Creep Contribution Research Articles

On the creep mechanisms and macroscopic creep rate modeling of high-uranium-density composite fuels

UN-U3Si2 composite fuel is a promising candidate of Accident Tolerant Fuels (ATFs). Its in-pile creep performance will have an important impact on the irradiation-induced thermo-mechanical coupling behavior and safety of fuel elements. Based on the existing mechanism-based creep models of UN and U3Si2, the finite element analysis was performed for the multi-scale creep behaviors of various UN-U3Si2 composite fuels, and the influences of the U3Si2 volume fraction, applied stress, temperature, fission rate, and grain size were investigated. The dominant creep mechanisms under different conditions were proposed, and the mechanism-based macroscopic creep rate model for the UN-U3Si2 composite fuels were correspondingly developed, quantitatively correlating with the macroscopic creep rate with the U3Si2 vol fraction, temperature, fission rate and equivalent stress. The creep rate predictions obtained by this model were in good agreement with the results of the finite element simulation. The research results indicate that: (1) the irradiation or thermal diffusion creep contributions from the UN and U3Si2 phases were dominated under various conditions; (2) the dislocation creep contributions from the dispersed-phase of U3Si2 were appreciable at the temperatures ranged from 500 K to 1300 K, while those of the matrix-phase of UN appeared only at an extremely high temperature of 1300 K; (3) different from the ordinary inert-matrix dispersion fuels, the macroscopic creep rates of UN-U3Si2 composite fuels were predicted to be almost independent of burnup, due to the slight difference in the irradiation swelling of UN and U3Si2; (4) the total creep contribution proportions of the U3Si2 or UN phase were almost equal to the initial volume fractions, while the creep contribution ratios within the UN or U3Si2 phase vary under different conditions. The evolutions of various creep contributions with the fission density were attributed to the fuel-swelling induced additional stresses.

Macro synthetic fiber pullout behavior in short- and long-term tests

The creep phenomenon in fiber-reinforced composites is particularly important when synthetic macro fibers are used. In addition to the low modulus of elasticity, the polymers used in fibers have pronounced viscoelastic behavior even at room temperature, which can lead to changes in the cracking control over time. The long-term fiber pullout response also results from fiber and interface creep contributions. To understand the above phenomena, pullout tests with short- and long-term loads were conducted on three types of polymeric macro fibers subject to different levels of long-term loads (20, 30, 40 and 50%). The results of the pullout tests were strongly dependent on the fiber configuration, surface corrugation and modulus of elasticity. Macro fibers with crimped surfaces and higher modulus of elasticity achieved higher bond stresses and lower creep deformations. The Burgers model was applied and showed a good agreement with the experimental creep curves, therefore consisting of a promising alternative for modeling long-term behavior. Microtomography and scanning electron microscopy images showed that a large portion of the strain in tension, under sustained load, can be attributed to the creep of the fiber itself, thus making it challenging to estimate the creep of this type of composite, given the considerable variability of fiber configurations.

Creep behavior and long-term strength characteristics of pre-peak damaged sandstone under conventional triaxial compression

A series of creep tests were carried out on sandstone specimens with different pre-peak instantaneous damage characteristics under different confining pressures. The results revealed that the creep stress was the key factor affecting the occurrence of the three stages of creep, and the steady-state creep rate increased exponentially with increasing creep stress. Under the same confining pressure, the larger the instantaneous damage of the rock specimen was, the more quickly creep failure occurred and the lower the creep failure stress was. For the pre-peak damaged rock specimens, the strain threshold for accelerating creep was the same for a given confining pressure. The strain threshold increased with increasing confining pressure. In addition, the long-term strength was determined using the isochronous stress–strain curve and the variation in the creep contribution factor. The results revealed that the long-term strength decreased gradually with increasing pre-peak instantaneous damage under lower confining pressures. However, the instantaneous damage had little effect on the long-term strength under higher confining pressures. Finally, the macro–micro-failure modes of the sandstone were analyzed according to the fracture morphology observed via scanning electron microscopy. It was found that the macroscale creep failure patterns of the sandstone specimens could be divided into a shear-dominated failure mode under high confining pressures and a mixed shear-tensile failure mode under low confining pressures. At the microscale, as the confining pressure increased, the micro-fracture mode of the sandstone changed gradually from a single brittle fracture to a mixed brittle and ductile fracture mode.

