Long-time Creep Research Articles

Microstructural evolution of 800H alloy after long-time creep rupture test at 675 °C

This study investigated the microstructural evolution of 800H alloy tubes and their welding through creep rupture test at 675 °C with different applied stresses. Vickers hardness testing was used to determine microhardness and optical microscopy, scanning electron microscopy, and transmission electron microscopy were used to examine the microstructure of the tubes. The microhardness of the base metal, heat-affected zones, and welding seams of the tubes after testing were all significantly higher than the as-received 800H tubes due to the combination of Cr23C6 precipitates, dislocation multiplication, and work-hardening effect. The base metal of all tubes had a coarse and equiaxed austenitic structure with large Ti-N-rich precipitates, parallel needle-like Cr23C6 precipitates within twins, and discontinuously dispersed chain-like Cr23C6 precipitates along grain boundaries. As testing progressed, spherical Cr23C6 were precipitated within grains then coarsened, and the intervals between parallel needle-like Cr23C6 within twins were increased due to their low thermodynamic stability and synergistic effect of time and stress. The heat-affected zones exhibited a low correlation with test time, with all samples displaying dense spherical and needle-like Cr23C6, whereas the welding seams showed insignificant microstructural evolution. γ΄ (Ni3(Al, Ti)) precipitates were observed along the sides of the Cr23C6, further coarsening of γ΄ was observed as the creep rupture test progressed.

Buckling of viscoelastic spherical shells

Viscoelastic spherical shells exhibit a wide range of time/rate-dependent buckling behaviors when subjected to pressure. For certain loadings, buckling can even occur after a significant time delay, termed creep buckling. To gain a thorough understanding of the nonlinear time-dependent buckling behavior of viscoelastic spherical shells, this work develops an analytical model employing the small-strain, moderate-rotation shell theory combined with a linearly viscoelastic material law. Numerical results are presented for axisymmetric spherical shells with geometric imperfections for two types of loading: a prescribed rate of volume change and a prescribed pressure that remains constant after it is applied. The first type reveals the rate-dependent behavior of viscoelastic buckling while the constant pressure loading is used to quantify creep buckling phenomena. The results show that viscoelasticity and loading rates play important roles in the load-carrying behavior of these shells, and the results for the constant pressure loading reveal an unexpected and important connection between the short-time elastic buckling limit and the long-time creep buckling limit. An imperfection sensitivity map is constructed for the constant pressure loading showing three regimes with qualitatively different behaviors: near-instantaneous buckling, creep buckling and no buckling.

Creep and Recovery Behavior of Vitrimers with Fast Bond Exchange Rate.

Vitrimers encompass the desirable mechanical properties of thermosets with the recyclability of thermoplastics. This ability arises from the rearrangement of the vitrimer covalent network upon heating via a bond shuffling mechanism while its cross-link density remains preserved. This unique feature makes vitrimers interesting candidates for the design of materials that combine dimensional stability at high temperatures and solvent resistance with the ability to be reshaped and processed. Despite these advantages, vitrimer exhibits significant creep at operating conditions where thermosets show little or no creep. As the mechanical properties of vitrimers not only depend on their chemical composition but also on the dynamics of the polymer chains, molecular dynamics (MD) simulations can provide detailed molecular mechanisms of the system of interest under macroscopic stress-induced deformations. In this regard, the recently developed MD/Monte Carlo simulation methodology capable of capturing the bond exchange mechanics in vitrimers is used to study the creep and recovery response of a coarse-grained model thermoset and vitrimer with a fast bond exchange rate. The time-stress superposition principle is then successfully applied to the creep response. The resulting universal curves enable us to predict the long-time creep behavior of both systems extending the timescale from 4 to over 10 orders of magnitude.

