Creep Properties Research Articles

Creep behavior and viscoelastic-plastic models for polymer-blend HDPE geocell sheets based on the stepped isothermal method

Polymer-blend High Density Polyethylene (PHDPE) sheet materials have been developed in China as an alternative to the High Density Polyethylene (HDPE) materials for the manufacture of geocells. It is necessary to evaluate the creep properties of the geocell-reinforced structures to provide a reference for design of their service life and improve their application potential. In this paper, the Stepwise Isothermal Method (SIM) based accelerated creep testing, in which the test temperature was gradually increased but the load was kept constant, was employed obtain the long-term creep behavior of PHDPE and HDPE geocell sheets. Conventional creep tests were also carried out to assess the accuracy of the creep prediction. Furthermore, the Modified Burgers (MB) model and the Mon-linear Three-component (NLTC) model were proposed to predict long-term creep deformations of the PHDPE geocell sheets. The experimental results indicated that SIM creep tests could predict long-term creep deformations. The creep resistance of all three PHDPE, PHDPE-A, PHDPE-B, and PHDPE-C, was better than that of HDPE due to the addition of nylon 6 for the polymer-blending modification in HDPE. Comparison of the experimental data with the predicted data also confirmed that the MB and NLTC models could accurately model the viscoelastic-plastic creep behavior of PHDPE geocell sheets.

Microstructures and Creep Properties of Type 316LN Stainless Steel Weld Joints

Microstructures and Creep Properties of Type 316LN Stainless Steel Weld Joints

Creep Performance and Life Prediction of Bamboo Scrimber under Long-Term Tension in Parallel-to-Grain

Creep performance is a crucial factor that must be considered in structural design. This paper aims to investigate the creep failure mode, creep strain, creep compliance, and other creep properties of bamboo scrimber under long-term tension in parallel-to-grain. To establish a general creep life prediction method for the full stress level of the bamboo scrimber, a multi-branch Kelvin–Voigt model, a generalized Maxwell model, and a creep finite element simulation were employed. The results showed that the creep strain curve of bamboo scrimber included the unsteady creep stage and the stable creep stage, but not the accelerated creep stage. When the stress ratio was less than 0.3, the residual strength decreased gradually. Below 70% of the ultimate load capacity, the creep characteristics of the bamboo scrimber were linear viscoelastic, and the creep compliance was generally independent of the load level. The creep finite element model of bamboo scrimber could accurately calculate the creep deformation of specimens. Based on this creep finite element model and creep failure rules, a life prediction model for the full stress level of bamboo scrimber was established, which could accurately predict the creep life. This paper provides theoretical guidance for the creep design of bamboo scrimber in engineering structures.

Influence of network structure determined by Time-domain [formula omitted] Double Quantum NMR on the creep properties of non-stoichiometric epoxy-amine resins aimed for chemical anchoring applications

Chemical anchors, used to fasten structural components, are prone to creep under sustained loading, which significantly affects their long-term performance and service life. In this study, the molar ratio, that determines the ultimate network structure of an epoxy-amine resin system, is systematically modified to investigate the creep properties. The percentage of physical entanglement and defects (free and dangling chains) for each formulation is obtained by Time-Domain 1H Double Quantum (DQ) Nuclear Magnetic Resonance (NMR) measurements. The crosslink density values from the dynamic mechanical analysis (DMA) and the network parameter from the NMR measurements show a strong linear correlation. Tensile tests were performed to determine the influence on the mechanical properties and set the load levels for tensile creep measurements of the selected formulations. Epoxy-rich systems (Epoxy-Amine adduct) show the highest modulus and tensile strength but inferior creep properties compared to stoichiometric and amine-rich (Amine-Epoxy adduct) compositions. This can be attributed to the build-up of several network structures with higher content of free and dangling chains leading to lower deformation resistance. At high loads, the entanglement density has an increasing impact on the creep behavior compared to defect-rich systems. Modeling using the Findley approach was carried out to link the obtained creep parameter with the material properties. Therefore, this work aims to deepen the understanding of the molecular-scale structures of epoxy resins required for material and design optimization, by investigating the correlation between network structure and creep properties.

