb & glide. However, crack evolution through M23C6-decorated grain boundaries within the carburized layer was observed in all the tested material's conditions at creep strains of 0.50%. The average crack length increases with creep strain and attains values close to the carburized layer thickness at creep strains of ε ≥ 1.00%. The prevalence and size of cracking increases with the local hardness carburized layer depth. At applied stresses σ ≥ 318 MPa, cracking on-loading prior to forward creep already occurs in the carburized layer, leading to a sharp increase in minimum creep rates of carburized specimens. These results demonstrate the impact a relatively thin layer of modified material can have on the materials creep performance; emphasising the requirement to consider the effects of the service environment in structural assessment procedures, particularly for high temperature applications including Generation IV and advanced modular reactor designs.

Effect of indentation depth in impression creep test: conversion relationships and correction functions

ABSTRACT The impression creep test method has been used extensively to determine the in-service creep properties of power plant components from a small volume of material. In the standardisation of the impression creep test, the determination of a maximum ‘allowable’ indentation depth is an important issue. This paper presents the results of finite element analyses obtained using a simplified method based on steady-state creep, for evaluating the variation of conversion factors with indentation depth. The results obtained show that the indentation depth has a significant influence on the conversion parameters and the steady-state creep deformation rate when the indentation depth is sufficiently large. On this basis, recommendation on practical application of these conversion relationships was made to the users, and empirical formulas were provided for the possible corrections of the conversion factors. The effect of plastic zone due to initial loading on the subsequent creep was found to be insignificant.

Characterization of thermal aging effects on the creep performance of T91 using small punch creep testing

AbstractThis paper is concerned with the development and application of a small punch creep testing method for assessment of the effects of thermal aging and associated precipitate coarsening on the high temperature creep life and deformation behavior of a 9Cr ferritic–martensitic steel. The tests are conducted on P91 steel, for both aged (up to 1 year) and unaged specimens, at temperatures of 600°C. The unaged test results are shown to be consistent with previously published creep tensile test rupture life data in terms of Chakrabarty membrane stress. Aging is shown to have a significant, detrimental effect on creep life and deformation rate. The steady‐state displacement rate, time to failure, and displacement at failure data, for both aged and unaged, were successfully correlated via the modified Monkman–Grant relationship.

Indenter misalignment in impression creep test: Uncertainty, correction and recommendation

The impression creep test (ICT) method, as a miniature specimen test technique, has been used extensively to determine the in-service creep properties of power plant components. However, the experiment results of the ICT can be affected by some uncertainties associated with the inaccuracies of measurement, which have not been studied before. This paper presents some results of finite element analyses, to evaluate the effect of indenter misalignment on the results of the ICT, such as the steady-state creep deformation rate and the conversion parameters. The results obtained have shown that the angular misalignments of the indenter around horizontal directions have a relatively significant influence on the conversion parameters, and the misalignment around vertical direction has a much less significant effect. Empirical formulas were developed for the possible correction of the conversion factors. Finally, some recommendations on the control of the indenter and specimen alignment were given.

INFLUENCE OF STEP-AGING ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF Al-Zn-Mg TERNARY ALLOYS

Al-Zn-Mg alloys were prepared using RF induction furnace. Two,out of 10 samples were left un-annealed and remaining were given step-aging in temperature range 125-180oC for 2-4 hours. Characterizations of thesamples were carried out by using optical microscopy, creep and hardness tests. The results of optical micrographs reveal an equiaxed grain structure formed in all step-aged specimens that becomes more obvious with increase of aging time and temperature. The creep deformation rate was found to be minimum in two step aged specimens, whereas, hardness of the two-step aged samples is maximum as compared to the others samples.

