Initial Shear Stress Research Articles

Experiments and molecular-dynamics simulation of elastic waves in a plasma crystal radiated from a small dipole source.

The radiation of elastic waves from a localized source is observed experimentally in a two-dimensional plasma crystal. An initial shear stress applied by a laser forms a small dipole source. The emerging complex wave pattern is shown to consist of outgoing compressional and shear wave pulses. Subsequent structures are identified as inward-going waves due to the finite size of the source region, which reappear on the opposite side. The compressional wave forms a trailing wave train due to strong dispersion, while the nondispersive shear wave evolves into a vortex-antivortex pair on either side. The experiments are compared with a molecular-dynamics simulation.

Post-failure mobility of saturated sands in undrained load-controlled ring shear tests

The undrained shear behavior of soils with progress of shear displacement is essential to the understanding of liquefied slope failures with long travel distance. In this paper, using a newly developed ring shear apparatus, a series of ring-shear tests were conducted on a silty sand to examine the undrained behavior of sand subjected to long shear displacement. Based on the test results, the undrained shear behavior of sands with a wide range of densities is discussed. A very low effective stress corresponding to liquefaction was observed at the steady state in all of the tests on loose, medium, as well as dense sand. The effects of stress state and shear history on the undrained shear behavior were examined by performing tests on a sample with different initial stress states and shearing the same specimen repeatedly (three times) at each initial stress state. The tests at different initial stress states proved that the initial stress state has an influence on static liquefaction resistance but has no effect on the steady-state shear strength. Repeated shear tests on the same specimen showed that with increasing shear times, both the peak shear strength and the steady-state shear strength for each specimen became greater. Detailed examination of the shear deformation revealed that the liquefaction phenomena in ring shear tests are localized in the shear zone, irrespective of the initial state of the sand. Grain crushing within the shear zone was examined. Finally, it was found that there was an optimal density at which the undrained brittleness index had a minimum value; meanwhile, the undrained brittleness index became greater with increasing initial normal and shear stresses, but decreased with shear times. These findings offer some basic understanding in assessing the postfailure mobility in landslides.Key words: excess pore pressure, localized liquefaction, shear resistance, ring-shear tests, grain crushing, silty sands.

Undrained Monotonic and Cyclic Shear Behaviour of Sand Under Low and High Confining Stresses

Monotonic and cyclic loading tests have been carried out on a silica sand over a wide range of stresses in order to compare non-crushing and crushing behaviour. Samples were sieved at several stages of testing to theoretically determine the increase in particle surface area and hence degree of crushing. Undrained shear behaviour was compared for crushing and non-crushing regions above 3 MPa. Samples consolidated to 0.1 MPa demonstrated strong dilative behaviour, while above the yield stress of 3 MPa dilation was suppressed and considerable particle crushing occurred. Shearing caused a marked increase in particle crushing particularly after the phase transformation point. Crushing at the steady state was similar for isotropic and anisotropically consolidated sands. At high confining pressures the cyclic strength curves were similar to those for loose sands except for confining pressure dependency due to particle crushing. For low confining stresses cyclic strength increased with initial stress ratio, while for high confining stresses it decreased with initial shear stress ratio. In the cyclic tests there was no significant crushing for 0.1 MPa. Crushing was seen to increase rapidly after the phase transformation point, where high strains developed and where particle rotation and translation contributed to the crushing process.

Energetics of Two-Level Shears and Hardening of Single Crystals

An energetic description of the hardening behavior of single crystals undergoing single slip is analyzed. Simultaneous macroscopic simple shear and mesoscopic slips are described by means of a class of structured deformations called “two-level shears,” along with recently proposed measures of separation of active slip-bands and the number of lattice cells traversed during slip. The energetics of two-level shears gives rise to a response consistent with the experimentally observed loading and unloading behavior of a single crystal in G. I. Taylor’s soft device, as well as with the Portevin-le Chatelier effect. The initial critical resolved shear stress, the flow stress, and the hardening response are obtained, and an application to aluminum single crystals is discussed.

