Creep Rig Research Articles

Experimental determination of creep coefficients for sintered flyash lightweight aggregate based concrete and verification with creep models

The experimental creep coefficient has been determined for two concrete strengths of about 40 MPa and 60 MPa using sintered flyash lightweight aggregate and granite aggregate in creep rig with capacity of 1500kN as per ASTM C-512. The creep results indicate that despite both density and modulus of elasticity being on lower side for sintered flyash lightweight based concrete, the creep coefficient has been lower than that of normal weight concrete with granite aggregate for same strength level. The comparison of existing creep models has been carried out with experimental results for both normal and lightweight concrete considering parameters in models such as relative humidity level, concrete mix ingredients, aggregate properties, oven dry density, elastic modulus of both concrete types etc. considered in study. The creep coefficients have been obtained experimentally at loading age of 28-day and testing period of 365 days and has been compared with existing creep models such as B-3, B-4, ACI-209, EN:1992/FIB. For the similar compressive strength level, creep coefficient of structural grade lightweight concrete is found to be lower in comparison to creep coefficient of normal weight concrete as the internally stored water in porous aggregates keeps capillary pores almost saturated and leaves minimal chance for seepage to occur

High-temperature creep of magnetite and ilmenite single crystals

We performed deformation experiments on dry natural single crystals of magnetite and ilmenite to determine the rheological behavior of these oxide minerals as a function of temperature, orientation, and oxygen fugacity. Samples were deformed at temperatures of 825–1150 ,^{circ }C to axial strains of up to 15–24% under approximately constant stress conditions up to 120 MPa in a dead-load-type creep rig at ambient pressure in a controlled gas atmosphere. Oxygen fugacity ranged from 10^{-9.4} to 10^{-4} atm. Ilmenite creep was insensitive to oxygen fugacity, while magnetite displayed a strong, non-monotonic oxygen fugacity dependence, with creep rates varying as f_{O_{2}}^{-0.7} and f_{O_{2}}^{0.4} at more reducing and more oxidizing conditions, respectively. Dislocation creep rates of magnetite single crystals were weakly dependent on crystallographic orientation with stress exponents that varied between 2.8 and 4.3 (mean 3.5 ± 0.4). Magnetite compressed parallel to <100>, <110>, and <111> axes exhibited apparent activation energies of 315±5, 345±30, and 290±5 kJ/mol, respectively. We estimated {f_O}_2-independent magnetite activation energies of 715 ± 150, 725 ± 145, and 690 ± 150 kJ/mol for <100>, <110>, and <111> orientations, respectively, in the region of negative {f_O}_2-dependence. Ilmenite single crystals were compressed parallel, normal, and inclined to the c-axis. Stress exponents of 3.4, 4.3, and 3.9 indicate dislocation creep with activation energies of 420 ± 35, 345 ± 30, and 360 ± 40 kJ/mol, respectively, for these orientations. Mechanical anisotropy in ilmenite is notably higher than in magnetite, as expected from its lower crystal symmetry. Constitutive equations were formulated for ilmenite and magnetite creep.

High‐Temperature Deformation Behavior of Synthetic Polycrystalline Magnetite

AbstractWe performed a series of deformation experiments on synthetic magnetite aggregates to characterize the high‐temperature rheological behavior of this mineral under nominally dry and hydrous conditions. Grain growth laws for magnetite were additionally determined from a series of static annealing tests. Synthetic magnetite aggregates were formed by hot isostatic pressing of fine‐grained magnetite powder at 1,100 °C temperature and 300‐MPa confining pressure for 20 hr, resulting in polycrystalline material with a mean grain size around 40 μm and containing 2–4% porosity. Samples were subsequently deformed to axial strains of up to 10% under constant load conditions at temperatures between 900 and 1,150 °C in a triaxial deformation apparatus under 300‐MPa confining pressure at applied stresses in the range of 8–385 MPa or in a uniaxial creep rig at atmospheric pressure with stresses of 1–15 MPa. The aggregates exhibit typical power‐law creep behavior with a mean stress exponent of 3 at high stresses, indicating a dislocation creep mechanism and a transition to near‐Newtonian creep with a mean stress exponent of 1.1 at lower stresses. The presence of water in the magnetite samples resulted in significantly enhanced static grain growth and strain rates. Best‐fit flow laws to the data indicate activation energies of around 460 and 310 kJ/mol for dislocation and diffusion creep of nominally dry magnetite, respectively. Based on the experimentally determined flow laws, magnetite is predicted to be weaker than most major silicate phases in relatively dry rocks such as oceanic gabbros during high‐temperature crustal deformation.