INFLUENCE OF STEP-AGING ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF Al-Zn-Mg TERNARY ALLOYS

Al-Zn-Mg alloys were prepared using RF induction furnace. Two,out of 10 samples were left un-annealed and remaining were given step-aging in temperature range 125-180oC for 2-4 hours. Characterizations of thesamples were carried out by using optical microscopy, creep and hardness tests. The results of optical micrographs reveal an equiaxed grain structure formed in all step-aged specimens that becomes more obvious with increase of aging time and temperature. The creep deformation rate was found to be minimum in two step aged specimens, whereas, hardness of the two-step aged samples is maximum as compared to the others samples.

Study on triaxial creep test and theoretical model of cemented gangue-fly ash backfill under seepage-stress coupling

In order to study the effects of seepage on the long-term stability of cemented gangue-fly ash backfill (CGFB), the triaxial creep test of CGFB under seepage-stress coupling was carried out in this paper. The creep deformation law of CGFB considering the seepage-stress coupling effect was explored and a creep damage constitutive model considering seepage-stress coupling effect was established. The results show that CGFB has obvious creep properties under the coupling action of seepage and stress and its creep generally has three stages: decelerating creep, constant velocity creep and accelerating creep; the higher the seepage water pressure, the larger the creep deformation and steady-state creep rate, and the shorter the time to enter the accelerated creep. The ratio of long-term strength to instantaneous strength under different seepage water pressure conditions is 0.56–0.75, and it shows that seepage has a significant effect on the long-term strength of CGFB. The improved Nishihara Masaki model considers the influence of seepage and stress level on creep parameters, and can better reflect the creep mechanical characteristics of CGFB under the underground seepage pressure and ground stress environment. The research results provide an important experimental basis for the safety and long-term stability analysis of backfill mining in underground coal mines.

Evaluation of Deformation and Permeability Behavior of Amygdaloidal Basalt under a Triaxial Cyclic Loading Creep Test

Amygdaloidal basalt is a typical rock mass in the dam foundation of the Baihetan hydropower project in southwest China. With rising and drawdown of the reservoir water level, the permeability and creep deformation characteristics of the amygdaloidal basalt are much complicated in the long-term cyclic loading processes. A cyclic loading-unloading creep test on the amygdaloidal basalt was performed to evaluate its deformation and permeability behavior. The results showed that Poisson’s ratio and elastic modulus of the rock specimen varied significantly under different loading processes with a relatively large irreversible deformation. The permeability and strain rates of rock changed in two phases under lower deviatoric stresses, while there are three typical stages of strain growth with the final stress level of 121.8 MPa. For axial stress of 128 MPa, the creep deformation and creep rate in the axial direction are smaller than these in the lateral direction. Before the sample failure, the lateral deformation accelerates earlier than the axial deformation. The results also suggested that the permeability of the rock specimens decreases considerably during each loading process and then tends to be constant with time. No apparent change in steady permeability is observed with variation of stress. For 128 MPa axial stress, the permeability first decreases, then tends to be in a stable value, and at last increases during the sample failure.

An enhancement in mechanical properties of grade 91 steel on microalloying with boron and nitrogen

ABSTRACT Effects of microalloying the grade 91 steel with boron along with control on nitrogen content (P91BN) on its microstructure, tensile and creep properties have been investigated and compared with those of the standard grade 91 steel (P91). The P91BN steel possesses finer martensite lath structure and M23C6 precipitates in comparison with the P91 steel. Tensile strengths of the P91BN steel were comparable with the P91 steel up to 823 K. The P91BN steel has possessed higher tensile strengths than the P91 steel at higher temperatures (above 823 K). Microalloying the steel with boron decreases the creep deformation rate with significant increase in creep rupture life. Finer lath width, higher number density of fine M23C6 precipitates in pre-creep condition along with sluggish coarsening of M23C6 precipitates resulting in better stabilisation of martensite structural features in the P91 steel on boron micro-alloying, leads to the significant improvement in high temperature tensile and creep strengths.