Statement of Retraction: Creep and mechanical properties of aluminium A356 composites reinforced with coated and un-coated MWCNTs fabricated using the stir casting method

Statement of Retraction: Creep and mechanical properties of aluminium A356 composites reinforced with coated and un-coated MWCNTs fabricated using the stir casting method

The Important Role Played by High-Temperature Tensile Testing in the Representation of Minimum Creep Rates Using S-Shaped Curve Models

It is important to characterize the creep life of materials used in power plants and aeroengines. This paper illustrates the important role played by a material’s tensile strength in enabling accurate creep property representations to be made. It also shows that published high-temperature tensile strength values are not always suitable for use in certain creep models due to its strain rate dependency. To deal with the absence of such suitable date on tensile strength, this paper estimates such values directly from minimum creep rate data. When this technique is applied to models that represent the relationship between minimum creep rates and normalized (with respect to tensile strength) stress using S-shaped curves, an improvement in the fit to data on 2.25Cr–1Mo steel was observed. The findings suggest an important need for future research into the most appropriate strain rates to be used in high-temperature tensile testing when the purpose is to use the resulting tensile strength values for use in creep modeling.

Effect of aging and α’ segregation on oxidation and electrochemical behavior of FeCrAl alloys

FeCrAl alloys are promising candidates for next generation nuclear fuel cladding applications due to their high resistance to hydrothermal corrosion, high temperature steam oxidation, fretting, and creep properties. However, a potential area of concern for FeCrAl alloys is the precipitation of Cr-enriched α’ phase which could have an implication on the mechanical, oxidative degradation as well as electrochemical behavior. Studies in this area are limited since most of the past applications of FeCrAl alloys have been in high temperatures above the miscibility gap. In this work, we investigate the high temperature oxidation and electrochemical behavior of a modified APMT (Fe21Cr5Al3Mo) alloy in unaged condition in comparison with a thermally aged condition having α’ phase segregation. The α’ is shown to have no major role in steam oxidation resistance of FeCrAl alloys. The electrochemical behavior in chloride environments shows that the presence of α’ slightly reduces the pitting potential but shows an overall increase in polarization resistance indicating that α’ may enhance corrosion resistance in chloride environments by increasing passivation.

Research Progress of ODS FeCrAl Alloys-A Review of Composition Design.

After the Fukushima nuclear accident, the development of new accident-tolerant fuel cladding materials has become a research hotspot around the world. Due to its outstanding corrosion resistance, radiation resistance, and creep properties at elevated temperatures, the oxide dispersion strengthened (ODS) FeCrAl alloy, as one of the most promising candidate materials for accident-tolerant fuel cladding, has been extensively studied during the past decade. Recent research on chemical composition design as well as its effects on the microstructure and mechanical properties has been reviewed in this paper. In particular, the reasonable/optimized content of Cr is explained from the aspects of oxidation resistance, radiation resistance, and thermal stability. The essential role of the Al element in oxidation resistance, high-temperature stability, and workability was reviewed in detail. The roles of oxide-forming elements, i.e., Y (Y2O3), Ti, and Zr, and the solid solution strengthening element, i.e., W, were discussed. Additionally, their reasonable contents were summarized. Typical types of oxide, i.e., Y-Ti-O, Y-Al-O, and Y-Zr-O, and their formation mechanisms were also discussed in this paper. All aspects mentioned above provide an important reference for understanding the effects of composition design parameters on the properties of nuclear-level ODS FeCrAl alloy.

