Effect of alternating stress on the creep of frozen soils

Cyclic creep behaviour of a frozen saturated sand and frozen clay was studied by superimposing alternating stresses over a mean static compressive stress on the samples. Cyclic loading increased the creep rate of these materials considerably, which could be due to the increase in unfrozen water content generated by the transient thermal energy produced by mechanical cycling. The strain increment per cycle was found to increase with increasing strain in the frozen sand; whereas it decreased in the frozen clay, showing a tendency for hardening. The difference could be due to the fact that the mean stress on the sample was larger than a threshold stress in the former and smaller in the latter material.     

Research on visco-elastic-plastic creep model of artificially frozen soil under high confining pressures

The triaxial creep experiment of artificially frozen soil in deep alluvium was performed by a self-developed machine of triaxial creep frozen soil. After analyzing the experiment results, applying parabolic yield criterion for improved viscoplasticity in the Nishihara model, a new creep constitutive model was established for describing frozen-soil's creep characteristics under high confining pressures. The secondary development tools and data interface had been used to add the visco-elastic-plastic creep constitutive model to standard ADINA FEM. Numerical simulation of the shaft well excavation process and field measurement displacements of frozen wall were performed in the mine; and the results showed that the visco-elastic-plastic creep constitutive model was suitable and reasonable. This constitutive model could be significance for the frozen soil structure long-term stability analysing and the displacement forecasting.     

Extended failure time in the creep of frozen soils

A new concept of defining the failure time in creep as that at which the slope of a straight line from the origin becomes a tangent to the creep curve in the tertiary creep region is proposed. Using this concept, the data from several creep tests carried out in frozen soils are analyzed. It is shown that the estimated useful life of a foundation for a structure built in perenially frozen ground is larger by a factor of at least 1.5 compared to that based on the conventional failure time taken as the time to reach the minimum creep rate or the end of the steady state creep. The exponent of the power law creep in the tertiary region is also calculated.     

Experimental study on the creep behavior of frozen clay with thermal gradient

Thermal gradient is one of the main features in frozen engineering, especially in artificial frozen wall (AFW) in deep alluvium. This paper investigated the creep behaviors of frozen soil with thermal gradient. A series of uniaxial creep tests were carried out on frozen saturated clay under various thermal gradients and creep stresses by GFC (freezing with non-uniform temperature without experiencing K₀ consolidation) method. Two stages were observed during the whole creep process, i.e., instantaneous elastic deformation and decaying creep deformation. Radial creep deformation of ε₃ almost increases linearly with an increase in axial creep deformation of ε₁, and the slope of ε₃–ε₁ curve increases as the thermal gradient (or creep stress) increased. Long-term strength decreases as the thermal gradient (or the creep time) increased. Considering the correction equation on thermal gradient, the generalized Kelvin model consisting of one Hooke element and two Kelvin elements has been developed to describe the axial creep deformation. The validity of the model is verified by comparing its calculated results with the results of creep tests under both low and high thermal gradient. It is found that the axial creep deformation behavior of frozen saturated clay can be represented by generalized Kelvin model, and the proposed model reflects thermal gradient effects to the creep deformation well.     

The relationship between creep and strength behavior of ice at failure

This work explores the correspondence between the results of creep and strength tests performed on isotropic polycrystalline ice. A unique experimental procedure, termed a two-mode test in the present work, allows the testing of a single specimen under conditions of constant deformation rate up to failure and constant load thereafter.Using this procedure, the prevailing values of stress, strain and strain rate can be compared at the failure point under the two test modes without the influence of specimen variation. The effect of the stress path prior to failure on the creep behavior after failure can also be investigated.Tests were performed at temperatures of ?5°C and ?10°C and initial strain rates ranged from 10^?6 to 2.2 × 10^?5 s^?1. Results indicate coincidence of the failure points from creep and strength tests in stress/strain-rate/strain space. Furthermore, it appears that within the range of variables tested, the creep behavior after the mode switch at failure is independent of the stress path experienced before failure.     

Tensile creep behavior of high strength concretes at early ages

Early age tensile creep of high strength concrete loaded during the first day after casting was studied. The effect of silica fume was evaluated, using two types of loading scheme: (i) creep which occurs in a restrained autogenous shrinkage test and (ii) creep measured in a conventional test under constant load. The two methods showed that the addition of silica fume increased the early age tensile creep of high strength concrete, when compared to concrete without silica fume but with similar water/binder ratio. This enhanced creep is of practical significance as it provides a mechanism to relieve some of the restraining stress that develops due to autogenous shrinkage.

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Résumé Dans cet article, on a étudié le fluage en traction d'un béton de haute résistance, pendant le premier jour suivant sa fabrication. On a caractérisé l'effet de la fumée de silice, à partir de deux modes de chargement: (1) fluage dans le cas d'un essai en retrait intrinsèque empêché et (2) fluage mesuré dans un essai conventionnel sous charge constante. Les deux méthodes indiquent que l'addition de fumée de silice augmente le fluage initial d'un béton de haute résistance, par rapport à celui d'un béton de même rapport eau/liant sans fumée de silice. Cette augmentation du fluage est d'une signification pratique, en fournissant un mécanisme qui compense en partie les contraintes générées par le retrait intrinsèque.

