Showing papers on "Creep published in 2017"

Polymer Interface and Adhesion

Abstract: "Interfacial Thermodynamics Molecular Interpretations Interfacial and Surface Tensions of Polymer Melts and Liquids Contact Angles of Liquids on Solid Polymers Surface Tension and Polarity of Solid Polymers Wetting of High-Energy Surfaces Dynamic Contact Angles and Wetting Kinetics Experimental Methods for Contact Angles and Interfacial Tensions Modifications of Polymer Surfaces: Mechanisms of Wettability and Bondability Improvements Adhesion: Basic Concept and Locus of Failure Formation of Adhesive Bond Weak Boundary Layers Effect of Internal Stress and Bond Strength Fracture of Adhesive Bond Fundamentals of Fracture Mechanics Analysis and Testing of Adhesive Bonds Creep, Fatigue, and Environmental Effects Creep and Fatigue of Adhesive Joints Environmental Effects Appendix I: Calculation of Surface Tension and Its Non-polar and Polar Components from Contact Angles By the Harmonic-Mean and the Geometric-Mean Methods Appendix II: Unit Conversion Tables "

Exceptional increase in the creep life of magnesium rare-earth alloys due to localized bond stiffening

Abstract: Several recent papers report spectacular, and unexpected, order of magnitude improvement in creep life of alloys upon adding small amounts of elements like zinc. This microalloying effect raises fundamental questions regarding creep deformation mechanisms. Here, using atomic-scale characterization and first principles calculations, we attribute the 600% increase in creep life in a prototypical Mg–rare earth (RE)–Zn alloy to multiple mechanisms caused by RE–Zn bonding—stabilization of a large volume fraction of strengthening precipitates on slip planes, increase in vacancy diffusion barrier, reduction in activated cross-slip, and enhancement of covalent character and bond strength around Zn solutes along the c-axis of Mg. We report that increased vacancy diffusion barrier, which correlates with the observed 25% increase in interplanar bond stiffness, primarily enhances the high-temperature creep life. Thus, we demonstrate that an approach of local, randomized tailoring of bond stiffness via microalloying enhances creep performance of alloys. Adding very small amounts of zinc to magnesium alloys containing rare earth elements dramatically improves their creep life. Here, the authors use first principles calculations and atomic-scale characterization to show that this is due to stiff covalent bonding of zinc and rare earth elements such as neodymium.

The High Temperature Tensile and Creep Behaviors of High Entropy Superalloy

Abstract: This article presents the high temperature tensile and creep behaviors of a novel high entropy alloy (HEA). The microstructure of this HEA resembles that of advanced superalloys with a high entropy FCC matrix and L12 ordered precipitates, so it is also named as “high entropy superalloy (HESA)”. The tensile yield strengths of HESA surpass those of the reported HEAs from room temperature to elevated temperatures; furthermore, its creep resistance at 982 °C can be compared to those of some Ni-based superalloys. Analysis on experimental results indicate that HESA could be strengthened by the low stacking-fault energy of the matrix, high anti-phase boundary energy of the strengthening precipitate, and thermally stable microstructure. Positive misfit between FCC matrix and precipitate has yielded parallel raft microstructure during creep at 982 °C, and the creep curves of HESA were dominated by tertiary creep behavior. To the best of authors’ knowledge, this article is the first to present the elevated temperature tensile creep study on full scale specimens of a high entropy alloy, and the potential of HESA for high temperature structural application is discussed.

Creep constitutive models for viscoelastic materials based on fractional derivatives

Abstract: To describe the time-dependent creep behavior of viscoelastic material, fractional constitutive relation models which are represented by the fractional element networks are studied. Three sets of creep experimental data for polymer and rock are employed to demonstrate the effectiveness of these fractional derivative models. The corresponding constrained problem of nonlinear optimization is solved with an interior-point algorithm to obtain best fitting parameters of these fractional derivative models. The comparison results of measured values and calculated values versus time are displayed through graphics. The results demonstrate that the fractional PoyntingThomson model is optimal in simulating the creep behavior of viscoelastic materials. And it also shows that the interior-point method is effective in the inverse problem to estimate parameters of fractional viscoelastic models.

