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To explore the creep characteristics of geomembrane under different tensile stresses, a series of creep tests were carried out on high-density polyethylene (HDPE) geomembrane specimens. For the interpretation and fitting of the experimental data, refined approximation functions were proposed. Particular attention was paid to the creep failure behavior under high tensile stresses, i.e., 70%, 80%, and 90% of maximum peak stress. To investigate the effects of size on the mechanical response, experiments with two different membrane thicknesses were conducted. The results obtained under high stress levels were compared with creep tests at medium and low stress levels. Depending on load level, different creep characteristics can be distinguished. In the secondary creep state, the creep velocity is higher for higher load levels. In contrast to the medium and low load levels, the geomembrane under high stresses underwent the tertiary creep stage after instantaneous deformation and primary and secondary creep stages. In some tests, it was observed that under very high stress levels, creep velocity does not necessarily follow the expected trend and creep rupture can occur within a short time. For numerical simulation, an improved mathematical model was proposed to reproduce in a unified manner the experimental data of the whole non-linear evolution of creep elongation under different stress levels. [ABSTRACT FROM AUTHOR]

In this paper, the nonlinear creep behaviour of additive-manufactured carbon fibre-reinforced polyethylene terephthalate (CF-PET) is characterised using experimental, theoretical and computational methods. The experimental approach investigates the influence of infill orientations on the creep deformation of the material. For the study, samples at 0°, 45○, and 90° infill orientations are produced with 90% infill density using fused filament fabrication (FFF). The infill orientation parameter highly influences the creep behaviour. Increasing the infill orientation from 0° to 90° monotonically improves the creep resistance of the material, which can be explained by orientation of the fibre-matrix reinforcement towards the uniaxial stresses. Surface examinations of creep-ruptured samples via scanning electron microscopy (SEM) reveal that a combination of matrix failure, fibre pull-out, fibre-matrix debonding, inter-layer debonding, and the presence of voids cause the fractures. Based on the experimental data, the primary and secondary creep responses are modelled theoretically and computationally. The theoretical model is based on the dependence of the material's creep on stress and time parameters at the transient and steady state stages. Combined stress and time functions are used to model the creep of the material. Parallelly, two-dimensional (2D) finite element (FE) analyses are made on COMSOL Multiphysics to model the creep computationally. The approach is based on the superposition of Norton's and Garofalo's creep models with predefined time hardening property. The results of the modelling are in good agreement with the experimental findings, showing a maximum of 1.04 % for the theoretical, and 2.9 % for the computational approaches.

Creep rupture tests were conducted on 67 thin-walled DD6 specimens, with thicknesses ranging from 0.35 mm to 1.50 mm, at temperatures of 760 °C, 850 °C, 980 °C, and 1100 °C, revealing a log-linear relationship between rupture lifetimes and specimen thicknesses. Micro-CT observations identified an inhomogeneous distribution of creep micropores around dendrites as a contributing factor to the thickness debit effect. Based on the reliability mathematical method, a possible explanation was proposed for the parabolic law relationship between rupture times and specimen thicknesses. In addition, a thickness debit characterization parameter (TDP) was derived to quantify the trend of creep property degradation as thickness decreases. The TDP equation effectively predicted the creep rupture lifetimes of DD6 thin-walled specimens mostly within a scatter band factor of 2.

By using state-of-the-art characterisation techniques including atom probe tomography and atomic resolved elemental mapping, we successfully probed the site occupation evolution associated with composition change in γ′, during 1100°C creep of a fourth generation Ru-bearing Ni-based superalloy. It is quantified that, W and Ru maintain unchanged site preference after creep rupture, while interestingly, the rest elements especially Co, Ta and Re show a weakened preference at both α- and β-sites in the γ′ lattice. This indicates the overall reduced γ′ ordering degree and thus the possible decrease in planar fault energies, which further facilitates dislocation shearing in γ′.

To explore the creep characteristics of geomembrane under different tensile stresses, a series of creep tests were carried out on high-density polyethylene (HDPE) geomembrane specimens. For the interpretation and fitting of the experimental data, refined approximation functions were proposed. Particular attention was paid to the creep failure behavior under high tensile stresses, i.e., 70%, 80%, and 90% of maximum peak stress. To investigate the effects of size on the mechanical response, experiments with two different membrane thicknesses were conducted. The results obtained under high stress levels were compared with creep tests at medium and low stress levels. Depending on load level, different creep characteristics can be distinguished. In the secondary creep state, the creep velocity is higher for higher load levels. In contrast to the medium and low load levels, the geomembrane under high stresses underwent the tertiary creep stage after instantaneous deformation and primary and secondary creep stages. In some tests, it was observed that under very high stress levels, creep velocity does not necessarily follow the expected trend and creep rupture can occur within a short time. For numerical simulation, an improved mathematical model was proposed to reproduce in a unified manner the experimental data of the whole non-linear evolution of creep elongation under different stress levels.

In this paper, a unified physics-informed machine learning (PIML) model is proposed for the accurate prediction of the creep rupture life of oxide/oxide ceramic matrix composites (CMCs) by incorporating prior physical information and microstructure features. The collected dataset undergoes missing value imputation and standardization methods for processing. Key feature parameters that influence rupture life are identified through global sensitivity analysis. Furthermore, a high-throughput design module is developed to explore optimal composition under specific creep conditions, providing design guidelines for new materials in oxide/oxide CMCs. This study demonstrates that the predicted results consistently fall within the ±3 error bands. Improved life expectancy is attained with fiber modulus (220–290 GPa) and matrix modulus (>310 GPa). The transformation from expensive creep testing to low-cost exploration of modulus and proportional limit is achieved, offering theoretical support and analytical methods for the design of CMC components and the development of new materials for high-temperature applications. [ABSTRACT FROM AUTHOR]

The theory and methods for constructing equations (functions) of evolutionary damage and rupture of materials in the Kachanov model (creep rupture and fatigue rupture) are presented. In general, it is proved that the factorized Kachanov model is identical to the Palmgren-Miner rule, which is often not confirmed experimentally. To construct damageability functions adequate to the experimental data, new mathematical objects (potential and normalized potential) are introduced. If the entire history of changes in the damage variable is known in experiments, then the use of the potential makes it possible to construct a damageability function of any complexity without integrating the evolutionary equation (explicit method). For cases where only rupture moments are recorded in experiments, a criterion for the adequacy of the normalized potential is formulated and an implicit method for its construction is developed. It is supplemented with a recursive algorithm that generates an unlimited number of such potentials. The implicit method is illustrated by examples, following which the reader can construct a damageability equation for his material without a thorough study of the theory. [ABSTRACT FROM AUTHOR]