Accelerated and long-time creep testing of extruded polystyrene using isothermal and stepped isothermal method

Seasonal Sensible Thermal Energy Storages based on existing inside structures are cost effective and require insulation materials with high performance. The insulation must endure high temperatures and high pressure for a long period. In this study, the creep effects of the insulation material XPS (extruded polystyrene foam) at high temperatures and pressure are investigated to determine whether XPS is a suitable insulation material to be applied on the inside of STES. Two studies were conducted: an isothermal long-term creep experiment and a SIM (Stepped Isothermal Method) experiment. The results of the first study were fitted using the Findley approach and extrapolated to 50 years. The SIM study is a method to accelerate creep effects using thermal effects and the Boltzmann superposition principle. Creep strain after 50 years is directly calculated by extrapolation. Both studies indicate that creep strain in XPS remains well below 6% with an applied load of 61.1 kPa at 60 °C after 50 years. Additionally, a material model that predicts creep strain at different or varying temperatures and stress was developed using the SIM data. • Creep behavior of XPS at elevated temperatures of 60 °C for 50 years. • Stepped Isothermal Method (SIM) and Findley Fit with thermal insulation material. • SIM with samples whose ratio of thickness to cross-sectional area is lower than one. • SIM with the duration and extent of the temperature steps individual for each step.

Accelerated and long-time creep testing of extruded polystyrene using isothermal and stepped isothermal method

Accelerated and long-time creep testing of extruded polystyrene using isothermal and stepped isothermal method

Yielding and resolidification of colloidal gels under constant stress

We examine the macroscopic deformation of a colloidal depletion gel subjected to a step shear stress. Three regimes are identified depending on the magnitude of the applied stress: (i) for stresses below yield stress, the gel undergoes a weak creep in which the bulk deformation grows sublinearly with time similar to crystalline and amorphous solids. For stresses above yield stress, when the bulk deformation exceeds approximately the attraction range, the sublinear increase of deformation turns into a superlinear growth which signals the onset of non-linear rearrangements and yielding of the gel. However, the long-time creep after such superlinear growth shows two distinct behaviors: (ii) under strong stresses, a viscous flow is reached in which the strain increases linearly with time. This indicates a complete yielding and flow of the gel. In stark contrast, (iii) for weak stresses, the gel after yielding starts to resolidify. More homogenous gels that are produced through enhancement of either interparticle attraction strength or strain amplitude of the oscillatory preshear, resolidify gradually. In contrast, in gels that are more heterogeneous resolidification occurs abruptly. We also find that heterogenous gels produced by oscillatory preshear at intermediate strain amplitude yield in a two-step process. Finally, the characteristic time for the onset of delayed yielding is found to follow a two-step decrease with increasing stress. This is comprised of an exponential decrease at low stresses, during which bond reformation is decisive and resolidification is detected, and a power law decrease at higher stresses where bond breaking and particle rearrangements dominate.

Predicting the early-stage creep dynamics of gels from their static structure by machine learning

Upon sustained loading, colloidal gels tend to feature delayed viscoplastic creep deformations. However, the relationship, if any, between the structure and creep dynamics of gels remains elusive. Here, based on accelerated molecular dynamics simulations and the recently developed softness approach (i.e., classification-based machine learning), we reveal that the propensity of a gel to exhibit long-time creep is encoded in its static, unloaded structure. By taking the example of a calcium–silicate–hydrate gel (the binding phase of concrete), we extract a local, non-intuitive structural descriptor (a revised version of the “softness” metric proposed by the pioneering work from Cubuk et al.) that is strongly correlated with the dynamics of the particles. Notably, the macroscopic creep rate exhibits an exponential dependence on the average softness. We find that creep results in a decrease in softness in the gel structure, which, in turn, explains the gradual decay of the creep rate over time. Finally, we demonstrate that the softness metric is strongly correlated with the average energy barrier that is accessible to the particles.

Nonlinear transient vibration of viscoelastic plates: A NURBS-based isogeometric HSDT approach

We present an efficient isogeometric analysis (IGA) formulation for nonlinear vibration problems of viscoelastic plates. The formulation is based on higher-order shear deformation theory (HSDT), which is guaranteed by p-order nonuniform rational B-splines (NURBS) resulting in Cp−1 continuity in the domain. The governing equations of motion are derived from Hamilton’s principle. The generalized multi-axial Maxwell model is used to express the viscoelastic constitutive equations in global coordinates. The performance of developed approach is demonstrated through several numerical examples. We show that our approach yields more realistic solutions than conventional FEA, especially for coarse discretizations. We find that C3-NURBS provide higher accuracy concerning the nonlinear vibration amplitude than C2-NURBS. Viscoelastic plates have several active vibration modes adjacent to the fundamental oscillation mode due to the combination of long-time creep and short-time shear moduli. Furthermore, the resultant creep strain leads to large deformations of viscoelastic plates and intensifies the dynamic stress concentration factors. The creep deflection of the plates rapidly increases with damping under sustained loads when the support restrains are released. We observe that the long-time stress-to-thickness ratio is constant for various slenderness ratios, which is relevant during creep.

