Axial Strain Rate Research Articles

Influence of Dynamic Disturbance on the Creep of Sandstone: An Experimental Study

Dynamic disturbance such as blasting may significantly affect the creep of rock. Quantification of the influence of dynamic disturbance on the creep behavior of rock is a prerequisite to the understanding of the long-term behavior of rock around excavations. In this study, a new creep-impact test machine that is capable of testing the rock failure behavior under combined creep loading and dynamic disturbance was introduced. We performed creep experiments on sandstone while a dynamic disturbance was applied. Axial strain, volumetric strain and acoustic emission (AE) events were measured. The results from the creep-impact experiment show that the rock creep was greatly affected by the dynamic disturbance. Dynamic disturbance may introduce further damage on rock and shorten the time-to-failure of creeping rock specimens. Combination of creep stress and dynamic disturbance resulted in two failure conditions: failure along with accelerating creep and failure during dynamic disturbance. If the dynamic disturbance was not followed by failure, the axial strain consisted of an instantaneous response as the dynamic disturbance was applied, followed by a primary phase of decelerating creep and a steady-state creep phase. This pattern was repeated after the next dynamic disturbance until the last dynamic disturbance that led to the failure. The creep resulted from micro-fracturing in the rock, which can be characterized by the cumulative AE energy if the AE events were monitored during the creep-impact test of rock. The creep behavior was more sensitive to dynamic disturbance under higher creep stress. Under the same creep stress, a dynamic disturbance with higher impact energy resulted in a higher axial strain rate, absolute volumetric strain rate and AE energy rate. Dynamic disturbance not only increased the axial strain rate but also promoted the dilatancy of the rock specimens. The failure of the rock specimens was mostly in the shear mode, and this failure pattern was merely affected by the dynamic disturbance, even though the specimens became more fragmented under the higher creep stress and higher impact energy.

Development of a novel plane strain test apparatus for frozen soils

In this paper, a novel apparatus enable of high controlling accuracy of temperature was developed for plane strain test of frozen soils. A cryostat was used to control test temperature. To improve temperature distribution uniformity of frozen soil specimens, two guiding plates were installed into the cryostat to force cooling air distribute uniformly surround specimen. According to test results, the temperature difference between different monitored points and average temperature can be controlled at ±0.2 °C. By taking Chinese standard sand as study object, compression tests under different axial strain rates were conducted. The images on the surface with strain restrained were captured during loading process, and strain localization evolution was analyzed with PIV technique. It was indicated that, with decrease of strain rate, shear band inclination and width do not change notably, while the strain field distributes more uniformly and concentration of which was decreased.

A micro-macro dynamic compressive-shear fracture model under static confining pressure in brittle rocks

A new dynamic compressive-shear fracture model under static confining pressure is proposed in brittle rocks containing numerous initial microcracks. The formulation of this model is based on the wing crack model, the Mohr-Coulomb failure criterion, the crack-strain relation, and the established dynamic fracture toughness relating axial strain rate. The crack-strain relation is obtained by correlating the micro- and macro damages. The dynamic fracture toughness relating axial strain rate is established by combining the crack-strain relation and dynamic fracture toughness relating crack growth speed. The intact dynamic stress-strain constitutive relationship describing pre-peak hardening phase and post-peak softening phase is presented. The effects of strain rate on dynamic stress-strain relationship, compressive strength, shear strength, cohesion and internal friction are predicted. A novel critical strain rate making the dynamic crack initiation stress equal dynamic compressive strength is found. Exceeding this critical strain rate, the dynamic compressive strength, shear strength, cohesion and internal friction angle have a larger change. The sensitivities of confining pressure, crack size, crack friction, crack inclination angle on dynamic stress-strain relationship, compressive strength, shear strength, cohesion and internal friction angle under high strain rate are also discussed. Rationality of this proposed compressive-shear fracture model is verified by comparing the experimental data.