By using state-of-the-art characterisation techniques including atom probe tomography and atomic resolved elemental mapping, we successfully probed the site occupation evolution associated with composition change in γ′, during 1100°C creep of a fourth generation Ru-bearing Ni-based superalloy. It is quantified that, W and Ru maintain unchanged site preference after creep rupture, while interestingly, the rest elements especially Co, Ta and Re show a weakened preference at both α- and β-sites in the γ′ lattice. This indicates the overall reduced γ′ ordering degree and thus the possible decrease in planar fault energies, which further facilitates dislocation shearing in γ′. Associated with the tremendous mass transfer during high-temperature creep, the site-occupation scenario in the γ′ lattice evolves as that, the preference at both α- and β-sites for Co, Ta and Re weakens while that of W and Ru remains nearly unchanged, indicating the overall decrease in ordering and thus the easier dislocation shearing in γ′. [ABSTRACT FROM AUTHOR]

Creep rupture as a failure mechanism during a severe accident in a PWR is of particular importance as it can lead to releasing radioactive materials into the environment. Following a severe accident, the decay heat transferred to other parts of the reactor cooling system can result in the heat-up of RCS structures and failure of vulnerable pressure boundaries. SBO without operator actions accident (TMLB sequence) is considered one of the most likely scenarios threatening the integrity of the RCS pressure boundary. In addition to PWR plant modeling, in this sequence, the location of rupture in the RCS depends on the type of natural circulation phenomenon of the reactor's primary side. This research investigates the sensitivity analysis of steam generator tube rupture (SGTR) and RCS hot-leg to the type of natural circulation, i.e. con-current and countercurrent, using the MELCOR code. The results of two models of con-current and countercurrent natural circulation show when the countercurrent natural circulation phenomenon is predominant, the RCS hot leg creep rupture occurs earlier than SGTR. However, the SGTR occurs earlier than any other part of RCS due to the concurrent natural circulation phenomenon. Moreover, the amount of materials released to different parts of the plant and the environment has been estimated for both models. The results show that about 18 kg and 145.76 kg of radioactive aerosol and fission product vapor materials are ejected to the containment following the rupture for con-current and countercurrent natural circulation models, respectively. On the other hand, in the con-current natural circulation, about 136.33 kg of radioactive aerosol and fission product vapor materials are released into the environment through the main steam line safety valve.

Fatigue strength, relaxation, and creep rupture strength of carbon-fiber-reinforced polymer (CFRP) strands were evaluated experimentally, and their impact on bridge beam design was investigated. The long-term relaxation of CFRP strands was evaluated by loading CFRP test specimens under different environmental conditions and monitoring prestress loss over time. Creep rupture strength of CFRP strands after I million hours of sustained stress exposure was predicted by loading and monitoring CFRP test specimens under a range of sustained stress levels for an extended time. The fatigue strength of CFRP strands was established by cyclically loading CFRP test specimens using different stress amplitudes. In addition, and as a benchmark for fatigue evaluation, low-relaxation steel and stainless steel strand test specimens were prepared and cyclically loaded within the fatigue test matrix. Test results showed that fatigue strength of CFRP strands is superior to that of low-relaxation steel and stainless steel prestressing strands. In addition, the one-million-hour relaxation loss of CFRP strands is approximately 2% for a wide range of initial stress levels. Furthermore, the one-million-hour creep rupture strength is at least 88% of the average tensile strength of the strands. Extended exposure to environmental conditions did not seem to affect the tensile capacity of CFRP strands. [ABSTRACT FROM AUTHOR]

• The remaining mechanical properties of bainitic 2.25Cr1Mo steel linearly decrease with Larson-Miller parameter. • The applied creep stress accelerates the softening of the steel. • The mechanism behind the degradation of the remaining strength of the steel is elucidated. • The application of remaining strengths in design and operation of SG is discussed. In this work, the microstructure and remaining mechanical properties of bainitic 2.25Cr1Mo steel plate used in steam generator of sodium cooled fast reactor during creep were systematically examined. Creep tests at 530 °C with a rupture time up to 18800 h reveals that the creep stress correlates linearly with the Larson-Miller parameter ( P LM ), while the creep plasticity is rather independent on P LM . The remaining strength parameters, such as yield strength (R P0.2), ultimate tensile strength (R m) and hardness (H), are found to linearly decrease with P LM , while the remaining plasticity is seen to increase. It suggests that the application of creep stress to the steel accelerates the softening of the material. During the creep process, the dislocation density in bainitic matrix decreases, while the number density of carbides increases, sacrificing the solution strengthening and leading to a continual reduction in the lattice parameter. Although abundant fine carbides precipitate after creep rupture, the remaining strength is however reduced. Instead, the weakening effect due to reduction in dislocation and solute content overwhelms the strengthening of precipitation, resulting in a degradation of the remaining strength. The obtained remaining mechanical properties are attempted to be applied in the design and operation assessment of steam generator. [ABSTRACT FROM AUTHOR]

The premature creep rupture of cast reformer tubes made of HP-Nb steel due to short-term overheating, was investigated. Investigation was performed on specimens from ruptured, cracked, and uncracked tubes and included metallography, mechanical testing, and SEM/EDX analysis. It was found that accidental short-term overheating, after 5 years of normal operation, caused rapid tertiary creep, manifested by the formation of dense cavities and cracking, which eventually led to the premature creep rupture of several tubes of the reformer. Creep cavities formed preferentially at M7C3 carbides but also at M23C6 and MC carbides while crack propagation took place along the interdendritic carbide network. It appears that the radial direction of the columnar grains from the inside to outside diameter orients the grain boundaries perpendicular to the hoop stress, a situation which degrades creep rupture resistance. An analysis based on the Larson–Miller parameter confirmed the premature creep rupture of the tubes due to overheating.

