Creep Experiments Research Articles

Creep constitutive law prediction via short-term evaluation using a small size testing method

To efficiently assess the mechanical reliability and long-term durability of solid oxide fuel cells (SOFCs) and solid oxide electrolyser cells (SOECs), a suitable creep test method and prediction of the material response is necessary. This study investigates the effectiveness of the small punch (SP) testing method for Ni–YSZ composites and in rapidly determining the constitutive creep law for SOFC materials. The creep properties of Ni–YSZ composites are evaluated under a 1% H2 environment at temperatures of 800–900 °C and stresses of 36–58 MPa. The finite element method is used to simulate the SP tests and generate numerical creep curves. These curves are fitted to the experimental curves to obtain the parameter set in the creep constitutive law of the strain hardening model. The numerical and experimental creep curves are well-fitted with only a 0.44% difference and convergent parameter sets equal to C1=1.0× 10−12, C2=1.03, C3 = -1.70, C4=96.78 in the strain hardening law, indicating the usefulness of the SP method and creep constitutive law for characterising and predicting the creep properties of SOFC constituent materials. This study highlights the importance of accurate characterisation of creep properties for predicting the long-term performance and reliability of SOFCs.

On the experimental characterization and response modelling of particulate composite: Application to flaky aluminium-epoxy composite

The epoxy-based composites are most commonly used now in the automobile and aerospace industries due to their low weight and high strength. The quasi-brittle nature of epoxy resin, which is not desirable in several applications, can be overcome by adding flaky aluminium additive resulting in a particulate composite. A small volume fraction of flaky aluminium influences the mechanics of epoxy resin in a significant manner (increase in tensile strength and strain at failure by 56 % and 200 %, respectively, comparing with neat epoxy) resulting in a ductile-type failure. The nonlinear, rate-independent (at low loading rate), rate-dependent (at higher loading rate), and inelastic (viscous effect) response, coupled with an internal damage, observed in epoxy-based flaky-aluminium composite is not fully understood due to the complex nature of resulting material. Much of the previous literature focused on modelling, either the rate-dependency or damage at low loading rate, but not capturing everything concurrently.The novelty of present work is a broad framework development for experimental characterization and calibration of particulate composite (flaky aluminium additives in the epoxy matrix) performing uniaxial tension, cyclic loading–unloading, and creep experiments at different displacement loading rates, and subsequent modelling of the constitutive response in a thermodynamically consistent manner. The macro-scale properties of the newly proposed particulate composite are estimated by mean-field homogenization and compared with the corresponding experimental values. The elasto-plastic-damage and visco-plastic-damage material constants are successfully obtained by a novel optimization approach coupling Levenberg–Marquardt algorithm with stress update algorithms. The practical applicability of the obtained material constants is finally demonstrated solving several boundary value problems employing Lemaitre’s ductile damage model.

A creep life prediction model of P91 steel coupled with back-propagation artificial neural network (BP-ANN) and θ projection method

In order to predict the long-term creep life of P91 steel, this work proposed a coupled model, combining the error-trained back-propagation artificial neural network (BP-ANN) and an improved θ model. Both the short-term rupture life obtained by creep experiments and the related creep data of National Institute for Materials Science (NIMS) database were used to validate the above model. In this model, creep parameters (temperature, stress) were used as the input and relative error of 5 θ model extrapolations as the output. Then, the prediction errors of BP-ANN method were introduced to correct the 5 θ model extrapolation results to achieve a more accurate prediction. Consequently, the long-term creep life of P91 steel was predicted. It demonstrated that with the proposed model, the prediction capability for long-term creep life of P91 steel with 200,000 h is much better than those of other θ methods, which has a prediction uncertainty less than 5%.