New results for the spin of the Eulerian triad and the logarithmic spin and rate

A new basis-free expression is derived for the logarithmic spin tensor, which assumes a unified form for all the three cases of coalescence of the three eigenvalues of the left Cauchy-Green deformation tensorB. It is shown that this new expression is endowed with particular continuity properties, i.e., each of its coefficients either accompanies the vanishing of its associated term or remains valid whenever the eigenvalues ofB become repeated. These favorable properties, which are not enjoyed by the expressions derived earlier, result in the foregoing unified form of expression which could not be achieved by means of the previous expressions. At the same time, a fully explicit, basis-free expression is obtained for the spin tensor of the Eulerian triad, which is expressed straightforwardly in terms of the three basic invariants ofB and hence renders computations of the eigenvalues ofB unnecessary. Moreover, remarks are given towards explaining and clarifying some issues concerning the hypo-elastic equation of grade zero with the logarithmic rate, including the exact integrability and unstable stress responses associated with initial shear stresses at simple shear deformations, etc.

Influence of fines content and initial shear stress on dynamic properties of hydraulic reclaimed soil

The hydraulic placement of sand fill is one of the most important methods of land reclamation. During the reclamation process, losses of fines in sand are induced by the transportation of soil which affects the dynamic properties of the soil materials. In this study, the reclaimed soil in the Yunlin area of Taiwan is adopted as the test material. Different fines contents, different relative densities, and initial stress ratio were taken as test conditions. Resonant column tests were performed to evaluate the shear modulus and damping ratio of the reclaimed soil under initial shear stress. The results show that the maximum shear modulus decreases as the fines content increases. The influences of initial shear stress are discussed. A prediction method for maximum shear modulus under different fines content and initial shear stress is proposed based on empirical equations obtained. The results can be helpful for land reclamation design and assessment.Key words: reclaimed soil, fines content, initial shear stress, dynamic properties.

Effect of normal stress during rupture propagation along nonplanar faults

We study a symmetrical three‐forked shear fault under simple triaxial stress. Rupture initiates in the central primary plane and propagates toward the branching point. One of the two branches is in the compressional quadrant, while the other is in the tensional domain. We study the following question: Does rupture always prefer propagating along the tensional branch because of the effect of normal stress? When the primary plane is located in the most favorable direction as determined by Mohr‐Coulomb criterion, one finds that the compressional branch is almost always preferred for various dynamic coefficients and confining pressures. Simultaneous rupture propagation on both branches appears only under a very special condition found by Aochi et al. [2000b]. These results indicate that shear stress changes produced by the rupture front dominate during dynamic rupture propagation, compared to normal stress changes. This result depends completely on the orientation of the whole fault system. In all cases, we can explain the branch selection by analyzing the applied initial shear stress and the fault properties. Change of normal stress may play a role similar to shear stress change for disjoint faults since the effect of normal stress change sometimes dominates.

FLOW PROPERTIES OF AFRICAN YAM BEAN (SPHENOSTYLIS STENOCARPA) AS AFFECTED BY ACCESSION AND MOISTURE CONTENT

Flow properties such as normal stress, shear stress, and initial shear stress, and hopper flow characteristics oftwo yam bean accessions (TSs 137 and TSs 138) were studied at four moisture content levels (4%, 8%, 12%, and 16 % wetbasis) to provide data for the design of handling and storage equipment. Two experimental designs were used: Randomizedcomplete block design (RCBD) with one observation per cell, and split plot in RCBD. The initial shear stress increased withmoisture content from 1.31 to 2.91 g/cm2 for TSs 137, while it increased from 1.58 to 3.09 g/cm2 for TSs 138. The hopper sidewall slope for the two accessions increased with moisture content from 60.21 to 64.06. The hopper opening size requiredincreased with moisture content and more than doubled at the highest value of moisture content studied, while the coefficientof mobility decreased with moisture content from 0.33 to 0.24. The accessions were found not to be statistically different interms of shear stress, coefficient of mobility, and side wall slope, but were statistically different in terms of normal stress, initialshear stress, and hopper opening dimensions. The methods of calculation of coefficient of mobility were observed not to bestatistically different, and the effect of normal loads and type of hopper opening was highly significant. There was also a highlysignificant effect of moisture content on all the flow properties (p < 0.05).