Experimental characterization and constitutive modeling of the non-linear stress\u2013strain behavior of unidirectional carbon\u2013epoxy under high strain rate loading

The mechanical response of IM7-8552 carbon epoxy was investigated for transverse tension and transverse tension/in-plane shear loadings at static and dynamic strain rates using transverse tension and off-axis tension specimens. The dynamic tests were carried out on a split-Hopkinson tension bar at axial strain rates from 113 to 300 hbox {s}^{-1}. With the already available off-axis and transverse compression test data for IM7-8552, a comprehensive data set is available now, which can be used for validation and calibration of numerical models. The measured axial stress–strain response was simulated using a fully 3D transversely isotropic elastic–viscoplastic constitutive model. The constitutive model represents a viscoplastic extension of the transversely-isotropic plasticity model developed by the authors (Vogler et al. in Mech Mater 59:50–64, 2013). An invariant based failure criterion is added to the model to be able to predict the strength for a given orientation and strain rate accurately. The strain rate dependency of the elastic and ultimate strength properties is introduced in the model through scaling functions. A good correlation between the measured and numerically predicted stress–strain response and failure of the specimens was achieved for all specimen types and both strain rate regimes.

Particle image velocimetry and infrared thermography of turbulent jet impingement on an oscillating surface

Jet impingement is widely used for forced-convection heat transfer applications and knowledge about its flow structure and heat transfer rate on a static surface are well established. However, the characteristics of jet impingement on an oscillating surface are relatively unknown. This study experimentally investigates the effect of surface oscillation on the fluid dynamics and heat transfer of an unconfined turbulent impinging jet. The Reynolds numbers of the axisymmetric jet are 5000 and 10,000, based on the jet-nozzle exit diameter, and the surface is placed at nominal standoff distances of 2 and 5 diameters from the jet-nozzle exit. The surface oscillates in a direction parallel to the jet axis at frequencies of 20 Hz and 50 Hz and at a peak-to-peak displacement amplitude of 0.2 times the jet-nozzle exit diameter. The phase-average and mean flow characteristics at six phases through the surface oscillation cycle, and the steady-state mean heat transfer rate at the oscillating surface, are determined using particle image velocimetry and infrared thermography respectively. These are analyzed and compared with the mean flow and heat transfer characteristics for jet impingement on a static surface. Surface oscillation directly affects the mean axial jet velocities and thence the mean radial velocities, and this effect is greater at locations in the flow-field closer to the surface. This gives rise to lower mean axial and radial strain rates in the impingement region and lower turbulence intensities in the wall-jet region when compared with those for a static surface. The frictional interaction between the impinging jet and oscillating surface induces higher surface temperatures than those on a static surface. These factors reduce the heat transfer rate for jet impingement on an oscillating surface when compared with that on a static surface. The reduction is greater in the impingement region than in the wall-jet region with the stagnation point Nusselt number for an oscillating surface being lower by a maximum value of 15%. Overall, for the range of parameters considered in this study, these findings suggest that surface oscillation in jet impingement weakens the transport phenomena capabilities from those present in the case of a static surface.

Forty years of paleostress analysis: has it attained maturity?

Powerful methods of paleostress analysis from fault-slip data have been used since the 1970's, and have demonstrated their capacity for reconstructing stress fields with tectonic significance. However, in some cases, these methods have been applied to samples that do not fulfill basic requirements, or developed on the basis of unsound conceptual models. Most methods of fault-slip are based on the Wallace-Bott principle (dynamic paradigm), while others invoke the necessity of remaining within the alternative kinematic paradigm, therefore inferring only finite strain or strain rate axes. The scientific community shares neither standard procedures for applying these methods, nor standard parameters for assessing the quality of results. This indicates that paleostress analysis has not attained maturity. It is necessary to see back to the conceptual and methological bases, following accurate procedures of data collection, using additional field observations, and critically assessing the results within the tectonic setting. Spatial and temporal heterogeneity of stress fields over all scales makes the mean stress tensor obtained from site analysis just an abstract notion, though indispensable for understanding lithosphere dynamics.