Contribution of high-temperature 3D-Digital Image Correlation to investigate the creep behaviour of refractory materials

Potential contributions of 3D-Digital Image Correlation are investigated to understand the dilatant behaviour of a refractory concrete during high-temperature creep experiments. An experimental analysis of high-temperature image quality is performed. The measurement uncertainty is assessed and compared to the sensitivity of computed kinematic fields to the parameters of the Drucker-Prager model that control the dilatancy. Promising results are found i) concerning deflection and neutral axis location measurement in bending creep tests and ii) contactless measurement of axial strain in compression creep tests, iii) to support the assumption of dilatant behaviour from bending and diametral compression tests and iv) to provide data for a rough parameter identification from compression creep tests.

A creep model for ultra-deep salt rock considering thermal-mechanical damage under triaxial stress conditions

To investigate the specific creep behavior of ultra-deep buried salt during oil and gas exploitation, a set of triaxial creep experiments was conducted at elevated temperatures with constant axial pressure and unloading confining pressure conditions. Experimental results show that the salt sample deforms more significantly with the increase of applied temperature and deviatoric loading. The accelerated creep phase is not occurring until the applied temperature reaches 130 °C, and higher temperature is beneficial to the occurrence of accelerated creep. To describe the specific creep behavior, a novel three-dimensional (3D) creep constitutive model is developed that incorporates the thermal and mechanical variables into mechanical elements. Subsequently, the standard particle swarm optimization (SPSO) method is adopted to fit the experimental data, and the sensibility of key model parameters is analyzed to further illustrate the model function. As a result, the model can accurately predict the creep behavior of salt under the coupled thermo-mechanical effect in deep-buried condition. Based on the research results, the creep mechanical behavior of wellbore shrinkage is predicted in deep drilling projects crossing salt layer, which has practical implications for deep rock mechanics problems.

Oxidation Effects on Short-Term Creep Response in Air of Commercially Pure Titanium (CP-2 Ti)

The creep response in the air of commercially pure titanium was investigated at 550, 600, and 650 °C to assess the effect of oxidation on the mechanical response. Experiments demonstrated that prolonged exposures at high temperatures produced a marked reduction in the minimum creep rate under a given applied stress. Microhardness measurements showed that a hardened zone formed in proximity to the surface due to oxygen penetration into the metal. A simplified composite model was then used to describe the creep response. In this model, the sample consisted of two zones, the hard case that was enriched in oxygen and the soft pure-titanium core, both creeping with similar strain rates. Calculations led to an estimation of the dependence of the minimum creep rate on stress and temperature for the hard high-oxygen zone. The simplified composite model presented here provided a good description of the experimental creep data for pure titanium, tested in its air, and a reliable picture of the effect of oxidation on complex Ti alloys.

Influence of the γ′ Volume Fraction on the High-Temperature Strength of Single Crystalline Co–Al–W–Ta Superalloys

Understanding the influence of γ′ and secondary-phase fractions on the mechanical properties of superalloys is very important to optimize these high-strength materials. So far, this has not been systematically investigated for the novel class of Co-based superalloys. In this study, a Co–Al–W–Ta model alloy series was designed with compositions of γ/γ′ on the tie-line and an increasing γ′ volume fraction of up to 70% after heat treatment at 900 °C, while a few alloys are unexpectedly out of γ/γ′ two-phase region with an additional secondary phase fraction of up to 15%. The high-temperature strength and creep properties were evaluated by compression tests up to 1050 °C and compressive creep experiments at 950 °C, respectively. At temperatures of up to 1050 °C, an increasing γ′ volume fraction consistently increased the yield strength, which was not dramatically changed by the presence of secondary phases. Significant work hardening was found in alloys with γ′ volume fractions of 65–70% during compression testing, but not in alloys with either a lower γ′ volume fraction (<50%) or a high fraction of secondary phases (~15%). Similar to the yield strength, the creep strength also increased continuously with the γ′ volume fraction, but was greatly reduced with an increasing fraction of secondary phases. The best creep performance at 950 °C and 200 MPa was found in the alloy with the highest γ′ volume fraction and no secondary phases. At higher creep stresses, rafting contributed significantly to the hardening and, again, the alloy with a high γ′ volume fraction and a small amount of secondary phases exhibited the highest strength.

