Primary Creep Stage Research Articles

Comparison of inelasticity creep failure evaluation codes for elevated-temperature non-nuclear pressure equipment

PurposeTo ensure the reliable and safe operation of elevated-temperature pipes and equipment in the long term, it is essential to thoroughly assess the creep rupture life. Nevertheless, there is currently no design code that specifies a creep rupture life evaluation method for non-nuclear elevated-temperature equipment. The paper aims to discuss this issue.Design/methodology/approachAn analysis was conducted to compare the differences and conservativeness in calculating creep strain using three major codes (ASME-CC-2843, API-579 and BS-7910) based on the results of the 316H creep constitutive model and creep strain prediction. In addition, the creep resistances of 316H, 304H and 347H were compared. Subsequently, the ANSYS Usercreep subroutine was developed to compare the discrepancies between different codes under multiaxial stress conditions using numerical simulations.FindingsBS-7910 employs the Norton creep model with calculation parameters for the average creep strain rate, which is not applicable for the engineering design stage. ASME-CC2843 code primarily focuses on the primary and secondary creep stages, making it more suitable for non-nuclear pipeline and equipment design. For 316H, the creep strain curves predicted by ASME-CC2843 and API-579 typically intersect at a specific point. By combining the creep strain predicted by ASME-CC2843 and API-579, 347H exhibits superior predicted creep resistance compared to 316H, whereas 316H exhibited better predicted creep resistance than 304H.Originality/valueThis study provides a guide for future evaluation methods and material choices for non-nuclear equipment and pipelines operating at elevated temperatures.

Effects of Ta on the high temperature creep behavior and deformation mechanism of a Ni-based single crystal superalloy

The effects of Ta on the high temperature creep behavior and deformation mechanism in a Ni-based single crystal (SX) superalloy were investigated at 1120 °C/137 MPa. Ta was regarded as an essential strengthening element of the γʹ phase in the alloy, and the results indicated that the increase of Ta prolonged the creep rupture life. In this work, the addition of Ta increased the volume fraction of the γʹ phase and narrowed the γ channels, making the dislocation bowing out at the primary stage of creep more difficult. As creep continued, with a lower effective diffusion coefficient of alloying elements and larger γʹ raft thickness, the alloy with 6.5 wt% Ta addition was considered to possess the lower dislocation climbing rate, which reduced the minimum creep rate of the alloys. Subsequently, it was noted that the density of superdislocations in γʹ phase was clearly reduced with the increase of Ta. This was supposed to be the elevated Ta content in the alloy, which increased the lattice misfit and led to the formation of a denser dislocation network, thereby enhancing the interfacial strengthening effect and effectively preventing the cutting of superdislocations. The Ta effect on high-temperature creep was systematically summarized in this study, which was an important guideline for further composition design and optimization of Ni-based SX superalloys.

Study on the fractional creep model of roadway filling paste under the coupling effect of pore water pressure and stress

Filling paste in deep roadways is significantly affected by groundwater, especially high pore water pressure, which increases the complexity and variability of the filling paste's mechanical properties. To explore the creep characteristics and long-term stability of filling paste under varying pore water pressure, the MTS815.03 test system is used to conduct creep tests under different pore water pressures and stress. Then the creep deformation of the filling paste under the complex pressure field of static pore water pressures is analyzed. Finally, through the one-to-one correspondence between the numerical simulation of the creep model and the characteristic points of the creep curve, a method for determining the creep parameters under the complex pressure field of static pore water pressures is proposed. Results show that the creep test curves of filling paste under different pore water pressures and stress are in good agreement with the model curves. This shows that the creep constitutive model in this research can better reflect the whole process of creep deformation of filling paste. The result also verifies the rationality of the proposed method to determine creep model parameters. The newly proposed creep model can effectively compensate for the traditional Nishihara model, which is inability to reflect the acceleration of creep and can more accurately describe the creep characteristics of the primary and steady-state creep stages.