Creep Behaviours of Argillaceous Sandstone: An Experimental and Modelling Study

Understanding the creep behaviours of rocks is essential for the long-term stability of underground excavations in mining engineering. Creep behaviours are more important when the mining depth is greater, which leads to the emergence of weak rock masses and high in situ stresses. In this study, the creep behaviours of argillaceous sandstone (AS) were systematically investigated. For the experimental investigation, creep tests were conducted on AS with different confining pressures (3, 6, 9, 12, 15, and 18 MPa) using an MTS815.02 rock mechanics test system. The mechanical characteristics of AS were analysed. For the numerical study, a nonlinear creep model of AS under equal and different confining pressures was established based on rock creep theory and plastic theory. The results showed that confining pressure could effectively improve the creep failure strength of AS, accelerating its creep deformation rate and process and reducing the final expansion volume. The nonlinear creep model was embedded in the FLAC3D software, and the experimental and numerical results agreed well. The experimental investigation and proposed creep model can provide important guidance in underground mines for safe long-term stability of underground excavations.

Fractional-order creep model for soft clay under true triaxial stress conditions

To investigate the creep characteristics of soft clay under true triaxial stress conditions, multistage loading creep tests using TSW-40 true triaxial apparatus were performed. The creep deformation characteristics, creep rate, and long-term strength of soft clay under a true triaxial stress path were studied. The results showed that the creep deformation and creep rate of soft clay increase with increasing major principal stress; the axial deformation is larger than the lateral deformation. The long-term strength and the equation for predicting the starting point of accelerated creep were obtained using the generalized shear stress. A four-element fractional-order creep (FFC) model based on the fractional Burgers creep model was established for soft clay and then extended to three dimensions. The model calculation results based on the parameters obtained using MATLAB’s Curve Fitting Toolbox were in good agreement with the experimental results. The comparison results showed that the 3D FFC model was more accurate at describing the entire creep curve of soft clay than the fractional Burgers model and the Burgers model, particularly regarding the accelerated creep characteristics. In conclusion, the results presented herein are expected to provide the scientific basis for determining soft clay creep instability under true triaxial stress conditions.

Creep Performance of Geosynthetic Reinforcements

Most geosynthetic materials exhibit rheological properties that lead to creep strain response when subjected to sustained loads, and consequently it is necessary to evaluate their long-term creep deformation before any real application. This paper presents the results of sustained loading tests conducted on large-scale geogrid soil reinforcement. The purpose of these laboratory tests was to identify the appropriate design parameters for geosynthetic-reinforced systems. The results of these tests demonstrate the continuous creep deformation characteristic of geogrid materials under constant sustained loading. The increase in the applied load led to a continuous increase in the amount and rate of the geogrid creep deformation. The data analysis method used in this investigation enabled the possibility of predicting the load-deformation-time behavior and the ultimate load of geosynthetic reinforcements.

Holistic Approach on the Research of Yielding, Creep and Fatigue Crack Growth Rate of Metals Based on Simplified Model of Dislocation Group Dynamics

The simplified model of numerical analyses of discrete dislocation motion and emission from a stressed source was applied to predict the yield stress, dislocation creep, and fatigue crack growth rate of metals dominated by dislocation motion. The results obtained by these numerical analyses enabled us to link various dynamical effects on the yield stress, dislocation creep, and fatigue crack growth rate with the experimental results of macroscopic phenomena, as well as to link them with theoretical results obtained by the concept of static, continuously distributed infinitesimal dislocations for the equilibrium state under low strain or stress rate conditions. This will be useful to holistic research approaches with concern for time and space scales, that is, in a time scale ranging from results under high strain rate condition to those under static or low strain rate condition, and in a space scale ranging from meso-scale to macro-scale mechanics. The originality of results obtained by these analyses were found by deriving the analytical formulations of number of dislocation emitted from a stressed source and a local dynamic stress intensity factor at the pile-up site of dislocations as a function of applied stress or stress rate and temperature material constants. This enabled us to develop the predictive law of yield stress, creep deformation rate, and fatigue crack growth rate of metals dominated by dislocation motion. Especially, yielding phenomena such as the stress rate and grain size dependence of yield stress and the delayed time of yielding were clarified as a holistic phenomenon composed of sequential processes of dislocation release from a solute atom, dislocation group moving, and stress concentration by pile-up at the grain boundary.