Failure mechanism of single-crystal superalloy NiAlReRu components with film cooling holes under the coupling high-speed rotating and high temperature condition

In order to make up for the deficiency that the laboratory testing of standard samples cannot approximately simulate the turbine blade geometry and service environment, the creep experiment of the nickel-based single crystal superalloy NiAlReRu components with film cooling holes (FCHs) is carried out under the coupling high-speed rotating and high temperature condition. The component with the FCHs exposed to the coupling high-speed rotating of 20000 rpm and high temperature of 1000 ℃ continuously lasts 255.92 h until it breaks. The microstructural evolution and fracture process are analyzed. The results indicate that the existence of the FCHs evidently changes the stress distribution of the region with the FCHs, which makes the microstructural evolution of γ′ phase significantly different. The component is fractured along the second row of the FCHs and the normal direction of the fracture surface is basically parallel to the centrifugal force direction. The fracture process can be divided into three typical stages. The coupling effect of temperature and stress on the failure mechanism is discussed.

Triaxial Deformation of the Goldwyer Gas Shale at In Situ Stress Conditions—Part II: Viscoelastic Creep/Relaxation and Frictional Failure

To understand the geomechanical implications of long-term creep (time-dependent deformation) response of gas shale, short-duration creep was recorded from laboratory triaxial tests on ten Goldwyer gas shale samples in the onshore Canning Basin at in situ stress conditions under constant differential axial stress. A simple power-law function captures primary creep behaviour involving elastic compliance constant B and time-dependent factor n. Experimental creep data revealed larger axial creep strain in clay and organic-rich rocks, than those dominated by carbonates. Anisotropic nature of creep was observed depending upon the direction of constant axial stress application (perpendicular or parallel to the bedding plane). Upon the application of linear viscoelastic theory on laboratory creep fitting coefficients, differential horizontal stress accumulation over a geological time scale was estimated from the viscoelastic stress relaxation concept. Further, this model was used to derive lithology-dependent least principal stress (Shmin) magnitude at depth for two vertical wells intersecting the Goldwyer gas shale formations. This newly proposed Shmin model was found to have a profound influence on designing hydraulic fracture simulation. Further, pore size distribution and specific surface area value SN2 were derived from low-pressure gas adsorption experiments. These physical properties along with weak mineral components were linked with creep constitutive parameters to understand the physical mechanisms of creep. A strong correlation was noted between SN2 and creep parameters n and B. Finally, an attempt was made to investigate how gas shale composition and failure frictional properties can influence shear fracturing.

Utilization of creep ringing and bioinformatic modelling in study of cold denatured pea protein emulsions

Pea protein is a popular and sustainable plant-based protein with broad applications in the food industry. However, functionalities such as emulsification are still lacking. Protein functionality is defined by its structure, and protein structure can be modified through denaturation. In this work, the impacts of cold denaturation on emulsifying abilities of pea protein are studied. Commercial pea protein is treated with ethanol, shear forces, and low temperatures. Emulsions are prepared and characterized using creep experiments over time. Creep data are modeled using the Burger model, and the often-overlooked creep ringing region is fit with the Jeffreys model. Using these methods together provides valuable insight into emulsion structure and flow. Furthermore, rheological parameters favorably correlate with bioinformatic estimates of emulsion network strength derived from surface hydrophobicity and zeta potential measurements. Findings show that protein pre-treatments improve rheological stability of emulsions, and these impacts are most obvious at high concentrations, making these treatments good for protein supplementation in the food industry.

An advanced approach to improve the high-temperature property for Ni-based superalloys: Interface segregation manipulation

In response to the requirements for the long-term service reliability of Ni-based superalloys, the correlations between the microstructure and creep property are investigated in present work. Creep experiments were conducted on a designed Ni-based single crystal superalloy at 900 °C/330 MPa, the results indicate that the creep rupture properties deteriorated following thermal exposure at both 900 °C and 1000 °C. The γ′-phase, γ-matrix and γ/γ′ interface undergo varying degrees of degradation, including the coarsening and the decrease in the volume fraction of γ′-phase, the decrease in the γ-matrix strength, the decrease in the lattice misfit and the increase in the dislocation network spacing. When the values of the microstructural parameters (γ′-phase size and volume fraction) were numerically similar for different thermal exposure times at 900 °C and 1000 °C, taking 900 °C/10,000h and 1000 °C/2000h samples as examples. The experimental alloy exhibited better creep resistance after being exposed to 1000 °C compared to 900 °C. Through the combination of atom probe tomography and first-principles calculations, an interface strengthening mechanism driven by interface segregation is identified. Finally, a design concept for high-strength metal alloys based on the decoration of solutes interface segregation is proposed, also called “Interface Segregation Manipulation”.

