Search Results (125)

The impact mechanism of long-term creep in gas-containing coal on coal and gas outbursts has not been fully elucidated and remains insufficiently understood for the purpose of disaster engineering control. This investigation conducted triaxial creep experiments on raw coal specimens under controlled confining pressures, axial stresses, and gas pressures. Through systematic analysis of coal’s physical responses across different loading conditions, we developed and validated a novel creep damage constitutive model for gas-saturated coal through laboratory data calibration. The key findings reveal three characteristic creep regimes: (1) a decelerating phase dominates under low stress conditions, (2) progressive transitions to combined decelerating–steady-state creep with increasing stress, and (3) triphasic decelerating–steady–accelerating behavior at critical stress levels. Comparative analysis shows that gas-free specimens exhibit lower cumulative strain than the 0.5 MPa gas-saturated counterparts, with gas presence accelerating creep progression and reducing the time to failure. Measured creep rates demonstrate stress-dependent behavior: primary creep progresses at 0.002–0.011%/min, decaying exponentially to secondary creep rates below 0.001%/min. Steady-state creep rates follow a power law relationship when subject to deviatoric stress (R² = 0.96). Through the integration of Burgers viscoelastic model with the effective stress principle for porous media, we propose an enhanced constitutive model, incorporating gas adsorption-induced dilatational stresses. This advancement provides a theoretical foundation for predicting time-dependent deformation in deep coal reservoirs and informs monitoring strategies concerning gas-bearing strata stability. This study contributes to the theoretical understanding and engineering monitoring of creep behavior in deep coal rocks. Full article

FeCrAl exhibits excellent resistance to high temperatures, corrosion, and irradiation, making it a prime candidate material for accident-tolerant fuel (ATF) cladding. This study investigates the high-temperature creep behavior of FeCrAl alloys with grain sizes of 12.0 μm and 9.9 μm under temperatures ranging from 450 °C to 650 °C and applied stresses between 75 and 200 MPa. The texture, grain morphology, grain orientation, and dislocation density of FeCrAl were characterized by electron backscatter diffraction (EBSD). The results indicate that temperature, applied stress, and grain size are the primary factors governing high-temperature creep behavior. The material texture showed no significant difference before and after creep. Large grains tend to engulf smaller ones during the creep process at lower temperatures and stresses, reducing the proportion of low-angle grain boundaries (LAGBs). In contrast, at higher temperatures or under higher stress, dislocations proliferate within grains, leading to a significant increase in the number of LAGBs. As the applied stress increases, the dominant creep mechanism tends to convert from grain boundary sliding to dislocation motion. Moreover, higher temperatures or smaller grain sizes lower the critical stress required to activate dislocation motion and significantly increase dislocation density, severely degrading the creep resistance. Full article

The rheological behavior of firefighting foam is the basis for analyzing foam flow and foam spreading. This experimental study investigates the complex rheological behavior of rapidly aging firefighting foams, specifically focusing on alcohol-resistant aqueous film-forming foam. The primary objective is to characterize the time-dependent viscoelasticity, yielding, and viscous flow of firefighting foam under controlled shear conditions, addressing the significant challenge posed by its rapid structural evolution (drainage and coarsening) during measurement. Using a cylindrical Couette rheometer, conductivity measurements for the liquid fraction, and microscopy for the bubble size analysis, the study quantifies how foam aging impacts key rheological parameters. The results show that the creep and relaxation response of the firefighting foam in the linear viscoelastic region conforms to the Burgers model. The firefighting foam shows ductile yielding and significant shear thinning, and its flow curve under slow shear can be well represented by the Herschel–Bulkley model. Foam drainage and coarsening have competitive effects on the rheology of the firefighting foam, which results in monotonic and nonmonotonic variations in the rheological response in the linear and nonlinear viscoelastic regions, respectively. The work reveals that established empirical relationships between rheology, liquid fraction, and bubble size for general aqueous foams are inadequate for firefighting foams, highlighting the need for foam-specific constitutive models. Full article

This study investigates the elevated-temperature mechanical and viscoelastic properties of a PDMS–silica-based superhydrophobic nanocomposite coating using nanoindentation and a nano-dynamic mechanical analysis over a temperature range of 24 °C to 160 °C. The nanoindentation load–displacement curves exhibited consistent hysteresis, indicating a stable energy dissipation across the temperature range. Creep tests revealed an increased displacement and accelerated deformation at elevated temperatures, displaying a two-stage creep profile characterized by rapid primary and steady-state secondary creep. The hardness decreased with the creep time, while the strain rate sensitivity remained relatively stable, suggesting consistent deformation mechanisms. A time-dependent creep model incorporating linear and logarithmic terms accurately captured the experimental data. The nano-dynamic mechanical analysis results showed a decrease in the storage modulus with depth, while the loss modulus and tan δ peaked at shallow depths. These findings are crucial for the evaluation and design of superhydrophobic nanocomposite coatings. Full article