Creep Behavior of Intact and Cracked Limestone Under Multi-Level Loading and Unloading Cycles

Abstract: A series of triaxial creep tests were carried out on intact and cracked Maokou limestone specimens under multi-level loading and unloading cycles. A new data processing algorithm is proposed to analyze the experimental data and divide the total strain into instantaneous and creep strains, with the instantaneous strain consisting of instantaneous elastic and plastic strains and the creep strain consisting of viscoelastic and visco-plastic strains. The results show that the viscoelastic strain converges to a certain value with time, but the visco-plastic strain keeps increasing with time, although both tend to increase with higher deviatoric stress. The ratio of the visco-plastic strain to the total creep strain also tends to increase when the deviatoric stress is higher. The steady-state creep strain rate increases with higher deviatoric stress or lower confining pressure, and the relation between the steady-state creep strain rate and the deviatoric stress can be well described by an exponential expression. The results also show that the preexisting cracks in the limestone have a great effect on its creep properties. At the same confining pressure and deviatoric stress, the cracked limestone shows larger instantaneous and creep strains (especially visco-plastic strains), longer duration of primary creep, and a higher steady-state creep strain rate than the intact limestone.

Slip-activated surface creep with room-temperature super-elongation in metallic nanocrystals.

Abstract: Nanoscale metallic crystals have been shown to follow a ‘smaller is stronger’ trend. However, they usually suffer from low ductility due to premature plastic instability by source-limited crystal slip. Here, by performing in situ atomic-scale transmission electron microscopy, we report unusual room-temperature super-elongation without softening in face-centred-cubic silver nanocrystals, where crystal slip serves as a stimulus to surface diffusional creep. This interplay mechanism is shown experimentally and theoretically to govern the plastic deformation of nanocrystals over a material-dependent sample diameter range between the lower and upper limits for nanocrystal stability by surface diffusional creep and dislocation plasticity, respectively, which extends far beyond the maximum size for pure diffusion-mediated deformation (for example, Coble-type creep). This work provides insight into the atomic-scale coupled diffusive–displacive deformation mechanisms, maximizing ductility and strength simultaneously in nanoscale materials. In situ atomic-scale imaging of deformation in silver nanocrystals reveals that it is possible to achieve deformability and high strength, attributed to a coupling mechanism between crystal slip and surface diffusional creep.

Catalyst Selection, Creep, and Stress Relaxation in High-Performance Epoxy Vitrimers

Abstract: Vitrimers were created from a commercial high-performance anhydride-cured epoxy in the presence of various metal transesterification catalysts. Compressive creep strains greater than 50% were observed in samples containing dibutyltin diacetate or dibutyltin bis(2,4-pentanedionate) via a compression set experiment. Stress relaxation experiments were carried out in a parallel plate geometry and analyzed with a newly modified exponential model proposed to better describe the relaxation process. The results demonstrate that it is not only possible to realize reworkability on relatively short time scales (hundreds of seconds) without compromising the mechanical properties of these networks at elevated temperatures but that, with proper catalyst selection, this may be accomplished with negligible activity under curing conditions. This effort also highlights differences in the behavior of different transesterification catalysts in this context. The approach to the selection and analysis of the materials reported...

Nonlinear creep damage constitutive model for soft rocks

Abstract: In some existing nonlinear creep damage models, it may be less rigorous to directly introduce a damage variable into the creep equation when the damage variable of the viscous component is a function of time or strain. In this paper, we adopt the Kachanov creep damage rate and introduce a damage variable into a rheological differential constitutive equation to derive an analytical integral solution for the creep damage equation of the Bingham model. We also propose a new nonlinear viscous component which reflects nonlinear properties related to the axial stress of soft rock in the steady-state creep stage. Furthermore, we build an improved Nishihara model by using this new component in series with the correctional Nishihara damage model that describes the accelerating creep, and deduce the rheological constitutive relation of the improved model. Based on superposition principle, we obtain the damage creep equation for conditions of both uniaxial and triaxial compression stress, and study the method for determining the model parameters. Finally, this paper presents the laboratory test results performed on mica-quartz schist in parallel with, or vertical to the schistosity direction, and applies the improved Nishihara model to the parameter identification of mica-quartz schist. Using a comparative analysis with test data, results show that the improved model has a superior ability to reflect the creep properties of soft rock in the decelerating creep stage, the steady-state creep stage, and particularly within the accelerating creep stage, in comparison with the traditional Nishihara model.

