Showing papers on "Creep published in 2014"

Thermo-mechanical behavior of energy piles in high plasticity clays

Abstract: Energy piles make use of constant and moderate ground temperature for efficient thermal control of buildings. However, this use introduces new engineering challenges because the changes of temperature in the foundation pile and ground induce additional deformations and forces in the foundation element and coupled thermo-hydro-mechanical phenomena in the soil. Several published full-scale tests investigated this aspect of energy piles and showed thermally induced deformation and forces in the foundation element. In parallel, significant progress has been made in the understanding of thermal properties of soils and on the effect of cyclic thermal load on ground and foundation behavior. However, the effect of temperature on the creep rate of energy piles has received practically no attention in the past. This paper reports the experimental results of an in situ tension thermo-mechanical test on an energy pile performed in a very stiff high plasticity clay. During the in situ test, the pile was subjected to thermal loading by circulating hot water in fitted pipes, simulating a thermal load in a cooling-dominated climate, at different levels of mechanical loading. The axial strain and temperature in the pile, and the load–displacement of the pile were monitored during the tension test at different locations along the center of the pile and at the pile head, respectively. The data showed that as the temperature increases, the observed creep rate of the energy pile in this high plasticity clay also increases, which will lead to additional time-dependent displacement of the foundation over the life time of the structure. It was also found that the use of geothermal piles causes practically insignificant thermally induced deformation and loads in the pile itself.

Timescales in creep and yielding of attractive gels

Abstract: The stress-induced yielding scenario of colloidal gels is investigated under rough boundary conditions by means of rheometry coupled with local velocity measurements. Under an applied shear stress σ, the fluidization of gels made of attractive carbon black particles dispersed in a mineral oil is shown to involve a previously unreported shear rate response (t) characterized by two well-defined and separated timescales τc and τf. First decreases as a weak power law strongly reminiscent of the primary creep observed in numerous crystalline and amorphous solids, coined the “Andrade creep”. We show that the bulk deformation remains homogeneous at the micron scale, which demonstrates that whether plastic events take place or whether any shear transformation zone exists, such phenomena occur at a smaller scale. As a key result of this paper, the duration τc of this creep regime decreases as a power law of the viscous stress, defined as the difference between the applied stress and the yield stress σc, i.e. τc ∼ (σ − σc)−β, with β = 2–3 depending on the gel concentration. The end of this first regime is marked by a jump of the shear rate by several orders of magnitude, while the gel slowly slides as a solid block experiencing strong wall slip at both walls, despite rough boundary conditions. Finally, a second sudden increase of the shear rate is concomitant with the full fluidization of the material which ends up being homogeneously sheared. The corresponding fluidization time τf robustly follows an exponential decay with the applied shear stress, i.e. τf = τ0 exp(−σ/σ0), as already reported for smooth boundary conditions. Varying the gel concentration C in a systematic fashion shows that the parameter σ0 and the yield stress σc exhibit similar power-law dependences with C. Finally, we highlight a few features that are common to attractive colloidal gels and to solid materials by discussing our results in the framework of theoretical approaches of solid rupture (kinetic, fiber bundle, and transient network models).

Time-Dependent Behavior of Diabase and a Nonlinear Creep Model

Abstract: Triaxial creep tests were performed on diabase specimens from the dam foundation of the Dagangshan hydropower station, and the typical characteristics of creep curves were analyzed. Based on the test results under different stress levels, a new nonlinear visco-elasto-plastic creep model with creep threshold and long-term strength was proposed by connecting an instantaneous elastic Hooke body, a visco-elasto-plastic Schiffman body, and a nonlinear visco-plastic body in series mode. By introducing the nonlinear visco-plastic component, this creep model can describe the typical creep behavior, which includes the primary creep stage, the secondary creep stage, and the tertiary creep stage. Three-dimensional creep equations under constant stress conditions were deduced. The yield approach index (YAI) was used as the criterion for the piecewise creep function to resolve the difficulty in determining the creep threshold value and the long-term strength. The expression of the visco-plastic component was derived in detail and the three-dimensional central difference form was given. An example was used to verify the credibility of the model. The creep parameters were identified, and the calculated curves were in good agreement with the experimental curves, indicating that the model is capable of replicating the physical processes.