One-Dimensional Compressibility and Creep Characteristics of Unsaturated Compacted Loess Based on Incremental Loading and Constant Rate of Strain Methods

In engineering practice, unsaturated compacted loess is often utilized as a filling material in the loess regions. The loess inevitably undergoes one-dimensional compressibility and creep deformation due to the long-term effects of the upper soil layers and buildings. When the deformation is large enough, it tends to damage buildings and threaten engineering safety. In this regard, the one-dimensional compressibility and creep properties of unsaturated compacted loess based on incremental loading (IL) and constant rate of strain (CRS) methods have been studied. First, soil materials with an initial moisture content of 15% were prepared and then compacted into soil samples with an 80 mm diameter and a 10 mm height. Second, the compressibility and creep properties of the compacted loess samples obtained via the IL and CRS compression tests were compared and analyzed. In this study, several parameters, including the primary compression index Cc and secondary compression index Cα, were derived. Meanwhile, the moisture content of the samples was measured via electrical resistivity methods. Finally, the microstructural characteristics were derived via nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM) tests. The results showed that Cc and Cα increased with the increase in moisture content and vertical stress; Cα/Cc ranged from 0.026 to 0.042. Compared with the compression parameters and deformation of the samples, those obtained via the CRS tests are a little larger than those obtained via the IL tests for a given loading and initial moisture content. The electrical resistivity depends on pore water-connected channels, which were deeply affected by the initial moisture content, vertical stress and loading duration (or strain rate). Moreover, as vertical stress increased, the pore size and pore area gradually decreased, the coarse particles were broken, and the fine particles increased. The contacts between particles changed from point-to-point contacts and edge-to-edge contacts to face-to-face contacts. Meanwhile, as vertical stress and loading rate increased, the loess particles were apt to vary from irregular elongated particles to equiaxial circular particles. This investigation can provide a theoretical base and experimental support for improving ground stability and preventing landslide disasters in loess regions.

Determination of the Rate Dependence of Damage Formation in Metallic‐Intermetallic Mg–Al–Ca Composites at Elevated Temperature using Panoramic Image Analysis

A cast Mg‐4.65Al‐2.82Ca alloy with a microstructure containing an α‐Mg matrix is studied, which is reinforced with a C36 Laves phase skeleton. Such ternary alloys are targeted for elevated temperature applications in automotive engines, since they possess excellent creep properties. However, in application, the alloy may be subjected to a wide range of strain rates, and thus accelerated testing is often essential. It is, therefore, crucial to understand the effect of such rate variations. Herein, their impact on damage formation is focused. The analysis is based on high‐resolution panoramic imaging using scanning electron microscopy, combined with automated damage analysis using deep learning for object detection and classification (YOLOv5). It is found that with decreasing strain rate the dominant damage mechanism for a given strain level changes: at a strain rate of 5 × 10−4 s−1, the evolution of microcracks in the C36 Laves phase dominates damage formation. However, when the strain rate is decreased to 5 × 10−6 s−1, interface decohesion, becomes equally important. A change in crack orientation is also observed, indicating an increasing influence of plastic codeformation of the α‐Mg matrix and Laves phase. This transition is attributed to the leading damage mechanism to thermally activated processes at the interface.

Small punch creep test reveals the differences of high-temperature creep behaviours for laser powder bed fusion and Rolled Inconel 718 alloys

The laser powder bed fusion (LPBF) additive super alloy is with the manufacturing advantages in high temperature equipment, but their creep behaviour especially for the effective heat treatment is not well understood. In this paper, small punch creep test (SPCT) is used to compare the creep properties of Inconel 718 fabricated by LPBF and the counterpart processed via rolling, considering different heat treatments. After aging heat treatment, both creep deformation and life of LPBF Inconel 718 are much lower than those of rolled nickel-base alloy. In contrast, after solution and aging heat treatment, the creep deformation capacity and life of LPBF Inconel 718 are significantly improved, and the difference of creep resistance between them is greatly reduced. Based on the Chakrabarty membrane stretching model, the equivalent creep strain and equivalent creep stress are estimated. The equivalent creep strain-time curve proves that the solution and aging heat treatment effectively improves the creep resistance of LPBF Inconel 718. Then, based on Norton creep model, Larson Miller model and Monkman Grant model, the creep life equations of LPBF Inconel 718 and Rolled Inconel 718 under two heat treatment conditions are obtained. Finally, the fracture mechanism of Rolled Inconel 718 under aging heat treatment is the creep induced brittle intergranular cracking, while LPBF Inconel 718 is dominated by the creep induced layered brittle cracking with serrated propagation along columnar grains. After solution and aging heat treatment, intergranular brittle cracking caused by microcracks at grain boundaries occurs in Rolled Inconel 718, accompanied by the appearance of creep micro-voids. In comparison, the fracture mode in LPBF Inconel 718 is the zigzag cracking along columnar grain with the elongation and stagger of long columnar grains.

