Showing papers on "Creep published in 2021"

Rare earth improves strength and creep resistance of additively manufactured Zn implants

Abstract: Poor mechanical strength and creep resistance limit the orthopedic application of biodegradable Zinc (Zn). In present work, cerium (Ce) was alloyed with Zn using laser additive manufacturing technique. As one kind of rare earth element, Ce possessed high surface activity, which effectively interrupted the grain growth and caused the formation of stable intermetallics, thus contributing to grain refinement strengthening and precipitate strengthening. More significantly, Ce alloying activated more pyramidal slip by means of reducing the critical resolved shear stress during plastic deformation, and resultantly formed the sessile dislocations, which caused the accumulated strain hardening and improved the creep resistance. As a result, Zn-Ce alloy exhibited a considerably improved ultimate tensile strength of 247.4 ± 7.2 MPa, and a reduced creep rate of 1.68 × 10−7 s−1. Moreover, it exhibited strong antibacterial activity, as well as favorable cytocompatibility and hemocompatibility. All these results demonstrated the great potential of Zn-Ce alloy as a candidate for bone repair application.

A triaxial creep model for salt rocks based on variable-order fractional derivative

Abstract: This paper intend to describe the entire creep process of a salt rock under triaxial loading, especially the creep characteristics in the accelerated creep stage, by replacing the Newtonian dashpot in the Maxwell model with the variable-order fractional derivative component and extending it from one to three dimensions. The experimental data were obtained from the creep of salt rock under multistage loading for approximately five months to examine the applicability of the new model. The fitting results show that the new model can accurately describe the creep behavior of the salt rock and can fully reflect the accelerative rheological property of the salt rock. Compared with the traditional nonlinear rheological model, the new model has fewer parameters and increased accuracy of the fitting results, and is anticipated to have an increased application area.

Tensile creep behavior of short-carbon-fiber reinforced polyetherimide composites

Abstract: Tensile creep behavior of short-carbon-fiber reinforced polyetherimide (SCF/PEI) composites is investigated at ambient temperature by short and long creep tests in this work Influences of carbon fiber concentration and applied tensile stress on the creep performance of SCF/PEI composites are studied Four viscoelastic models are employed to quantify the viscoelastic behavior of SCF/PEI composites and the effect of fiber surface treatment is discussed on the creep behavior Short-term creep results show that a relatively high carbon fiber content and fiber surface thermal treatment lead to a relatively high creep resistance In order to accelerate the short-term creep test, the time–temperature superposition principle (TTSP) is employed to detect the long-term creep behavior of SCF/PEI composites at ambient temperature In addition, an attempt has been made to generate a master curve to approximate the long-term creep behavior of the SCF/PEI composites at ambient temperature

Study on the Triaxial Unloading Creep Mechanical Properties and Damage Constitutive Model of Red Sandstone Containing a Single Ice-Filled Flaw

Abstract: The freezing method is an effective approach for constructing coal mine shafts through water-rich soft rock strata. The frozen wall produced by this technique stops the movement of water and offers temporary support, ensuring the safety and stability of the shaft working face. However, most frozen rock masses generally comprise ice-filled flaws and frozen intact rock. A frozen fissured rock mass undergoing the long-term effects of excavation unloading under in situ stress gradually accumulates damage or creep deformation over time; this damage is mainly responsible for the significant deformation, cracking and even instability of the frozen rock wall. To study the creep characteristics of ice-filled fissured rock masses under unloading conditions, a series of conventional triaxial compression tests and triaxial unloading creep tests were performed on ice-filled flawed red sandstone specimens from the Shilawusu mine in Northwest China using a self-developed DRTS-500 subzero rock triaxial testing system. In conjunction with the experimental data, the instantaneous strength and deformation characteristics of the rock specimens were analyzed, and their creep deformation characteristics and damage evolution characteristics were discussed. In this paper, based on the nonlinear creep characteristics of frozen rock, fractional calculus theory and damage theory, a new nonlinear creep damage model of frozen fissured red sandstone was defined in series with the improved Burgers model and elastoplastic damage model. The proposed creep model can reasonably describe the creep deformation of frozen fissured red sandstone. Classical elastoplastic mechanics and creep theory were used to derive the three-dimensional creep damage constitutive equation, and the results of creep experiments and simulation results of the creep damage constitutive model were very consistent in this study. The new creep model not only reflects the whole creep deformation process of frozen fissured red sandstone but also exhibits better performance than the classical Burger model and improved Nishihara model in describing the primary creep stage and accelerating creep stage. Therefore, the proposed nonlinear creep damage model is suitable for studying the mechanical creep properties of frozen fissured rock under unloading conditions. The results can provide an important reference for the long-term stability assessment of frozen rock walls of coal mine shafts.

