Search Results (36)

The creep properties of 15Kh2NMFAA nuclear WWER (water–water energetic reactor) vessel steel in the range of 500–1200 °C temperatures, which may appear during severe nuclear reactor accidents, were investigated. The present paper attempts to analyze the creep curves obtained from tensile testing at high temperatures using the Larson–Miller parametric technique. The power law rate and material coefficient of Norton’s equation with the Monkman–Grant relationship coefficient were found for each test temperature. It is shown that in accordance with the Monkman–Grant relationship coefficient values, changing the creep type from dislocation glide to high temperature dislocation climb occurs in the temperature range of 600–700 °C, which leads to a slope change in the Larson–Miller parameter plot and the conversion of steel creep behavior. It is also shown that in the range of $A_1$–$A_3$ temperatures, a stepwise change in creep characteristics occurs, which is associated with phase transformations. In addition, the constancy of the product of the time to rupture ${\tau }_r$ and the minimum creep rate ${\dot{ϵ}}_{min}$ in the ranges of 600–700 °C and $A_3$—1200 °C was noted. The proposed approach improves the accuracy of time to rupture estimation of 15Kh2NMFAA steel by at least one order of magnitude. Based on the research results, the calculated dependence of the steel’s long-term strength limit on temperature was obtained for several time bases, allowing us to increase the accuracy of material survivability prediction in the case of a severe accident at a nuclear reactor. Full article

This paper summarizes the results of investigations into heterogeneous P23/P91 welds after long-term creep exposure at temperatures of 500, 550 and 600 °C. Two variants of welds were studied: In Weld A, the filler material corresponded to P91 steel, while in Weld B, the chemical composition of the consumable material matched P23 steel. The creep rupture strength values of Weld A exceeded those of Weld B at all testing temperatures. Most failures in the cross-weld samples occurred in the partially decarburized zones of P23 or WM23 steel. The results of the investigations on the minor phases were in good agreement with kinetic simulations that considered a 0.1 mm fusion zone. Microstructural studies proved that carburization occurred in the P23/P91 weld fusion zones. The partial decarburization of P23 steel or WM23 was accompanied by the dissolution of M₇C₃ and M₂₃C₆ particles, and detailed studies revealed the precipitation of the Fe₂ (W, Mo) Laves phase in decarburized areas. Thermodynamic simulations proved that the appearance of this phase in partially decarburized P23 steel or WM23 is related to a reduction in the carbon content in these areas. According to the results of creep tests, the EBSD investigations revealed a better microstructural stability of the partially decarburized P23 steel in Weld A. Full article

Corrosion in reinforced concrete (RC) structures has led to the increased adoption of non-corrosive materials, such as carbon fiber-reinforced polymers (CFRPs), as replacements for traditional steel rebar. However, ensuring the long-term reliability of CFRP grids under sustained stress is critical for achieving safe and effective designs. This study investigates the long-term tensile creep rupture behavior of CFRP grids to establish a design threshold for their tensile strength under sustained loading conditions in demanding structural applications. A comprehensive laboratory experiment was conducted over 10,000 h, during which CFRP grid specimens were subjected to constant stress levels ranging from 92% to 98% of their ultimate tensile strength. The results confirm the excellent creep rupture resistance of CFRP grids. Specimens subjected to a sustained stress ratio of 92% of their ultimate tensile strength remained intact throughout the testing period, with minimal creep strain ranging from approximately 1% to 4% of the initial strain. The mean extrapolated creep rupture factors were found to be 92.1% and 91.7% of their ultimate tensile strength for service lives of 50 and 114 years, respectively. Based on the results of this study, a tensile stress limit of 48% of the ultimate tensile strength is recommended for CFRP grids to ensure long-term creep rupture resistance over a 100-year service life. Full article

In order to have a better understanding of the shear creep characteristics of muddy interlayer in unstable landslides, we took the more “inferior” clay muddy interlayer as the research object, and shear creep experiments under different normal stress levels were carried out by means of hierarchical loading. This paper focuses on the variation law of creep curve and its long-term strength in the clay-type muddy interlayer under different normal stresses. The results showed that the creep characteristics of clay-type muddy interlayer were obvious: at the same normal stress level, instantaneous deformation, initial creep and stable creep appeared at lower shear stress level; at the level of rupture shear stress, there were two cases: the creep curve included three stages of typical initial creep, stable creep and accelerated creep failure, or directly entered the accelerated creep stage until the specimen’s failure. The average shear and stable creep rate of the muddy interlayer specimen increased exponentially with the increase in shear stress. The empirical formula u = u₀ + A [1 − e^(−Bt)] + Ctⁿ of shear strength could better reflect the creep deformation law of muddy interlayer, and the correlation coefficient R² varies from 0.90 to 0.99. Based on the definition of long-term ultimate strength, the long-term strength of clay-type muddy interlayer was determined. Full article