Abstract Cross-laminated timber (CLT) has been used extensively in timber construction. CLT panels are typically used in roofs and floors that carry a continuous load, and it is important to examine the long-term loading capacity of CLT. However, studies that focus on the long-term loading capacity of CLT are limited. To this end, we conducted long-term out-of-plane bending tests on seven-layer CLT made from Japanese larch (Larix kaempferi) under constant environmental conditions, investigated creep performance and duration of load, and experimentally analyzed creep rupture behavior. The mean estimated relative creep after 50 years was 1.49. The sample showed a satisfactory resistance to creep as a building material. The duration of load of most of the specimens in this study was shorter than the conventional value of small clear wood specimens. Specimens had a lower duration of load capacity than solid lumber. According to the results of survival analysis, a loading level of 70% or more caused the initial failure of specimens. Creep rupture of most of the specimens occurred at less deflection than displacement at failure in the short-term loading test. Additional studies focusing on the effects of finger joints, transverse layers, and width of a specimen on creep rupture behavior are suggested.

HR3C (25Cr-20Ni-Nb-N) is a key material used in heat exchangers in supercritical power plants. Its creep properties and microstructural evolution has been extensively studied at or below 650 °C. The precipitation evolution in HR3C steel after creep rupture at elevated temperatures of 700 °C and 750 °C with a stress range of 70~180 MPa is characterized in this paper. The threshold strength at 700 °C and 750 °C were determined by extrapolation method to be σ 10 5 700 = 57.1 MPa and σ 10 5 750 = 37.5 MPa, respectively. A corresponding microstructure investigation indicated that the main precipitates precipitated during creep exposure are Z-phase (NbCrN), M23C6, and σ phase. The dense Z-phase precipitated dispersively in the austenite matrix along dislocation lines, and remained stable (both size and fraction) during long-term creep exposure. M23C6 preferentially precipitated at grain boundaries, and coarsening was observed in all creep specimens with some continuous precipitation of granular M23C6 in the matrix. The brittle σ phase formed during a relatively long-term creep, whose size and fraction increased significantly at high temperature. Moreover, the σ phases, grown and connected to form a large "island" at triple junctions of grain boundaries, appear to serve as nucleation sites for high stress concentration and creep cavities, weakening the grain boundary strength and increasing the sensitivity to intergranular fracture. [ABSTRACT FROM AUTHOR]

The initiation, expansion, and penetration of microscopic cracks in rock is the macroscopic manifestation of creep. This paper investigates mechanical creep characteristics and fracture evolution processes in rock masses with different fracture angles, lengths, and rock bridge dip angles. Single fractures, dual parallel fractures, and fracture groups are considered. The approach comprises discrete element simulation based on continuum mechanics, utilizing the continuous and discontinuous software, GDEM. Single-fracture rock masses are characterized by a progressive fracture development mode dominated by tensile shear failure. The rate of creep and fracture magnitude both increase according to fracture length. With increasing fracture inclination angle, creep rate and fracture magnitude increase and decrease. The creep rate and degree of rupture are highest for fractures inclined at 30°. The dual-fracture rock mass exhibits both tensile crack failure and compressional shear failure. Creep rates are highest, and rupture effects are most apparent at rock bridge inclination angles of 90°. If the rock bridge is too long or too short, the stable creep stage is prolonged, but the creep acceleration stage intensifies due to interaction between fracture-bounded rock masses. The failure mode, in this case, involves collective failure by tension fractures and compressional shear. Creep rate and fracture magnitude increase with the number of fractures, which accelerates rock mass deformation to a certain extent. However, when the number of fractures reaches a certain threshold, a relatively stable structure may become established, slowing down the creep rate, especially during the creep acceleration stage. This study can provide a theoretical basis and reference for investigating the creep rupture law of rock mass engineering and the prevention and control of fractured rock mass geological disasters.

• Investigated the influence of sliver defects on the creep rupture life of nickel-based single crystal superalloys. • Revealed the damage mechanism of sliver defects on the creep life of single crystal superalloys through SEM and EBSD observations. • Developed creep constitutive equations for specimens containing sliver based on crystal plasticity theory, explaining the reasons for the decrease in sample life caused by sliver. • Predicted the damage tolerance of sliver defects with different orientations. Sliver defects are common casting imperfections in the preparation of single crystal blades, significantly impacting the high-temperature mechanical performance of blades and potentially leading to blade failure. A detailed study was conducted on the microstructure of sliver in the second-generation nickel-based single crystal superalloy CMSX-4, focusing on its influence on creep performance at 980 °C/250 MPa. Results indicate a substantial impact of sliver on the creep performance of single crystal superalloys, with a 26.37 % reduction in creep rupture time for samples with sliver compared to those without. Fracture analysis reveals that sliver primarily contributes to performance deterioration by promoting the extension of grain boundary microcracks through grain boundary sliding during the creep process, accelerating sample fracture. Utilizing crystal plasticity theory, a successful fit was achieved for the creep rupture life of samples with sliver, allowing the prediction of fracture life for samples with different orientations of slivers. Predictive results suggest that, for CMSX-4 alloy, the orientation of the sliver should be restricted to within 8°. [ABSTRACT FROM AUTHOR]

In the field of nuclear engineering, Printed Circuit Heat Exchangers (PCHEs) have become increasingly popular and the structural integrity assessment of these key power plant components is crucial. As part of the structural integrity assessment, creep rupture analysis considers the interaction of cyclic plasticity and creep behaviour, which is vital for components subjected to cyclic thermal-mechanical loads. The Linear Matching Method (LMM) framework has included a creep rupture module based on an extended shakedown algorithm, which has been adopted by several researchers. However, the current LMM framework mainly relies on linear extrapolation and requires users to provide a large amount of data points to estimate the rupture stress. This paper introduces a Unified Creep Rupture Equation (UCRE) for characterizing the creep rupture curves of diverse steel types. The UCRE is implemented in an extended shakedown algorithm and integrated into the LMM framework. The numerical method is validated through a comparative analysis, wherein the estimated rupture curves are compared with those provided in the ECCC data sheet, as well as against alternative numerical models. Shakedown and creep rupture analyses are then performed on a PCHE core. The proposed method facilitates the parametric study of changing materials and the process of material selection. Various geometric configurations are also considered and the proposed unitary cell model is verified by comparing the results with ones from full FE models. The UCRE has been proved to be an accurate engineering tool for the prediction of rupture strengths while the LMM framework has gained improved usability and versatility for engineering applications. • A numerical procedure to predict rupture strengths for a wide range of steel. • Proposed method is verified by comparing with alternative numerical models. • Direct method-based creep rupture analysis using the Linear Matching Method framework. • Parametric study on a Printed Circuit Heat Exchanger core based on a unitary cell model. [ABSTRACT FROM AUTHOR]