Research on thermal creep behavior of Chinese low-activation ferritic/martensitic steel

A series of thermal creep experiments are conducted on Chinese low-activation ferritic/martensitic steel (CLF-1 steel) at a temperature of 550, 575, and 600 °C with various normal stress (from 150 to 300 MPa), respectively. The results are analyzed by creep curves, microstructure, and creep properties. Finished samples display adequate fracture surface in short creep rupture tests (within 3000 h). The comparative creep curve shows no difference related to the thickness of CLF-1 steel plate (10 and 30 mm). Microstructure images before and after creep reflect a tempered lath martensite structure and carbides decorating both in prior austenitic grain boundaries and lath boundaries. Comparison is discussed between 30 mm plate and 10 mm plate. All current results reflect that all CLF-1 steel samples shows good creep behavior and proves that a series excellent progress of CLF-1 steel R&D work has been successfully achieved. Besides, collected data will contribute a lot to the design and R&D work of the Chinese Helium Cooled Ceramic Breeding Test Blanket Module (CNHCCB-TBM).

Evaluation of time dependent mechanical properties through nanoindentation of different compositions of 2D Ruddlesden Popper and 3D lead halide perovskites

Hybrid organic-inorganic perovskite (HOIPs) films' simplicity in manufacturing, both in solution and at low temperatures, foresees its application in deformable technologies including flexible photovoltaics, sensors and displays. The next generation of flexible optoelectronic devices may be possible owing to the special characteristics of hybrid organic-inorganic perovskites (HOIPs). However, it is necessary to have a thorough knowledge of the mechanical response of hybrid organic-inorganic perovskites(HOIPs) to dynamic strain to successfully use them in deformable devices. In the current study, a range of perovskites: 3D, butylammonium(BA) based 2D and quasi-2D, phenylethylammonium(PEA) based 2D and quasi-2D perovskite have been fabricated with particle sizes ranging in 5–10 nm as found using TEM. The nanoindentation creep and stress relaxation experiments prove the time and rate-dependent mechanical properties of this 2D, quasi-2D, and 3D HOIPs crystal though varying in their magnitudes. We observe that the 3D as well as BA based perovskite show strain rate sensitivity whereas PEA based perovskite samples were relatively insensitive towards the rate of loading. Propagation and interaction of dislocation are much more difficult in PEA-based perovskite, which has a triclinic crystal structure and are less symmetrical as compared to the BA based and 3D perovskite orthorhombic and tetragonal structure respectively. The knowledge offered by this work is crucial for creating perovskite devices that can endure mechanical deformations.

Using Selective Electron Beam Melting to Enhance the High‐Temperature Strength and Creep Resistance of NiAl–28Cr–6Mo In Situ Composites

By increasing the density of interfaces in NiAl–CrMo in situ composites, the mechanical properties can be significantly improved compared to conventionally cast material. The refined microstructure is achieved by manufacturing through electron beam powder bed fusion (PBF‐EB). By varying the process parameters, an equiaxed or columnar cell morphology can be obtained, exhibiting a plate‐like or an interconnected network of the (Cr,Mo) reinforcement phase which is embedded in a NiAl matrix. The microstructure of the different cell morphologies is investigated in detail using scanning electron microscope, transmission electron microscopy, and atom probe tomography. For both morphologies, the mechanical properties at elevated temperatures are analyzed by compression and creep experiments parallel and perpendicular to the building direction. In comparison to cast NiAl and NiAl–(Cr, Mo), the yield strength of the PBF‐EB fabricated specimens is significantly improved at temperatures up to 1,027 °C. While the columnar morphology exhibits the best improved mechanical properties at high temperatures, the equiaxial morphology shows nearly ideal isotropic mechanical behavior, which is a substantial advantage over directionally solidified material.