Scaling properties towards vortex reconnection under Biot–Savart evolution

Reconnection of a vortex filament under the Biot–Savart law is investigated numerically using a vortex ring twisted in the form of a figure-of-eight. For the numerical method, the vortex ring is divided into piecewise linear segments, and the Biot-Savart integral is approximated by a summation over the segments with a cut-off method to deal with the singular terms. It is demonstrated that the centre part of the skewed vortex ‘chopsticks’, where the interaction is maximal, tends to approach and accelerate to form a singularity while making a ‘tent-like’ structure as shown by de Waele and Aarts (1994 Phys. Rev. Lett. 72 pp 482–5). The minimum separation of the chopsticks, the maximum velocity and the maximum axial strain rate show clear scaling exponents near the singularity consistent with Leray scaling for self-similar solutions of the Navier–Stokes equations.

A tent model of vortex reconnection under Biot–Savart evolution

Vortex reconnection under Biot–Savart evolution is investigated geometrically and numerically using a tent model consisting of vortex filaments initially in the form of two tilted hyperbolic branches; the vortices are antiparallel at their points of nearest approach. It is shown that the tips of these vortices approach each other, accelerating as they do so to form a finite-time singularity at the apex of the tent. The minimum separation of the vortices and the maximum velocity and axial strain rate exhibit nearly self-similar Leray scaling, but the exponents of the velocity and strain rate deviate slightly from their respective self-similar values of $-1/2$ and $-1$; this deviation is associated with the appearance of distinct minima of curvature leading to cusp structures at the tips. The writhe and twist of each vortex are both zero at all times up to the instant of reconnection. By way of validation of the model, the structure of the eigenvalues and eigenvectors of the rate-of-strain tensor is investigated: it is shown that the second eigenvalue $\unicode[STIX]{x1D706}_{2}$ has dipole structure around the vortex filaments. At the tips, it is observed that $\unicode[STIX]{x1D706}_{2}$ is positive and the corresponding eigenvector is tangent to the filament, implying persistent stretching of the vortex.

Structural Properties and Simple Compression Behavior of Laboratory-Made Polycrystalline Isotropic Ice at High Temperatures

Polycrystalline isotropic ice was selected as the material of choice for this fundamental study on the mechanical behavior of ice. Two essential properties of the ice structure are the porosity and degree of anisotropy (DA). On the one hand, it is clear that these two factors have a great influence on the mechanical properties of the material. On the other hand, however, they are strongly dependent on the laboratory procedure used to fabricate the ice samples. Thus, in this work, three procedures to produce ice samples are analyzed. For this purpose, the structural and mechanical properties observed in uniaxial compression tests are discussed for each sample fabrication procedure. Then, after the most suitable fabrication procedure has been determined, the viscous behavior of isotropic ice is analyzed and discussed using the results of simple compression test at different temperatures and axial strain rates.

Ratcheting behavior of zirconium alloy tubes under combined cyclic axial load and internal pressure at 350 °C

Zirconium alloy is used as nuclear fuel cladding in nuclear reactors because it offers the merits of low neutron absorption cross-section, high corrosion resistance, excellent mechanical properties, and satisfactory creep resistance under operating conditions. However, the cladding experiences uniaxial and multiaxial stresses, which can lead to material failure due to the ratcheting effect. In this context, in this study, a series of uniaxial and multiaxial ratcheting tests were conducted on zirconium alloy tubes at 350 °C. The experimental results show that the temperature significantly affects the properties of zirconium alloy, and its ductility at 350 °C is poorer than that at room temperature. The axial ratcheting strain rate greatly increases with increase in the mean stress, stress amplitude, and peak stress holding time. In addition, the loading history also greatly affects the uniaxial and multiaxial ratcheting of zirconium tubes. A lower stress level after a loading history with higher stress level leads to shakedown of the ratcheting, while a higher mean stress after a loading history with lower mean stress brings down the ratcheting strain rate. The existence of a constant internal pressure significantly retards the failure of the zirconium tubes.