The relevance of incorporating "threshold stress" in models for the dependence of rupture time on applied stress is discussed in the context of data for 2.25Cr1Mo and 1CrMoV steels, with particular reference to a recent publication on this subject (Evans, 2023) [1]. • Highlights of "Concerning the use of 'threshold stress' in the modelling of creep rupture data". • Reviews recently published rupture models including threshold stress. • Examines validity of threshold stress in those models of rupture data. • Presents and compares alternative models excluding threshold stress. [ABSTRACT FROM AUTHOR]

The effect of grain size on the mechanical properties and creep rupture of 253 microalloyed (MA) austenitic stainless steel (ASS) was investigated. The cold rolling process with a 53% reduction in thickness was applied to the steel followed by annealing at 1100 °C over 0, 900, 1800, and 3600 s to obtain grain sizes of 32.4, 34.88, 40.35, and 43.77 µm, respectively. Uniaxial tensile and micro-Vickers hardness tests were carried out to study the effect of grain size on mechanical properties at room temperature. The creep rupture test was performed at 700 °C under a load of 150 MPa. The results showed that there was a correlation between grain size, mechanical properties, and creep rupture time. The fine initial grain size showed relatively good mechanical properties with a short creep rupture time, while the coarse initial grain size produced low mechanical properties with a long creep rupture time. The initial grain size of 40.35 µm was the optimum grain size for a high value of creep rupture time due to the low hardness and elongation values at room temperature and low creep ductility value. The intergranular fracture was found on the initial grain size below 40.35 µm, and a mixed mode of intergranular and transgranular fracture was found on the initial grain size above 40.35 µm after the creep rupture test. [ABSTRACT FROM AUTHOR]

The fatigue properties of lithium hydride (LiH) are crucial to its application as neutron shielding and moderating at elevated temperatures. The dynamic fatigue tests of LiH were investigated with the notched 3-point bend (3 PB) specimens over ranges of loading rates at RT up to 400 °C. At RT, the results showed that slow crack growth (SCG) occurred prior to failure as the minor deviation from linearity to nonlinearity in the load-deflection curves. In addition, the fracture strength of LiH decreased with decreasing stress rate and the SCG zone gradually became smaller with higher stress rates, indicating evident dynamic fatigue phenomenon. However, the trends were quite different at 200, 300 and 400 °C due to accumulative creep damage for low stress rates at elevated temperatures. With increasing temperature and decreasing stress rate, there existed a transition of the dominated failure mechanism, from SCG to creep rupture. Evidence of very small SCG zone could also be detected at the notch for the failure dominated by creep rupture. • Lithium hydride (LiH) shows obvious dynamic fatigue phenomenon and the slow crack growth parameter is determined to be 15.787 at room temperature. • A transition in the dominated failure mechanism, from slow crack growth (SCG) to creep rupture, is found with increasing temperature and decreasing stress rate in the dynamic fatigue tests. • Evidence of very small SCG zone could also be detected at the notch for the failure dominated by creep rupture. [ABSTRACT FROM AUTHOR]

The alloying elements that control the creep rupture life of low carbon steel in the region of inherent creep strength were investigated by a data-driven model selection method. The experimental data published in NRIM Creep Data Sheet No. 7B was used. A model in which the Larson–Miller parameter is expressed as a linear sum of the logarithmic stress and the chemical composition of alloying elements was proposed on the basis of the analysis of the experimental data. There were 1023 ($${ = 2^{10}} - 1$$) candidate models containing one or more of the 10 alloying elements, and they were compared using two model selection methods: 10-fold cross validation (CV) and posterior probability based on Bayesian inference. The 10-fold CV suggested that Mo must be included in the model to improve the predictivity of the unknown data. A comparison among the Bayesian posterior probabilities of the candidate models showed that the Mo-only model has a predominantly high posterior probability of 79.76% compared with the other models. Furthermore, other element terms that co-occur with Mo evenly appeared in the top list after the Mo-only model, indicating that only Mo is crucial for increasing the posterior probability. From these results, it is concluded that the Mo-only model is the most appropriate as a data generation model. The selected Mo-only model was confirmed to have high predictivity of the creep life.

The mechanism governing de-oxidation of vanadium metal is regarded as fundamental knowledge; however, it has not been elucidated in existing literature. In this paper, the thermodynamic data of V-H-O systems were summarized, and the Gibbs free energies of the main compounds were calculated. Consequently, the de-oxidation limits of different reductants in a VO system were evaluated, namely: Si, Al, and Mg. It was observed that Si could not remove an O content of less than 7.27 wt% from V. However, Al was the stronger reducing agent; it could remove O contents of up to 0.01 and 0.1 wt% at 800 and 1050°C, respectively. Nevertheless, Mg exhibited the best reducing properties as it could remove less than 0.01 wt% of O at 1100°C. The addition of H2 rendered the V-O solid solution unstable to a certain extent, thereby indicating that H2 facilitated de-oxygenation. Furthermore, the results obtained by analyzing the equilibrium conditions were in accordance with the results of the de-oxidation limit in the V-O system. In other words, this study demonstrated that oxygen in vanadium can be effectively controlled by changing the reductant dosage and temperature.