Micromechanical analysis of crack initiation and hysteresis loops of aluminium single crystals under high-cycle fatigue

Abstract The micromechanical model of extrusion formation in a polycrystal under high-cycle fatigue is briefly reviewed. Following the same general approach, and guided by the observations of Mecke and Blochwitz on the subgrain boundary displacement in a single crystal, a micromechanic model of extrusions and intrusions in an aluminum single crystal with multiple fatigue bands under stress-controlled loading is presented. The microstress and strain fields in the crystal are calculated by the boundary element method for the three-dimensional elasto-plastic solids. From these microfields, the macroscopic stress and strain of the crystal at different stages of loading are calculated. The numerical analysis gives the changes in hysteresis loop shape with loading cycles. The incremental plastic strain distribution and the incremental residual stress in each cycle depend on the initial shear stresses. Two sets of initial stresses are taken to calculate the hysteresis loops. This shows the dependence of the shap...

REFINED HIGHER ORDER FINITE ELEMENT MODELS FOR THERMAL BUCKLING OF LAMINATED COMPOSITE AND SANDWICH PLATES

Two refined higher order theories, one that neglects and the other that takes into account the effect of transverse normal deformation, are used to develop two discrete finite element models for the thermal buckling analysis of composite laminates and sandwiches. The two models, one with nine degrees of freedom per node and the other with eleven degrees of freedom, are based on a nine-node Lagrangian isoparametric element. The geometric stiffness matrices are developed by taking into consideration the effects of the higher order terms on the initial in-plane and transverse shear stresses. The accuracy of the present formulations is first evaluated by analyzing sample problems for which analytical three-dimensional solutions exist in the literature. Numerical results are presented for the first time for sandwich plates, demonstrating the importance and accuracy of the higher order theory in comparison to first-order theory. Some new results are also given for sandwich plates with angle-ply composite face sheets, showing the effects of various boundary conditions and of variations in geometric and lamina tion parameters on critical temperature.

The development of transient fingering patterns during the spreading of surfactant coated films

The spontaneous spreading of an insoluble surfactant monolayer on a thin liquid film produces a complex waveform whose time variant shape is strongly influenced by the surface shear stress. This Marangoni stress produces a shocklike front at the leading edge of the spreading monolayer and significant film thinning near the source. For sufficiently thin films or large initial shear stress, digitated structures appear in the wake of the advancing monolayer. These structures funnel the oncoming flow into small arteries that continuously tip-split to produce spectacular dendritic shapes. A previous quasisteady modal analysis has predicted stable flow at asymptotically long times [Phys. Fluids A 9, 3645 (1997)]. A more recent transient analysis has revealed large amplification in the disturbance film thickness at early times [O. K. Matar and S. M. Troian, “Growth of nonmodal transient structures during the spreading of surfactant coated films,” Phys. Fluids A 10, 1234 (1998)]. In this paper, we report results of an extended sensitivity analysis which probes two aspects of the flow: the time variant character of the base state and the non-normal character of the disturbance operators. The analysis clearly identifies Marangoni forces as the main source of digitation for both small and large wave number disturbances. Furthermore, initial conditions which increase the initial shear stress or which steepen the shape of the advancing front produce a larger transient response and deeper corrugations in the film. Disturbances applied just ahead of the deposited monolayer rapidly fall behind the advancing front eventually settling in the upstream region where their mobility is hampered. Recent findings confirm that additional forces which promote film thinning can further intensify disturbances [O. K. Matar and S. M. Troian, “Spreading of surfactant monolayer on a thin liquid film: Onset and evolution of digitated structures,” Chaos 9, 141 (1999). The transient analysis presented here corroborates our previous results for asymptotic stability but reveals a source for digitation at early times. The energy decomposition lends useful insight into the actual mechanisms preventing efficacious distribution of surfactant.

