Compressive Creep Research Articles

Impact of structural anisotropy on compressive creep behaviors of composite rocks based on digital image correlation technology

Impact of structural anisotropy on compressive creep behaviors of composite rocks based on digital image correlation technology

Experiment and analysis of physical, mechanical, and viscoelastic properties of the roots and stalks of green leafy vegetables.

Green leafy vegetables are an essential component of Chinese leafy vegetables. Due to their crisp stems and tender leaves, orderly harvester generally causes significant mechanical clamping damage. The physical and mechanical properties of green leafy vegetables are one of the important basis to design the orderly harvester. At the same time, they provide important parameters for the simulation and optimization of harvester. So, this paper measured the physical characteristic parameters of roots and stems of green leafy vegetables. Then, based on the TMS-Pro texture analyzer, the elasticity modulus of the roots and stems of green leafy vegetables were measured. The static friction coefficient, dynamic friction coefficient, and restitution coefficient of green leafy vegetables root-root, stem-stem, root-steel, and stem-steel were measured separately using a combination method of inclined plane and high-speed photography. Uniaxial compression creep experiments were carried out on whole and single leaf of green leafy vegetables using the TA.XT plus C universal testing machine. The constitutive equation of the four-element Burgers model was used to fit the deformation curve of the sample with time during the constant-pressure loading stage. The fitting determination coefficients R2 were all higher than 0.996, which verified the reasonable validity of the selected model. The above experimental results provide a parameter basis and theoretical support for the design and discrete element simulation optimization of orderly harvester critical components of green leafy vegetables.

Compression creep behaviors of GH4169 cylinder entangled wire material at elevated temperatures

As a nickel-based alloy, GH4169 has the properties of excellent corrosion resistance, high temperature oxidation resistance and high creep resistance. In this paper, the compression creep behaviors of cylinder entangled wire materials (CEWMs) made from metal wires (GH4169 nickel-based alloy, 304 stainless steel) were investigated at elevated temperatures (from 400 °C to 500 °C). The performance degradation of the two materials was evaluated by the variation amplitude of four mechanical properties parameters and material characterization methods. The results indicated that both of 304 CEWMs and GH4169 CEWMs suffered a significant performance degradation at elevated temperatures, and both of the two CEWMs showed a much serious performance degradation above 450 °C (tempering temperature). Compared to the 304 CEWMs at the tested temperatures, the GH4169 CEWMs obtained better creep resistance. It is therefore concluded that the GH4169 CEWM is an excellent material that can replace the commonly used 304 CEWM at elevated temperature work conditions.

Deformation behaviour of concrete with different moisture contents subjected to compressive creep and cyclic loading

The expected long-term deformations of concrete structures are calculated using creep models, derived from experiments performed with constant mechanical loads. However, in the majority of real structures, such as bridges, constant creep loads are superimposed with cyclic loads of substantial magnitude. Additionally, such structures are subject to changes in environmental conditions (temperature and humidity). Deformation measurements of existing bridges have shown significant underestimations by established creep models, which might be traced back to the superimposition of cyclic loads and different moisture contents. Therefore, the developments of strains, viscoplastic strains and modulus of elasticity under creep and cyclic loading of a normal strength concrete have been comparatively investigated for two different pore moisture contents (approx. 100 and 75%). The results show that viscous strains due to cyclic loading are significantly higher than those due to creep loading at the mean stress level of cyclic loading. Furthermore, the strains are higher for the higher moisture content. The differences in the development of the modulus of elasticity and viscoplastic strains of both load types give clear indication for load type dependent microstructural deformation mechanisms. The results obtained concerning the influence of the load type and the moisture content need to be considered for the improvement of existing models.