The alloying elements that control the creep rupture life of low carbon steel in the region of inherent creep strength were investigated by a data-driven model selection method. The experimental data published in NRIM Creep Data Sheet No. 7B was used. A model in which the Larson–Miller parameter is expressed as a linear sum of the logarithmic stress and the chemical composition of alloying elements was proposed on the basis of the analysis of the experimental data. There were 1023 ( = 2 10 − 1) candidate models containing one or more of the 10 alloying elements, and they were compared using two model selection methods: 10-fold cross validation (CV) and posterior probability based on Bayesian inference. The 10-fold CV suggested that Mo must be included in the model to improve the predictivity of the unknown data. A comparison among the Bayesian posterior probabilities of the candidate models showed that the Mo-only model has a predominantly high posterior probability of 79.76% compared with the other models. Furthermore, other element terms that co-occur with Mo evenly appeared in the top list after the Mo-only model, indicating that only Mo is crucial for increasing the posterior probability. From these results, it is concluded that the Mo-only model is the most appropriate as a data generation model. The selected Mo-only model was confirmed to have high predictivity of the creep life. [ABSTRACT FROM AUTHOR]

Cross-laminated timber (CLT) has been used extensively in timber construction. CLT panels are typically used in roofs and floors that carry a continuous load, and it is important to examine the long-term loading capacity of CLT. However, studies that focus on the long-term loading capacity of CLT are limited. To this end, we conducted long-term out-of-plane bending tests on seven-layer CLT made from Japanese larch (Larix kaempferi) under constant environmental conditions, investigated creep performance and duration of load, and experimentally analyzed creep rupture behavior. The mean estimated relative creep after 50 years was 1.49. The sample showed a satisfactory resistance to creep as a building material. The duration of load of most of the specimens in this study was shorter than the conventional value of small clear wood specimens. Specimens had a lower duration of load capacity than solid lumber. According to the results of survival analysis, a loading level of 70% or more caused the initial failure of specimens. Creep rupture of most of the specimens occurred at less deflection than displacement at failure in the short-term loading test. Additional studies focusing on the effects of finger joints, transverse layers, and width of a specimen on creep rupture behavior are suggested. [ABSTRACT FROM AUTHOR]

In this study, an experimental creep crack growth rate (CCGR) of austenitic steel AISI 316L was predicted from the creep constant values and Nibkin Smith Webster (NSW) method. The experimental CCGR data were obtained from the CCG test under four different load conditions ranging from 6000 to 7000 N at 525 oC. The creep constants, A and n were obtained from the uniaxial creep rupture test under various load conditions, ranging from 180 to 225 Mpa at 525 oC. The creep constants were determined both from the minimum and average creep strain rate data. By applying the creep constants and NSW method, the predicted CCGR curve was generated and compared with the experimental CCGR curve. The result showed that the predicted CCGR curve based on the minimum creep strain rate data was found to be close to the experimental CCGR curve. The significant portion of creep crack growth’s lifetime of austenitic steel AISI 316L was occupied by the crack propagation (steady stage) period rather than crack initiation and/or fracture periods. In addition, plane stress and strain conditions could also be determined from the predicted CCGR curve. It was observed that the experimental CCGR curve was located near the plane strain condition where no deformation occurred in the lateral direction.

HR3C (25Cr-20Ni-Nb-N) is a key material used in heat exchangers in supercritical power plants. Its creep properties and microstructural evolution has been extensively studied at or below 650 °C. The precipitation evolution in HR3C steel after creep rupture at elevated temperatures of 700 °C and 750 °C with a stress range of 70~180 MPa is characterized in this paper. The threshold strength at 700 °C and 750 °C were determined by extrapolation method to be σ105700= 57.1 MPa and σ105750=37.5 MPa, respectively. A corresponding microstructure investigation indicated that the main precipitates precipitated during creep exposure are Z-phase (NbCrN), M23C6, and σ phase. The dense Z-phase precipitated dispersively in the austenite matrix along dislocation lines, and remained stable (both size and fraction) during long-term creep exposure. M23C6 preferentially precipitated at grain boundaries, and coarsening was observed in all creep specimens with some continuous precipitation of granular M23C6 in the matrix. The brittle σ phase formed during a relatively long-term creep, whose size and fraction increased significantly at high temperature. Moreover, the σ phases, grown and connected to form a large “island” at triple junctions of grain boundaries, appear to serve as nucleation sites for high stress concentration and creep cavities, weakening the grain boundary strength and increasing the sensitivity to intergranular fracture.

In the present study, the creep rupture behaviour and microstructural evolution of 2195-T84 Al-Li alloy are investigated at different tensile stresses. It is found that as the applied stress during the creep rupture process increases, the corresponding creep strain and creep rate significantly increase. Moreover, the evolution of microstructures, including precipitates, dislocation density and creep cavities at different stages is characterized using a transmission electron microscope (TEM), X-ray diffractometer (XRD) and scanning electron microscopy (SEM). The main strengthening precipitates are identified as T1 (Al2CuLi) and θ′ (Al2Cu) phases. Obtained results show that as the applied stress increases, T1 and θ′ phases are gradually coarsened. This coarsening is more pronounced for θ′ phase. Furthermore, the creep cavities are mainly distributed at the interface between the insoluble second phase particles and the matrix, and their average sizes gradually increase as the applied stress increases. Meanwhile, the density and size of dimples on the fracture surface gradually decrease as the applied stress increases. Moreover, the main fracture mechanism changes from transgranular dimple fracture to quasi-cleavage fracture. Based on the microstructural evolution, a novel set of unified creep rupture constitutive model is proposed. The established model incorporates the evolution of microstructures, including the dislocation density, average length of T1 and θ′ precipitates and creep cavitation, and correlates microstructural variables with creep rate. The results calculated by the constitutive model are in good agreement with the experimental data, which validates the proposed model. [ABSTRACT FROM AUTHOR]

The effect of grain size on the mechanical properties and creep rupture of 253 microalloyed (MA) austenitic stainless steel (ASS) was investigated. The cold rolling process with a 53% reduction in thickness was applied to the steel followed by annealing at 1100 °C over 0, 900, 1800, and 3600 s to obtain grain sizes of 32.4, 34.88, 40.35, and 43.77 µm, respectively. Uniaxial tensile and micro-Vickers hardness tests were carried out to study the effect of grain size on mechanical properties at room temperature. The creep rupture test was performed at 700 °C under a load of 150 MPa. The results showed that there was a correlation between grain size, mechanical properties, and creep rupture time. The fine initial grain size showed relatively good mechanical properties with a short creep rupture time, while the coarse initial grain size produced low mechanical properties with a long creep rupture time. The initial grain size of 40.35 µm was the optimum grain size for a high value of creep rupture time due to the low hardness and elongation values at room temperature and low creep ductility value. The intergranular fracture was found on the initial grain size below 40.35 µm, and a mixed mode of intergranular and transgranular fracture was found on the initial grain size above 40.35 µm after the creep rupture test.