CREEP CHARACTERISTICS OF CHALK MARL UNDER UNIAXIAL AND CONFINED COMPRESSION STATE STRESS

This paper studies the time-dependent deformation of chalk marl under uniaxial state of stress in a specially built creep rigs, and confined compression stress state. The tests carried out include: six uniaxial creep tests with vertical and lateral deformation measurements, at different stress levels which lasted for 75 to 268 days, six oedometer creep tests under different stress intensities and different loading conditions which lasted for 80 to 250 days. All tested samples were trimmed from 113 mm diameter cores. For uniaxial tests 0.5% axial strain occurred almost instantaneously in the first series subjected to a stress level of 80%. In the second series subjected to a stress level of 40%, the average initial strain was 0.25% which indicate clearly their stress level dependency. After 75 to 150 days creep period, the measured average creep strain was 0.33% for the first series, which represents 57% of the consolidation strain. For series 2 the average creep strain = 0.26%, which represents 54% of the consolidation strain. This implies that the proportional creep ratio Cr = (Et-E1) / E1 is independent of the applied stress level, where Et strain at any time t, E1 strain at one day. For 1-D creep tests E1 and the proportional strain (Et-E1) are not affected significantly by the intensity of applied creep stress, while both of them are highly influenced by the method of load application. From these results, it can be concluded that creep deformation constitutes an important part of the total deformation of chalk marl.

Creep testing of concrete since setting time by means of permanent and repeated minute-long loadings

Monitoring the evolution of early age properties of concrete is necessary to provide input data to the models of prediction of the behavior of concrete structures since setting time. The experimental challenge lies in the fact that this monitoring must be fully automatic since the earliest age because the hardening process of the concrete takes place continuously over a period counted in hours and even in days after the casting time. This research paper presents a new methodology developed at ULB and IFSTTAR to monitor the creep and relaxation of an ordinary concrete chosen as reference concrete since setting time. Compressive creep rigs and two test devices were used: at ULB, a Temperature Stress Testing Machine (TSTM) specifically designed for testing concrete since setting time under free and restraint conditions and at IFSTTAR, a test set up called BTJASPE developed to monitor, in compression, the modulus of elasticity, the creep and the relaxation of a concrete since very early age. For the sake of the study, the same concrete has been used in the two laboratories. In addition, this study is performed at a constant temperature to exclude this parameter, at this step of the study. The methodology is based on experimental measurements with two kinds of test. A classical creep test with permanent loading during one week and a repeated minute-long loading test for which every 30 min, a loading is applied and kept constant during 5 min and finally totally removed. The classical test is used to characterize the non-aging creep function and the repeated minute-long loading test is used to quantify the aging creep functions.

Long-Term Behaviour of Prestressed Basalt Fibre Reinforced Polymer Bars

The opportunity to use basalt fibre reinforced polymer (BFRP) bars as internal reinforcement for concrete could be beneficial in aspect of limiting the relatively high deformability. The information about long-term behaviour of such reinforcement under prestressing loading would be helpful to estimate prestress losses, which have to be expected. Obtaining such experimental data and analysing it could be used for further developing of the design principles and procedure for prestressed elements with BFRP reinforcement.A testing rig (referred to as creep rig) has been build to evaluate the behaviour of BFRP rebars in constant loading conditions with a direct correlation to steel rebars and cables. All samples have been put in to equal tension of 16 kN and have been monitored over a long period of time. The constant level of loading was kept and corresponding deformations have been measured. This study aims to investigate the behaviour of BFRP rebars under long term static loading.