A Study on the Characteristics of High-Solid-Water Filling Materials with Different Water Contents Based on the Boltzmann Superposition Principle

High-solid-water material is a kind of soft rock-like material with significant plastic deformation characteristics, and creep performance is its important material property. In order to study the effect of creep on the properties of high-solid-water materials, this paper first conducts creep tests of high-solid-water filling materials under different loading methods, performs creep data analysis based on the Boltzmann superposition principle, carries out the creep experiment of high-solid-water materials under different water contents to analyze the creep characteristics of high-solid-water materials, and then processes the curve of graded loading based on the Boltzmann superposition principle to obtain the strain–time curve of axial creep under different constant stress. Finally, the instantaneous curve is compared with the creep stress–strain curve. It is concluded that the high-solid-water filling materials with water content of 61%, 65% and 69% meet the transportation requirements and filling requirements, and the total coagulation time increases slightly with the increase in water content. The curve treated by the Boltzmann superposition principle has the same trend in the overall creep process and the creep curve loaded separately. With the increase in loading stress, the total strain of the material gradually increases, and with the increase in the water content of the material, the rate of change in the total strain of the material also increases. The long-term strength of high-solid-water materials decreases with the increase in water content. The specimen will eventually cause irreversible failure under long-term stress during the fourth-stage loading.

Mechanical properties of polyurethane foam for potential application in the prevention and treatment of pressure ulcers

The viscoelastic property is a crucial parameter for evaluating the feasibility of the foam to be used as pressure relief. This work aims to perform experimental measurements to determine the dependence of the viscous behavior of polyurethane foam on density and energy dissipation. Experimental measurements were realized for three types of polyurethane foams with different densities soft, medium and hard. The compression tests at room temperature characterized the mechanical properties of these samples. Furthermore, the structures of the samples were characterized using scanning electron microscopy (SEM). A proportional relationship was observed between the density of polyurethane (PU) foam and the tolerated strength at 90% compression. This material has an excellent remanence (persistence) which showed by the creep experiments at constant stress (2, 5, 10 N) for 6 h. Young Modulus, relaxation time and dissipation coefficient (DE) were also calculated from the stress-strain curve. Moreover, softening behavior, which is responsible for pressure relief, was measured by a cyclic compression test. This paper assesses the capacity of polyurethane to effectively maintain interfacial pressure below the critical threshold of 32 mm of mercury, which represents a significant risk level for the development of pressure sores.

Creep Deformation Behavior, Microstructure Evolution, and Damage Mechanism of Super304H ODS Steel

In this paper, the creep deformation behavior, microstructure evolution, and damage mechanism of Super304H oxide dispersion strengthened (ODS) steel have been systematically investigated at 650 °C. The creep behavior of the ODS steel could be understood by virtue of a dislocation creep deformation mechanism. Interrupted creep experiments under 100 MPa were conducted to further study the microstructure evolution during long-term creep deformation. The grains began to refine at the initial stage, and some M23C6 phases were observed at grain boundaries, which enhanced the microhardness during the first creep stage. Along with creep, both Y2O3 and Cu-rich precipitates exhibited good coherence with the matrix. The Y2O3 precipitates showed better thermal stability than the Cu-rich phase. During the second creep stage, some cavities emerged around the M23C6 phase and at grain boundaries. The cavities gradually developed into significant cracks, causing the steel to fracture. The creep damage due to cavity growth could be determined according to the creep damage tolerance factor value.

Calculation of Short-Term Creep of Concrete Using Fractional Viscoelastic Model.