Effect of He Plasma Exposure on Recrystallization Behaviour and Mechanical Properties of Exposed W Surfaces—An EBSD and Nanoindentation Study

Fusion reactors are designed to operate at extremely high temperatures, which causes the plasma-facing materials to be heated to 500 °C to 1000 °C. Tungsten is one of the target design materials for the plasma-facing diverter components in Tokamak designs, such as ITER, because of its excellent high-temperature strength and creep properties. However, recrystallization due to high temperatures may be detrimental to these superior mechanical properties, while exposure to He plasma has been reported to influence the recrystallization behaviour. This influence is most likely due to the Zener effect caused by He bubbles formed near the surface, which retard the migration of grain boundaries, while at the same time modifying the surface microstructure. This paper reports a study of the effect of plasma exposure at different sample temperatures on the recrystallization behaviour of W at different annealing temperatures. The characterization after plasma exposure and annealing is pursued through a series of post-exposure annealing, followed by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) characterization and nanoindentation to determine the mechanical properties. Here, it is shown that the hardness is closely related to the recrystallization fraction, and that the plasma exposure at a sample temperature of 300 °C slows down the recrystallization more than at higher sample temperatures of 500 °C and 800 °C. Atomic force microscopy (AFM) was subsequently used to determine any changes in pile-up height around the nanoindents, to probe any indication of changes in hardenability. However, these measurements failed to provide any clear evidence regarding this aspect of mechanical behaviour.

Experimental analysis and response surface optimization of the influence of Zn/Al mass ratio on the creep resistance and tensile properties of As-cast Mg-Zn-Al alloy

Mg-Zn-Al (ZA) alloys are intended to exhibit creep resistance and high temperature mechanical capabilities due to appropriate Zn/Al mass ratio control. The effect of altering Al concentration (1, 2, 3, 4, 5, 6) on the tensile and creep resistance properties of Mg-9Zn-xAl alloy was determined experimentally and quantitatively optimized using response surface method (RSM). It was observed that 2 wt% Al improves both room and elevated temperature tensile strength, and ductility of the considered ZA alloy. The creep rate sharply increased when the mass fraction of aluminum rose above 4 wt%. For the purpose of forecasting the tensile and creep properties of the ZA alloy at any given aluminum mass percentage, quartic models were generated, with all model terms significant. Strong room and high-temperature tensile strength, %elongation, and creep resistance were all reached in the low Al-containing ZA alloys, with 1.597 wt% anticipated ideal Al mass concentration, according to RSM optimization.

Buckling behavior of Formed-in-Place-Pipe (FIPP) liners under groundwater pressure: An experimental investigation for buried municipal pipelines

Specifications and models for FIPP lining structures are conservative, with the El-Sawy model providing the most accurate prediction of critical buckling pressure. Initial elliptical defects significantly reduce the liner's resistance to external groundwater pressure, and these defects, along with pipeline constraints, influence the deformation processes. This study emphasizes the necessity of strict quality control measures, the consideration of initial ovality during design, and the exploration of reinforcement strategies for FIPP liners. Future research should investigate the long-term creep properties of the material, assess the impact of various types and sizes of initial defects, and develop effective reinforcement technologies for FIPP liners to advance pipeline rehabilitation technologies and practices.