Study on Tensile Creep Behavior of 12Cr1MoV Alloy Steel under High-Temperature Alkali Metal Salt Environment for Solar Thermal Power Generation

The heat exchange tubes of solar thermal power generation work in molten salt environment with periodic temperature change. In order to reveal the tensile creep behavior of 12Cr1MoV pipeline steel under high-temperature alkali metal salt environment, the tensile creep behavior of 12Cr1MoV alloy under different applied load and reaction temperature in high-temperature alkali metal chloride salt environment was studied. The results show that the deformation of 12Cr1MoV alloy in 600°C, NaCl-35%KCl mixed salt environment is mainly controlled by diffusion creep; with the increase of stress, the creep life of 12Cr1MoV alloy decreases. The creep fracture mechanism of 12Cr1MoV alloy in 600°C, NaCl-35%KCl mixed salt environment is intergranular ductile fracture; the increase of temperature will enhance the activation and oxidation of the chlorine atoms, thereby accelerating the corrosion of the base metal and increasing the spheroidization speed of the pearlite matrix, and the creep deformation rate of the alloy increases with increasing temperature.

On the Room-Temperature Creep Behavior and Its Correlation with Length Scale of a LiTaO3 Single Crystal by Spherical Nanoindentation.

Relying on nanoindentation technology, the room-temperature creep behavior of a LiTaO3 single crystal in the typical orientation (012), i.e., Y-42° plane was investigated. Three kinds of spherical tips with the radii of 0.76, 2.95 and 9.8 μm were respectively applied to detect nanoindentation length scale effect on creep deformation at both elastic and plastic regions. Superficially, both creep displacement and rate were nearly linearly increased with increasing holding depth and independent of tip size, which could be ascribed to the simultaneously enlarged holding strain and deformation volume beneath the indenter. At a similar holding strain, creep deformation, i.e., creep strain and strain rate were more pronounced under smaller spherical tips. Strain rate sensitivities of creep flows under different spherical tips and holding strains were also estimated. The potential room-temperature creep mechanism of LiTaO3 under high shear compression stress was discussed.

Anisotropic creep characteristics and mechanism of shale under elevated deviatoric stress

The time-dependent characteristics of anisotropic shale are important for accurately predicting a reservoir's behavior over a long period of time. To study the transverse isotropic creep characteristics of shale, a series of shale creep tests using specimens with 4 different bedding layer orientations (0°, 45°, 75° and 90°) under multiple levels of deviatoric stress were conducted. The experimental results indicate that shale presents creep behavior even when the deviatoric stress is relatively low. Anisotropy has significant influences on the creep deformation and steady creep rate. When the bedding plane inclined angle is 45° and under the same deviatoric stress, the creep deformation and steady creep rate are the highest, while the smallest creep deformation and steady creep rate are generated when the bedding plane orientation is 90°. Time-dependent deformation appears the tertiary creep stage when the deviatoric stress is crack damage threshold, which implies that crack damage stress can be regarded as the shale's long term strength. The steady creep rate increases exponentially with increasing deviatoric stress as the stress is larger than the crack initiation threshold. The rationality of the empirical creep law is evaluated. It is concluded that the empirical creep model can only fit the existing creep data, but it is almost impossible to predict the creep deformation. The mechanism for generating anisotropic creep behavior of shale is analyzed in detail, and three basic creep patterns for shale are proposed based on the action mechanism of the stress tensor components. A general methodology for proposing the anisotropic creep model is finally suggested.