Microstructure-driven self-assembly and rheological properties of multi-responsive soft microgel suspensions

HypothesesThe deformation and swelling ability of microgels is influenced by the crosslinking distribution. Varying microgels microstructure is expected to obtain suspensions with different flow behavior and thereby, different rheological properties. ExperimentsDifferent multi-responsive microgels were synthesized using two different crosslinkers and varying their amounts: N,N-methylene bis-acrylamide (MBA) and oligo(ethylene glycol) diacrylate (OEGDA). The rheological results were obtained by zero-shear viscosity and long-time creep measurements on concentrated microgel suspensions Microgel microstructure was analyzed by 1H nuclear magnetic resonance transverse relaxation measurements. FindingsAt a constant crosslinking rate, we show that the viscosity of OEGDA-crosslinked microgels diverges at a higher concentration than MBA ones, suggesting a looser shell and less restricted dangling chains at the periphery for the later. By scaling with the effective volume fraction, the viscosity curves of the different microgel suspensions reduce into a single curve and closely follow hard sphere models up to ϕeff < 0.45. The results from creep tests revealed a much higher yield stress for MBA-crosslinked microgels, strengthening the hypothesis of a looser shell for the later. Finally, transverse relaxation (T2) NMR measurements demonstrated that, although all microgels exhibit a core–shell microstructure, MBA samples present a less crosslinked shell corroborating with the rheological results.

Strengthening mechanisms and creep rupture behavior of advanced austenitic heat resistant steel SA-213 S31035 for A-USC power plants

A newly advanced heat-resistant steel SA-213 S31035 (22Cr25Ni3W3CuCoNbN), is currently the strongest commercially available austenitic heat resistant steel and has been recognized as one of the main candidate materials for A-USC plants operating at 700 °C. Up to now, the reason for which has excellent creep performance is not clear yet. In this paper, the S31035 steels were crept at 700 °C with applied stresses in the range of 140–220 MPa up to 15627 h, and during creep the microstructure evolution including the precipitation of second phases as well as their strengthening mechanisms were deeply investigated. Creep tests results show that S31035 steel has much higher creep strength than S30432 steel, with extrapolated creep rupture strength of about 118.0 MPa at 700 °C for 50,000 h. Furthermore, it has great rupture ductility with an area reduction of more than 50%. The excellent creep strength of S31035 steel was produced by the combined precipitation strengthening of nano-scaled Cu-rich phase, needle-shaped Laves phase, and secondary Z phase. The nano-scaled Cu-rich phase was very stable during long-time creep and acted as the dominant precipitation strengthening. Unlike the coarse and granular Laves phase in ferritic P92 steel (9CrW), the spindly needle-shaped Laves phase in austenitic S31035 steel could effectively impede the moving of dislocations and greatly increase the creep strength. And the coarsening of Laves phase in width was slight but in length was apparent, which was conducive to the strengthening effect. The fine secondary Z phase also showed relatively slow coarsening rate, contributed an important part of strengthening enhancement. The precipitation of detrimental σ phase is effectively suppressed through the increased content of Ni and the addition of Co. Besides, the grain-boundary sliding and grain rotation occurred during creep. Under high stresses (≥170 MPa) and short-time creep, intragranular cracks dominated the creep failure. The cracks preferred to produce inside the grains with higher Taylor factor which are hard to deform. Under relatively lower stresses (≤140 MPa), intergranular cracks dominated the creep failure and cracks mainly generate on the grain boundaries between the grains with greatly different deformations. The texture <111>//RD formed during creep, suppressing the proceeding plastic deform and improving the creep resistance. Under lower stress and long-time creep, a new texture <001>//RD occurred, which improves the creep rupture ductility of the S31035 steel.

High-temperature Creep Behavior of 12Cr1MoVG Steel

Heat-resistant steels are key materials for the construction of boilers and high-temperature pipes. Herein, high-temperature creep behavior of 12Cr1MoVG steel has been studied. Creep tests over a long period of time were carried out for 12Cr1MoVG steel samples at varied temperatures. The activation energy for the creep of 12Cr1MoVG steel was evaluated based on the changes of creep rate with temperature when the steel was subject to various stresses. The creep mechanism of the steel has been elucidated by analyzing the relations between the minimum creep rate and the stress. In addition, continuum damage mechanics (CDM) model-based calculation agree well with the strain versus creep time curves obtained from the creep tests. The creep behavior of the 12Cr1MoVG steel at high temperatures has been perfectly described by means of long-time creep tests and mathematical modelling.

