Volumetric Strain Behavior Research Articles

Volumetric strain and strength behavior of frozen soils under confinement

Triaxial compression tests of frozen sand and silty clay were conducted under confining pressures varying between 0.0 and 18.0 MPa, at temperatures of − 4 and − 6 °C, and a strain-rate of 1.67 × 10− 4 s− 1. The results show that the volume of the specimen nonlinearly changes with axial deformation, and cross-section (modified by the volumetric value) of specimen has large influence on stress–strain curve pattern and compressive strength.

Behaviour of tire shred – sand mixtures

Tire shreds and tire shred – soil mixtures can be used as alternative backfill material in many geotechnical applications. The reuse of tire shreds may not only address growing environmental and economic concerns, but also help solve geotechnical problems associated with low soil shear strength. In this study, an experimental testing program was undertaken using a large-scale triaxial apparatus with the goal of evaluating the optimum dosage and aspect ratio of tire shreds within granular fills. The effects on shear strength of varying confining pressure and sand matrix relative density were also evaluated. The tire shred content and tire shred aspect ratio were found to influence the stress–strain and volumetric strain behaviour of the mixture. The axial strain at failure was found to increase with increasing tire shred content. Except for specimens of pure tire shreds and with comparatively high tire shred content, the test results showed a dilatant behaviour and a well-defined peak shear strength. The optimum tire shred content (i.e., the one leading to the maximum shear strength) was approximately 35%. For a given tire shred content, increasing the tire shred aspect ratio led to increasing overall shear strength, at least for the range of tire shred aspect ratios considered in this study. The shear strength improvement induced by tire shred inclusions was found to be sensitive to the applied confining pressure, with larger shear strength gains obtained under comparatively low confinement.Key words: tire shreds, shear strength, reinforcement, triaxial testing, stress–strain behaviour.

Triaxial Testing of Frozen Sand: Equipment and Example Results

This paper describes a high-pressure, low-temperature triaxial testing system that has been specifically designed and built at the Massachusettes Institute of Technology in order to study the physical mechanisms controlling the strength-deformation behavior of frozen sands. The computer controlled system is capable of performing high-stress monotonic triaxial compression tests on specimens measuring D=3.6cm×H=7.6cm and incorporates advanced technology for temperature, strain rate, and loading control. One of the most important and unique features of the system is the technology used for the measurement of small axial strains that makes use of two miniature submersible linear variable differential transformers mounted on a pair of yokes that clamp to the specimen. A description of the specimen preparation equipment and triaxial testing techniques is presented to aid in developing standardized testing approaches, which are currently lacking in the literature. Representative test results on frozen Manchester fine sand are presented to demonstrate the capability of this system to accurately characterize the entire stress-strain and volumetric strain behavior in triaxial compression. Results are provided to illustrate the influence of relative density, confining pressure, strain rate, and temperature.

The effects of salinity and temperature on the behaviour of polyacrylamide gels

Polymer gels are capable of undergoing large volume changes under the influence of solvent composition and temperature. Studies have been conducted on the effects of salinity and temperature on hydrolysed polyacrylamide gels. Three salinities were investigated from 5 parts per thousand (ppt) to 35 ppt, which is approximately the salinity of natural seawater in temperate waters. For each of the salinities, the effect of temperature from 5°C to 40°C with 5°C increments was investigated. It was found that hydrolysed polyacrylamide gels shrank in all the solutions, this effect being most pronounced at the high salinity (35 ppt), with a smaller volume decrease noted in 20 ppt and 5 ppt salinities, respectively. The effect of temperature was minimal, with all solutions promoting a decreasing volume change as the temperature increased. The polyacrylamide gels remained whole in the experiments with no visible signs of degradation. The cyclical volumetric strain behaviour of the gels was also investigated by alternate exposure to saline solutions and distilled water. Cyclical swelling and deswelling of the gels was observed which, in some cases, was fully reversible.