Indentation creep characterization of plasma-sprayed nanostructured La2(Zr0.75Ce0.25)2O7 thermal barrier coatings

Nano-indentation technology can not only analyze the nano-hardness and elastic modulus, but also extract the creep properties. In the current study, the nanostructured La2(Zr0.75Ce0.25)2O7 (LCZ) coating were successfully constructed by atmospheric plasma spraying (APS). Furthermore, the indentation creep characterization of LCZ coating was systematically analyzed by nano-indentation. Results imply that power-law formula can well fit the curve relationship between indentation depth and time in the creep process of LCZ coating. The determined strain rate sensitivity of LCZ coating is 0.0134 ± 0.0045. The study provides support for further research on creep properties of nanostructured LCZ coating in the future.

Time-dependent creep analysis of ultra-high-temperature functionally graded rotating disks of variable thickness

Functionally graded materials (FGMs) are high temperature-resistant materials that can simultaneously maintain metallic tenacity and anti-corrosive properties. Nevertheless, using FGMs during a multi-year service life at ultrahigh temperatures is crucial. Hence, the time-dependent creep response of variable-thickness rotating disks made of FGM is investigated. Four different disk profiles of linear, concave, convex, and uniform are considered. The material's creep properties are defined by the Bailey-Norton creep law. Loading is a rotation-based mechanical body force and a uniform temperature throughout the disk. Simultaneous solution of equilibrium, stress-strain, and strain-displacement equations yields a non-homogenous differential equation containing variable and time-dependent coefficients. In an attempt to optimize the computation cost, Bat and Fish algorithms were used to optimize the initial strain presumptions. Semi-analytical solution of this differential equation gives radial and circumferential stress histories and displacement histories. To confirm the solution method, initial thermo-elastic radial stress, and the effective stress history are validated with the existing literature; there is a good agreement between the results. In addition, the finite element software ABAQUS was used to model the FGM disk thermo-elastic behavior, and the result was compared with the semi-analytical solution results. This study emphasizes the significance of accounting for creep effects in the design of FGM rotating disks, as remarkable changes in their displacements and stresses occur over time. This study emphasizes the significance of accounting for creep effects in the design of FGM rotating disks, as notable changes in their displacements and stresses occur over time.

Upscaling the creep behavior of clay-rich and quartz-rich shales from nanoindentation measurements: Application to the Wufeng-Longmaxi shale, China

Creep of shale controls the stability of fault gouge in sedimentary rocks, aseismic deformation in the upper crust, and several engineering operations, such as proppant embedment, reservoir subsidence, and wellbore stability during unconventional shale gas exploitation. However, measuring the creep properties of shale is challenging due to the multi-scale structure of this rock. Here, our goal is to assess how creep parameters measured at the core sample scale depend on those measured at the nanoscale in shales. We selected three clay-rich and quartz-rich shale samples and conducted a series of experiments on shale samples at two scales: nanoindentation creep measurements on thin sections, and uniaxial creep tests on core samples. The creep parameters, including creep displacement (Δh), indentation creep (CIT), creep strain rate sensitivity (m), contact creep modulus (C), and viscoelastic parameters (E1, E2, η1, and η2) of the Burgers' model, were calculated by fitting the nanoindentation creep data of each single-phase component of shale matrix. From these data, we upscale the creep parameters (E1, E2, and C) of shale matrix to those of the bulk shale. Upscaled E2 and C values based on the Mori-Tanaka scheme are found to be relatively close to those measured in previous microindentation study on Longmaxi shale and are consistent with our measurements in uniaxial creep tests on core samples of clay-rich shale and quartz-rich shale. The upscaling value of the elastic parameters, E1, is almost twice larger than the results from uniaxial creep test. We interpret this difference as due to low elastic stiffness along grain contacts of the shale that nanoindentation measurements underestimate. Our results show a consistency of some creep parameters upscaled from nanoindentation data and those measured at a macroscopic scale on core samples.