Creep failure mechanism of -oriented thin-wall Ni3Al-based single crystal superalloys

Currently, thin-wall structures are widely employed in aviation engine turbine blades to enhance cooling efficiency. However, the mechanical properties of thin-walled structures exhibit significant sensitivity to thickness. This study investigates the creep behavior of a <111>-oriented Ni3Al-based single-crystal superalloy with different thicknesses (0.3, 0.5, 0.8 mm) at 1100 °C/137 MPa. The results indicate a significant reduction in creep life with a decrease in wall thickness. The mechanism of various influencing factors on different creep stages was analyzed from both microscopic and macroscopic perspectives combined with finite element modeling. The alteration in stress state from thick-wall to thin-wall influences the number of slip systems that are activated, which mainly affects the primary and steady-state creep stages. Surface oxidation as time-dependent damage leads to non-invasive crack formation, somewhat increasing the steady-state creep strain rate of thin-wall samples. The accumulation and propagation in varying wall thicknesses of cleavage microcracks primarily influence the steady-state and tertiary creep stages, respectively, thereby exacerbating the thickness debit effect.

Nonlinear Nishihara model of soft rock based on damage mechanics and its parameter identification

The traditional Nishihara model is difficult to describe the nonlinear primary and accelerated creep characteristics of soft rock. Based on this, an improved nonlinear Nishihara model is established by modifying the unsteady dashpot and a nonlinear damaged viscoplastic body in series. The one-dimensional and three-dimensional creep constitutive equations of soft rock are derived. Levenberg-Marquardt algorithm is used to identify model parameters of creep test data of Cretaceous soft rock and red sandstone. The results show that the correlation coefficient between the fitted results of the improved nonlinear Nishihara model and the creep test data is above 0.97. The sensitive parameters λ,η′1 and a of the model are all within a reasonable range. It shows that the model can accurately reflect the characteristics of the unsteady primary creep stage and the nonlinear damage accelerated creep stage of soft rock, which is better than the traditional Nishihara model. The research results can provide theoretical guidance for similar soft rock creep model research.

Characteristics of negative creep aging and its microstructure-oriented tensile behavior

UNS S17400 steel was creep-aged under a constant stress of 300 MPa at 480 °C and 600 °C for 1, 3, and 5 h, and its microstructural phase evolution was investigated. During the initial creep aging at 480 °C for 15 min, negative creep strain occurred because of the stress-assisted precipitation, which resulted in Cu-rich precipitates impeding the movement of dislocations. By contrast, the sample creep-aged at 600 °C had a typical creep-strain curve comprising a primary creep stage and secondary steady-state stage. The creep stress accelerated the growth of Cu-rich precipitates and austenite γ phase reversion, which were clearly observed in samples creep-aged at 600 °C. The reverted γ phase formed due to the stress-assisted aging at 480 °C for 3 h, as well as at 600 °C due to the elevated temperature aging. The samples with the longer creep aging times decreased in tensile yield strength but increased in strain hardening rate. Strain hardening after strain softening was observed in the sample creep-aged at 600 °C for 5 h, which had the highest strain hardening rate. Strain hardening was attributed to the dislocations becoming entangled with nanoscale Cu-rich precipitates, the formation of microtwins, and the presence of deformation-induced martensite.

Investigation of small punch creep damage and rupture behavior in 347H stainless steel by means of experiments and a numerical simulation

The creep behavior of 347H stainless steel was investigated by small punch creep test (SPCT). Finite element simulations coupled with a strain damage model were employed to reveal the creep deformation mechanism of 347H stainless steel. The experimental results showed that the SPC curves of 347H stainless steel had a stagnation stage before the primary creep stage, and the duration of the accelerated stage of SPCT was shorter than that of uniaxial creep tests. The competition between precipitation strengthening and dislocation motion causes the deflection rate in the stagnation stage decreases first and then increases. A similar change of deflection rates was observed in the following creep stages, which can be attributed to the stress redistribution and crack propagation in the SPCT specimen. The present work provides a comprehensive understanding of the creep behavior and fracture mode of 347H stainless steel under multiaxial stress, and could promote the development of a new method for characterizing material properties through SPCT.