A new method to determine the beam bending creep critical displacement of three-point bending specimen with fixed constraints

Three-point bending specimen with fixed constraints (TPBSF) can be used to obtain uniaxial creep parameters from small samples of material. Conversion relationships between TPBSF and uniaxial creep data are based on the beam bending model. For large loads and longtime creep test, the specimen deformation can be large, which may be contrary to the small deformation assumption of the beam bending model. So it is necessary to define the critical deformation, below which the beam bending model holds. In this study, a new method to define this critical deformation was proposed by displacement. Based on the relationship between the maximum rotation angle and displacement at the elastic and creep stages, three procedures to determine the critical displacement were first proposed. Then it was verified by the creep test of P91 steel at 566 °C and 66 N. The results show that the critical displacement obtained by finite element method is in good agreement with the experimental result, and the maximum error is 4.5%. Creep parameters were regressed and compared with those in the literature on the basis of critical displacement, it can be seen that creep parameters regressed by the new proposed method are much closer to uniaxial creep than the literature, which indicates that beam bending model holds below this critical displacement. Finally, the finite element method was used to investigate the relationship between the time correction parameter and specimen geometric parameters. It can be found that the specimen width has little influence on the time correction parameter. However, the influence of specimen thickness and span can be significant.

High-temperature creep behaviour of 15CrMoG heat-resistant steel

Rapid increase in power plant efficiency requires development of heat-resistant steels for use in the boiler and piping systems. In this study, the creep activation energy of 15CrMoG steel was first investigated on the basis of the relationship between creep rate and temperature subjected to different stresses. Long-time creep tests at selected temperatures were performed for the 15CrMoG steel. Manson-Haferd model was utilized to predict the creep behavior of the steel, the results of which indicate that the experimental creep data agreed well with the data from the Manson-Haferd model prediction. Besides, the strain vs. creep time curves obtained from the creep tests were in good accordance with theoretical calculation based on the CDM model over a long creep time. Both Manson-Haferd model and CDM model are powerful tools in studying the high-temperature creep behaviour of the 15CrMoG steel.

Reducing uncertainties in estimating long-time prestress losses in concrete structures using a hygro-thermo-chemo-mechanical model for concrete

Prestress losses occur in concrete structures due to long-time creep and shrinkage in concrete, and stress relaxation in prestressing strands. The existing formulations for creep and shrinkage modeling in concrete are empirical in nature, which have been developed based on a particular set of measured data. In this study a coupled hygro-thermo-mechanical formulation is used to model the physics associated with creep and shrinkage in concrete structures. Uncertainty tools based on a probabilistic framework are used to model the possible variations in the input variables in the hygro-thermo-mechanical formulation. A distance measure based global response sensitivity analysis is adopted to identify the parameters having maximum impact on the output responses. Uncertainties in the selected set of important variables are reduced using Bayesian inference and short-time measured responses. For validating efficiency of the uncertainty reduction technique adopting Bayesian inference and global response sensitivity, the predictions of creep and shrinkage in concrete are compared with a few experiments from the North-Western University (NU) data base. The uncertainty reduction technique is then used to predict the long-time prestress losses in post-tensioned concrete beams and slabs cast in the laboratory.

Anomalous reduction in the long-time flux creep relaxation in superconducting Ca10(Pt4As8)((Fe1−xPtx)2As2)5 (x ≈ 0.05) single crystals

We report the vortex dynamics of superconducting a Ca10(Pt4As8)((Fe1−xPtx)2As2)5 (x ≈ 0.05) single crystal with Tc = 26 K investigated by performing magnetic measurements. The field dependence of the magnetization displays a second peak (SPM), typically related to a crossover between elastic and plastic vortex relaxation in a weak pinning scenario. Long-time flux creep relaxation measurements for fields smaller that of the SPM show that the vortex dynamics can be separated in two different regions. For magnetic fields smaller than the lower end of the SPM, glassy relaxation (with a characteristic glassy exponent μ) is observed. For magnetic fields between the lower end and the SPM, the flux creep rate decreases systematically to values below to the ones predicted by the collective theory. This effect can be understood by considering a stable vortex lattice configuration. As the field position of the SPM can be adjusted by modifying the quenched potential, our results suggest that extremely low flux creep relaxation rate may be tuned in many other superconducting materials.

