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.     

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.     

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.     

Thermal interaction between gas line and frozen soils

The article examines the numerical solution of the problem of heat exchange during the flow of gas through an underground pipeline taking into account the phase transitions in the soil under various cooling regimes of the gas.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 37, No. 3, pp. 489–496, September, 1979.     

Experimental and numerical study of creep and creep rupture behavior of PA6

Polyamide 6 (PA6) is a semi-crystalline polymer utilized in many structural components working under steady load. At room temperature, PA6 exhibits time dependent (viscoplastic) deformation. The aim of this work is to investigate the mechanical response and the crack growth of PA6 under creep conditions.The experimental database consists of tests carried out on smooth and notched round bars. Load versus displacement curves were recorded for monotonic tests tensile at various crosshead speeds. Then, creep tests at constant load were performed allowing the record of the creep strain history according to the applied load.Microscopic observations highlight the initial spherulitic structure.Smooth and notched specimens were utilized in order to identify and to validate material coefficients dedicated for analytical modeling. The non linear fracture mechanics for creeping solids was applied to results on “pre-cracked” specimens. For this kind of loading, use of the load parameter C^∗ is recommended.By plotting C^∗ values versus time to failure, a unique correlation was obtained. The knowledge of this master curve allows lifetime assessment of PA6 cracked bodies.     

Effect of microstructural texture on the creep behavior

The two steels were prepared with the same composition but different rolling processes. The equiaxed grain of TMCP EH36 steel was produced by thermo-mechanical control rolling (TMCP) with an accelerated cooling process. The banded structure of SM490C steel was produced using the conventional hot rolling process. After creep the results show that the apparent activation energy and apparent stress exponent in the band structure of SM490C steel are much higher than that in the equiaxed grain of TMCP EH36 steel. The second phase distribution and morphology have an important effect on the creep behavior in this study.     

Effects of gage length and stress concentration on the compressive strength of a unidirectional CFRP

Compression tests for a unidirectional CFRP were carried out for a wide range of gage lengths, three different configurations of end tab edges and two different end tab materials under the conditions of constant specimen thickness. The relation of σ_c-L is to be divided into two parts, namely, a part where the compressive strength, σ_c, is nearly constant independent of gage length, L, and beyond that, a part where σ_c decreases with increasing L. The apparent compressive strength measured by Celanese test method was lower than the true compressive strength because of the stress concentration near the end tab edges of the specimen. The true compressive strength was obtained by using the specimen where the gage lengths of 3.2 mm and 6.4 mm, and the material of end tab is stainless steel.