Uniaxial compressive creep tests by spark plasma sintering of 70% theoretical density α-uranium and U-10Zr

Metallic fuels hold numerous advantages over conventional uranium dioxide fuels and are a key component of several liquid metal-cooled advanced reactor concepts including sodium fast reactors. These fuels undergo rapid swelling during early burnup; consequently, they spend most of their reactor lifetime in a porous state. The presence of this porosity alters many of the mechanical properties of the fuel including creep impacting fuel deformation during axial swelling. This work investigates the creep behavior of the porous fuel using a spark plasma sintering technique. Creep tests were performed for the first time on porous α-phase uranium and uranium with 10 wt. % zirconium (U-10Zr) samples. The samples of α-phase uranium and U-10Zr were fabricated from depleted uranium by spark plasma sintering and subjected to uniaxial compressive creep testing. Calculated stress exponents were found to be 2.6±1.6 and 5.7±1.4 for α-U and U-10Zr, respectively, and calculated activation energies were found to be 61.6±1.1kJ/mol for α-U. The creep data were also used to evaluate existing porosity inclusive in creep models.

Synchronously enhancing the creep performance along two different loading directions in hot-rolled Mg-2wt%Sn alloy plate

To save rare-earth (RE) resource and develop creep-resistant Mg alloys in a low-cost way, RE-free Mg alloys are strongly recommended recently. However, serious creep anisotropy and poor creep resistance are still the main problems of safely using RE-free Mg alloys. In the present research, one binary Mg-2wt%Sn hot-rolled plate was employed for studying the compressive creep performance at 393 K. It was found that different activities of basal slip, twinning and pyramidal slip weakened the creep performance along the rolling direction and caused the inevitable creep anisotropy. To weaken the creep anisotropy between the rolling direction and normal direction, multi-directional forging (MDF) with three passes was performed at 393 K. Through inducing high-density twins before creep loading, detwinning was frequently activated during creep loading instead of inducing new twins. As a result, the creep anisotropy was successfully weakened in the MDF processed alloy. Moreover, the creep performance was synchronously enhanced along both loading directions rather than deteriorating the creep performance along the normal direction. It is thus expected that this RE-free way of preparing creep-resistant Mg alloys can be widely accepted in the relative application fields.

CREEP Coefficient and Specific Creep of Engineered Cementitious Composite -Bendable Concrete

Abstract Creep is a complex behavior of concrete since it induces different stages of response under loading and unloading with time. It represents the time-dependent strain as a result of a constant sustained load applied with time. In cementitious materials such as engineered cementitious composite concrete, ECC, the loaded cement paste is the principal source of creep strain. Therefore, investigating the overall creep behavior, under the loading and unloading stages, will be beneficial in providing data for the performance of ECC concrete. The test for compressional creep strain is done according to the ASTM C- 512 under controlled temperature (21 oC) and relative humidity (40%). In this research, the loading stage lasted for 9 months, and the unloading stage lasted 3 months. The total creep strain, creep coefficient, and specific creep were recorded for six ECC concrete mixes. The mixes have two strength levels, 30 and 60 MPa at 28 days, and contain polypropylene and polyvinyl alcohol fibers. Results revealed significant enhancement, and lower creep behavior, in mixes including fibers compared to plain mixes. The best promising results for the creep coefficient and specific creep were recorded when using mixes of 60 MPa containing polyvinyl alcohol fibers compared to plain mixes.

Short term creep properties of WPC at different temperatures—An experimental and numerical investigation at different stress levels

AbstractUsing a two‐step molding process, wood plastic composite (WPC) were prepared under temperatures ranging from 160 to 180°C. Uniaxial compression tests of WPC were conducted at temperatures of −20°C, 0°C, 20°C, and 40°C, determining stress levels of 30%, 45%, and 60% based on the test results. Subsequently, 24‐hour compression creep tests were performed for WPC under 12 different conditions. The Findley power law model was employed to fit the experimental results, indicating that creep deformation increased with both stress level and environmental temperature. Notably, specimens at 0°C failed after sustaining a load for 13.6 hours at 60% stress, while those at 20°C failed after only 0.72 hours under the same conditions. Overall, the Findley model effectively predicted the creep behavior of WPC under all temperature conditions and stress levels.Highlights The influence of temperature variation on the strength of WPC. Short term creep of WPC under different stress levels. Creep of WPC at different temperatures. Prediction of creep performance under different operating conditions.