Effect of activator properties on drying shrinkage of alkali-activated fly ash and slag

Alkali-activated fly ash and slag (AAFS) pastes with a CaO content of 10%–20% are cementitious materials with high strength and good durability. The correlation between the drying shrinkage and microscopic characteristics of the pastes is investigated. By affecting the reaction degree of the pastes, the alkali content and activator modulus cause different pore-size distributions and deformation abilities, leading to different capillary pressures and shrinkage of the pastes during drying. The elastic deformation of the pastes under the capillary pressure during drying only accounts for a small part of the whole drying shrinkage. The long-term development of drying shrinkage mainly depends on the viscous behavior of the pastes under capillary pressure. The change law of the elastic deformation part with the activator modulus and alkali content is positively correlated with that of the corresponding capillary pressure. Moreover, the change law of the creep contribution part with the activator modulus and alkali content negatively correlates with that of the corresponding strength.

Mesoscale investigation on concrete creep behaviors based on discrete element method

Studying creep behaviors of concrete materials at a mesoscale level is crucial to understanding their mechanism. Discrete element method (DEM) could provide an effective numerical tool to explain the mesoscale mechanism of concrete materials. In this study, the creep characteristics of concrete are analyzed by combining DEM and Burgers model. Firstly, a multi-phase concrete numerical model consisting of matrix, aggregate, and interfacial transition zone (ITZ) was developed. Then, the sensitivity of the Burgers model parameters was conducted. Finally, to investigate concrete creep behaviors at a mesoscale level, a series of numerical simulations were carried out. Simulation results show that Burgers model can reflect the creep behavior of concrete well. The porosity gradually decreased as the loading duration increase, and deceleration became stable at the later stage. The creep contribution of ITZ decreases with the increase of the ratio of ITZ strength to matrix strength. Force-chain strength in ITZ was greater than that in matrix, and viscoelastic sliding or squeezing occurs at Burger contact. Meanwhile, the direction distribution of force-chain strength in the matrix is regular and the main direction of the contact distribution is about 90° or 270°. In addition, the probability function of contact force is exponential, and the strength probability distribution of matrix and ITZ contact reached the peak at 0.7 f/f¯ and then decreased, and the proportion of strong contact and weak contact in matrix and ITZ was about 70% and 30%, respectively. The current study can bridge the gap between simulation and experimental research.

Crack growth in rotor P2M steel at elevated temperature

The paper presents a comprehensive computational and experimental study of the crack growth rate during the creep-fatigue interaction in compact specimens of P2M steel at a temperature of 550 °C. The theoretical part of the study consisted in the formulation of the fracture resistance parameters through the classical and new constitutive equations of the cracked body state taking into account the accumulation of damage. Numerical calculations included the determination of the stress-strain state fields for the conditions of elasticity, plasticity, and creep, as well as the distributions of nonlinear stress intensity factors and the C*-integral along the crack length and along the crack front for each tested compact specimen. The interpretation of the experimental results for specimens of the same shape in plan but different thicknesses is given in terms of elastic and nonlinear stress intensity factors, taking into account the accumulation of creep damage. It was found that the crack growth rate under the fatigue and creep interaction increases monotonically as the crack size increases in comparison with harmonic fatigue by an order of magnitude or more on specimens of the same geometry. Taking into account the damage through the creep stress intensity factors causes differences in the cyclic fracture diagrams. The superposition of fatigue and creep contributions in terms of load holding time shows an order of magnitude increase in the total crack growth rate compared to the interpretation of the experimental data in terms of pure creep.