Frequency-dependent attenuation in water-saturated cracked glass based on creep tests

We estimate attenuation at subseismic frequencies from an experimental creep test performed on a thermally cracked water-saturated glass sample. The time-dependent axial stress and strain rates are used to infer the attenuation and Young’s modulus as functions of frequency. Attenuation is characterized by a pronounced frequency dependence between [Formula: see text] and [Formula: see text]. A corresponding frequency-dependent behavior of the Young’s modulus is observed with an increase from 60 to 70 GPa, which is consistent with the measured static and ultrasonic values. These observations are interpreted as being due to fluid flow between interconnected cracks in the mesoscopic scale range. To test this hypothesis, we compare the analytical characteristic frequency for the presumed mesoscopic squirt-type flow with its experimental counterpart. We also compare the experimentally observed attenuation characteristics with results of numerical simulations. For the latter, a thin section of the cracked glass sample has been digitized to provide essential information with regard to the geometry of the crack network. Together with the known physical properties of the intact glass matrix, this then allows for deriving a first-order 2D poroelastic model for the cracked sample based on Biot’s quasi-static equations.

Creep behaviour of intact and remoulded fibrous peat

This paper presents the creep behaviour of intact and remoulded specimens of fibrous peat obtained from a field site near Anzac, Alberta, Canada. The creep behaviour was investigated by means of long-term drained and undrained triaxial tests. The development of volumetric, axial, and undrained axial strain and strain rate during drained and undrained creep tests under variable stress conditions is presented. The stress – strain – strain rate (p′–e v– $$\dot{\varepsilon }_{\text{v}}$$ ) relationship is found to be unique for different stress and loading durations. The p′–e v– $$\dot{\varepsilon }_{\text{v}}$$ relationship is analysed and represented by creep isotaches. The applicability of different creep models developed for normally consolidated clay is discussed and applied to define the development of creep strain in fibrous peat under varying isotropic and deviator stresses. The secondary consolidation coefficient for evaluating the volumetric strain rate of peat is found to be applicable with some limits. The drained creep behaviour of remoulded peat specimens differs from the behaviour shown by Shelby tube specimens, whereas the undrained creep behaviour in remoulded and Shelby tube specimens is similar.

Discrete element modeling of the single-particle crushing test for ballast stones

In this paper, a discrete element modeling approach for the single-particle crushing test for irregularly shaped ballast stones is presented. Bonded spherical particles are used to represent test specimens. Parametric studies focusing on particle size, axial strain rate, particle aggregate size and number of bonds are performed. The selection criteria of these parameters are discussed from the perspective of railway engineering. The results indicate that the proposed modeling approach is reliable for simulating railway ballast stones and can thus be further used for simulations of ballast aggregations.

Non-Invasive Identification of Vulnerable Atherosclerotic Plaques Using Texture Analysis in Ultrasound Carotid Elastography: An In Vivo Feasibility Study Validated by Magnetic Resonance Imaging

The aims of this study were to quantify the textural information of strain rate images in ultrasound carotid elastography and evaluate the feasibility of using the textural features in discriminating stable and vulnerable plaques with magnetic resonance imaging as an in vivo reference. Ultrasound radiofrequency data were acquired in 80 carotid plaques from 52 patients, mainly in the longitudinal imaging view, and axial strain rate images were estimated with an ultrasound carotid elastography technique based on an optical flow algorithm. Four textural features of strain rate images—contrast, homogeneity, correlation and angular second moment—were derived based on the gray-level co-occurrence matrix in plaque regions to quantify the deformation distribution pattern. Conventional elastographic indices based on the magnitude of the absolute strain rate, such as the maximum, mean, median, standard deviation and 99th percentile of the axial strain rate, were also obtained for comparison. Composition measurement with magnetic resonance imaging identified 30 plaques as vulnerable and the other 50 as stable. The four textural features, as well as the magnitude of strain rate images, significantly differed between the two groups of plaques. The best performing features for plaque classification were found to be the contrast and 99th percentile of the absolute strain rate, with a comparative area under the receiver operating characteristic curve of 0.81; a slightly higher maximum accuracy of plaque classification can be achieved by the textural feature of contrast (83.8% vs. 81.3%). The results indicate that the use of texture analysis in plaque classification is feasible and that larger local deformations and higher level of complexity in deformation patterns (associated with the elastic or stiffness heterogeneity of plaque tissues) are more likely to occur in vulnerable plaques.