ISRM Suggested Methods for Determining the Creep Characteristics of Rock

Abstract: It is important to note that creep is only one aspect of the time-dependent behavior of rocks. In Fig. 1, three cases are illustrated with respect to the complete stress–strain curve: creep, i.e., increasing strain when the stress is held constant; stress relaxation, i.e., decreasing stress when the strain is held constant; and a combination of both, when the rock unloads along a chosen unloading path. This ISRM suggested method deals only with the case of creep, which is particularly relevant for cases where the applied load or stress is kept constant. Creep tests have also been carried out on soft rocks such as tuff, shale, lignite, and sandstone, medium-hard rocks such as marble, limestone, and rock salt, and hard rocks such as granite and andesite (i.e., Akagi 1976; Akai et al. 1979, 1984; Ito and Akagi 2001; Berest et al. 2005; Doktan 1983; Passaris 1979; Serata et al. 1968; Wawersik 1983; Okubo et al. 1991, 1993; Masuda et al. 1987, 1988; Ishizuka et al. 1993; Lockner and Byerlee 1977; Boukharov et al. 1995; Fabre and Pellet 2006; Aydan et al. 1995; Chan 1997; Cristescu and Hunsche 1998; Hunsche 1992; Hunsche and Hampel 1999; Ito et al. 1999; Mottahed and Szeki 1982; Perzyna 1966; Slizowski and Lankof 2003; Yang et al. 1999). These experiments were mostly carried out under compressive loading conditions. There are few studies on rocks using creep tests under a tensile loading regime (Ito and Sasajima 1980, 1987; Ito et al. 2008; Aydan et al. 2011). In particular, shallow underground openings may be subjected to a sustained tensile stress regime, which requires the creep behavior of rocks under such conditions. Please send any written comments on this ISRM suggested method to Prof. Resat Ulusay, President of the ISRM Commission on Testing Methods, Hacettepe University, Department of Geological Engineering, 06800 Beytepe, Ankara, Turkey.

Co-optimization of wrought alumina-forming austenitic stainless steel composition ranges for high-temperature creep and oxidation/corrosion resistance

Abstract: A series of candidate alumina-forming austenitic (AFA) stainless steels designed to evaluate the effects of variation in Al, C, Cr, Mn, Nb, and Ni content on high-temperature tensile properties, creep, and oxidation/corrosion resistance were studied. The compositions assessed were based on medium Ni (20–25 wt%) and low Ni (12 wt%) AFA variations strengthened primarily by MC and/or M23C6 carbide precipitates, and a high Ni (32 wt%) AFA superalloy variation strengthened primarily by γ′-Ni3Al intermetallic precipitates. Tensile and creep properties were measured at 650 and 750/760 °C, oxidation resistance from 650 to 900 °C in air with water vapor and steam environments, and sulfidation–oxidation resistance in Ar–20%H2–20%H2O–5% H2S at 550 and 650 °C. Optimized composition ranges for different use temperatures ranges based on these evaluations are presented.

Creep and fracture of a protein gel under stress.

Abstract: Biomaterials such as protein or polysaccharide gels are known to behave qualitatively as soft solids and to rupture under an external load. Combining optical and ultrasonic imaging to shear rheology we show that the failure scenario of a protein gel is reminiscent of brittle solids: after a primary creep regime characterized by a power-law behavior whose exponent is fully accounted for by linear viscoelasticity, fractures nucleate and grow logarithmically perpendicularly to shear, up to the sudden rupture of the gel. A single equation accounting for those two successive processes nicely captures the full rheological response. The failure time follows a decreasing power law with the applied shear stress, similar to the Basquin law of fatigue for solids. These results are in excellent agreement with recent fiber-bundle models that include damage accumulation on elastic fibers and exemplify protein gels as model, brittlelike soft solids.