Size effect in creep–fatigue crack growth interaction in P2M steel

AbstractAn investigation of the crack propagation under the creep–fatigue interaction in C(T) specimens of P2M steel was carried out. Numerical calculations included the determination of the stress–strain state fields for the viscoelastic material, as well as the distributions of stress triaxiality, nonlinear stress intensity factors, and the C‐integral along the crack front in specimen with different thickness. The interpretation of the experimental results under fatigue–creep interaction is given by the elastic and nonlinear stress intensity factors in terms of the traditional and new normalized cyclic fracture diagrams. Considering the size effects through the creep‐dominated stress intensity factors caused differences in the crack growth behavior. In specimens with the same geometry, the crack growth rate under the creep–fatigue interaction increased monotonically compared with harmonic fatigue, and the superposition of fatigue and creep contributions shows an order of magnitude increase in the total crack growth rate with respect to the pure creep.

Reconstructing the slip velocities of the 1202 and 1759 CE earthquakes based on faulted archaeological structures at Tell Ateret, Dead Sea Fault

Archaeological structures built across active faults and ruptured by earthquakes have been used as markers to measure the amount of displacement caused by ground motion and thus to estimate the magnitude of ancient earthquakes. The example used in this study is the Crusader fortress at Tel Ateret (Vadum Iacob) in the Jordan Gorge, north of the Sea of Galilee, a site which has been ruptured repeatedly since the Iron Age. We use detailed laser scans and discrete element models of the fortification walls to deduce the slip velocity during the earthquake. Further, we test whether the in-situ observed deformation pattern of the walls allows quantification of the amount both sides of the fault moved and whether post-seismic creep contributed to total displacement. The dynamic simulation of the reaction of the fortification wall to a variety of earthquake scenarios supports the hypothesis that the wall was ruptured by two earthquakes in 1202 and 1759 CE. For the first time, we can estimate the slip velocity during the earthquakes to 3 and 1 m/s for the two events, attribute the main motion to the Arabian plate with a mostly locked Sinai plate, and exclude significant creep contribution to the observed displacements of 1.25 and 0.5 m, respectively. Considering a minimum long-term slip rate at the site of 2.6 mm/year, there is a deficit of at least 1.6 m slip corresponding to a potential future magnitude 7.5 earthquake; if we assume ~5 mm/year geodetic rate, the deficit is even larger.

Asymmetric creep modeling of common refractory ceramics with high temperature wedge splitting test

Common refractory ceramics display significant asymmetric creep behavior at high temperatures under uniaxial loading conditions. This study introduces an advanced asymmetric creep model based on the stress path in the hydrostatic pressure–von Mises stress diagram. The developed creep model was applied in the modeling of high-temperature wedge splitting test procedure, where tensile and compressive stresses simultaneously occur perpendicular to the loading direction during testing at high temperatures. A case study of magnesia chromite material at different temperatures was performed to assess the creep contribution to the nominal total fracture energy at different temperatures and determine the pure mode I fracture parameters. Under the present testing conditions, the creep contribution is 13% at 1300 °C and 1400 °C and 5% at 1500 °C for the magnesia chromite material. The ratio of the nominal notch tensile strength to the pure tensile strength is 1.54–2.11, which decreases monotonically with respect to the brittleness number. The necessity to determine creep functions in multiaxial compression, multiaxial tension or tension–compression loading area shall be identified case by case.

Effect of fracture voids on the swelling behaviour of Callovo-Oxfordian claystone

The Callovo-Oxfordian (COx) claystone has been considered as a possible host rock for deep geological repository of high-level radioactive waste in France. When constructing the repository, it is unavoidable to generate a fractured zone in the vicinity of excavated repository walls. In the assessment of the repository performance, it is important to investigate the role of the fractures on the swelling capacity of COx claystone and to further characterize the fractured zone evolution. In this study, the effect of fracture voids on the swelling behaviour of COx claystone was investigated by simulating the fracture voids with a space created by a limited amount of grains sandwiched between claystone disks, and by performing a series of swelling tests in oedometer under a low vertical stress (0.05 MPa) and swelling pressure tests under constant-volume condition. Results showed that, with water infiltrating into the sample with fracture voids, the grains collapsed before the swelling of claystone disks. The whole sample was composed of swelling zones corresponding to the claystone disks with higher dry density than the expected final value, and of compression zones corresponding to the initial fractures with lower dry density. Moreover, the axial strain and swelling pressure decreased as the fracture void increased. The maximum fracture voids that might be fully filled by the swelling of COx claystone (sealing) were found to be 5.14%–7.65% without considering the long-term creep contribution in the field condition.