Low oxygen fugacity dependency for the deformation of partially molten lherzolite

We present the results of an experimental investigation of the influence of oxygen fugacity on the deformation of a partially molten spinel lherzolite using a 0.1MPa gas-media creep rig under temperatures of 1160–1190°C, stresses of 4–74MPa and well-controlled oxygen fugacities of 10−7 to 10−11MPa. The partially molten spinel lherzolite was deformed by a dislocation-mediated creep process such as dislocation creep or dislocation-accommodated grain boundary sliding (DGBS) (n=3.5±0.3) as well as in the diffusion creep regime (n=1.2±0.2). The average oxygen fugacity exponent of lherzolite is 0.04±0.02, which is significantly smaller than the values measured for olivine single crystals (0.10–0.36) and dunite rocks (0.20) deformed by dislocation creep. We attribute the low oxygen fugacity exponent of partially molten lherzolite samples either to the operation of grain boundary sliding, if DGBS dominates the deformation, or to the presence of pyroxenes whose deformation has a weak or no dependency on oxygen fugacity, if dislocation creep dominates the deformation. In the latter case, the oxygen fugacity exponent decreases rapidly with decreasing volume fraction of olivine. The low oxygen fugacity exponent for our partially molten lherzolite samples implies that dislocation creep or DGBS of mantle peridotite will most likely be insensitive to variations of oxygen fugacity in the upper mantle.

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All in an engineer’s life

High‐temperature deformation of calcite single crystals

To investigate the high‐temperature plasticity of calcite, we performed creep experiments in a 0.1‐MPa, dead‐load creep rig at temperatures T = 690°–880°C, oxygen fugacities fO2 = 10−20 − 10−2 MPa, and a constant CO2 fugacity fCO2 ≈ 10−1 MPa. Single crystals from four different sources were compressed along . Applied stress, σ, was controlled at 5 to 75 MPa to yield strain rates, ε°, between 10−7 and 10−4 s−1. Inductive coupled plasma mass spectrometry analysis indicated that Mn is the major impurity in the four crystals; concentrations varied between 0.002 and 0.06 wt %. Two deformation regimes were revealed: a power law regime and a power law breakdown regime. At low stress (σ ≤ 30 MPa), steady state creep data yielded a stress exponent of ∼3.5 ± 0.5. Optical microscopy showed few twins. Transmission electron microscopy revealed that f and c were the major slip systems. The majority of dislocations in deformed samples are curved, suggesting that dislocation climb is the rate‐limiting step within the low stress regime. Three deformation mechanisms with different flow laws operated. For the samples with Mn concentration &lt;200 ppm, the oxygen fugacity exponent, m, is 0 and the activation energy for creep, Q, is 305 kJ/mol. For the samples with higher Mn concentration (≥600 ppm), creep depends on both fO2 and temperature. A deconvolution of the fO2 ‐ ε° data and the ε° ‐ 1/T data yielded that, at low fO2 and low T, m = 0 and Q = 200 kJ/mol, while at high fO2 and/or high T, m = 1/6 and Q = 400 kJ/mol. At similar temperature and stresses, samples with low Mn concentration deform substantially slower than those with high Mn concentration. Appararently, variations in Mn concentration affect creep through the charge neutrality conditions governing point defect structure. Based on the mechanical data, published diffusion data and point defect chemistry calculations, three rate‐controlling mechanisms for creep are proposed. At high stress (σ ≥ 30 MPa), deformation rate is much more sensitive to stress, indicating power law creep down break. The microstructure showed increased activity of dislocation glide, cross slip, and twinning. The value of the stress separating the two deformation regimes is about 10−3 when normalized by the shear modulus, in agreement with those for some other geological materials.