This paper aims to develop a rheological model with fewer parameters that accurately describes the primary and secondary creep behavior of wood materials. The models studied are grounded in Riemann–Liouville fractional calculus theory. A comparison was conducted between the constant-order fractional Zener model and the variable-order fractional Maxwell model, with four parameters each. Using experimental creep data from four-point bending tests on two tropical wood species, along with an optimization algorithm, the variable-order fractional model demonstrated greater effectiveness. The selected fractional derivative order, modeled as a linearly increasing function of time, helped to elucidate the internal mechanisms in the wood structure during creep tests. Analyzing the parameters of this order function enabled an interpretation of their physical meanings, showing a direct link to the material’s mechanical properties. The Sobol indices have demonstrated that the slope of this function is the most influential factor in determining the model’s behavior. Furthermore, to enhance descriptive performance, this model was adjusted by incorporating stress non-linearity to account for the effects of the variation in constant loading level in wood. Consequently, this new formulation of rheological models, based on variable-order fractional derivatives, not only allows for a satisfactory simulation of the primary and secondary creep of wood but also provides deeper insights into the mechanisms driving the viscoelastic behavior of this material. Full article

Directionally solidified (DS) superalloys have become a primary material choice for turbine blade applications. Due to the complex shape of the blades, certain regions inevitably experience stress axes oriented orthogonally to the crystal growth direction. Therefore, this study explores the creep characteristics of a DS superalloy in different orientations (transverse (T) versus longitudinal (L) with respect to grain growth direction) under intermediate and high temperatures (980 °C and 1070 °C), while simultaneously analyzing their respective deformation mechanisms and microstructural transformation behaviors. Experimental findings reveal pronounced orientation-dependent variations in creep performance, deformation modes, and microstructural development. Notably, the T specimen exhibits higher creep resistance at 980 °C, which can provide a basis for the design of some components that require high creep resistance and maintain small deformation. At 980 °C, L specimens primarily undergo γ′ phase shearing via antiphase boundaries (APBs) pairs, whereas T specimen exhibits APB pairs and superlattice intrinsic stacking faults (SISFs) shearing mechanisms. At 1070 °C, the L specimen exhibits dislocation shearing of γ′ alongside dislocation bypassing of tertiary γ′, while the T specimen demonstrates dislocation climbing within the γ channels. Additionally, the L specimen exhibits significant N-type rafting, while the T specimen shows significant Ostwald ripening characteristics, with an Ostwald ripening rate constant of 1.04 × 10⁻²⁰ m³/h. Full article

Background and Objectives: Vertebral metastases often lead to pathological fractures and spinal cord compression, thus impacting patient quality of life. This study aimed to quantify the poroelastic mechanical properties of vertebral metastatic tissue explanted during spine surgery and correlate it with clinical data. Methods: Nine patients (61.7 ± 13.1 years) were prospectively recruited from April 2021 to February 2022, where 78% had a vertebral fracture. Demographic and metastases data, including primary origin, spinal level, lesion volume, and SINS score, were collected, and tissue specimens were explanted during surgery. Using a newly developed portable experimental setup, confined compression creep tests were conducted to measure the aggregate modulus and permeability of each sample. Dealing with limited samples, the results were expressed as the median (min; max). Results: Specimens from the unfractured vertebrae had higher aggregate modulus (200.35 (149.80; 250.90) kPa vs. 14.47 (8.27; 35.89) kPa) and higher permeability (0.02 (0.01; 0.03) mm⁴/N·s vs. 0.41 (0.10; 4.68) mm⁴/N·s) compared with the specimens from the fractured vertebrae. Histology revealed prominent levels of neoplastic cell infiltration and disruption of typical bone matrix architecture. Specimens with low neoplastic cellularity had comparable or slightly higher poroelastic properties compared to high cellularity. No clear trend was observed between the mechanical properties and SINS score, nor between the mechanical properties, percentage lesion volume, and fractures. Conclusions: Despite the limited sample size, the poroelastic characterization revealed relevant insights to investigate in future research. A transdisciplinary assessment of vertebral metastases, incorporating poroelastic testing, deserves further attention and could enhance the treatment options. Full article