The effect of grain boundary water on deformation mechanisms and rheology of rocksalt during long-term deformation

Abstract: Reliable modeling of the deformation of rocksalt under the very low strain rates characterizing long term engineering conditions or natural halokinesis requires extrapolation of experimentally-derived flow laws to rates much lower than those attainable in the laboratory. This extrapolation must be based on an understanding of the microscale deformation mechanisms operating under these conditions, from studies of natural laboratories. The engineering creep laws generally used in the salt mining industry are based on dislocation creep processes quantified in laboratory experiments of necessarily limited duration. However, a large body of evidence clearly demonstrates that under conditions of long-term deformation, grain boundary dissolution-precipitation processes, such as solution-precipitation creep (or “pressure solution”) and dynamic recrystallization, play a significant role. In this contribution, we briefly review the microphysics of grain boundary water related, solution-precipitation processes in halite, together with the flow behaviour associated with these processes, and we discuss the contribution of these mechanisms to the strain rate during long-term creep.

Microstructure evolution in HR3C austenitic steel during long-term creep at 650 °C

Abstract: The creep behavior of HR3C austenitic steels was investigated at 650 °C and over the stress range from 150 to 250 MPa for up to 13,730 h. The corresponding microstructure evolution was characterized by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In the initial stage of the creep process, the creep-resistance of HR3C steel is enhanced by the precipitation of second-phases particles in the grain and at the grain boundary. Compared with the precipitates inside the grain, the higher nucleation and growth rate of precipitates at the grain boundary is related to the higher interfacial energy and diffusion rate of atoms. The high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) results show that the precipitates inside the grain may initially nucleate at dislocation pile-up sites, and the interface coherency between the precipitate and the matrix can be destroyed after a long-term creep process. The TEM morphology indicates that the agglomerated tiny particles interact with the dislocations, contributing mostly to the precipitation strengthening inside the grain during the long-term creep process at 650 °C, while the growth of chain-like M23C6 precipitates at the grain boundary increases the tendency of intergranular cracking as the creep time increased.

The Influence of Temperature on Time-Dependent Deformation and Failure in Granite: A Mesoscale Modeling Approach

Abstract: An understanding of the influence of temperature on brittle creep in granite is important for the management and optimization of granitic nuclear waste repositories and geothermal resources. We propose here a two-dimensional, thermo-mechanical numerical model that describes the time-dependent brittle deformation (brittle creep) of low-porosity granite under different constant temperatures and confining pressures. The mesoscale model accounts for material heterogeneity through a stochastic local failure stress field, and local material degradation using an exponential material softening law. Importantly, the model introduces the concept of a mesoscopic renormalization to capture the co-operative interaction between microcracks in the transition from distributed to localized damage. The mesoscale physico-mechanical parameters for the model were first determined using a trial-and-error method (until the modeled output accurately captured mechanical data from constant strain rate experiments on low-porosity granite at three different confining pressures). The thermo-physical parameters required for the model, such as specific heat capacity, coefficient of linear thermal expansion, and thermal conductivity, were then determined from brittle creep experiments performed on the same low-porosity granite at temperatures of 23, 50, and 90 °C. The good agreement between the modeled output and the experimental data, using a unique set of thermo-physico-mechanical parameters, lends confidence to our numerical approach. Using these parameters, we then explore the influence of temperature, differential stress, confining pressure, and sample homogeneity on brittle creep in low-porosity granite. Our simulations show that increases in temperature and differential stress increase the creep strain rate and therefore reduce time-to-failure, while increases in confining pressure and sample homogeneity decrease creep strain rate and increase time-to-failure. We anticipate that the modeling presented herein will assist in the management and optimization of geotechnical engineering projects within granite.