In order to reveal the creep failure behavior of novel modified 310S austenite steel welded joints, the creep life and microstructure evolution of the 310S austenite steel welded joints were investigated in this study. The rupture life was assessed to estimate the damage of the welded joint based on creep rupture tests performed at 600 °C in the stress range of 170–238 MPa. Compared with WM, HAZ facilitated the occurrence of creep failure in long term creep due to the combination of a smaller hardness value, a more heterogenous microstructure accompanied by coarsened M23C6, a larger grain size, higher KAM and Schmid factor. Discontinuous Laves phases appeared near the boundaries between the δ-ferrite and γ-austenite grains in the WM, and dislocation strengthening and precipitation strengthening were observed near the boundary in the BM. Furthermore, segregation elements were detected by APT and EDS adjacent to the boundary. Cr and C segregation near grain boundaries weaken the creep resistance in long term creep service.

Creep tests were carried out on notched plate specimens of nickel-based superalloy GH4169 with different stress concentration coefficients. It was found that the duration of the first stage of the creep curve increases with the increase of stress concentration coefficient, while the fracture ductility decreases with the increase of stress concentration coefficient. To predict the life of notched plate specimens, four constitutive models were used to analyze the stress and strain of the notches. It was found that the average Von Mises equivalent stress (AVES) on the minimum notch section first decreases and then increases with the creep time, resulting in a minimum value. The minimum average Von Mises equivalent stress (MAVES) is considered as the characteristic stress of notched specimens in this paper. The creep life equation is fitted according to the results of creep tests of smooth specimens, and then the predicted life of notched specimens is obtained by substituting the minimum average Von Mises equivalent stress of notched specimens into the creep equation. The prediction results of the four constitutive models are within 2 times the dispersion band, and the three-stage model is within the 1.5 times dispersion band.

This paper presents a finite element analysis of the small punch creep test for 316L(N), which is compared with experimental data for 650 and 700 °C. Special emphasis is placed on (i) assessing the influence of friction and (ii) comparing two different creep models: the simple Norton creep and the more general creep model. The computed normalized deflection rate versus time is almost identical for all cases, which allows for scaling of the results. The computed time to rupture increases linearly with the friction coefficient due to a reduction in the mean stress. There is a good overall agreement between the experimental values and the computed deflection rate for a friction coefficient of around 0.3. It is shown that the initial reduction in deflection rate is due to stress relaxation and homogenization, and is only marginally affected by primary creep hardening. The computed results are compared with the equivalent stress and strain rates in the recently published small punch standard (EN 10371). The computed von Mises stresses at minimum deflection decrease linearly with the friction coefficient but are consistently slightly higher than the equivalent stress in the standard. For the strain rates, the computed values are significantly higher than the equivalent values in the standard. The presented simulations give a deeper insight of the small punch creep and impact of key parameters such the friction coefficient and in general as a guidance to refinement and improvement of the empirically based formulae in the standard.

In a number of circumstances, the Kachanov–Rabotnov isotropic creep damage constitutive model has been utilized to assess the creep deformation of high-temperature components. Secondary creep behavior is usually studied using analytical methods, whereas tertiary creep damage constants are determined by the combination of experiments and numerical optimization. To obtain the tertiary creep damage constants, these methods necessitate extensive computational effort and time to determine the tertiary creep damage constants. In this study, a curve-fitting technique was proposed for applying the Kachanov–Rabotnov model into the built-in Norton–Bailey model in Abaqus. It extrapolates the creep behaviour by fitting the Kachanov–Rabotnov model to the limited creep data obtained from the Omega-Norton–Bailey regression model and then simulates beyond the available data points. Through the Omega creep model, several creep strain rates for SS-316 were calculated using API-579/ASME FFS-1 standards. These are dependent on the type of the material, the flow stress, and the temperature. In the present work, FEA creep assessment was carried out on the SS-316 dog bone specimen, which was used as a material coupon to forecast time-dependent permanent plastic deformation as well as creep behavior at elevated temperatures and under uniform stress. The model was validated with the help of published experimental creep test data, and data optimization for sensitivity study was conducted by applying response surface methodology (RSM) and ANOVA techniques. The results showed that the specimen underwent secondary creep deformation for most of the analysis period. Hence, the method is useful in predicting the complete creep behavior of the material and in generating a creep curve.

In the present study, creep rupture behavior of a bagasse fiber- polypropylene composite was investigated. Two accumulated damage model (EDRM and Wood) and an energy based failure model (R-W) were adopted to describe the load duration influence on the studied composite. Results have shown that at very high stress levels, the EDRM and Wood models underestimated and overestimated the time to failure than static value respectively. This was attributed to the difference between application of static loading and creep loading. Findings indicated that the all three models were able to describe the creep rupture behavior of the studied composite and among this; the Wood model has shown a better fitting with experimental data, statistically. Based on findings in the present research, it can be concluded that the R-W model was more conservative in predicting time to failure in comparison with the two other models, and the reason can be attributed to the difference between failure criteria in energy based models and accumulated damage models. Results also showed that with increasing stress levels, the secondary stage of creep will shorten and at high stress level, the tertiary stage of creep will be more dominant

Materials for core components of fusion reactors and fast reactors, such as blankets and fuel cladding tubes, must offer the best possible high temperature strength and irradiation resistance because they will be exposed to high heat flux and heavy neutron irradiation. Japan Atomic Energy Agency (JAEA) has been developing 9 and 11 chromium (Cr) oxide dispersion strengthened (ODS) steels as candidate materials for advanced fast reactor cladding tubes. In this study the JAEA 11Cr-ODS steel was rolled in order to evaluate its anisotropy. The tensile tests and creep tests were carried out at 700°C in longitudinal and transverse orientations. The anisotropy of the tensile strength was negligible, though that of the creep strength was distinct. The observation results and chemical composition mapping suggested that the cause of the anisotropy in the creep strength was a previously formed columnar boundary, that is, a prior particle boundary including Ti-rich sub-micro metric precipitates.