Strengthening mechanisms of reduced activation ferritic/martensitic steels: A review

Abstract: This review summarizes the strengthening mechanisms of reduced activation ferritic/martensitic (RAFM) steels High-angle grain boundaries, subgrain boundaries, nano-sized M23C6, and MX carbide precipitates effectively hinder dislocation motion and increase high-temperature strength M23C6 carbides are easily coarsened under high temperatures, thereby weakening their ability to block dislocations Creep properties are improved through the reduction of M23C6 carbides Thus, the loss of strength must be compensated by other strengthening mechanisms This review also outlines the recent progress in the development of RAFM steels Oxide dispersion-strengthened steels prevent M23C6 precipitation by reducing C content to increase creep life and introduce a high density of nano-sized oxide precipitates to offset the reduced strength Severe plastic deformation methods can substantially refine subgrains and MX carbides in the steel The thermal deformation strengthening of RAFM steels mainly relies on thermo-mechanical treatment to increase the MX carbide and subgrain boundaries This procedure increases the creep life of TMT(thermo-mechanical treatment) 9Cr-1W-006Ta steel by ∼20 times compared with those of F82H and Eurofer 97 steels under 550°C/260 MPa

Creep properties and resistivity-ultrasonic-AE responses of cemented gangue backfill column under high-stress area

Abstract: To investigate the creep and instability properties of a cemented gangue backfill column under a high-stress area, the uniaxial compression creep tests were conducted by single-step and multi-step loading of prismatic samples made of cemented gangue backfill material (CGBM) under the high stress-strength ratio The creep damage was monitored using an electrical resistivity device, ultrasonic testing device, and acoustic emission (AE) instrument The results showed that the CGBM sample has a creep hardening property The creep failure strength (CFS) is slightly larger than the uniaxial compressive strength (UCS), ranging in ratio from 1089% to 1165% The instantaneous strain, creep strain, and creep rate increase with increasing stress-strength ratio in the single-step loading creep tests The instantaneous strain and creep strain decrease first and then increase during the multi-step loading creep process The axial creep strain of the CGBM column can be expressed by the viscoelastic-plastic creep model Creep instability is caused by the accumulation of strain energy under multi-step loading and the continuous lateral expansion at the unconstrained middle position during the creep process The creep stability of a CGBM column in a high-stress area can be monitored based on the variation of electrical resistivity, ultrasonic pulse velocity (UPV), and AE signals

The concept of function creep

Abstract: Function creep – the expansion of a system or technology beyond its original purposes – is a well-known phenomenon. Correction: it is a well-referenced phenomenon. Yearly, hundreds of publications ...

Damage evolution characteristics of saw-tooth joint under shear creep condition:

Abstract: The creep characteristics of joint have an important influence on the long-term stability of rock mass engineering such as tunnels and slopes. In this paper, the sawtooth angle α is taken as the va...

Evaluating the Microstructure Evolution Behaviors of Saturated Sandstone Using NMR Testing Under Uniaxial Short-Term and Creep Compression

Abstract: Understanding the micromechanical mechanism of the rock creep process is of great importance for studying the macroscopic time-dependent behavior of rocks. In this study, the evolution characteristics of the microstructure (cracks and pores) of saturated sandstones under short term and creep uniaxial compression conditions were investigated with the nuclear magnetic resonance (NMR) technique. The samples were first loaded to different stress levels and creep stages and then completely unloaded for NMR testing. Based on the testing results, the macroscopic deformation behavior, moisture migration law, pore size distribution, porosity, and microstructure change of the each sample under the short-term loading or different stages of creep were quantitatively analyzed. After that, by introducing a nonlinear elasto-viscoplastic damage creep model (EVP) by Zhao et al. (18:04017129, 2018), the relationships between the macroscopic irreversible strains and microscopic porosity increments were established. Overall, it was observed that: (1) regardless of the stress level, the magnitudes of the axial and lateral critical strains of samples at the onset of the accelerating creep stage are both relatively constant, and the axial strain is almost comparable to that at the peak stress in the short-term test, while the lateral strain is larger than that of the short-term test. (2) During the mechanical tests, the moisture in the samples migrates from large pores into small pores, and after mechanical tests, the porosities of the samples increase, in which the small pores always account for a larger proportion. (3) Corresponding to the three creep stages, the porosity of the sample increases slightly after the transient stage, increases to a constant value that is largely independent of stress after the steady stage, and increases significantly after the creep failure. In particular, compared to the initial porosity of 6.7%, the average porosities of samples taken to the onset of the tertiary stage and creep failure is 7.49% and 8.71%, increasing by 16.7% and 29.8%, respectively. (4) The porosity growth of sandstone during the brittle creep is mainly driven by the microscopic subcritical crack growth along the grain boundaries.