Fractional differential viscoelastic calculus was used to develop a model for predicting the primary to tertiary creep in the tensile creep deformation of various polypropylenes (PPs). The primary and secondary creep were described via simple fractional differential viscoelasticity with an empirical formula for the stress and temperature dependence of the fractional differential order. Tertiary creep was treated as a pure viscous body with damage. The temperature dependence is treated simply, and Arrhenius’s law is applied. As for stress dependence, the Eyring law of the sinh function was applied to the primary and secondary creep processes, while the WLF-type shift function was adopted for tertiary creep. The primary and secondary creep behaviors of each model material showed creep growth rates according to the rigidity of each material. As for the tertiary creep, the homo PP showed a little damage progression with a damage index of 0.17, while the impact-resistant PP showed faster damage progression with a damage index of around 0.5. The three types of post-consumer recycled PPs showed intermediate properties between these virgin PPs, and no peculiarities were confirmed in the static creep behaviors. It was confirmed that the creep experimental results for all model materials fell on the same Monkman–Grant law. The presented creep model can predict the creep strain transition and minimum strain rate well and is effective in predicting the creep characteristics of PPs. Full article

Pressure hulls are the primary pressure-bearing structures in submersibles and deep-sea space stations, which are essential for marine scientific research. Due to repeated dive cycles and extended operational periods, these hulls undergo creep deformation over time, posing risks to their structural integrity. This paper provides a comprehensive review of research on the creep behavior of pressure hulls, focusing on three key aspects: creep testing, creep constitutive models, and numerical simulation techniques. Initially, various creep testing methodologies are presented, with the experimental data serving as a foundational basis for subsequent analyses. Experimental data from creep tests form the foundation for constructing and validating constitutive models, which are critical for predicting long-term deformation. The review also explores advanced numerical simulation techniques, such as user subroutines and multiscale modeling, to analyze creep in complex pressure hull structures. Finally, based on the insights from the reviewed studies, the paper proposed potential directions for future research to address current challenges and enhance the design and maintenance of pressure hulls. Full article

Primary and low-strain creep represents a very important integrity challenge to large, complex structures, like fusion reactors. Here, we develop a predictive empirical primary creep model for 9Cr tempered martensitic steels (TMS), relating the applied stress (σ) to strain (ε), time (t) and temperature (T). The most accurate model is based on the applied σ normalized by the steel’s T-dependent ultimate tensile stress (σ_o), σ/σ_o(T). The model, fit to 17 heats of 9Cr TMS, yielded a σ root mean square error (RMSE) of ≈±11 MPa. Notably, the model also provides robust predictions for all the other TMS, when calibrated only by the fusion candidate Eurofer97 database. The model was extended to explore two possible effects of neutron irradiation, which produces both displacements per atom (dpa) and helium (He in atomic parts per million, appm) damage. These effects, which have not been previously considered, include: (a) softening, as a function of dpa, at T > ≈400–450 °C, in low-He fission environments (<1 He/dpa); and (b) subsequent re-hardening in high-He (≥10 He/dpa) fusion first-wall environments. The irradiation effect models predict (a) accelerated primary creep due to irradiation softening; and (b) fully arrested creep due to high-He re-hardening. Full article

For many years, granular fill has been the preferred fill material in reinforced fill structures (RFSs) due to its high strength and drainage properties. However, the global scarcity of granular fill has necessitated the exploration of alternative fill materials. This study aims to evaluate the performance of three different alternative fill materials: (i) weak onsite fill (fill 1), (ii) lime-stabilized onsite fill (fill 2), and (iii) recycled construction and demolition (C & D) waste (fill 3). A finite element analysis (FEA) was conducted to assess the stability and horizontal displacement of an RFS and the long-term creep deformation of geogrid using viscoelastic (time-dependent) model in Plaxis. This RFS comprised a combination of wire mesh and geogrids, serving as primary and secondary reinforcement materials, respectively. The results indicate that fill 1, with low shear strength and stiffness, induces excessive lateral displacement and was unstable, making it unsuitable for RFS applications. In contrast, Fill 2 and Fill 3 achieve Eurocode-based safety factors of 1.12 and 1.19, respectively, while significantly reducing horizontal displacement. The long-term creep deformation analysis of geogrid in the case of fill 1 exceeds the prescribed serviceability strain limit threshold, while in the cases of fill 2 and fill 3, it conforms to the serviceability strain limit, which indicates effective mobilization of tensile resistance without excessive elongation. Finally, an analysis was conducted to optimize the geogrid length and to see its impact on cost and performance. The results revealed up to a 29% cost reduction while ensuring performance criteria. These findings validate lime-stabilized onsite fill and recycled C&D waste as viable, cost-effective alternatives to conventional granular backfill, ensuring not only stability and serviceability but also the long-term performance of geogrids in RFSs. Full article