This addendum to Reliable analysis and extrapolation of creep rupture data, Int J Press Vessels Pip 2017 , covers matters that were insufficiently addressed in the original article. Uncertainties that obscure evaluation of the P-NID procedure by analysis of multi-batch data are addressed by blind testing the procedure for individual batches of an 18%Cr steel, Type 316 SS. Uncertainties that arise in the extrapolation of near-linear rupture curves are addressed by the inclusion of temperature dependence in parametric stress equations. The use of Tensile Strength data as equivalent short-term rupture data is demonstrated and discussed. • Discusses testing the reliability of an extrapolation procedure by modelling data for individual batches of material. • Shows that P-NID models of short-term data for individual batches of 18%Cr steel predict long-term data accurately. • Discusses difficulties in extrapolating data that show a near-linear relationship between log stress and log rupture time. • Illustrates the advantages of a model that includes a temperature-dependent parametric stress. • Discusses and demonstrates the use of UTS data as equivalent short-term rupture data. [ABSTRACT FROM AUTHOR]

Highlights • A new model was proposed to reasonably predict creep life. • The notch weakening effect was found in the brazed joint. • The creep life increases with notch angle increases while it decreases with notch radius increases. • The brazed joint under multiaxial stress was mainly fractured by intergranular brittle mode. Abstract The brazed components operating at high temperature inevitably suffer from multiaxial creep under multiaxial stress. In this study, the creep strain, fracture and failure mechanisms of HastelloyC276-BNi2 brazed joint under multiaxial stress were studied by experiment and finite element modeling (FEM). A new damage evolution model was put forward to estimate the creep fracture behavior of brazed joint under multiaxial stress. And the influences of notch angle and radius were also discussed. The notch weakening effect was exhibited in the brazed joints. The creep life is not only dependent on the creep fracture strain but also on notch type. The proposed creep damage evolution model can reasonably reflect the relationship between creep fracture ductility and multiaxiality. The predicted creep life by proposed model agrees very well with the experimental results. Most importantly, the fracture mechanisms of brazed joint under multiaxial stress were revealed and the creep damage evolution was well interpreted by FEM. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

The influence of fireside corrosion on creep rupture of a Ni-Fe based model alloy was investigated by comparing the rupture life, fractography and microstructure of specimens in fireside corrosion and air. Results showed that the rupture life at 140 MPa in fireside corrosion is only 40% of that in air. The internal sulfidation along grain boundaries promoted the initiation of intergranular cracks, and was responsible for the premature rupture. The surface collapse in fireside corrosion was attributed to the outward propagation of internal cracks, while surface cracks in air originated from the formation of recrystallized grains in the surface layer. • The creep rupture life of a new Ni-Fe based superalloy in different environment was investigated. • The internal cracks caused by internal sulfidation accounts for the surface collapse in fireside corrosion. • The premature rupture in fireside corrosion is attributed to the increase of internal creep cracks. • The surface intergranular cracks initiate in the recrystallization layer in air. [ABSTRACT FROM AUTHOR]

The nonlinear Maxwell-type constitutive relation with two arbitrary material functions is formulated for viscoelastoplastic materials and studied analytically in uni-axial case to reveal capabilities of the model and its applicability scope. Its coupling with a number of fracture criteria is analyzed in order to simulate creep rupture under constant and piecewise-constant loading and to compare creep life estimates arising as a result. The limit strain criterion, the critical dissipation criterion and two proposed new families of failure criteria taking into account a strain history (i.e. a whole creep curve) are considered. Long-term strength curves equations generated by each one of the four chosen failure criteria are derived. Their general qualitative properties are analyzed and compared to each other under minimal restrictions on material functions of the constitutive relation. It is proved that qualitative properties of all theoretic long-term strength curves coincide with basic properties of typical test long-term strength curves of viscoelastoplastic materials. For every failure criteria considered herein, rapture time under step-wise loading is evaluated for arbitrary material functions and compared to the lifetime yielding from the linear damage accumulation rule (i.e. “Miner’s rule”). General formulas for cumulative damage (“Miner’s sum”) deviations from unity are obtained for all failure criteria coupled with the nonlinear Maxwell-type constitutive relation. Their dependences on material functions and loading program parameters are examined. In particular, it is proved that the linear damage rule is exactly valid for the critical dissipation criterion whatever material functions, number of loading steps and stress levels are chosen. On the contrary, for the limit strain criterion, the linear damage rule is never valid for two-step loading and cumulative damage at rapture instant is greater or less than unity depending on the sign of stress jump.

The present work deals with the development of material constitutive models for creep-deformation and creep-rupture of SiC/SiC ceramic-matrix composites (CMCs) under general three-dimensional stress states. The models derived are aimed for use in finite element analyses of the performance, durability and reliability of CMC turbine blades used in gas-turbine engines. Towards that end, one set of available experimental data pertaining to the effect of stress magnitude and temperature on the time-dependent creep deformation and rupture, available in the open literature, is used to derive and parameterize material constitutive models for creep-deformation and creep-rupture. The two models derived are validated by using additional experimental data, also available in the open literature. To enable the use of the newly-developed CMC creep-deformation and creep-rupture models within a structural finite-element framework, the models are implemented in a user-material subroutine which can be readily linked with a finite-element program/solver. In this way, the performance and reliability of CMC components used in high-temperature high-stress applications, such as those encountered in gas-turbine engines can be investigated computationally. Results of a preliminary finite-element analysis concerning the creep-deformation-induced contact between a gas-turbine engine blade and the shroud are presented and briefly discussed in the last portion of the paper. In this analysis, it is assumed that: (a) the blade is made of the SiC/SiC CMC; and (b) the creep-deformation behavior of the SiC/SiC CMC can be represented by the creep-deformation model developed in the present work.

The effects of thermal simulation on the short-term creep fracture of modified 91 (mod. 91) weld metal (WM) were evaluated at elevated temperature. The reheated zones in the WM during multiple passes were simulated by an infrared heater. The simulated WM specimens after post-weld tempering at 1023 K/2 h were loaded with dead weight either at 903 K/120 MPa or 933 K/80 MPa. In this work, the simulated WM specimens after tempering were loaded either at 903 and 933 K during the tests. The loss in creep lives of various specimens at elevated temperature was determined accordingly and further compared with the Gr. 91 steel base metals, which were normalized either at 1213 K or 1333 K and then tempered at 1033 K for 2 h. The coarse, solidified structure of the WM had much better creep resistance than the base metal even that of the base metal normalized at 1333 K. However, the imposed welding thermal cycles would cause a significant decrease in creep resistance of the WM. Creep lives were shortened obviously in the simulated WM samples, especially in the simulated sample that underwent partial transformation. The combination of a fine-grained structure and soft ferrite present in the simulated WM was responsible for their huge decline in creep resistance, as compared with the WM in the as-tempered condition.