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Editorial Note Dr. K. Kovler and Prof. A. Bentur are RILEM Senior Members. Technion is a RILEM Titular Member. Prof. Amon Bentur is a Fellow of RILEM and a member of the RILEM Coordinating Committee. He is the Chaiman of TC 159-ETC: ‘Engineering of the interfacial transition zone in cementitious composites’ (co-chaired with Prof. Alxander) and of TC EAS: ‘Early age shrinkage induced stresses and cracking in cementitious systems’. Dr. K. Kovler is a Member of the TC EAS and of TC 162-TDF: ‘Test and design methods for steel fiber reinforced concrete’.     

Effects of creep ductility and notch constraint on creep fracture behavior in notched bar specimens

The creep rupture life of U-type notched specimens and smooth specimens has been calculated based on the ductility exhaustion damage model using stress-dependent creep ductility. Effects of creep ductility and notch constraint on creep fracture behaviour in notched bar specimens have been investigated. The results show that the U-type notch exhibits notch strengthening effect under a wide range of stress level and notch constraint condition (notch acuity) for creep ductile materials. The lower equivalent stress in notched specimens plays main role for reducing creep damage and increasing rupture life. The rupture life of notched specimens of creep brittle materials (with lower creep ductility) decreases with the increase in stress level and notch constraint. With increasing creep ductility and decreasing notch constraint, the degree of the notch strengthening effect increases. In creep life designs and assessments of high-temperature components containing notches, the material creep ductility, notch constraint and stress levels need to be fully considered.     

含盐量对冻结粉土单轴抗压强度影响的试验研究

采用取自甘肃省白银市平川区的典型天然盐渍粉土,洗去土中的盐分,在含水量为20%的情况下,以NaCl和Na₂SO₄为研究对象,研究了含盐量对冻结粉土单轴抗压强度和破坏应变的影响。试验结果表明：当含盐量介于0～3%时,随着NaCl含量的增加,冻土中的未冻水含量增加,土颗粒间的距离增大,凝聚力和内摩擦角减小,冻结粉土的单轴抗压强度以指数规律减小,破坏应变则呈现出先增加后减小的变化趋势。由于Na₂SO₄易于结晶析出,当含盐量小于3%时,盐分含量不同,盐分在土体中发挥着不同的作用,致使含盐粉土的单轴抗压强度随着含盐量的增加呈先减小后增大的变化趋势;破坏应变则呈现出先增大后减小的趋势。     

Effects of brine content on the strength of frozen Ottawa sand

Frozen saline Ottawa sand was tested in unconfined compression to establish the effects of the volumetric brine content, v_b, on its mechanical behavior. Values of v_b were computed from equilibrium phase relations for NaCl ice, and it was found that the failure strength, the failure strain, and the initial tangent modulus all correlate well with this parameter. The applicability of the equation σ_f = σ_o(1 - cv^k_b) in describing the relation between brine content and failure strength, σ_f, is examined. This equation is often used to describe the failure strength of sea ice. It is found that the equation works well for saline Ottawa sand, but that the values of the constants σ_o, c, and k depend on the assumptions made in fitting the data.     

Effects of seasonally frozen soil on the seismic behavior of bridges

Many of the broad cold regions are located in seismic active zones. Little research has been conducted to investigate the seasonally frozen soil effects on the seismic performance of bridge structures. An analytical investigation of the seasonally frozen soil effects on the seismic behavior of a soil–pile–bridge pier system is presented. Elastic–plastic Finite Element (FE) analyses of a soil–pile model were conducted to obtain the cyclic behavior of the soil–pile system under the unfrozen and frozen soil condition. The equivalent foundation spring coefficients widely used for design practices were developed for the frozen soil condition. A simplified FE model of the soil–pile–bridge pier system was built by representing the soil–pile system with the derived equivalent foundation springs. Modal and push-over analyses were conducted to investigate the effects of seasonally frozen soil on the seismic behavior of the bridge bents. It is found that seasonally frozen soil causes significant change in the stiffness and damping ratio of the soil–pile system, and may have greatly impact the dynamic response of the bridge piers; the influence increases with decreasing pier height-span ratio or increasing overall pier stiffness. It is also found that seasonally frozen soil may impact bridge seismic behavior; the lateral displacement capacity of the pier decreases and the shear demand increases in the frozen soil condition.     