Creep rupture behavior of 9Cr–1.8W–0.5Mo–VNb (ASME grade 92) ferritic steel weld joint

Abstract: Creep rupture behavior of 9Cr–1.8W–0.5Mo–VNb (ASME grade 92) ferritic steel weld joint fabricated by activated TIG (A-TIG) welding process have been investigated at 923 K over a stress range of 80–150 MPa. The weld joint was comprise of fusion zone, heat affected zone (HAZ) and base metal. The HAZ consisted of coarse prior-austenite grain (CGHAZ), fine prior-austenite grain (FGHAZ) and intercritical (ICHAZ) regions in an order away from the fusion zone to base metal. A hardness trough was observed at the outer edge of HAZ of the weld joint. TEM investigation revealed the presence of coarse M23C6 precipitates and recovery of martensite lath structure into subgrain in the ICHAZ of the weld joint, leading to the hardness trough. The weld joint exhibited lower creep rupture lives than the base metal at relatively lower stresses. Creep rupture failure location of the weld joint was found to shift with applied stress. At high stresses fracture occurred in the base metal, whereas failure location shifted to FGHAZ at lower stresses with significant decrease in rupture ductility. SEM investigation of the creep ruptured specimens revealed precipitation of Laves phase across the joint, more extensively in the FGHAZ. On creep exposure, the hardness trough was found to shift from the ICHAZ to FGHAZ. Extensive creep cavitation was observed in the FGHAZ and was accompanied with the Laves phase, leading to the premature type IV failure of the steel weld joint at the FGHAZ.

Mechanisms of time‐dependent deformation in porous limestone

Abstract: We performed triaxial deformation experiments on a water-saturated porous limestone under constant strain rate and constant stress (creep) conditions. The tests were conducted at room temperature and at low effective pressures Peff=10 and Peff=20 MPa, in a regime where the rock is nominally brittle when tested at a constant strain rate of 10−5 s−1. Under these conditions and at constant stress, the phenomenon of brittle creep occurs. At Peff=10 MPa, brittle creep follows similar trends as those observed in other rock types (e.g., sandstones and granites): only small strains are accumulated before failure, and damage accumulation with increasing strain (as monitored by P wave speeds measurements during the tests) is not strongly dependent on the applied stresses. At Peff=20 MPa, brittle creep is also macroscopically observed, but when the creep strain rate is lower than ≈10−7 s−1, we observe that (1) much larger strains are accumulated, (2) less damage is accumulated with increasing strain, and (3) the deformation tends to be more compactant. These observations can be understood by considering that another deformation mechanism, different from crack growth, is active at low strain rates. We explore this possibility by constructing a deformation mechanism map that includes both subcritical crack growth and pressure solution creep processes; the increasing contribution of pressure solution creep at low strain rates is consistent with our observations.

Rate- and strain-dependent brittle deformation of rocks

Abstract: We develop a unifying framework to quantify rate-dependent deformation in the brittle field and establish links between the microscale time-dependent crack growth processes and the macroscopically observed rate dependency Triaxial deformation experiments have been performed under both constant strain rate and constant stress (creep) conditions on three types of sandstone The measured relative evolution of P wave speeds as a function of inelastic axial strain is similar for both types of test, despite differences in strain rate of up to 3 orders of magnitude This similarity indicates that there exists a direct, time-independent link between the microstructural state (as reflected by the variations in P wave speed) and the inelastic axial strain Comparison of applied stresses between constant strain rate and creep experiments as a function of inelastic strain indicates that creep deformation requires less mechanical work to bring the sample to failure This energy deficit corresponds to a stress deficit, which can be related to a deficit in energy release rate of the microcracks We establish empirically that the creep strain rate is given by e∝exp(ΔQ/σ∗), where ΔQ is the stress deficit (negative) and σ∗ is an activation stress This empirical exponential relation between creep strain rate and stress deficit is analogous to rate-and-state friction law We develop a micromechanical approach based on fracture mechanics to determine the evolution of an effective stress intensity factor at crack tips during creep deformation and estimate the activation volume of the stress corrosion reaction responsible for brittle creep