External reactor vessel cooling (ERVC) for in-vessel retention (IVR) has been considered one of the most useful strategies to mitigate severe accidents. However, reliability of this common idea is weakened because many studies were focused on critical heat flux whereas there were diverse uncertainties in structural behaviors as well as thermal–hydraulic phenomena. In the present study, several key factors related to molten corium behaviors and thermal characteristics were examined under multi-layered corium formation conditions. Thereafter, systematic finite element analyses and subsequent damage evaluation with varying parameters were performed on a representative reactor pressure vessel (RPV) to figure out the possibility of high temperature induced failures. From the sensitivity analyses, it was proven that the reactor cavity should be flooded up to the top of the metal layer at least for successful accomplishment of the IVR-ERVC strategy. The thermal flux due to corium formation and the relocation time were also identified as crucial parameters. Moreover, three-layered corium formation conditions led to higher maximum von Mises stress values and consequently shorter creep rupture times as well as higher damage factors of the RPV than those obtained from two-layered conditions.

Background The creep-fatigue damage in low-cycle regime has been described by a strain-based creep-fatigue equation through integrating creep effect into fatigue damage. Need There is a need to develop a creep-fatigue equation which describes the stress-controlled creep-fatigue behaviour in high-cycle regime. Approach This stress-based creep-fatigue equation was developed through superposing a fatigue mechanism with a creep mechanism. This creep-fatigue equation was then validated on GP91 casting steel. The creep-fatigue data were transformed to the reference condition and collapsed into one power-law curve with good quality. Outcomes This result verified the formulas of the fatigue component and the creep component, and demonstrated the method of extracting the coefficients. The full-range characteristic of this creep-fatigue equation is discussed. In addition, the introduction of the compatibility presents a better description of the reference condition. Originality A new creep-fatigue equation is provided with demonstrably good ability to cover the full range of conditions from the pure-fatigue condition to the pure-creep condition for the high-cycle regime. The method of extracting the coefficients is also provided. [ABSTRACT FROM AUTHOR]

A micro-mechanism based void growth model involving the void shape change during the creep deformation is proposed, referring to the traditional Cocks–Ashby (C–A) model framework, for multiaxial power-law creep rupture prediction. Accordingly, an improved prediction for creep lifetime and multiaxial creep ductility are derived. Results indicate that the creep life and creep ductility are significantly affected by the void shape evolution in the low stress triaxiality range. By contrast, such an effect could be neglected under the situation of high stress triaxiality. In addition, a large fraction of creep ductility is consumed during the tertiary creep stage. A good agreement has been achieved between the proposed model and the collected experimental data. [ABSTRACT FROM AUTHOR]

Background Creep and fatigue damages in metals are known to interact and then lead to aggregated damage. While models exist for fatigue, creep and creep-fatigue, no models cover all 3 load regimes. Also, a heat treatment-related parameter is not well included in most creep-fatigue models. Need There is a need to develop a creep-fatigue equation, which covers the full loading regime from pure fatigue to pure creep, and creep-fatigue. Also needed is inclusion of a heat treatment-related parameter. Approach The unified creep-fatigue equation was started from the Coffin-Manson equation and integrated with the Manson-Haferd parameter. This equation was validated on Inconel 718. Outcomes The method of deriving the coefficients and the formula of the creep function are demonstrated, and the resulting equation shows a good ability to describe the grain-size effect and the fully integrated characteristics. Originality Original contributions of this work are the development of a new formulation to represent creep, fatigue and creep-fatigue in metals. Also the inclusion of grain size-which is a proxy for heat treatment-in the formulation of this equation and in a proposed modified Manson-Haferd parameter. [ABSTRACT FROM AUTHOR]

The present study examines the physical models for predicting long-term creep strengths at different application temperatures using creep parameters determined from short term creep tests for the modified 9Cr-1Mo steel, which is a candidate material for design and construction of Gen-IV nuclear power plants with a specified service life of minimum 60 years. It is demonstrated that the complex stress and temperature dependence of minimum creep rate of this steel grade can be rationalised in a simple manner on the basis of the new power law creep equation that incorporates the tensile strength σ TS . The activation energy of creep determined on this basis does not depend on stress and the stress exponent depends only on the stress ratio σ/σ TS region but not on temperature. This allows the new power law equation to be used in combination with Monkman-Grant relationship to predict long-term (60 years) creep strengths and lifetimes at different application temperatures using creep parameters determined from creep tests lasting less than 6000 h. The reliability of these predictions is discussed and the causes for possible over-predictions are considered. It is also shown that the previously observed stress dependence of activation energy of creep is simply the consequence of the conventional method by which this parameter is determined and hence has no physical significance. The change in stress exponent in respective stress ratio σ / σ TS regions is not due to the change in creep mechanism but may be attributed to microstructure changes under creep load in respective σ / σ TS ranges. [ABSTRACT FROM AUTHOR]

A two-dimensional (2D) finite element framework for creep damage simulation at the grain boundary level was developed and reported. The rationale for the paper was that creep damage, particularly creep rupture, for most high temperature alloys is due to the cavitation at the grain boundary level, hence there is a need for depicting such phenomenon. In this specific development of the creep damage simulation framework, the material is modeled by grain and GB (grain boundary), separately, where smeared-out grain boundary element is used. The mesh for grain and grain boundary is achieved by using Neper software. This paper includes (1) the computational framework, the existing subroutines, and method applied in this procedure; (2) the numerical and programming implementation of the GB; (3) the development and validation of the creep software; and (4) the application to simulate plane stress Copper−Antimony alloy. This paper contributes to the development of finite element simulation for creep damage/rupture at a more realistic grain boundary level and contributes to a new understanding of the intrinsic relationship of stress redistribution and creep fracture.