Inconel 740H superalloy is commonly used in advanced ultra-supercritical power plants since it possesses excellent strength and creep resistance. This study investigates the microstructure and mechanical properties of Inconel 740H superalloy fabricated using wire-arc additive manufacturing. The as-printed microstructure consisted of columnar γ grains with the Laves phase and (Nb, Ti)C carbides as secondary phases. The anisotropy in grain structure increased from the bottom to the top regions, while the hardness was highest in the middle portion of the build. To guide the post-heat treatment design, thermodynamic and kinetic simulations were employed to predict the temperature and time. Complete recrystallization with the Laves phase dissolution occurred throughout the build after homogenization at 1200 °C for 2 h. The peak hardness was achieved after aging at 760 °C for 12 h with the M₂₃C₆ carbides decorating the grain boundaries and γ’ precipitates in the grain interior. The yield strength (655 MPa) and ductility (29.5%) in the post-heat treated condition exceeded the design targets (620 MPa, 20%). Stress rupture tests at 750 °C showed that the high-temperature performance was at par with the wrought counterparts. The fracture mode after rupture was identified to be intergranular with the presence of grain boundary cavities along with grain boundary sliding. Full article

The integration of mesoscale modeling and macroscale experimentation has emerged as a promising approach for understanding and predicting the mechanical behavior and fatigue performance of fiber-reinforced polymer composites. In this work, the mean field homogenization technique is implemented to predict the fatigue performance of the carbon-fiber-reinforced polymer composites under cyclic loading conditions. To predict the number of fatigue cycles, Modified Gerber criteria are used with the stress-based Tsai–Hill failure indicator. Fatigue strength factor (α) and creep rupture strength factor (β) are experimentally evaluated and further implemented in a computational approach to predict fatigue life cycles of the composite. The effect of composite constituents, stress ratio, and loading direction are investigated in detail against the fatigue performance of the composite. Fatigue cycles are predicted at individual matrix and fiber levels at various stress ratios of 0.2, 0.4, 0.6, and 0.8 at different loading inclinations. The experimental results are compared with the mesoscale S–N curves. Full article

In our previous works, the effects of forging and heat treatment variables on microstructure evolution and mechanical properties have been studied for an ingot-metallurgy Re-bearing nickel base superalloy. To overcome the issues associated with the production of large-scale ingots and fine-grained workpieces, in the present work, the effect of hot forging and heat treatment variables was studied in a Re-bearing nickel base superalloy prepared via powder metallurgy. The purpose of the study was to reach the properly balanced mechanical properties for the potential use of the superalloy as a disc material. The initial as-HIPed workpieces were subjected to different hot forging and post-forging heat treatment or only to heat treatment (no forging). For the processed workpieces, the recrystallization behavior, size, morphology and volume fraction of γ′ precipitates were evaluated by scanning electron microscopy followed by a study of mechanical properties. The most properly balanced mechanical properties (strength, ductility, creep resistance and creep rupture lifetime) were reached for the γ grain size of d_γ ≈ 13.6 µm. A finer and coarser γ grain size (down to d_γ ≈ 2.6 and up to d_γ ≈ 37.5 µm) even when superimposed with a higher volume fraction of dispersed secondary γ′ precipitates (in the case of d_γ = 27–37.5 µm) was associated with worse mechanical properties. Full article

High-performance Ferritic (HiperFer) steels are a novel class of heat-resistant, fully ferritic, Laves phase precipitation hardened materials. In comparison to conventional creep strength-enhanced 9–12 wt.% Cr ferritic–martensitic steels, HiperFer features increased mechanical strength, based on a thermodynamically stable distribution of small (Fe,Cr,Si)₂(Nb,W) Laves phase precipitates, and—owing to its increased chromium content of 17 wt.%—improved resistance to steam oxidation, resulting in superior temperature capability up to 650 °C. Previous publications focused on alloying, thermomechanical processing, and basic mechanical property evaluation. The current paper concentrates on the effect of heat treatment on microstructural features, especially Laves phase population, and the resulting creep performance. At 650 °C and a creep stress of 100 MPa, an increase in rupture time of about 100% was achieved in comparison to the solely thermomechanically processed state. Full article