Numerical analysis of extensional flow through the pharyngeal duct

The flow through the pharynx from the glossopalatal junction (GPJ) to the upper esophageal sphincter (UES) has been numerically investigated with a non-Newtonian fluid obeying the power-law with similar rheological indices to a contrast medium used in videofluroscopy. For that purpose, a three-dimensional model of the transport of food bolus along the pharynx has been proposed using the immersed boundaries method, which allow representing the shape of the pharynx using Cartesian grids. The pharyngeal wall has been considered to be an elastic membrane. Flow fields in terms of the axial velocity, pressure, shear rate and strain rate were obtained. Results show that the highest velocity concentrates in the central stream as the fluid enters into the pharynx. In addition, as the flow quits the pharynx, a recirculation zone appears inside the cavity, resulting in low velocity zone, which increases with the coefficient of elasticity. A strong dependence on the coefficient of elasticity was observed on the pressure fields; so that as such a coefficient increases, the pressure in the pharyngeal wall will increase. It has been also observed that the bolus head travels faster than the bolus tail, which indicates that the bolus is not only subjected to shear but also to elongation. Results from this work can be further used for a rheological characterization (shear and extension) of oral nutritional supplements for patients suffering from swallowing disorders.

Strength Behavior, Creep Failure and Permeability Change of a Tight Marble Under Triaxial Compression

The coupled hydro-mechanical behaviors of a tight marble are investigated by a series of laboratory tests with continuous gas injection during the hydrostatic compression, triaxial compression and compressive creep tests. Hydrostatic compression tests are firstly carried out in three steps to identify the viscous effect of hydrostatic stress on deformation and permeability of the marble. Coupled triaxial tests are then conducted at a constant axial strain rate under five different confining pressures (P c) with continuous gas injection. Coupled creep behaviors of the marble are also characterized by a constant deviatoric stress test under P c = 30 MPa with gas flowing at a constant injection pressure. The high-stress unloading failure behavior of the marble is finally investigated by an unloading test with a previous multi-step creep phase to realize a high-stress state as well as to investigate the time-dependent deformation of marble under different deviatoric stresses. Experimental results reveal that gas permeability of the marble shows an evident rate-dependent effect in hydrostatic compression. Mechanical behaviors of the tight marble are closely depended on the applied P c in triaxial tests, and its permeability exhibits a decrease phase at initial deviatoric loading and turns to increase at a critical stress corresponding to the initial yield stress. Marble can withstand more important plastic deformation under high P c than under lower ones. Gas flow seems to be more sensitive than the strains to characterize the creep behaviors of the marble. No time-dependent strains are observed when deviatoric creep stress is lower than 50% of its peak strength under P c = 30 MPa.

Experimental study of brittleness anisotropy of shale in triaxial compression

Brittleness of rock plays a key role in petroleum related rock mechanics. Compression brittle failure of shale is more relevance to directional wellbore stability. The bedding angle (0°, 30°, 45°, 60°, 90°) dependent brittleness of anisotropic Longmaxi shale samples was studied by conducting triaxial experiments under various confining pressure (15, 25, 40 MPa). A servo-controlled loading system was used to keep axial loading strain rate as constant (8.33 × 10−5/s) at room temperature. The stress and strain was synchronously and independently recorded with time. We used five definition of brittleness indices that were based on laboratory stress-strain curves to characterize the anisotropy of brittleness. The results show that for 30°-sample and 45°-sample under relative low confining pressure (15 MPa), the stress-strain curves deviate from stress-time relations during instant rupture of post-failure. Under confining pressure of 25 MPa, both pre-failure and post-failure indices of brittleness remain highly consistent along bedding angle as a sine curve. When the confining pressure is increased to 40 MPa, variation patterns of brittleness indices clearly differentiate into pre-failure or post-failure type. It seems that the unstable recording of servo system during explosive failure of samples can be considered as a relative brittleness indicator. The results indicate that the brittleness of samples in some certain bedding angle don't decrease with increasing confining pressure, instead the post-failure type indices may increase. The brittleness anisotropy of Longmaxi shale samples is bedding angle (structure) dependent, and it decreases with increasing confining pressure.