Creep Tests and Modeling Based on Continuum Damage Mechanics for T91 and T92 Steels

9-11%Cr ferritic steels play an important role in high-temperature and high-pressure boilers of advanced power plants. In this paper, a continuum damage mechanics (CDM)-based creep model was proposed to study the creep behavior of T91 and T92 steels at high temperatures. Long-time creep tests were performed for both steels under different conditions. The creep rupture data and creep curves obtained from creep tests were captured well by theoretical calculation based on the CDM model over a long creep time. It is shown that the developed model is able to predict creep data for the two ferritic steels accurately up to tens of thousands of hours.

Uncertainty based model averaging for prediction of long-time prestress losses in concrete structures

Prestressed concrete structures are widely used in long span bridge girders and nuclear power plants. Long-time creep and shrinkage in concrete cause loss of prestress forces in the concrete structures. The creep and shrinkage models existing in the literature are empirical in nature. Each model has been developed on the basis of an arbitrary selection of data sets among all the data sets available. In this study it is claimed that each model contains some information about the creep and shrinkage patterns in the concrete structures. A single best model is developed in this study using all existing models, based on the concept of model averaging. In model averaging all the models are updated independently based on the short-time measurements. Weights are assigned to each model based on the likeliness of predictions from that model to the actual measurements. The time period for short-time measurements is evaluated using stochastic simulations. The proposed single best model for creep and shrinkage, and the time period for short-time measurements are (i) first validated against experiments reported in the Northwestern University (NU) database, and (ii) then used to predict the long-time losses of prestress forces in the concrete beams and slabs cast in the laboratory.

Polymer Osmotic Pressure in Hydrogel Contact Mechanics

Simple, semi-dilute hydrogels made from flexible polymers are often used as material surrogates for biological tissues, despite the dramatic differences between gels and tissues in their micro- and nano-structure, osmotic properties, and fluid permeability. Moreover, these simple hydrogels are often treated as poroelastic, even when applied pressures are below the hydrogel's osmotic pressure. Here we investigate the role of polymer osmotic pressure in hydrogel contact mechanics with a series of local indentation tests and bulk compression tests. Performing hydrogel indentation atop an inverted confocal microscope and applying surface pressures less than the hydrogel osmotic pressure, we find that hydrogel deformation behavior agrees with the Hertz model, observing no evidence of fluid flow or volumetric gel compression. A long-time creep of the hydrogel is also found, which can be predicted from a model of diffusive relaxations within the gel. In bulk compression tests, the gel is found to be incompressible, and therefore water cannot be driven out of the gel, unless the applied pressure exceeds the hydrogel osmotic pressure.

Prediction of creep curve of HP40Nb steel using artificial neural network

Simulation of creep curves using data obtained from a limited number of short-time creep tests is helpful for predicting the long-time creep life of materials by extrapolation techniques. The present paper demonstrates the application of artificial neural network (ANN) for the prediction of creep curves for HP40Nb micro-alloyed steel. The network consists of stress, temperature and time as the input parameters and the creep strain as the output parameter. The data used are taken from accelerated creep tests carried out at constant temperatures in the range 650–1050 °C and constant stresses 47–120 MPa. The network was trained using a three-layer feed-forward back-propagation network, having a 15-neuron hidden layer, using the Levenberg–Marquardt optimization algorithm. After successful network training, the model was subjected to several tests to demonstrate consistent prediction capability. 98% of the creep strain data could be predicted within an error of ±10% deviation from the experimental values. An additional experiment carried out to check the prediction capability of the model confirms very good prediction capability, with a correlation coefficient of 0.994, by ANN modeling.

Mechanical properties of Precontraint 1202 S2 based on uniaxial tensile and creep tests

The purpose of the paper is the estimation of the polyvinyl chloride – polyester-coated fabric (Precontraint 1202 S2) mechanical properties under uniaxial tensile tests as well as short- and long-time creep tests. The uniaxial tests are the basis of non-linear elastic description while the creep tests are used for the evaluation of the stiffness parameters in time and for the identification of the standard viscoelastic model. The paper also includes a short survey of literature concerning the coated woven fabric description.