Compression creep characteristics of crushed sugarcane end-leaves

The creep behavior of crushed sugarcane end-leaf (SEL) was studied, with consideration of different loading force, moisture content, and feeding amount. Statistical analysis software was used to develop and fit the regression data to a strain change law as a function of time. The four-element Burgers model was used. A further goal was to analyze the effect of different test conditions on the fitted creep characteristic parameters. The instantaneous elasticity coefficient E0 was found to increase when the loading force increased; the value of delayed elasticity coefficient E1 increased and the value of cohesion coefficient η0 decreased when the feeding amount increased; the value of delayed elasticity coefficient E1 and cohesion coefficient η0 decreased when the moisture content was increased. Therefore, the loading force, moisture content, and feeding amount of crushed SEL all affected the creep capacity of crushed SEL in compression. The results can provide substantial theoretical reference for the silage production of crushed SEL.

Creep Characteristics of Reconstituted Silty Clay under Different Pre-Loading Path Histories

Due to the long-term deformation settlement of foundations, issues such as damage and functional failure of buildings and structures have long been a concern in the engineering field. The creep of soil is one of the primary causes leading to long-term deformation of foundations. In this paper, the consolidation deformation, creep characteristics, and creep model of reconstituted saturated silty clay were studied using the isotropic consolidation creep test and triaxial compression creep test. The results show that for the isotropic consolidation creep test, although the applied load adopted different stages of loading, as long as the final applied confining pressure was the same, the number of stages applied by the confining pressure had little effect on the final isotropic consolidation deformation of the sample and the triaxial undrained shear strength after creep. However, for the triaxial shear creep test, it was found that under the same final deviatoric stress, the final deviatoric strain of the sample was closely related to the number of loading stages of deviatoric stress. The test showed that the more loading stages with the same deviatoric stress, the smaller the final deviatoric strain, and the triaxial undrained shear strength of the sample after creep increased. In addition, it was reasonable to set the pore pressure dissipation of the sample at 95% ((u0 − u)/u0 = 95%) as the time (t100) at which the primary consolidation of the soil sample was completed. The isotropic consolidation creep curves and the triaxial compression creep curves showed certain non-linearity. Then, the logarithmic model and the hyperbolic model were used to fit the creep curves of the samples. It was found that the hyperbolic model had a better fitting effect than the logarithmic model, but for the triaxial compression creep test, the creep parameters of the sample changed greatly. Therefore, studying the creep characteristics of soil under different pre-loading steps is of significant engineering importance for evaluating the long-term deformation of underground structures.

Microstructure and mechanical properties of cast, eutectic Al-Ce-Ni-Mn-Sc-Zr alloys with multiple strengthening mechanisms

The effects of Ni additions on microstructure and microhardness of near-eutectic Al-9Ce-xNi-0.75Mn-0.18Sc-0.12Zr (x = 2.5, 3.2, and 3.9 wt.%) alloys are investigated for various cooling rates. The as-cast microstructure consists of fine, micron-scale Al11Ce3 and Ni-rich phases formed during eutectic solidification. Two Ni-rich phases are observed: (i) Al27Ce3Ni6 at higher Ni contents and slower solidification rates, and (ii) Al9(Ni,Mn,Fe)2 at lower Ni contents and faster solidification rates. While these Ni-containing alloys have higher microhardness than a Ni-free control alloy, varying the Ni concentration in the 2.5–3.9 wt.% range does not significantly affect the microhardness, indicative of competing strengthening and weakening effects from adding Ni. The alloy with intermediate Ni content (3.2 wt.%) is selected for further microstructural and mechanical characterization. It contains four coarsening-resistant strengthening constituents: (i) micron-scale Al11Ce3 and (ii) Al27Ce3Ni6 or Al9(Ni,Mn,Fe)2 platelets, all formed during eutectic solidification, (iii) L12-Al3(Sc,Zr) nanoprecipitates formed on aging, and (iv) Mn atoms in solid solution in the α-Al matrix. The combined strengthening mechanisms impart high strength at ambient and elevated temperatures, as measured by microhardness (as a function of aging time), by compression and tensile experiments, and by compressive creep measurements. Alloys previously reported in the literature - with various combinations of one, two or three of these four strengthening phases - show lower hardness and creep resistance, indicating that cumulative strengthening can be achieved when combining mechanisms; the present alloy containing all four phases shows a remarkably high creep threshold stress of 62 MPa at 300 °C.