Modelling thermal and irradiation creep with crystal plasticity theory and self-consistent method

In order to help comprehend the underlying deformation mechanisms related to irradiation creep of metallic polycrystalline materials, the crystal plasticity theory and viscoplastic self-consistent method are combined in this work. In the developed constitutive laws for single crystals, the influence of irradiation-induced defects on the creep strain rate has been effectively addressed by considering dislocation climb and glide. For the former, dislocation climb with irradiation effect can be enhanced through the absorption of point defects by dislocations, where the steady-state concentration of point defects is affected by the defect sinks including dislocations and defect clusters. As a comparison, dislocation glide may be retarded through the impediment of mobile dislocations by irradiation-induced defects. More importantly, the effect of dislocation static recovery has been incorporated into the dislocation evolution law, which is noticed to play a deterministic role for the annihilation of dislocations during the long-term creep process. In addition, the viscoplastic self-consistent method is taken as a cross-scale way to predict the irradiation creep properties of polycrystals. To validate the developed creep model, experimental data of both single crystalline and polycrystalline nickel has been considered under both thermal and irradiation creep. A good agreement between the theoretical results and experimental data is achieved, which offers a solid basis to further analyze the macroscopic irradiation creep deformation from the aspect of microstructure evolution.

Creep behavior and permeability evolution of red sandstone in three Gorges Reservoir area subjected to cyclic seepage pressure

The creep behavior of the rock mass in an anchoring section affects the control effect of the stabilizing piles and the long-term stability of the landslide-stabilizing pile system. Additionally, periodic reservoir water level fluctuations cause cyclic water-seepage pressure changes in the rock mass of bank slopes, which may promote rock creep deformation, further compromising the stability of the slope and increasing the risk of landslides. In this experimental study, the creep properties and permeability of red sandstone from the bedrock of the Majiagou landslide in the Three Gorges Reservoir area were investigated under cyclic seepage pressure conditions. Four seepage pressures (0, 1, 2, and 1-2 MPa) were selected for comparison. The results demonstrated that creep strain curves, especially volumetric strain, exhibited fluctuations under cyclic seepage pressure conditions. The creep deformation of red sandstone was enhanced by cyclic seepage pressure. Creep strain increased with increasing deviatoric stress, except at 60 MPa deviatoric stress. Furthermore, the creep time to failure at 108 MPa deviatoric stress was inversely proportional to the seepage pressure. The steady-state creep rate exponentially increased with deviatoric stress, and the steady-state creep rate under cyclic seepage pressure was higher than that under constant seepage pressures of 1 and 2 MPa. Finally, the permeability of the red sandstone fluctuated in accordance with the fluctuating seepage pressure. The permeability evolution of the red sandstone samples under different deviatoric stresses was attributed to the competing processes of preexisting microcrack closure and the propagation of new microcracks. Accurate evaluation of the long-term performance of stabilizing piles and the stability of stabilized landslides necessitates consideration of the influence of cyclic seepage pressure on rock mass creep behavior.

Microstructure, creep properties, and electrical resistivity of magnetron sputtering deposited SAC305 thin films

This paper investigates the surface morphology, mechanical properties, and electrical resistivity of 96.5Sn–3.0Ag–0.5Cu (SAC305) thin films deposited on Si and SiO2 substrates through RF magnetron sputtering. Various deposition parameters were tested using both DC and RF power sources at different pressures and powers to produce robust continuous films. The most optimal surface morphology, with an average grain size of ∼1 μm and a thickness of ∼2.2 μm, was accomplished at a pressure of 2.4 mTorr and 200 W power. After polishing, a uniform thickness of 1800 nm with a mean roughness (Ra) of 14.9 nm was obtained. The samples contained polycrystalline β-Sn grains at (200) diffraction planes with a preferred orientation 2θ of 30.70°. Although the XRD pattern did not indicate any Ag peaks, weak peaks of Ag3Sn were observed at 2θ of 37.60° and 39.59°, corresponding to diffraction planes (020) and (211), respectively. The electrical resistivity of the SAC305 thin film deposited on the SiO2 substrate and of the bulk SAC305 samples were measured as 19.6 and 13.7 μΩ cm, respectively. It was noted that changes in hold time at peak loads or the rate of loading in the creep experiments did not significantly influence the creep properties of the SAC305 bulk or thin film material.