Behavior of a fine-grained soil during the Loma Prieta earthquake: Discussion

583 The authors introduced a very interesting and valuable case history of the behaviour of a fine-grained soil during a large earthquake. In addition, they suggested issues to be addressed by further research into cyclic softening of finegrained soils. The Discusser is encouraged to learn that a silty clay or a clayey silt might contribute to lateral deforma tion of ground beneath buildings during earthquakes, because he has been involved in studies of the contributions of fine-grained soils to damages during earthquakes (Yasuhara et al. 1992; Yasuhara 1994; Yasuhara and Hyde 1996; Yasuhara and Toyota 1997; Yasuhara et al. 1998). Based on the above-mentioned findings on cyclic loading of cohesive soils, the following questions associated with the paper have arisen: 1. According to previous studies (Dobry and Vucetic 1986; Matsui et al. 1992; Yasuhara and Hyde 1996), cohesive soils undergoing undrained cyclic loading exhibit more marked degradation in stiffness than in undrained strength. In other words, deformation of fine-grained soils is more sensitive to undrained cyclic loading than instability is. As an example, post-cyclic degradations in undrained strength and stiffness obtained from direct-simple shear (DSS) tests are shown in Fig. D1 for a reconstituted plastic silt ( Ip = 17) with almost the same plasticity index as that experienced in the paper. The initial vertical stress applied to each specimen before cyclic loading was 198 kPa in DSS tests. The Discussers’ opinion is that deformation and settlement due to cyclic softening of fine-grained soils depend more heavily on decreases in stiffness than on changes in strength. However, the paper does not specify whether the term “cyclic softening” refers to degradation in stiffness or strength associated with lateral deformation of ground trig gered during or after earthquakes. To clarify, it would be useful if post-cyclic undrained strength and Young’s modulus or shear modulus were determined for the silt existing in the actual site and were plotted against each other. The results obtained using the method proposed by the Discussers (Yasuhara and Hyde 1996; Yasuhara and Toyota 1997) for clayey silt are shown in Fig. D2. 2. In analysing the behaviour of fine-grained soils under undrained cyclic loading, it is important to take the effect of initial shear stress into consideration. The results from the research by Hyodo et al. (1994) in cyclic triaxial tests and by the Discussers in DSS tests as shown in Fig. D3 indicate that the larger initial shear stress gives rise to less cyclic strength. This condition is not beneficial for stability of finegrained soils during earthquakes although this tendency is

A cyclic elasto-plastic constitutive model for sand considering a plastic-strain dependence of the shear modulus

A cyclic elasto-plastic constitutive model based on a non-linear kinematic hardening rule for sand is proposed. Three points are incorporated into the model: a new flow rule, a cumulative strain-dependent characteristic of the plastic shear modulus and a fading memory characteristic of the initial anisotropy of the constitutive model. In order to verify its effectiveness, the proposed model was evaluated by means of the results of a series of hollow-cylinder torsional shear tests with and without an initial shear stress after isotropic and anisotropic consolidation. The liquefaction strength curve, the effective stress path and the stress—strain relation during cyclic loading are well reproduced by the proposed model. KEYWORDS: constitutive relations; liquefaction; plasticity; repeated loading; sands. Nous proposons ici un modèle constitutif élastoplastique cyclique basé sur une règle de durcissement cinématique non linéaire du sable. Trois points sont incorporés dans le modèle: une nouvelle règle d'écoulement, une caractéristique cumulative dépendant de la déformation du module de cisaillement plastique et une caractéristique de mémoire évanescente de l'anisotropie initiate du modèle constitutif. Afin de vérifier son efficacité, nous évaluons le modèle proposé en utilisant les résultats d'une série d'essais de cisaillement torsionnel cylindrique creux avec et sans contrainte tangentielle initiale après consolidation isotrope et anisotrope. La courbe de résistance à la liquéfaction, le chemin de contrainte effective tout comme la relation contrainte-déformation pendant le chargement cyclique sont bien reproduits par le modèle proposé.

SHEAR MODULUS AND DAMPING OF CLAYEY SANDS

A series of cyclic triaxial tests on clayey sands was carried out and attempts were made to evaluate the strain dependency of shear modulus and damping. Although the strain dependency of the shear modulus of clayey sands was similar to that of clay in the low strain range, G/G 0 became close to that of clean sand in the high strain range. The damping ratio was less for the clayey sand containing more fines, especially in the medium to large strain range. It was shown that the change in the effective confining stress with loading cycles in the undrained shear test needed to be considered, particularly in the large strain range. The consideration can be made by normalising G with G 0' instead of using G 0. When an initial shear stress was applied to the specimen, the amount of residual strain, which was induced with cycles, had to be added to the normal strain amplitude in order to determine the shear modulus. If the effects of excess pore pressure and the amount of residual strain were taken into account properly, it was suggested that the shear modulus of clayey sand in irregular loading could be evaluated reasonably well by using the average G/G 0—γcurve obtained by cyclic loading tests on isotropically-consolidated specimens.