This article compares the creep testing behavior of AISI 4340 high-strength steel in the as-received and coated conditions. The coating material used was a NiCrBSi self-fluxing alloy. The microstructural characterization was carried out using optical and scanning electron microscopy. The creep tests were conducted at a temperature of 550 °C and with loads of 200, 250, and 300 MPa. The microstructure analysis of the deposited layer revealed some inclusions, very low porosity, and good adhesion to the substrate. The results of the creep tests indicated a decrease in the time to rupture under loads of 250 and 300 MPa for the coated steel. At a load of 200 MPa, the coated steel presented longer times to rupture and higher yield strength, demonstrating an improvement over the uncoated steel under these test condition. The fracture surface inspection showed a failure by a ductile fracture in both samples, with and without coating. Full article

Fiber-reinforced polymer (FRP) composites have gained increasing recognition and application in the field of civil engineering in recent decades due to their notable mechanical properties and chemical resistance. However, FRP composites may also be affected by harsh environmental conditions (e.g., water, alkaline solutions, saline solutions, elevated temperature) and exhibit mechanical phenomena (e.g., creep rupture, fatigue, shrinkage) that could affect the performance of the FRP reinforced/strengthened concrete (FRP-RSC) elements. This paper presents the current state-of-the-art on the key environmental and mechanical conditions affecting the durability and mechanical properties of the main FRP composites used in reinforced concrete (RC) structures (i.e., Glass/vinyl-ester FRP bars and Carbon/epoxy FRP fabrics for internal and external application, respectively). The most likely sources and their effects on the physical/mechanical properties of FRP composites are highlighted herein. In general, no more than 20% tensile strength was reported in the literature for the different exposures without combined effects. Additionally, some provisions for the serviceability design of FRP-RSC elements (e.g., environmental factors, creep reduction factor) are examined and commented upon to understand the implications of the durability and mechanical properties. Furthermore, the differences in serviceability criteria for FRP and steel RC elements are highlighted. Through familiarity with their behavior and effects on enhancing the long-term performance of RSC elements, it is expected that the results of this study will help in the proper use of FRP materials for concrete structures. Full article

The present study focused on the development of the novel heat resistant cast Al-Cu-Yb(Gd)-Mg-Mn-Zr alloys based on the prevue investigations. Microstructures and mechanical properties were investigated by optical, scanning and transmission electron microscopy, hardness measurements, and tensile and creep tests at room and elevated temperatures. Ytterbium in combination with Zr and Ti provide greater Al grain refining than gadolinium. The L1₂-Al₃(Zr,Yb) or L1₂-Al₃(Zr,Gd) and Al₂₀Cu₂Mn₃ phase precipitates were nucleated during solution treatment. The average sizes of L1₂-Al₃(Zr,Yb) and L1₂-Al₃(Zr,Gd) are 28 ± 6 nm and 32 ± 4 nm, respectively. Al₂₀Cu₂Mn₃ phase precipitates formed with a more coarse size of 100–200 nm. The highest hardening effect was achieved after 3 h of aging at 210 °C in both alloys due to S’(Al₂CuMg) precipitates. The ultimate tensile strengths (UTS) of the AlCuYbMg and AlCuGdMg alloys at room temperature are 338 and 299 MPa, respectively. The UTS decreases to 220–272 MPa when increasing the temperature of the tensile test to 200–250 °C. The rupture stress at 250 °C for 100 h under stress is 111–113 MPa. The contribution from different structure parts in the yield strength was calculated. The main strengthening effects of 54–60 MPa and 138–153 MPa were achieved from L1₂ and S’ precipitates, respectively. The calculated values of yield strength (YS) are consistent with the experimental data. Novel AlCuYbMg and AlCuGdMg alloys are a potential option for castings for high temperature application. Full article