Utilization of Bracing Arms as Additional Reinforcement in Pultruded Glass Fiber-Reinforced Polymer Composite Cross-Arms: Creep Experimental and Numerical Analyses

Abstract: The application of pultruded glass fiber-reinforced polymer composites (PGFRPCs) as a replacement for conventional wooden cross-arms in transmission towers is relatively new. Although numerous studies have conducted creep tests on coupon-scale PGFRPC cross-arms, none had performed creep analyses on full-scale PGFRPC cross-arms under actual working load conditions. Thus, this work proposed to study the influence of an additional bracing system on the creep responses of PGFRPC cross-arms in a 132 kV transmission tower. The creep behaviors and responses of the main members in current and braced PGFRPC cross-arm designs were compared and evaluated in a transmission tower under actual working conditions. These PGFRPC cross-arms were subjected to actual working loads mimicking the actual weight of electrical cables and insulators for a duration of 1000 h. The cross-arms were installed on a custom test rig in an open area to simulate the actual environment of tropical climate conditions. Further creep analysis was performed by using Findley and Burger models on the basis of experimental data to link instantaneous and extended (transient and viscoelastic) creep strains. The addition of braced arms to the structure reduced the total strain of a cross-arm’s main member beams and improved elastic and viscous moduli. The addition of bracing arms improved the structural integrity and stiffness of the cross-arm structure. The findings of this study suggested that the use of a bracing system in cross-arm structures could prolong the structures’ service life and subsequently reduce maintenance effort and cost for long-term applications in transmission towers.

The effect of homogenization temperature on the microstructure and high temperature mechanical performance of SLM-fabricated IN718 alloy

Abstract: Microstructure, mechanical performances at elevated temperature (650 °C) and their correlation of Inconel 718 manufactured by Selective Laser Melting (SLM) were investigated in this paper. Four kinds of heat treatment schemes including homogenization, solution treatment and conventional aging process were carried out to regulate the microstructure in order to obtain the optimum comprehensive mechanical performances at high temperature. The dimension and morphology of grains, subgrains, different precipitates including δ phase, strengthening phase, carbides and nitrides were investigated. In addition, their evolution mechanism was also analyzed in detail. Furthermore, for figuring out the effects of microstructures on the mechanical performance, creep rupture test and tensile test were carried out to compare the performance differences of heat-treated samples. The results demonstrated that the heat-treated samples show the better creep rupture performance and higher tensile strength at elevated temperature than that of the wrought. In addition, the sample under homogenization heat treatment in 1080 °C, solution treatment in 980 °C and conventional aging treatment shows the optimum mechanical properties at 650 °C.

Analysis of Thermal Creep Effects on Fluid Flow and Heat Transfer in a Microchannel Gas Heating

Abstract: Thermal creep effects on fluid flow and heat transfer in a microchannel gas flow at low velocities are studied numerically. The continuity and Navier–Stokes equations in vorticity–stream function form, coupled with the energy equation, are solved, considering the thermal creep effect due to the longitudinal temperature gradient along the channel wall in addition to the combined effects of viscous dissipation, pressure work, axial conduction, shear work, and nonequilibrium conditions at the gas–wall interface. The governing equations are also solved without thermal creep, and comparisons between the two solutions are presented to evaluate the thermal creep effect on the flow field in the slip flow regime at relatively low Reynolds numbers. The results presented show that the thermal creep effect on both velocity and temperature fields become more significant as the Reynolds number decreases. Thermal creep effect on the velocity field also extends a longer distance downstream the channel as the Reynolds number decreases, hence increasing the hydrodynamics entrance length. Thermal creep can cause high positive velocity gradients at the upper channel wall for gas heating and hence reverse the flow rotation in the fluid layers adjacent to the wall. Thermal creep also results in a higher gas temperature in the developing region and higher heat exchange between the fluid and the channel wall in the entrance region. Thermal creep effect on heat exchange between the gas and the channel wall becomes more significant as the Knudsen number decreases.

Understanding tensile and creep properties of WC reinforced nickel-based composites fabricated by selective laser melting

Abstract: In this study, submicron-WC reinforced Inconel 718 composites were fabricated by selective laser melting (SLM). They were investigated regarding forming quality, microstructure evolution, tensile and creep properties. The results demonstrated that SLM-processing with decreasing volumetric energy density (Ed), entailed the fabrication of composites with also decreasing density, due to the formation of more pores and cracks. The microstructure of the prepared composites mainly consisted of two different types, namely cellular and columnar dendrites, which formed within the molten pool. The microstructure was more heterogeneous at smaller length scale, whereby reduced grain size and enhanced high angle grain boundaries (HAGBs) were observed. At the optimal Ed of 120 J/mm3, the SLM-fabricated composite was highly dense and exhibited small stress concentration, so that it showed the highest microhardness (361.7 HV0.2), ultimate tensile strength (1030.5 MPa) and elongation (24.8%). Owing to elongated grain boundaries, which were characteristic for the respective microstructure, the composite displayed reduced creep life and ductility. The deformation behaviors during tension and creep were analyzed and they were interrelated to the microstructural defects, grain size, grain boundary morphology, and dislocation density.