Low-temperature cracking is a primary failure mode of asphalt pavement. The poker chip test provides a straightforward and efficient approach to simulating the film state of asphalt binders in asphalt structures. By measuring the tensile strength and ultimate tensile strain of the binder film, this test can effectively evaluate the cracking resistance and ductility of asphalt binders. Accordingly, this study employed the poker chip test to analyze the evolutions of low-temperature cracking resistance under various aging levels. To ensure the reliability of tensile strength and ultimate tensile strain, a Pearson correlation analysis was conducted between the two indicators and the traditional low-temperature performance evaluation indicators: stiffness modulus, creep rate, and the Glover-Rowe (G-R) parameter. The results indicate that the tensile strength and ultimate tensile strain of styrene–butadiene–styrene (SBS)-modified asphalt are higher than those of 70# base asphalt at the same aging level. With increasing aging time, the tensile strength of both SBS-modified asphalt and 70# base asphalt increases, while the ultimate tensile strain decreases. Additionally, the tensile strength and ultimate tensile strain are sensitive to changes in asphalt binder types and aging levels. They have a good linear correlation with stiffness modulus and creep rate, with correlation coefficients exceeding 0.9. Due to the distinct characteristics represented, the correlation between the two indicators and the G-R parameter is relatively weaker, with correlation coefficients exceeding 0.7. The findings of this study demonstrate that tensile strength and ultimate tensile strain are effective indicators for assessing the low-temperature performance of asphalt binders. They can serve as substitute indicators of stiffness modulus and creep rate, respectively. Full article

In the present work, the effect of different casting processes on the microstructure and creep properties of the second-generation single-crystal superalloy DD419 was investigated. Under conventional production conditions and a contour-suited thermal insulation method, single-crystal rods of types A and B were fabricated, respectively. In comparison to rod type A, the solidification process of rod type B featured a 1.6-fold increase in the temperature gradient and a 32% reduction in primary dendrite spacing. The γ/γ′ eutectic in the as-cast microstructure, the residual eutectic phase, and porosity after heat treatment were also significantly reduced, resulting in the improved homogeneity of the single crystal castings. Under the testing conditions of 850 °C/650 MPa and 1050 °C/190 MPa, the stress rupture life of sample B was enhanced by 25% and 5.2%, respectively, compared to sample A. Therefore, due to dendrite structure refinement, the stress rupture life of the superalloy was evidently improved, especially at medium temperatures. Full article

A novel nickel-based powder metallurgy superalloy was processed using two different thermal–mechanical processes, including hot isostatic pressed (As-HIP) and hipped + hot extruded + isothermally-forged (IF) heat treatments following two processed alloys, designated as As-HIP-HT and IF-HT. The objective of this study is to investigate the microstructure and mechanical property evolution in a nickel-based powder disk alloy fabricated by two processes. The findings revealed that both As-HIP and IF alloys underwent substantial recrystallization, with grains in the IF alloy being finer. Notable Prior Particle Boundaries (PPBs) were identified in the As-HIP samples. The IF-HT alloy exhibited a larger grain size due to a greater amount of stored energy. Significant differences in the secondary γ′ precipitates were observed between the two processes. More uniform substructures in the IF-HT alloy led to a higher density of finer γ′ precipitates. At temperatures of 704 °C and 760 °C, the As-HIP-HT alloy displayed a higher yield strength, but its plasticity significantly declined as temperature increased, while the IF-HT alloy showed a relatively stable plasticity. The presence of PPBs in the As-HIP-HT alloy minimally affected the alloy’s strength but reduced its plasticity. The creep property of the two processes was compared at 800 °C/330 MPa; the IF-HT alloy demonstrated lower creep rates and a longer creep life, which was attributed to its finer γ′ precipitates. Dominant creep deformation mechanisms in the As-HIP-HT alloy included Orowan dislocation loops and deformation twinning, while the primary mechanisms in the IF-HT alloy involved dislocation cutting through γ′ precipitates, dislocation slip, and micro-twins. These findings support the use of isostatic pressing + hot extrusion+ isothermally-forging process for critical high-temperature components. Full article

This study evaluates the perfection of the crystal structure of single-crystal turbine blade castings made from the CMSX-4 nickel superalloy. The analysis included primary and secondary crystal orientation measurements using the Ω-scan method and the novel OD-EFG X-ray diffractometer. The selected microstructural parameters of the single crystals were also analyzed, including the assessment of stereological parameters and the degree of porosity. A creep test was performed according to standard procedures and under conditions simulating real operational environments. The model single-crystal turbine blades were manufactured using the Bridgman–Stockbarger method, with variable withdrawal rates of 1 and 3 mm/min. Heat treatment of the single-crystal castings involved solution treatment followed by double aging. The evaluation of structural perfection was carried out in three states: as-cast, after solution heat treatment, and after double aging. The crystallographic orientation of the blades was determined on both the airfoil and the root part. The study determined how crystallographic orientation and microstructural parameters influence the creep resistance of the castings. It was found that in the as-cast condition, the greatest influence on high creep strength has a small deviation of the primary and constant value of secondary crystal orientation along the height of the blade casting. After heat treatment, the highest creep resistance was obtained for the blade manufactured at a withdrawal rate at 1 mm/min. Full article