Creep damage constitutive model of rock based on the mechanisms of crack-initiated damage and extended damage

Since the classical element model cannot describe the nonlinear characteristics of rock during the entire compressive creep process, nonlinear elements and creep damage are generally introduced in the model to resolve this issue. However, several previous studies have reckoned that creep damage in rock only occurs in the accelerated creep stage and is only described by the Weibull distribution. Nevertheless, the creep damage mechanism of rocks is still not clearly understood. In this study, a reasonable representation of the damage variables of solid materials is presented. Specifically, based on the Gurson damage model, the damage state functions reflecting the constant creep stage and accelerated creep stage of rock are established. Further, the one-dimensional and three-dimensional creep damage constitutive equations of rock are derived by modifying the Nishihara model. Finally, the creep-acoustic emission tests of phyllite under different confining pressures are conducted to examine the creep damage characteristics of phyllite. And the proposed constitutive model is verified by analyzing the results of creep tests performed on saturated phyllite. Overall, this study reveals the relationship between the creep characteristics of rocks and the corresponding damage evolution pattern, which bridges the gap between the traditional theory and the quantitative analysis of rock creep and its damage pattern.

Experimental study on I/II/III mixed mode fracture characteristics of a combined rock mass under creep loading

I/II/III mixed mode fractures of intersecting joint fissures often occur in natural rock masses, and jointed rock masses are prone to rockbursts in deep underground engineering when subjected to long-term crustal stresses. However, most studies of the mechanical mechanisms of these intersected joints have been conducted by simplifying two-dimensional joint model tests. Furthermore, the fracture mechanisms of two-dimensional intersected joints under tension and compression are completely different from those of three-dimensional joints. This paper presents a novel prefabricated specimen with combinations of intersecting joints capable of detecting the failure behaviours of rock I/II/III mixed mode fractures under creep loading. Uniaxial compression and multistage creep tests are performed on prefabricated sandstone specimens with intersecting joints of 0°/0°, 0°/30°, 0°/60°, and 0°/90°. The experimental results show that with the increase in the number of prefabricated intersecting joints, the uniaxial compressive strength and elastic modulus values of the sandstone specimens gradually decrease. In addition, the sandstone specimens experience relatively few AE events and minor axial strain variations in the first creep stage and the second creep stage of the multistage creep test. The axial strain increases sharply due to the sharp increase in the number of AE events in the third creep stage. The 0°/60° sandstone specimen undergoes accelerated creep failure, resulting in mixed X-shaped tensile‒shear rupture. The RA value is high based on the quantification of the creeping cracks using the acoustic emission parameters of the rise angle (RA) and average frequency (AF). The AF values of the 0°/0°, 0°/30°, and 0°/90° sandstone specimens are high. The experimental results show that a larger joint intersection angle leads to greater mutual restraints and greater effects of prefabricated crack propagation in the rock specimens, thus increasing the final failure strength. Finally, based on the acoustic emission count, a characteristic variable D suitable for characterizing the creep damage evolution of a joint rock mass is established. The findings of this paper can facilitate an effective understanding of the creep effect of I/II/III mixed mode fracture and its micromechanism. The research results will have a certain reference value for the detection and risk mitigation of instantaneous and time-delayed rockbursts.