Experimental investigation of the long-term behavior of reconstituted bamboo beams with various loading levels

Abstract: To study the creep behavior of bamboo beams, the long-term bending creep tests of twelve bamboo beams with different loading levels were carried out in an indoor environment. The following results were obtained. The creep of reconstituted bamboo beams under different loading levels exhibits the three-stage characteristics of short rapid creep, sustained moderate-speed creep, and stable low-speed creep; the growth ratios of creep deflection are between 30.9% and 39.1% for the reconstituted bamboo beams at the 10–50% loading level. The creep and the creep rate increase with an increase in the loading level. Meanwhile, the long-term creep prediction model of the reconstituted bamboo beam is derived based on the Burgers model; the proposed model may reflect the significant effects of the loading level and is able to accurately evaluate the deflection-time behavior of the recombinant bamboo by taking the loading level into consideration. Based on the prediction, the creep value of reconstituted bamboo beams is large and develops rapidly, and the creep deflection of the reconstituted bamboo beams at high loading level (30–50%) will exceed the maximum deflection corresponding to the ultimate bending capacity within 5 years.

Dynamic mechanical relaxation and thermal creep of high-entropy La30Ce30Ni10Al20Co10 bulk metallic glass

Abstract: Dynamic mechanical relaxation is a fundamental tool to understand the mechanical and physical properties of viscoelastic materials like glasses. Mechanical spectroscopy shows that the high-entropy bulk metallic glass (La30Ce30Ni10Al20Co10) exhibits a distinct β-relaxation feature. In the present research, dynamic mechanical analysis and thermal creep were performed using this bulk metallic glass material at a temperature domain around the β relaxation. The components of total strain, including ideal elastic strain, anelastic strain, and viscous-plastic strain, were analyzed based on the model of shear transformation zones (STZs). The stochastic activation of STZ contributes to the anelastic strain. When the temperature or external stress is high enough or the timescale is long enough, the interaction between STZs induces viscous-plastic strain. When all the spectrum of STZs is activated, the quasi-steady-state creep is achieved.

Influence of Additional Bracing Arms as Reinforcement Members in Wooden Timber Cross-Arms on Their Long-Term Creep Responses and Properties

Abstract: Previously, numerous creep studies on wood materials have been conducted in various coupon-scale tests. None had conducted research on creep properties of full-scale wooden cross-arms under actual environment and working load conditions. Hence, this research established findings on effect of braced arms on the creep behaviors of Virgin Balau (Shorea dipterocarpaceae) wood timber cross-arm in 132 kV latticed tower. In this research, creep properties of the main members of both current and braced wooden cross-arm designs were evaluated under actual working load conditions at 1000 h. The wooden cross-arm was assembled on a custom-made creep test rig at an outdoor area to simulate its long-term mechanical behaviours under actual environment of tropical climate conditions. Further creep numerical analyses were also performed by using Findley and Burger models in order to elaborate the transient creep, elastic and viscoelastic moduli of both wooden cross-arm configurations. The findings display that the reinforcement of braced arms in cross-arm structure significantly reduced its creep strain. The inclusion of bracing system in cross-arm structure enhanced transient creep and stress independent material exponent of the wooden structure. The addition of braced arms also improved elastic and viscoelastic moduli of wooden cross-arm structure. Thus, the outcomes suggested that the installation of bracing system in wooden cross-arm could extend the structure’s service life. Subsequently, this effort would ease maintenance and reduce cost for long-term applications in transmission towers.

Damage Evolution and Deformation of Rock Salt Under Creep-Fatigue Loading

Abstract: During the operation of a compressed air energy storage (CAES) salt cavern, the surrounding rock experiences creep damage during the stages of constant internal pressure and undergoes fatigue damage due to the periodical injection-production. To describe the damage evolution of salt rock under creep-fatigue loading, a novel damage accumulation model based on the ductility exhaustion concept is proposed by applying a nonlinear summation method to represent the synergistic effect of creep and fatigue damage. Low-cycle fatigue (LCF) and creep-fatigue tests of rock salt were conducted under stress-control mode for various cycle stress amplitudes and hold times. Results show that the deformation of rock salt under creep-fatigue loading consists of initial, steady and accelerated phases. The proposed model matches well with the test data and can accurately describe the damage evolution as the applied stress amplitudes and dwell times change. The introduction of the hold times at the upper limit stress causes a strain increment and life reduction, which become more evident as the duration periods prolong and can be understood by the dislocation theory of crystals.