Semi-empirical elastic–thermoviscoplastic model for clay

Clays exhibit creep in compression and shear. In one-dimensional compression, creep is commonly known as “secondary compression” even though it is also a significant component of deformations resulting from shear straining. It reflects viscous behaviour in clays and therefore depends on load duration, stress level, the ratio of shear stress to compression stress, strain rate, and temperature. Research described in the paper partitions strains into elastic (recoverable) and plastic (nonrecoverable) components. The plastic component includes viscous strains defined by a creep rate coefficient ψ that varies with plasticity index and temperature (T), but not with stress level or overconsolidation ratio (OCR). Earlier elastic–viscoplastic (EVP) models have been modified so that ψ = ψ(T) in a new elastic–thermoviscoplastic (ETVP) model. The paper provides a sensitivity analysis of simulated results from undrained (CIŪ) triaxial compression tests for normally consolidated and lightly overconsolidated clays. Axial strain rates range from 0.15%/day to 15%/day, and temperatures from 28 to 100 °C.

Controlling micro‐ and nanofibrillar morphology of polymer blends in low‐speed melt spinning process. Part III: Fibrillation mechanism of PLA/PVA blends along the spinline

ABSTRACTThe effects of spinning conditions on the fibrillation process of poly(lactic acid) (PLA) and poly(vinyl alcohol) (PVA) polymer blends in an elongational flow within the fiber formation zone are systematically and thoroughly investigated. By considering the relationship between the changes in filament parameters with the focus on the maximum axial strain rate (ASR) and tensile stress at maximum ASR and the morphological evolution of the dispersed PLA phase along the spinline, the fibrillation process from rod‐like to nanofibrillar structures of the dispersed PLA phase in a binary blend with PVA matrix is elucidated. The final morphology of the dispersed PLA phase in PLA/PVA blends is controlled by the changes in the spinning conditions. The lengths and diameters of the PLA fibrils are caused not only by the deformation of their initial sizes but also by the combination of the deformation, coalescence, and break‐up process. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 44259.

A new perspective in the evaluation of the mixing of biopolymer solutions with different coaxial mixers comprising of two dispersing impellers and a wall scraping anchor

The hydrodynamic performances of coaxial mixers consisting of a close clearance impeller and dual central impellers in the agitation of the xanthan gum solution, a yield-pseudoplastic biopolymer solution, were explored in the laminar-transitional regimes in the co-rotating mode. The different coaxial mixers utilized in this study were dual Scaba impellers, dual Rushton turbines, or dual pitched blade turbines in combination with an anchor impeller. Considering both numerical and experimental approaches, three techniques were employed: computational fluid dynamics (CFD) to simulate the flow domain of the fluid, electrical resistance tomography (ERT) to measure mixing time and visualize flow behavior inside the stirred vessel, and design of experiments (DOE) combined with the response surface methodology (RSM) to analyze the data. The experimental and numerical data were collected to assess the performances of the coaxial mixers in the mixing of the complex fluids with respect to the power consumption, mixing time, velocity vector plots, streamline plots, axial, radial and tangential velocity profiles, axial shear strain rate profiles, and flow number. It was found that among the three coaxial mixers considered in this research work, the mixing time achieved by the double Scaba-anchor coaxial (DSAC) mixer was the lowest. Furthermore, the DSAC coaxial mixer created a more uniform velocity profile throughout the tank compared to the other two coaxial mixers. The results also demonstrated that the DSAC mixer was the most efficient mixing system for the mixing of the highly viscous non-Newtonian fluids.