Resonant irradiation creep of 316 stainless steel under pulsed deuteron bombardment

Pulsing effects on irradiation creep of type 316 stainless steel have been investigated using a well-controlled tensile creep rig and a 10 MeV deuteron beam generated by a cyclotron. Frequency dependence of pulse-induced strain has been measured applying square waveforms over a frequency range of 50 to 5 × 10 −4 Hz. Solution-annealed 316SS shows a significant increase in strain by the pulsation, much larger than the steady-state creep, while the cold-worked one gives no creep enhancement. This pulse-induced strain has a resonant feature around a frequency of 5 × 10 −3 Hz, and may indicate a technological importance of pulsing effects on solution-annealed 316SS in fusion reactors, even at such low frequencies and low doses. A point defect model suggests that transient interstitial enrichment plays an important role in the creep enhancement, and the results may be in favor of a creep mechanism of stress-induced preferred nucleation.

Irradiation creep of Fe-25Ni-15Cr during 10 MeV deuteron bombardment

Abstract Irradiation creep of Fe-25Ni-15Cr-0.07C and a Ti-modified counterpart has been investigated using a tensile creep rig and a 10 MeV deuteron beam generated by a small cyclotron. The creep rates found in the Fe-25Ni-15Cr-0.07C alloy are relatively large compared to those of 316 stainless steels, while the Ti-modified alloy shows a smaller creep rate. The result suggests that the Ni content of 25% solely does not improve the creep resistance but that the minor elements such as Ti would play an important role in suppressing the creep. The stress exponent of creep rate tends to change from linear to quadratic at about 150 MPa. The observed stress dependence may be qualitatively explained in terms of the SIPA and the PAG models, but, quantitatively, there exists considerable disagreement especially for the high stress region.

The effect of mechanical deformation on the magnetic properties of amorphous alloys

Amorphous alloys can deform in two distinct modes, one homogeneous and the other inhomogeneous, and it has been known for some time that inhomogeneous deformation produces a severe degradation of soft magnetic properties. We report d.c. measurements on METGLASR 2826, VITROVACR 0040 and VITROVACR 6010 that have been cold rolled up to a 55% thickness reduction. By separately considering the effect of surface roughness on Hc, we are able to give clear evidence for pinning of the domain walls at the bulk shear bands. Measurements on magnetostrictive and non-magnetostrictive materials provide evidence that the major source of pinning after inhomogeneous deformation is associated with localized stress fields in the sample. As a counterpart to our earlier a.c. measurements on homogeneously deformed ribbon, we report d.c. measurements on samples homogeneously deformed at low strains in a creep rig. Hc in this case remains close to that of a ribbon given the same heat treatment but not crept. The results after deformation in both regimes are discussed in terms of the current models of the effects of deformation.

Tensile creep study of copper with alumina dispersions prepared by high rate physical vapour deposition

Mott has suggested that the ideal creep-resistant material will be one with a fine grain size in which the grain boundaries are filled with some substance, say a refractory oxide, to inhibit the motion of grain boundaries. Such a system, alumina-dispersed copper, was prepared by high rate physical vapour deposition. The process parameters and their effect on structure and texture have been studied. The room temperature mechanical properties have also been reported. This paper deals with a high temperature mechanical property, i.e. tensile creep. Tests were made on a constant-stress vacuum creep rig with a Ferrometic feedthrough to ensure a zero leakage rotary seal. A vacuum of 1.33 X 10 −3 Pa was maintained. The test temperatures were 500°C (0.57 T m) and 700°C (0.72 T m). The stresses applied were 2.07, 3.45, 4.14 and 6.89 X 10 7 Nm −2. Tests were made on as-deposited films and on cold-rolled condensates. Minimum creep rate curves showed the effect of the alumina content in raising the creep resistance of copper. Cold rolling also reduced the minimum creep rate markedly. Varying the temperature and stress affected the shape of these curves. Stress-rupture plots were used to summarize the data. Grain refinement together with a fine stable dispersion seem to give improved creep strength. The critical barrier of the Orowan stress was noted. The stress exponent for a low alumina deposit (0.21 vol.%) was about 8, and the apparent activation energy for creep was about 202 kJ mol −1 (0.13 vol.%). There was an inverse relationship between the rupture life and the minimum creep rate, their product being constant (about 0.2).