Some observations on the effects of shear stress on a polymorphic transformation in perovskite‐structured lead‐zirconate‐titanate ceramic

We performed a series of hydrostatic and constant‐stress‐difference (CSD) experiments at room temperature on modified lead‐zirconate‐titanate (PZT 95/5‐2Nb) ceramic in order to quantify the influence of shear stress on the displacive, and possibly martensitic, first‐order, ferroelectric/rhombohedral → antiferroelectric/orthorhombic phase transformation. In hydrostatic compression, the transformation began at approximately 260 MPa and was incompletely reversed upon return to ambient conditions. Strains associated with the transformation were isotropic, both on the first and subsequent hydrostatic cycles. Results for the CSD tests were quite different. First, the confining pressure and mean stress at which the transition begins decreased approximately linearly with increasing stress difference. Second, we observed that the rate of transformation apparently decreased with increasing shear stress and the accompanying purely elastic shear strain. This result contrasts with the almost universal assertion that shear stresses accelerate reaction and transformation kinetics. Finally, strain was not isotropic during the transformation: axial strains were greater and lateral strains smaller than for the hydrostatic case, though volumetric strain behavior was comparable for the two types of tests. However, this last effect does not appear to be an example of transformational plasticity but, rather, a “one‐time” occurrence: no additional unexpected strains accumulated during subsequent cycles through the transition under nonhydrostatic loading. If subsequent hydrostatic cycles were performed on samples previously run under CSD conditions, strain anisotropy was again observed, indicating that the earlier superimposed shear stress produced a permanent mechanical anisotropy in the material. The mechanical anisotropy probably results from a crystallographic preferred orientation that developed during the transformation under shear stress.

Hydrostatic and triaxial compression experiments on unpoled PZT 95/5–2Nb ceramic: The effects of shear stress on theFR1→AOpolymorphic phase transformation

We conducted a series of hydrostatic and constant shear stress experiments at room temperature on three different sintering runs of unpoled, niobium-doped lead-zirconate-titanate ceramic (PZT 95/5–2Nb) in order to quantify the influence of shear stress on the displacive (possibly martensitic), first-order, rhombohedral → orthorhombic phase transformation. Inter- and intra-batch variations were detected, but some generalizations can be made. In hydrostatic compression at room temperature, the transformation began at approximately 260 MPa, and was usually incompletely reversed upon return to ambient conditions. Strains associated with the transformation were isotropic, both on the first and subsequent hydrostatic cycles. Results for the constant shear stress tests were very different. First, the confining pressure and mean stress at which the transition begins decreased systematically with increasing shear stress. Second, we observed that the rate of transformationdecreasedwith increasing shear stress and the associated elastic shear strain. This result contrasts with the typical observation that shear stresses increase reaction and transformation kinetics. Third, strain was not isotropic during the transformation: axial strains were greater and lateral strains smaller than for the hydrostatic case, though volumetric strain behavior was comparable for the two types of tests. However, this effect does not appear to be an example of transformational plasticity: noadditionalunexpected strains accumulated during subsequent cycles through the transition under deviatoric loading. If subsequenthydrostaticcycles were performed on samples previously subjected to shear stress, strain anisotropy was again observed, indicating that the earlier superimposed shear stress produced a permanent mechanical anisotropy in the material. The mechanical anisotropy probably resulted from a crystallographic preferred orientation that developed during the transformation under shear stress. Finally, in a few experiments on specimens from one particular sintering run, volume strain was often completely recovered and sporadic evidence for a shape memory effect was observed.

Further observations on creep enhanced by a liquid phase in porous potassium chloride

The enhancement of creep by the introduction of aqueous solutions into the porosity of nonfully dense, cold compacted potassium chloride powder has been characterized in room temperature experiments. This phenomenon was first investigated by Pharr and Ashby, who concluded that the mechanism of creep is diffusionally controlled, with the liquid acting as a rapid diffusion path. Current results show that at high stresses, a second mechanism becomes important. This conclusion is based on differences in the stress exponent for creep, the strain at the onset of tertiary creep, and the volumetric strain behavior at low and high stresses. The mechanism is shown to involve grain boundary sliding accommodated by the opening of intergranular cracks and voids.