Experimental study on thermal properties of frozen soils

A series of measurements of the effective thermal conductivity, specific heat and unfrozen-water content of various soils exposed to a cold environment have been carried out by a transient probe method and a calorimetric technique. The present experiments were performed at a very slow rate of cooling of the moist-soil and using four kinds of soil samples from fine to coarse grain-size. It was established that the amount of unfrozen-water in the frozen soil was dependent on the initial moisture content, the grain-size of the soil and the temperature (below 0°C). Measured values of the effective thermal conductivity and specific heat show a strong dependence on temperature, initial moisture content and grain-size of soil in the temperature range T = 0°C to ?10°C. The anomalous behavior of a decrease in the effective thermal conductivity of frozen soil for the finest grain-size with decreasing temperature (below 0°C) was observed for a large initial moisture content w = 30–40%.     

Effects of grain-boundary configuration on the high-temperature creep strength of cobalt-base L-605 alloys

The effects of grain-boundary configuration on the high-temperature creep strength are investigated using commercial cobalt-base L-605 alloys with low carbon content in the temperature range 816 to 1038° C (1500 to 1900° F). Serrated grain boundaries are formed principally by the precipitation of tungsten-rich b c c phase (the same as ₂ phase found in Ni-20Cr-20W alloys) on grain boundaries by a relatively simple heat treatment in these alloys. The creep rupture properties are improved by strengthening of grain boundaries by the precipitation of tungsten-rich bcc (₂) phase. The specimens with serrated grain boundaries have longer rupture lives and higher ductility than those with normal straight grain boundaries under low stress and high-temperature creep conditions, while the rupture lives and the creep ductility of both specimens are almost the same under high stresses below 927° C. The matrix of the alloys is strengthened by the precipitation of carbides at temperatures below 927° C and by the precipitation of tungsten-rich ₂ phase at 1038° C during creep. It is found that there is an orientation relationship between tungsten-rich a2 phase particles and-Co matrix, such that (0 1 1)₂ ¶ (1 1 1) _-Co and [1 1]₂ ¶ [1 0] _-Co. The fracture surface of specimens with serrated grain boundaries is a ductile grain-boundary fracture surface, while typical grain-boundary facets prevailed in specimens with straight grain boundaries.     

Coupled modeling of steady-state creep rate and creep rupture strength for metals

New methods of coupled mathematical modeling of steady-state creep rate and creep rupture strength of metals in tension have been devised. Two nonlinear fractional power functions with four material constants are used as basic dependences of the steady-state creep and creep rupture life on stress. Computation of the functions is based on optimally solving two nonlinear and inconsistent — in the conventional meaning — equations sets by the method of minimization of quadratic residuals. The authors outline the methods for calculating material constants, which were used to derive analytical expressions that optimally approximate the test results for 10Kh15N27T3MR steel at 600°C under various stresses. A method of piecewise-linear approximation of creep rupture strength test results, which involves the use of a two-segment broken line, is put forward. It implies that the locations of kinks as well as other numerical characteristics of the broken line are determined from the condition of the line’s optimal arrangement relative to the experimental data points. The method takes a more comprehensive account of various damage accumulation mechanisms in steel under various stress levels. __________ Translated from Problemy Prochnosti, No. 4, pp. 25–35, July–August, 2008.     

Hot strength of creep resistant ferritic steels and relationship to creep rupture data

Abstract

Experimental data on the tensile strength of ferritic steels designed for prolonged service at elevated temperatures have been assessed as a function of many variables, including the testing temperature. The resulting model has been combined with other data on the intrinsic strength of pure ferritic iron and substitutional solute strengthening to show that there is a regime in the temperature range 780–845 K beyond which there is a rapid decline in the microstructural contribution to strength. This decline cannot be attributed to changes in microstructure, but possibly to the ability of dislocations to overcome obstacles with the help of thermal activation. There is evidence of an approximate relationship between the temperature dependence of hot tensile strength and creep rupture stress.     

Short-term creep and strength of fibrous polypropylene structures

The short-term creep and strength of fibrous polypropylene structures are investigated. On the basis of these characteristics, we develop the models of linear and nonlinear viscoelastic deformation of materials, specify the fields of their applicability, and study criteria used for the evaluation of the static strength and durability of these composites. __________ Translated from Problemy Prochnosti, No. 6, pp. 77–90, November–December, 2007.     

Effects of Re content and crystallographic orientation on creep behavior of aluminized Ni-base single crystal superalloys

Effects of Re content and crystallographic orientation on creep behavior of aluminized Ni-base single crystal superalloys have been investigated in the present study. The addition of Re seemed to be effective for the creep strength improvement of Ni-base single crystal superalloys. The creep strength was significantly decreased in aluminized specimens due to the change in microstructure under coating layer by the formation of the interdiffusion zone (IDZ) and substrate diffusion zone (SDZ). The specimens with {100} side-surface showed longer creep rupture lives than the specimens with {110} side-surface which means that the anisotropic creep behavior took place. The anisotropic creep behavior was more evident in aluminized CM186LC than in aluminized PWA 1480. This anisotropic creep behavior was mainly induced by the discrepancy of fracture modes due to the different arrangement of slip systems, the geometry as well as the depth penetration of topologically close-packed (TCP) phases between the two side-surface orientations.