The study of short-term creep is essential for understanding the concrete creep process and deformation under alternating stress. Researchers are concentrating on the nano- and micron-scale creep of cement pastes. In the latest RILEM creep database, short-term concrete creep data at hourly or minutely levels are still rare and scarce. In order to describe the short-term creep and creep-recovery behavior of concrete specimens more accurately, the short-term creep and creep-recovery experiments were carried out firstly. The load-holding time varied from 60 s to 1800 s. Secondly, the accuracy of current creep models (B4, B4s, MC2010, and ACI209) in predicting the short-term creep of concrete was compared. It was discovered that the B4, B4s, and MC2010 models all overestimate concrete's short-term creep, and the ACI model does the opposite. Thirdly, the applicability of the fractional-order-derivative viscoelastic model (with a derivative order between 0 and 1) in the calculation of the short-term creep and creep recovery of concrete is investigated. The calculation results show that the fractional-order derivatives are more suitable for analyzing the static viscoelastic deformation of concrete while the classical viscoelastic model requires a large number of parameters. Therefore, a modified fractional-order viscoelastic model is proposed considering the residual deformation characteristics of concrete after unloading, and the values of the model parameters under different conditions are given with the experimental data.

Experimental study on the shear creep characteristics of joints under wetting-drying cycles

To better understand the shear creep characteristics of joints under wetting-drying cycles, multi-stage shear creep experiments are firstly conducted on the joints subjected to different wetting-drying cycles. Then, the shear creep damage characteristics of joints are investigated from the microscopic and macroscopic perspectives through wave velocity tests, scanning electron microscope observations, and failure morphologies. The investigation demonstrates that the increasing wetting-drying cycles lead to a porous and disintegrated microstructure of joints, which further results in increases in the shear deformation and steady-creep rate, and decreases in the failure strength and long-term strength. With increasing wetting-drying cycles, the wear rate of joints decreases and the damaged region of joints becomes smaller and invisible. Furthermore, a classification of the shear creep process of joints containing four stages is firstly proposed based on the test results. It is found that the presence of asperities on the rough joints is the physical basis of the existence of strengthening and cutting stages of the shear creep process. Finally, based on the experimental results, a new fractional-order damage creep model of joints considering wetting-drying cycles is proposed.

Nonlinear time-dependent mechanical behavior of mammalian collagen fibrils.

The viscoelastic mechanical behavior of collagenous tissues has been studied extensively at the macroscale, yet a thorough quantitative understanding of the time-dependent mechanics of the basic building blocks of tissues, the collagen fibrils, is still missing. In order to address this knowledge gap, stress relaxation and creep tests at various stress (5-35 MPa) and strain (5-20%) levels were performed with individual collagen fibrils (average diameter of fully hydrated fibrils: 253±21 nm) in phosphate buffered saline (PBS). The experimental results showed that the time-dependent mechanical behavior of fully hydrated individual collagen fibrils reconstituted from Type I calf skin collagen, is described by strain-dependent stress relaxation and stress-dependent creep functions in both the heel-toe and the linear regimes of deformation in monotonic stress-strain curves. The adaptive quasilinear viscoelastic (QLV) model, originally developed to capture the nonlinear viscoelastic response of collagenous tissues, provided a very good description of the nonlinear stress relaxation and creep behavior of the collagen fibrils. On the other hand, the nonlinear superposition (NSP) model fitted well the creep but not the stress relaxation data. The time constants and rates extracted from the adaptive QLV and the NSP models, respectively, pointed to a faster rate for stress relaxation than creep. This nonlinear viscoelastic behavior of individual collagen fibrils agrees with prior studies of macroscale collagenous tissues, thus demonstrating consistent time-dependent behavior across length scales and tissue hierarchies. STATEMENT OF SIGNIFICANCE: Pure stress relaxation and creep experiments were conducted for the first time with fully hydrated individual collagen fibrils. It is shown that collagen nanofibrils have a nonlinear time-dependent behavior which agrees with prior studies on macroscale collagenous tissues, thus demonstrating consistent time-dependent behavior across length scales and tissue hierarchies. This new insight into the non-linear viscoelastic behavior of the building blocks of mammalian collagenous tissues may serve as the foundation for improved macroscale tissue models that capture the mechanical behavior across length scales.