Damage evolution in flax fibre composite under creep load

This work aims to reveal the damage mechanisms and evolution in unidirectional flax fibre biocomposites when subjected to creep load. X-ray micro computed tomography and acoustic emission (AE) was used to monitor the failure progress during flexural creep tests. A correlation between the event clusters and directly observed damage modes was established based on coupons with expected failure mechanisms and then validated by computed tomography observations. The damage initiated from matrix cracking in the primary creep stage, and then fibre–matrix interface debonding combined with fibre pull-out occurred sequentially during the steady creep stage. In the tertiary creep stage, the explosive fibre fracture emerged and eventually triggered catastrophic failure. Considerably more AE events were detected during creep tests compared to those in quasi-static tests, which indicates that intensive damage is generated under creep load, and therefore causes the strength degradation. A good agreement was observed between the cumulative number of AE events and the increasing damage volume fraction over time determined by X-ray micro computed tomography during multi-step creep tests.

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.

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.

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 damage mechanism of 20Cr32Ni1Nb steel at high temperatures

In the petrochemical industry, the centrifugally cast austenitic 20Cr32Ni1Nb stainless steel used in steam reformer furnace's hot outlet manifold experiences creep failure and microstructural degradation, impacting its structural integrity. Understanding the causes of these issues is crucial for addressing the associated challenges. This study examines the creep deformation behaviour, damage mechanisms, and microstructural evolution of 20Cr32Ni1Nb steel under stresses of 22–50 MPa at temperatures of 1163 K (890 °C) and 1223 K (950 °C). Analysis of creep and creep strain rate curves revealed the presence of both primary and tertiary creep stages, with the power law relationship governing the stress-dependent minimum creep rate and rupture life. The variance of creep damage tolerance factor was also analysed, with results showing that in high-stress regimes, loss of section and necking along with cavity growth may initiate tertiary creep, while in low-stress regimes, microstructural degradation, specifically G-phase formation, may initiate tertiary creep. As the duration of creep exposure increases, microstructures were observed to degrade, with the growth and coarseness of NbC and M23C6 carbides. The structure of the space group for the Ni–Nb–Si enriched G-phase in 20Cr32Ni1Nb steel was found to be Fm3‾m, with a lattice parameter of about 1.14 nm.

Determination of the parameters of rock viscoelastic creep model and analysis of parameter degradation

Different stress creep tests are conducted on the sandstone in this study to better describe the creep properties of rocks under different stress states. A model that describes the rock creep process is established. The various stages of creep can be described by combining the creep properties of the creep elements of the model. A new method for determining creep parameters is proposed by using the special point on the creep curve and the definition of creep deformation. The relationship between the creep parameters, stress, and time is analyzed. An improved creep model that considers the effects of stress state and time on the creep parameters is developed. This model is verified using experimental data and calculation results. Results shows that the improved creep model better describes the creep properties of rocks and provides a new method for determining future model parameters. The shear modulus of elastic model controls the instantaneous deformation. The shear modulus of viscoelastic model governs the limit of viscoelastic deformation. The shear viscoelastic coefficient of viscoelastic model increases with the increase in stress. The coefficient of viscoplastic model controls the viscoplastic creep rate. The coefficient of a nonlinear Newtonian dashpot mainly controls the accelerated creep deformation of rock. The calculation results of the proposed model agree well with the experimental data under the action of different stress levels. This model accurately reflects the creep characteristics of the primary and steady-state creep stages, and overcomes the shortcomings of the traditional Nishihara model in describing accelerated creep.

Effect of Ru on Deformation Mechanism and Microstructure Evolution of Single-Crystal Superalloys under Medium-Temperature and High-Stress Creep.

In this work, the effect of the Ru element on the γ'-phase evolution and deformation mechanism in the fourth-generation Ni-based single-crystal superalloy was investigated. Results show that the Ru element alters the distribution coefficient of other elements in the alloy to produce reverse partitioning behavior, which leads to a difference in microstructure between 0Ru and 3Ru. The addition of Ru triggered the incubation period before the beginning of the primary creep stage, which depends on the creep temperature and stress during creep deformation. TEM results revealed that Ru addition inhibits the slip system {111}<112> at medium-temperature (760-1050 °C) and high-stress (270-810 MPa) creep, which brings a considerably low creep rate and high creep life to the Ru-containing alloy.