Alloys meeting the requirements of “700 °C and above” advanced ultra-super-critical technology, with higher thermal efficiency, have been developed in recent years. Here, a new wrought Ni-based superalloy with excellent high-temperature creep strength based on Waspaloy has been developed and is proposed as a candidate material for application in 700 °C class advanced ultra-super-critical steam turbine blades. In this new alloy, the Molybdenum (Mo) in Waspaloy is partially replaced by Tungsten (W). Creep tests have shown that this new Ni-based alloy has a 70 MPa higher creep-rupture strength than that of Waspaloy at 700 °C by extrapolating the experimental data. Detailed creep-rupture mechanisms have been analyzed by means of scanning electron microscopy, transmission electron microscopy, and chemical phase analysis with a view to devising potential approaches for performance improvements. The results showed that the partial replacement of Mo by W had negligible effect on the composition of carbides precipitated in the alloy. Instead, the amount of the γ′ phase was significantly increased, and mismatch between the γ and γ′ phases was reduced. In this way, the stability of the γ′ phase was increased, its coarsening rate was reduced, and its critical shear stress was increased. As a result, the high-temperature creep-fracture strength of the new alloy was increased. Full article

A new, wrought Ni-Fe-based alloy with excellent creep rupture life has been developed for 700 °C-class advanced ultra-supercritical (A-USC) steam turbine rotor application. In this study, its tensile deformation behaviors and related microstructure evolution were investigated. Tensile tests were carried out at room temperature, 700 °C, and 750 °C. The results show that the Ni-Fe-based alloy has excellent yield strength at 700 °C, which is higher than that of some other Ni-based/Ni-Fe-based alloys. The fracture surface characteristics indicate trans-granular and intergranular fracture modes at room temperature, 700 °C, and 750 °C. However, the intergranular fraction mode became dominant above 700 °C. Dynamic recrystallization occurred at 700 °C and 750 °C with increasing average misorientation angles. The volume fraction of the γ′ precipitate was around 20%, and the average size of the γ′ precipitates was around 30 μm, which had no noticeable change after the tensile tests. The predominant deformation mechanisms were planar slip at room temperature, bypassing of the γ′ precipitates by the Orowan mechanism, and dislocation shearing at 700 °C and 750 °C. The tensile properties, fracture characteristics, and deformation mechanisms have been well-correlated. The results are helpful in providing experimental evidence for the development and optimization of high-temperature alloys for 700 °C-class A-USC applications. Full article

In the present study, the first tailored steel based on HiperFer (high-performance ferrite) was developed specifically for the additive manufacturing process. This steel demonstrates its full performance potential when produced via additive manufacturing, e.g., through a high cooling rate, an in-build heat treatment, a tailored microstructure and counteracts potential process-induced defects (e.g. pores and cavities) via “active” crack-inhibiting mechanisms, such as thermomechanically induced precipitation of intermetallic (Fe,Cr,Si)₂(W,Nb) Laves phase particles. Two governing mechanisms can be used to accomplish this: (I) “in-build heat treatment” by utilizing the “temper bead effect” during additive manufacturing and (II) “dynamic strengthening” under cyclic, plastic deformation at high temperature. To achieve this, the first HiperFer^AM (additive manufacturing) model alloy with high precipitation kinetics was developed. Initial mechanical tests indicated great potential in terms of the tensile strength, elongation at rupture and minimum creep rate. During the thermomechanical loading, global sub-grain formation occurred in the HiperFer^AM, which refined the grain structure and allowed for higher plastic deformation, and consequently, increased the elongation at rupture. The additive manufacturing process also enabled the reduction of grain size to a region, which has not been accessible by conventional processing routes (casting, rolling, heat treatment) so far. Full article

The excavation of tunnels in brittle rocks with high in-situ strengths under large deviatoric stresses has been shown to exhibit brittle failure at the periphery of tunnels parallel to the maximum in-situ stress. This failure can either occur instantaneously or after several hours due to the strength degradation that is implicitly and indirectly considered in typical brittle constitutive models. While these models are powerful tools for engineering analyses, they cannot predict the time at which brittle rupture occurs, but rather, they show a possible failure pattern occurring instantaneously. In this paper, a model referred to as the long-term strength (LTS) model is introduced and implemented into FLAC2D. The model is built as a modified version of the CVISC model, introduced by Itasca, by adding a strength decay function. This function is developed from lab-scale time-to-failure (TTF) data. The LTS model is verified against its corresponding analytical solution using a constant stress creep lab test and implemented into a tunnel-scale model using the geometry, stress, and geologic conditions from the Atomic Energy of Canada Limited Underground Research Laboratory (AECL URL). The results of the LTS tunnel model are then compared to an identical model using the Cohesion Weakening Friction Strengthening (CWFS) approach. Full article