Anisotropic characteristics and creep model for thin-layered rock under true triaxial compression

The failure phenomenon of thin-layered rock tunnels not only exhibits asymmetric spatial characteristics, but also significant time-dependent characteristics under high in-situ stress, which is attributed to the time-dependent fracture of thin-layered rocks. This paper conducted a series of true triaxial creep compression tests on typical thin-layered rock siliceous slate with acoustic emission technique to reveal its anisotropic time-dependent fracture characteristics. The anisotropic long-term strength, creep fracturing process, and fracture orientation characteristics of thin-layered rocks under different loading angles (β, ω) and intermediate principal stress were summarized. A three-dimensional (3D) non-linear visco-plastic creep model for thin-layered rock was developed to simulate its anisotropic creep behavior. The time-dependent fracturing of rocks during true triaxial creep loading is reflected through the change of equivalent strain based on an improved Euler iteration method. By constructing the plastic potential function and overstress index related to loading angles and stress state, the anisotropic time-dependent fracturing process and propagation of thin-layered rocks under different loading angles and intermediate principal stress are expounded. The model was validated experimentally to show it can reflect the long-term strength and creep deformation characteristics of thin-layered rocks under true triaxial compression.

Identifying the activated slip modes and twin variants in compressive creep deformation of Mg-8.0Al-1.0Nd-1.0Gd

The compressive creep deformation mechanism of as-cast Mg-8.0Al-1.0Nd-1.0Gd alloy at 180 °C under constant stress of 60 MPa and 90 MPa was novelly analyzed through the distribution characteristics of in-grain misorientation axes (IGMA). Activated slip modes in the creep process was accurately identified and the structure of dislocations is revealed by the weak beam dark-field (WBDF) technology. The intensities of IGMA are concentrated on 011̅0 and 1̅21̅0 axis demonstrates that 0001112̅0(basal slip) and 112̅2112̅3̅ (pyramidal slip) dominate the deformation mechanism. Tensile twins 101̅21̅011 were extensively activated in compression-creep process and the 101̅21̅011 variants with a twinning trace angle close to 90 deg are 1̅012101̅1 and 101̅21̅011 with para-position. The creep mechanism of the alloy crept at 180°C/60 MPa is dominated by grain boundary sliding triggered by the precipitation of β-Mg17Al12, and pipe diffusion-controlled boundary sliding is the dominant creep mechanism of the alloy under 180 °C/90 MPa. In addition, the results reveal that tensile twins induced by dislocation slip contribute to coordinate the grain c-axis deformation, release the stress concentration caused by dislocation accumulation at the grain boundary.

Quantitative investigation on the individual contribution of twinning and dislocation slip to creep in dilute Mg-Y alloys with various grain sizes

This work investigated the individual contribution of twinning and dislocation slip to creep in Mg-2 wt%Y alloys with different grain sizes. Three samples with the grain sizes of d = 31.2 μm, d = 63.5 μm and d = 127.7 μm were prepared by regulating the annealing treatment conditions, and the compressive creep tests were conducted at 473 K under 80 MPa along the rolling direction. It was found that the volume fraction of twins and the density of dislocations were continuously increased with increasing creep time regardless of grain size. During the first 50 h, the contribution of twinning was gradually increased while the contribution of dislocation slip was gradually decreased. Afterwards, the contributions of twinning and dislocation slip remained stable with increasing creep time in all samples. In addition, it was suggested that decreasing grain size could reduce the contribution of twinning but enhance the contribution of dislocation slip to creep. The results may provide reliable reference to the understanding and modeling of creep behaviors in Mg-RE alloys.

Improved creep resistance of Mo-V-Si-B alloys with a density below 8 g/cm³

Improving the efficiency of turbines for power plants and aircraft engines is an increasingly important research subject. Ternary Mo-Si-B alloys, consisting of a molybdenum solid solution (Moss) phase and two intermetallic phases Mo5SiB2 (T2) and Mo3Si, are able to combine balanced room temperature fracture toughness, high temperature creep strength and good oxidation performance. However, the high density (typically > 9 g/cm3) of this class of alloys is a drawback when used as structural materials for rotating components. In this study, high potential Mo-V-Si-B materials which provide a reduced density by about 17 % as compared with the reference alloy Mo-9Si-8B were investigated. A comparative characterization was carried out on sintered (FAST) and arc-melted Mo-40V-9Si-8B alloys. Compressive creep tests reveal competitive creep resistance of this new type of alloys in the application relevant temperature regime.