A novel method for load line displacement rate partitioning in creep crack growth tests on Type 316H stainless steel

Characterising the creep crack growth behaviour of Type 316H stainless steel is vital in obtaining accurate predictions for the lifetime of high temperature components, for example in UK advanced gas cooled reactors. The correlation between creep crack growth rates and the fracture mechanics parameter C∗, considered to govern the crack growth process, is obtained from creep crack growth tests. The C∗ parameter is experimentally determined using an expression which requires knowledge of the load line displacement rate due to creep. Historically this has been calculated by subtracting values for the elastic and plastic contributions to the load line displacement, obtained from available solutions, from the total experimentally measured load line displacement. However, the solutions available to determine the plastic contribution rely on generating a power-law fit to uniaxial tensile data, which is difficult to accomplish accurately over a large stress range. In addition, these expressions cannot account for strain history effects during crack growth. Consequently the elastic and plastic contributions are often erroneously large and can even be in excess of the experimental total load line displacement. A novel technique has been proposed to provide improved estimates of the creep contribution to the load line displacement rates during creep crack growth tests. This technique employs finite element analysis that incorporates material specific uniaxial tensile test data to simulate crack growth in an experimental test. A single elastic-plastic-creep simulation is used to determine the separate elastic-plastic and creep contributions to the load line displacement, meaning that, unlike historic analyses, creep stress relaxation and strain history effects can now be accounted for. The results have demonstrated that advanced predictions of the creep contributions to the load line displacement can be obtained using this technique.

Concrete creep and shrinkage effect in adhesive anchors subjected to sustained loads

The safe design of fastening systems requires an accurate understanding of all time-dependent phenomena that may lead to damage or even failure during its service life of typically 50 years. Generally, safety and serviceability requirements are ensured through rigorous experimental testing during the approval phase. In the case of time-dependent phenomena, or more specifically the behavior under sustained load, the common practice of directly testing all critical service conditions fails. The long-term response can only be approximated e.g. by short-term structural system tests that are extrapolated to the full life-time utilising empirical models as required by current guidelines. These are based on the assumption of uniformly distributed bond stress along the anchor, attribute all creep deformations to the adhesive mortar, neglect the concrete creep contribution, and assume a stationary creep process represented by a power-law. Generally, it can be assumed that reliable predictions of the time-dependent behavior of complex systems can only be obtained by models that are derived from a systematic investigation of all involved factors and their interaction. In this study, specifically the effect of concrete creep and shrinkage on the long-term behavior of adhesive anchor systems, based on extensive experimental data and numerical analyses, is investigated. A state-of-the-art computational framework is applied which can explicitly model the underlying physical and chemical mechanisms at early age and beyond. The well-established Lattice Discrete Particle Model (LDPM) describes the time-dependent mechanical response of the structure considering the local maturity of concrete, temperature, and moisture content. After calibration and validation of the numerical framework, structural deformations owing to concrete creep, bond stress redistributions over time, and the gradual development of damage at higher load levels are discussed.

Understanding reversible Maxwellian electroactuation in a 3M VHB dielectric elastomer with prestrain

Dielectric elastomers can undergo a large strain under a high driving electric field; however, significant creep is observed due to the viscoelasticity under the Maxwellian pressure. In this work, dielectric properties of a VHB 4905 tape with different prestrains were studied using low-field broadband dielectric spectroscopy and high-field electric displacement-electric field loop measurements. The dielectric constant first increased (below 200% prestrain) and then decreased (above 200% prestrain) upon increasing the prestrain. Electroactuation of prestrained VHB films was studied using both direct current (DC) and alternating current (AC) driving fields. Only the AC actuation method could be used to subtract the creep contribution and obtain reversible Maxwellian actuation. In addition to removing the boundary constraint, prestrain had more influence on the actuation property of VHB. The Young's modulus reached the minimum level (∼0.05 MPa) between 100% and 400% areal prestrain, and increasing the prestrain could enhance electroactuation. Above 500% prestrain, the electroactuation started to decrease because of significant strain-hardening in the sample. Different nonlinear mechanical models were used to fit the experimental data of electroactuation. This study provides a viable standard for the future investigation of reversible Maxwellian actuation in various dielectric elastomers.