Triaxial Creep Mechanical Behaviors and Creep Damage Model of Dolomitic Limestone Material under Multi-Stage Incremental Loading

Dolomitic limestone is the main surrounding rock material in Yangzong tunnel engineering; the instantaneous mechanical properties and creep behaviors of limestone are significant for stability evaluation during the stages of tunnel excavation and long-term maintenance. Herein, four conventional triaxial compression tests were carried out to explore its instantaneous mechanical behavior and failure characteristics; subsequently, the creep behaviors of limestone subjected to multi-stage incremental axial loading at the confinements of 9 MPa and 15 MPa were studied by employing an advanced rock mechanics testing system (i.e., MTS815.04). The results reveal the following. (1) comparing the curves of axial strain-, radial strain-, and volumetric strain-stress under different confining pressures shows that these curves present a similar trend, whereas the stress drops during the post-peak stage decelerate with the increase in confining pressure, suggesting that the rock transits from brittleness to ductility. The confining pressure also has a certain role in controlling the cracking deformation during the pre-peak stage. Besides, the proportions of compaction- and dilatancy-dominated phases in the volumetric strain-stress curves differ obviously. Moreover, the failure mode of the dolomitic limestone is a shear-dominated fracture but is also affected by the confining pressure. (2) When the loading stress reaches a creep threshold stress, the primary and steady-state creep stages occur successively, and a higher deviatoric stress corresponds to a greater creep strain. When the deviatoric stress surpasses an accelerated creep threshold stress, a tertiary creep appears and then is followed by creep failure. Furthermore, the two threshold stresses at 15 MPa confinement are greater than that at 9 MPa confinement, suggesting that the confining pressure has an obvious impact on the threshold values and a higher confining pressure corresponds to a greater threshold value. Additionally, the specimen's creep failure mode is one of "abrupt" shear-dominated fracturing and is similar to that under a conventional triaxial compression test at high confining pressure. (3) A multi-element nonlinear creep damage model is developed by bonding a proposed visco-plastic model in series with the Hookean substance and Schiffman body, and can accurately describe the full-stage creep behaviors.

Uniaxial Creep Test Analysis on Creep Characteristics of Fully Weathered Sandy Shale

The creep damage behavior of rocks is very important for evaluating the stability and safety of key rock engineering. Based on the Lianhua Tunnel Project in China, this paper aims to study the creep damage mechanics, the influencing factors and the creep constitutive models of sandy shale. In order to achieve these goals, a uniaxial compressive strength test and a creep test under different moisture contents and load levels were carried out. According to the test results, the creep parameters (elastic coefficients E1 and E2 and viscosity coefficients η1 and η2) of the Burgers Model were achieved, and the relationship between the creep parameters and moisture content, ω, was established accordingly (E1 = f(ω), E2 = f(ω), η1 = f(ω), η2 = f(ω)). A fully weathered sandy-shale creep constitutive model considering moisture content was finally obtained. Test results showed that creep deformation increases with any increase in load level or moisture content, and the influence of moisture content is more significant. For instance, creep deformation increased by 35% when the load increased by 50%, and creep deformation increased by 82% when the moisture content increased by 45%. In addition, the creep rate in the steady stage and the duration of the primary creep stage increased with any increase in moisture content or load level. The higher the moisture content, the greater the influence of creep deformation on the total deformation. The creep model of fully weathered sandy shale showed that the elastic coefficients (E1, E2) and the viscosity coefficients (η1, η2) are negatively correlated to moisture content; E1 is negatively correlated to load level; and E2, η1 and η2 are positively correlated to load level. Qualitative and quantitative analysis of fully weathered sandy shale can improve the existing research of creep properties and is expected to provide theoretical support for treatment of large deformation disasters in the fully weathered sandy-shale stratum.

Comparative Study of θ Projection Method and Its Modified Forms on Creep Life Prediction

The phenomenological characteristics of θ projection method constrain its prediction accuracy, especially at the primary creep stage. Herein, the mathematical features of the classical θ projection method and its modified forms based on a group of creep curves of a newly designed low‐cost heat‐resistant steel are analyzed. A detailed data processing method is introduced, and the prediction accuracy is comparatively studied with respect to the relative error on different creep stages. Results reveal that the simplified 3‐parameter method brings a balanced improvement on the residual errors by sacrificing the prediction accuracy on primary creep. However, the 5‐parameter method can precisely predict the characteristics of primary creep, and the tolerance fitting process can further reduce the deviation at the tertiary creep stage. Furthermore, the 5‐parameter method shows a larger reliable prediction region than the 3‐parameter method, and the two methods present good consistency when the applied stress is larger than 134 MPa. Herein, theoretical guidance for the selection of different θ projection methods is provided.