Exploring the evolution of microstructure and mechanical property of a γʹ-strengthened Co-based single crystal superalloy during creep and thermal exposure at 1000 °C

The study provided a systematic investigation into the creep behavior of a γʹ-strengthened Co-based single crystal superalloy Co-7Al-8W-1Ta-4Ti (at.%) under compressive creep conditions at 1000 °C and 137 MPa. Simultaneously, the microstructural changes during thermal exposure at 1000 °C were thoroughly examined. The microstructure analysis of the creep samples revealed the presence of γ/γʹ rafts, twins, stacking faults, and dislocation networks. Statistical analyses were conducted to assess the dimensions and volume fractions of γʹ, alongside Vickers hardness tests and nanoindentation experiments. Results show that γʹ dissolution in the creep samples with substantially higher dissolution rates compared to thermal exposure. Localized Co depletion and Ti enrichment in twin is observed in creep sample, potentially initiating localized phase transformations. Concurrently, lattice rotation occurred during creep, resulting in a progressive deviation of crystal orientation away from the [001] direction. Stacking faults transitioned from network-like structures to parallel configurations, and twins played a significant role in creep deformation, particularly in samples subjected to 382 h creep. The study also explored the evolution of Vickers hardness and the elastic modulus, introducing a systematic linear model to describe Vickers hardness changes during both thermal exposure and creep conditions.

Analysis on the compressive creep behaviors of Mg–Y alloys with various grain sizes

The creep behaviors of a hot-rolled Mg-4.5 wt%Y alloy sheet in the grain size ranging from d = 10 μm to d = 160 μm were carefully investigated in this work. It was shown that both loading direction and grain size exhibited effects on the creep behavior. The creep resistance along the ND was always greater than that along the RD, and the increased grain size contributed to the improved creep resistance along both directions. For the fine-grained samples (d = 10 μm to d = 25 μm), the dominant mechanisms were independent of the loading direction, including grain boundary migration (GBM), cross-slip made by basal dislocations and prismatic dislocations, and <c + a > dislocations slipping on the prismatic planes. The undifferentiated dominant mechanisms led to the extremely low creep anisotropy. For the coarse-grained samples (d = 60 μm to d = 160 μm), the dominant mechanisms were different along two loading directions. Specifically, all RD samples shared the dominant mechanisms of cross-slip and {101‾2} twinning, while the dominant mechanisms of all ND samples were dislocation climb and <c + a > dislocations slipping on pyramidal planes. The easy activation of {101‾2} twinning replaced GBM and induced the creep anisotropy, resulting in a low incremental speed of creep resistance along the RD. Differently, the primary dislocation climb and pyramidal <c + a > slip brought a high incremental speed of creep resistance along the ND. Consequently, the coarsen-grained samples had a strong creep anisotropy.

Investigation of the influence of pre-crack number on acoustic emission characterization of red-sandstone short-term creep damage and failure precursors

Rock crack is one of the main factors responsible for rock failure. Uniaxial compression creep tests are performed using acoustic emission techniques, a high-sensitivity, non-radiative, non-destructive testing method to understand the influence of crack number on the precursor characteristics of short-term creep damage in the fractured rock mass. Based on the Grassberger-Procaccia (G-P) algorithm, the calculation step size for the correlation dimension value (D 2) of the acoustic emission ringing count rate is consistent with that for the acoustic emission b-value. The influence of the number of pre-cracks on the Acoustic emission precursor characteristics of red sandstone creep is analyzed. The results show that near the destabilization of the specimen, the Acoustic emission accumulative ringing count surges in a stepwise manner, the Acoustic emission b-value decreases, the D 2-value increases, the Acoustic emission amplitude shows high intensity and high frequency, and the ringing count increases sharply, all with the characteristics of failure precursors. During the accelerated creep stage of the specimens, with the increase of pre-cracks number, the precursory time points of acoustic emission b-value and D 2-value advance, and their acoustic emission ringing counts increase sharply.