Angular flux creep contributions in YBa2Cu3O7\u2212\u03b4 nanocomposites from electrical transport measurements

The shape of the electric-field—current-density (E-J) curve is determined by flux pinning and also by dynamics of vortices. Here, we propose a novel methodology to study the normalized flux creep rate S in YBa2Cu3O7−δ measured from E-J curves obtained by electrical transport measurements that provides a fast and versatile way to foresee the flux magnetic relaxation in films and disentangle angular flux creep contributions by the scaling of the isotropic contribution of S. After a detailed comparison of various pristine and nanocomposite films with differentiated nanostructures, we focus on the roles that intrinsic pinning and stacking faults (YBa2Cu4O8-intergrowths) play when the magnetic field is applied parallel to the superconducting CuO2 planes. This study reveals that the emerging intergrowths provide advanced pinning properties that additionally reduce the thermal activated flux magnetic relaxation. For this purpose, creep analysis becomes a very appropriate tool to elucidate the dominance of the different pinning sites at different regions of the magnetic-field—temperature diagram.

鉛フリーはんだにおけるクリープ・疲労き裂進展の微視的観察

Creep-fatigue crack propagation tests of lead-free solder were conducted under various loading waveforms. The waveforms adapted in displacement-controlled tests were four triangular waveforms: pp waveform having fast loading-unloading, cc waveform having slow loading-unloading, cp waveform with slow loading followed by fast unloading, and pc waveform with fast loading followed by slow unloading, and three waveforms with hold time: cc-h waveform having a hold time under tension and compression, cp-h waveform having a hold time under tension, and pc-h waveform having a hold time under compression. In load-controlled conditions, three waveforms are adapted: pp, cc-h and cp-h. Microscopic observation using SEM and EBSD was conducted near the crack tip region and on fracture surfaces. Plenty of microcracks were observed near the main crack tip on the surfaces of specimens, while almost all of them disappeared after removing the surface layer of about 0.4mm. On the removed surfaces, microcracks were observed near the main crack tip for unsymmetrical waveforms, cp, pc, cp-h, while no microcracks for symmetrical waveforms, pp, cc, cc-h. The existence of microcracks is responsible for crack acceleration under unsymmetrical waveforms. EBSD observation showed the formation of subgrains within original grains of Sn and eutectic phases near the crack tip, and the grain size decreased with increasing crack propagation rates. The grain average of GROD also decreased with increasing crack propagation rate, while that of KAM was nearly constant without respect to the crack propagation rate. Striations, fragmentation, and intergranular fracture facets are three main features of creep-fatigue fracture surfaces. The features of striations were clear for the cases of pp, cc, pc, while vague for the other cases. The spacing of striations was nearly equal to the crack propagation rate, supporting the linear summation rule of creep and fatigue crack propagation rates. Plenty of intergranular facets were observed for cp, cp-h. Fragmentation was abundant for waveforms including creep contribution.

Influences of pore pressure on short-term and creep mechanical behavior of red sandstone

Short-term and creep tests for saturated red sandstone under different pore pressures were carried out by a rock servo-controlled triaxial equipment. Based on the experimental results, the influences of confining pressure and pore pressure on short-term mechanical behavior of red sandstone are firstly analyzed. The results show that the peak strength of dry red sandstone increases with the confining pressure, which can be better expressed by the nonlinear Hoek–Brown criterion than the linear Mohr–Coulomb criterion. But with the increase of pore pressure, the peak strength and elastic modulus of saturated red sandstone all decrease step by step. The short-term failure mode of red sandstone is dependent of the confining pressure, but independent of the pore pressure. And then, the influence of pore pressure and axial deviatoric stress on the creep mechanical behavior of saturated red sandstone is analyzed quantitatively. Creep contribution to rock deformation increases with the pore pressure, and the specimens show significant time-dependent effect at higher deviatoric stresses. However, the steady-state creep rate of saturated red sandstone increases nonlinearly but the viscosity coefficient decreases gradually with increasing axial deviatoric stress and pore pressure. An exponential function is suggested to characterize the relationship between the creep parameters (including creep strain, steady-state creep rate and viscosity coefficient) and the axial deviatoric stress, and pore pressure. The calculated curves using the proposed function are compared with the experimental results, which is in a good agreement with the experimental data. In the end, the mechanism that the pore pressure and deviatoric stress affect the creep deformation of saturated red sandstone is discussed in detail. The presented experimental results on short-term and creep mechanical behavior of red sandstone under different pore pressures are very significant for evaluating the long-term stability and safety of deep underground rock mass engineering.