The Role of Thermomechanical Processing in Creep Deformation Behavior of Modified 9Cr-1Mo Steel

In this study, to refine the microstructure and enhance the mechanical properties, thermomechanical treatment (TMT) was performed on modified 9Cr-1Mo steel. The creep deformation behavior of TMT processed steel and the steel in its normalized and tempered (NT) state were studied for different stress levels at 923 K (650 °C). Transient, secondary, and tertiary creep regimes were analyzed for both conditions of the steel based on the empirical equation \( \varepsilon = \varepsilon_{0} + \varepsilon_{\text{t}} \left( {1 - e^{ - rt} } \right) + \varepsilon_{\text{s}}^{ \cdot } t + \varepsilon_{\text{L}} e^{{p\left( {t_{\text{t}} - t_{\text{r}} } \right)}} \). The rate of exhaustion of primary creep (r), r with time to reach the minimum creep rate, minimum creep rate\( \left( {\varepsilon_{ \hbox{min} }^{ \cdot } } \right) \), \( \varepsilon_{ \hbox{min} }^{ \cdot } \) with time spent in the secondary creep regime, rate of acceleration of tertiary creep (p), p with time to reach the onset of tertiary creep, and creep rate with applied stress exhibited a proportional relationship in both conditions of the steel. This proportionality existence in the transient and tertiary creep deformation obeyed the first-order reaction rate kinetic theory. The enhanced MX (M = V, Nb; X = C, N) precipitation in the TMT steel significantly decreased the creep deformation rate and extended the secondary stages of deformation. TMT processing of modified 9Cr-1Mo steel led to a significant increase in the creep rupture strength through the stable microstructure.

The effect of cyclic-loading generated intergranular strains on the creep deformation of a polycrystalline material

Intergranular strains are generated due to the incompatible deformations at grain length-scales during elastic and inelastic loading in a polycrystalline material. Estimating the effects of intergranular strains on the creep life of the material is of interest for accurate life prediction of high-temperature structural systems. In this study, the effect of the cyclic loading generated intergranular strains on the creep deformation behaviour of Type 316H austenitic stainless steel was studied using in-situ neutron diffraction. The load-controlled creep dwells introduced at various positions during tension-compression cyclic loading with different intergranular strain state but under the same applied stress showed markedly different behaviours. It is inferred that the intergranular strains are a significant contributor to the observed differences in creep deformation behaviour. Comparing the evolution of intergranular strains in various grain families during plastic and creep deformation, it was found that the grain families which deformed relatively more or less during plastic deformation behaved similarly during creep deformation. The present work shows that intergranular strains, which contribute to accelerating/decelerating creep deformation rates, need to be accounted for in current creep life assessment procedures, to obtain a more realistic creep deformation prediction under cyclic loading conditions.

Accounting for crack tip cyclic plasticity and creep reversal in estimating (Ct)avg during creep‐fatigue crack growth

AbstractCreep‐fatigue crack growth (C‐FCG) rates in a P91 steel at 625°C were correlated as the average time rate of crack growth during hold time, (da/dt)avg, with (Ct)avg. At 60‐second hold time, the rates were lower than for 600‐second hold time. At 600‐second hold time, the crack growth rates converged on to the creep crack growth rate (CCGR) trend. Thus, the CCGR trend represents the upper bound for time‐dependent crack growth rates in P91 materials. The analytical expressions based on considering just the elastic and secondary creep deformation rates overestimated the magnitudes of (Ct)avg by as much as a factor of 10 for the 600‐second hold time tests. After accounting for the effects of cyclic plasticity during unloading, and accounting for only partial reversal of creep strains accumulated during hold time, the estimates of (Ct)avg compared well with the measured values. CR represents the extent of crack tip creep strain reversal, and tpl is the time required for the crack tip creep zone during the hold time to become equivalent in size to the cyclic plastic zone in terms of stress carried by that region. Together, these parameters accurately account for the effects of crack tip cyclic plasticity on the magnitude of (Ct)avg.Both tpl and CR depend on material properties, and the latter also depends on the hold time. A parameter ϕ is introduced that is dependent only on material properties and from which CR can be estimated for a given hold time. tpl and ϕ can be reported as part of the test results from C‐FCG testing.