A low-cost creep rig for concrete

Synopsis A low-cost creep rig based on a commercially available flexible-diaphragm jack (flat jack) was developed by the authors. It was designed for creep tests on 380 × 130 × 130 mm specimens at a maximum load of 170 kN. A portable hydraulic hand pump is used with the flat jack to apply the load. Thirty rigs of this type have been used, six of them for continuing tests over a period of three years. The paper describes the methods used to calibrate the jack and shows that the rig is capable of maintaining a constant load to the accuracy required by the Australian Standard.

Graphite high temperature creep rigs

A description is given of two high temperature tensile creep rigs, for irradiating pyrocarbon and graphite specimens. PIRITHOOS is a creep rig operating at 1100°C and utilizing three in line pyrocarbon specimens. These have different cross sections giving three stress values. Unstressed specimens are placed close to the tensile ones. Dimensional measurements: length, thickness, width are made in hot cells, after each reactor shut down. FLACH is a graphite creep rig allowing continuous length measurement to be made in comparison with the length of two reference specimens. These rigs consist of two main parts: the creep capsule including: specimens, loading bellows, microwave measuring apparatus, which is introduced into a standard gas gap furnace regulating the temperature by gas mixture and electrical heating.

Irradiation induced creep experiments in the BR2 reactor using the resonant cavity method

A 16Cr16NiNb stainless steel (steel 1.4981) will be used as wrapper material for SNR 300. Therefore, some in-pile creep tests have been performed with this material in the temperature range 420–700°C. The main objective of this programme was to see, whether the creep rates of steel 1.4981 followed at low fluences ( ⩽ 2.5 × 10 22 n/cm 2 , E > 0.1 MeV ) the same rules as for other austenitic stainless steels. The experiments were performed in the BR2 reactor at Mol/Belgium, using creep rigs which were developed and manufactured by CEN Grenoble. The creep strains were measured by the resonant cavity method. The paper describes the main characteristics of the creep capsules, and reports on the performance of those types of rigs. Finally, the experimental results are presented and discussed.

In-pile creep measuring rigs for metallic specimens

Two types of creep rigs are described using stainless steel or Zircaloy specimens. First, a tensile creep rig allowing continuous length measurement of tubular or solid specimens. The measurement of the specimen length is compared with that of a reference specimen, situated as close as possible to the tensile one. The second rig is used for continuous measurement of the radial strain of pressurised tubes. The measurement is made by a cone and ball system, transforming diameter changes into axial displacements. These rigs are made in two parts: a capsule with NaK, including: specimens, loading bellows, microwave measuring system. And a standard “Chouca” furnace with electrical heating.

Verification of a model for in-reactor creep transients in zirconium

Abstract Time-dependent solutions to the rate equations describing the vacancy and interstitial concentrations in an irradiated metal have been used to predict the transients in the in-reactor creep rate of zirconium that would occur during reactor start-up or shut-down. It is predicted that both the application and removal of flux causes the in-reactor creep rate to exceed the steady-state rate. Experiments using a creep rig designed to be able to move in and out of the reactor core have shown that the flux enhancement of creep begins immediately and that the creep rate decreases continuously until steady state is reached. Similarly, the removal of flux causes the creep rate initially to increase, before decreasing to a negligible value. The good agreement between theory and experiment gives strong support to a model of flux-enhanced climb plus glide for the in-reactor creep of zirconium.

A creep rig for testing concrete under multiaxial compressive stress

Summary The paper describes the design and testing of apparatus for the study of time-dependent strains in concrete under sustained multiaxial loading. The apparatus is designed to measure the three orthogonal strains under three independent compressive stresses. Special attention is given to the problem of frictional effects between the loading platens and the concrete specimens.