Contribution of Primary Creep in Modeling the Mechanical Behavior of Polycrystalline Ice

In most of the models proposed for deformation of ice, in addition to instantaneous elastic and viscoelastic parts, a viscoplastic part is also considered. An expression used for material secondary creep is usually employed to describe the viscoplastic component. In this study, however, another viscoplastic deformation of primary creep type is also considered in addition to the secondary creep. Therefore the permanent contribution of deformation is suggested to consist of primary and secondary creep parts. The existence of the primary creep contribution is investigated and characterized by using experimental results reported in the literature. The identified primary creep contribution is then validated by other available experimental results. Finally, the significance of primary creep in the inelastic behavior will be discussed.

Investigation of isochronous stress-strain formulations for elevated temperature structural design

For elevated temperature design evaluations by the ASME-NH rules, the most important material data is the isochronous stress-strain curves, which can provide design creep information. The main purpose of this paper is to investigate appropriate formulations to be able to generate the isochronous stress-strain curves and implement it to the computer program which is coded the ASME-NH design evaluation procedures. To do this, formulations by the strain-time relationship are investigated in detail and the sensitivity studies for rapid initial transient creep contributions, slower and longer transient creep contribution, and secondary creep contributions are carried out for type 316 austenitic stainless steel. From the results of this study, it is found that the strain-time relationship formulations can well describe the isochronous stress-strain curves with the transient creep contributions.

Strong pinning and elastic to plastic vortex crossover in Na-doped CaFe2As2single crystals

We study the vortex dynamics of NaxCa1-xFe2As2 single crystals with x = 0.5 (underdoped) and x = 0.75 (optimally doped), having T-c approximate to 19.4 and 33.4 K, respectively, by performing magnetization measurements of the critical current density J(c) and flux creep rate S. We find that the J(c) versus temperature, T, dependence is consistent with delta T-c pinning, indicating strong pinning associated with randomly distributed defects larger than the coherence length xi. The temperature dependence of S shows a crossover between glassy (elastic) and plastic creep regimes. The boundary T-cr(H) between both creep regimes coincides with the upper limit of the regime of strong pinning by nanoparticles. The glassy exponent mu in the optimally doped crystal is consistent with the thermal collective creep theory previously applied to cuprate superconductors, but in the underdoped sample the plateau in S(T) indicates that mu approximate to 3-3.3, a value larger than the existing theoretical predictions. We discuss the quantum creep contributions in both samples.

Computation of diametral deformation of irradiated Zr-2.5 Nb pressure tubes from RAPS-2

The pressure tubes in Pressurised Heavy Water Reactors (PHWRs) are subjected to high stresses, temperatures and fast neutron fluxes which cause changes in dimensions and material properties. To ensure the safe, reliable and economic performance of the reactor, it is important that these changes are known and that the rate of change can be predicted and demonstrated to remain within the design basis. The diametral expansion of pressure tubes due to irradiation creep and growth is an important property that may limit the useful life of the tube. A semiempirical predictive model has been developed to calculate the irradiation creep (diametral) and irradiation growth (diametral) contributions and total diametral change in irradiated pressure tubes. The model can account for variations arising from tube orientation. The sixteen numbers of Zr-2.5Nb pressure tubes (PTs) of RAPS-2 selected had undergone 7.15 Effective Full Power Years (EFPY) of reactor operation. The diametral changes at discrete locations along the length of the pressure tubes are estimated by using the model developed. The maximum of (along the tube length) measured internal diameter data of those fifteen pressure tubes have been compared with those obtained from the model.