Creep of Posidonia Shale at Elevated Pressure and Temperature

Abstract: The economic production of gas and oil from shales requires repeated hydraulic fracturing operations to stimulate these tight reservoir rocks. Besides simple depletion, the often observed decay of production rate with time may arise from creep-induced fracture closure. We examined experimentally the creep behavior of an immature carbonate-rich Posidonia shale, subjected to constant stress conditions at temperatures between 50 and 200 °C and confining pressures of 50–200 MPa, simulating elevated in situ depth conditions. Samples showed transient creep in the semibrittle regime with high deformation rates at high differential stress, high temperature and low confinement. Strain was mainly accommodated by deformation of the weak organic matter and phyllosilicates and by pore space reduction. The primary decelerating creep phase observed at relatively low stress can be described by an empirical power law relation between strain and time, where the fitted parameters vary with temperature, pressure and stress. Our results suggest that healing of hydraulic fractures at low stresses by creep-induced proppant embedment is unlikely within a creep period of several years. At higher differential stress, as may be expected in situ at contact areas due to stress concentrations, the shale showed secondary creep, followed by tertiary creep until failure. In this regime, microcrack propagation and coalescence may be assisted by stress corrosion. Secondary creep rates were also described by a power law, predicting faster fracture closure rates than for primary creep, likely contributing to production rate decline. Comparison of our data with published primary creep data on other shales suggests that the long-term creep behavior of shales can be correlated with their brittleness estimated from composition. Low creep strain is supported by a high fraction of strong minerals that can build up a load-bearing framework.

Subduction megathrust creep governed by pressure solution and frictional-viscous flow

Abstract: Subduction megathrust slip speeds range from slow creep at plate convergence rates (centimetres per year) to seismic slip rates (metres per second) in the largest earthquakes on Earth. The deformation mechanisms controlling whether fast slip or slow creep occurs, however, remain unclear. Here, we present evidence that pressure solution creep (fluid-assisted stress driven mass transfer) is an important deformation mechanism in megathrust faults. We quantify megathrust strength using a laboratory-constrained microphysical model for fault friction, involving viscous pressure solution and frictional sliding. We find that at plate-boundary deformation rates, aseismic, frictional–viscous flow is the preferred deformation mechanism at temperatures above 100 °C. The model thus predicts aseismic creep at temperatures much cooler than the onset of crystal plasticity, unless a boundary condition changes. Within this model framework, earthquakes may nucleate when a local increase in strain rate triggers velocity-weakening slip, and we speculate that slip area and event magnitude increase with increasing spacing of strong, topographically derived irregularities in the subduction interface.

Microstructure, Fatigue Behavior, and Failure Mechanisms of Direct Laser-Deposited Inconel 718

Abstract: Inconel 718 is considered to be a superalloy with a series of superior properties such as high strength, creep resistance, and corrosion resistance at room and elevated temperatures. Additive manufacturing (AM) is particularly appealing to Inconel 718 because of its near-net-shape production capability for circumventing the poor machinability of this superalloy. Nevertheless, AM parts are prone to porosity, which is detrimental to their fatigue resistance. Thus, further understanding of their fatigue behavior is required before their widespread use in load-bearing applications. In this work, the microstructure and fatigue properties of AM Inconel 718, produced in a Laser Engineered Net Shaping (LENS™) system and heat treated with a standard heat treatment schedule, are evaluated at room temperature. Fully reversed strain controlled fatigue tests were performed on cylindrical specimens with straight gage sections at strain amplitudes ranging from 0.001 mm/mm to 0.01 mm/mm. The fracture surfaces of fatigue specimens were inspected with a scanning electron microscope. The results indicate that the employed heat treatment allowed the large, elongated grains and dendritic structure of the as-built material to break down into smaller, equiaxed grains, with some dendritic structures remaining between layers. The AM specimens were found to possess lower fatigue resistance than wrought Inconel 718, and this is primarily attributed to the presence of brittle metal-carbide/oxide inclusions or pores near their surface.

Effect of normalization and tempering on microstructure and mechanical properties of V-groove and narrow-groove P91 pipe weldments

Abstract: In Very High Temperature Reactor (VHTR), The Nb-V modified 9Cr-1Mo (P91) creep strength enhanced ferritic (CSEF) steel is currently considered as a candidate material for reactor internals and reactor pressure vessels (RPVs). After the welding of P91 steel, the inhomogeneous microstructure of weldment is a serious issue because it promotes the well-known Type IV cracking in P91 weldments. The present research work is focused on how the microstructure evolve in various zone of P91 pipe weldment during the sub-critical post weld heat treatment (PWHT) and normalized and tempered (N&T) heat treatment. The effect of PWHT and N&T heat treatment are also considered on tensile properties and hardness variation of P91 weldments. To characterize the sample scanning electron microscope (SEM), X-ray diffraction (XRD) and optical micrograph was used. It was observed that the N&T heat treatment provides the homogeneous microstructure compared to PWHT. The superior mechanical properties was also measured in N&T condition compared to PWHT. Study of fracture surface morphology of tensile tested specimen in different heat treatment condition is also presented.