Tensile creep properties and damage mechanisms of 2D‐woven C/HfC–SiC composites in high temperature

Abstract2D‐C/HfC–SiC composites were prepared by a combination of precursor infiltration and pyrolysis (PIP) and chemical vapor infiltration (CVI). Creep tests were performed at 1100°C in air under different stress conditions. Unlike most, C/SiC and SiC/SiC ceramic matrix composites only underwent primary and secondary creep stages, and the C/HfC–SiC composites underwent tertiary creep stage in the creep process. The reason was that the mechanical properties of C/HfC–SiC materials prepared by PIP + CVI methods were different from those prepared by traditional methods. The microscopic morphological analysis of the sample fracture showed that the oxidation products SiO2 and Hf–Si–O glass phases of the HfC–SiC matrix played a crack filling role in the sample during creep. In turn, it provided effective protection to the internal fibers of the sample. The creep failure of C/HfC–SiC composites in a high‐temperature oxidizing atmosphere was caused by the oxidation of the fibers. The total creep process was dominated by the oxidation of carbon fibers. It is noteworthy that there was the generation of HfxSiyOz nanowires in the samples after high‐temperature creep. The analysis of the experimental data showed that the creep stress had a linear negative correlation with the creep life.

Impact of the load level on the creep behaviour of adhesively bonded fastener in tension: experimental investigations, analytical and numerical modelling

The use of structural adhesive bonding is increasing in structural assembly. It has indeed several advantages compared to other assembly techniques such as welding and riveting: uniform stress distribution, assembly of materials with different natures, no heat production. This last aspect is particularly interesting for the offshore activities. The firm Cold Pad has developed adhesively bonded fastener solutions, whose aim is to replace welding by structural bonding for the reinforcement of steel structures. Yet, some issues remain about the appraisal of the durability for such assembly and particularly creep. To address this issue, experimental creep investigations were carried out at real scale for one geometry of a fastener bonded with a methacrylate adhesive, and at different tension load levels. An inverse identification method has been developed in a previous study to identify the material creep properties from these investigations, independently of the fastener geometry. Considering the industrial applications of this fastener, the aim of this modelling approach is to describe correctly the primary and secondary creep stages. In this article, this identification method was applied to all the experimental results, using the Burger creep model. The dependency of the parameters with the load level is highlighted, which led to a nonlinear model. These results were then compared to finite element investigations to verify the consistency of the method and its ability to correctly identify the adhesive creep behaviour. This offers good perspectives in the transposition of the study to alternative fastener geometries using simple analytical design approaches. Highlights Bonded fastener’s creep behaviour experimentally investigated under different load levels. Creep modelling using a nonlinear and analytical approach with Burger model Identification of the material creep properties from experimental investigations at real scale on the bonded assembly. Numerical model developed to assess the consistency of the analytical approach.

Experimental characterization of flat indentation creep behaviour of Inconel 617

To analyse the creep deformation characteristics of Inconel 617, vacuum flat indentation creep test and microstructure analysis were completed in this study. With an increase in the creep load and temperature, limiting primary creep deflection and power law coefficient increased. An apparent indentation primary creep stage was observed and proven to be about 15–25 h in flat indentation creep test here. To describe the macro–deformation of Inconel 617, a modified theta–projection model was proposed and proven to be a feasible method to predict flat indentation creep curve accurately. Subsequently, a creep life prediction method based on deformation control was proposed. Based on the Larson – Miller parameter, reasonable conversion relationship between indentation creep stress and uniaxial creep equivalent stress was established. Finally, the microstructure analysis was completed to investigate the creep deformation characteristic. A hemispherical region along with significant extended grains and increase in hardness were observed. Considerable amount of Cr-rich precipitates and a small amount of Mo-rich precipitates were observed. Meanwhile, the precipitates were found to be discontinuous and granular for short indentation creep times.