Some characteristics of the stress field of the 1995 Hyogo‐ken Nanbu (Kobe) earthquake

We investigate the characteristics of the stress field associated with the 1995 Hyogo‐ken Nanbu (Kobe) earthquake. We use for the study a tomographic model of the fault slip inferred from the numerous near‐field recordings, and we calculate the space and time evolution of shear stress on the fault during the earthquake. We show that the spatial distribution of stress drop is very heterogeneous. The apparent strength of the fault at the onset of the earthquake is low (of the order of 1 MPa or less), which indicates that the pre‐earthquake tectonic shear stress was close to the static friction over most of the fault. At many locations the stress drop rotates significantly during sliding. As was originally proposed by Spudich [1992], we use the requirement of colinearity between the directions of maximum shear stress and instantaneous slip to determine the initial stress on the fault at the onset of the earthquake. The pre‐earthquake tectonic stress varies greatly over the fault and ranges from about 1 to 10 MPa. Average values of the initial and final shear stresses over the fault are 3.3 and 1.6 MPa, respectively, indicating that about half of the pre‐earthquake tectonic stress was released during the earthquake. There is a relatively strong correlation between the initial and final stress distributions, which suggests that intrinsic fault properties, not modified by the earthquake, control the spatial distribution of tectonic stress over the fault. We show that on average over the fault the dynamic friction coefficient is equal to about 40% of the static friction coefficient. Diagrams depicting the evolution of shear stress as a function of slip are consistent with slip‐weakening behavior, but their interpretation is questionable because of the poor resolution of the data at high frequency and because of the constraints imposed on the model to perform the inversion.

Coseismic temporal changes of slip direction: The effect of absolute stress on dynamic rupture

AbstractWe investigate the dynamics of rupture at low-stress level. We show that one main difference between the dynamics of high- and low-stress events is the amount of coseismic temporal rake rotation occurring at given points on the fault. Curved striations on exposed fault surfaces and earthquake dislocation models derived from ground-motion inversion indicate that the slip direction may change with time at a point on the fault during dynamic rupture. We use a 3D boundary integral method to model temporal rake variations during dynamic rupture propagation assuming a slip-weakening friction law and isotropic friction. The points at which the slip rotates most are characterized by an initial shear stress direction substantially different from the average stress direction over the fault plane. We show that for a given value of stress drop, the level of initial shear stress (i.e., the fractional stress drop) determines the amount of rotation in slip direction. We infer that seismic events that show evidence of temporal rake rotations are characterized by a low initial shear-stress level with spatially variable direction on the fault (possibly due to changes in fault surface geometry) and an almost complete stress drop.Our models motivate a new interpretation of curved and cross-cutting striations and put new constraints on their analysis. The initial rake is in general collinear with the initial stress at the hypocentral zone, supporting the assumptions made in stress-tensor inversion from first-motion analysis. At other points on the fault, especially away from the hypocenter, the initial slip rake may not be collinear with the initial shear stress, contradicting a common assumption of structural geology. On the other hand, the later part of slip in our models is systematically more aligned with the average stress direction than the early slip. Our modeling suggests that the length of the straight part of curved striations is usually an upper bound of the slip-weakening distance if this parameter is uniform over the fault plane, and the direction of the late part of slip of curved striations should have more weight in the estimate of initial stress direction.

Thermal aging behavior of an SiC-fiber reinforced glass matrix composite in a non-oxidizing atmosphere

The thermal aging in argon of a commercially available SiC-fiber reinforced glass matrix composite was investigated at temperatures in the range 500–700 °C for exposure duration of up to 1000 h. An inert atmosphere was used to study the effects of temperature alone, thus minimizing and neglecting the effects of oxidation. The mechanical properties of aged samples were evaluated at room-temperature by using four-point flexure strength and three-point flexure chevron-notch techniques. The interfacial properties were determined by push-out indentation measurements using an in-situ SEM indentation apparatus. The fracture toughness values determined by the chevron-notch tests were little affected by the aging conditions and were in the range 19–26 MPa√m . The frictional interfacial shear stress was not affected by the aging conditions either. For the most severe aging conditions investigated (1000 h at 600 °C and 100 h at 700 °C), a significant loss of flexure strength and stiffness of the samples was detected, which has been ascribed to microstructural changes that occurred in the material during aging as a consequence of the softening of the glass matrix. At these aging conditions, a lower interfacial shear stress for fiber-matrix debonding initiation was measured, which may be explained also by the occurrence of matrix softening and void formation.

Buckling Analysis of Prestressed Rotating Circular Cylindrical Shells.

Buckling phenomenon of prestressed rotating circular cylindrical shells is investigated. As prestresses, the initial external pressure, axial compression stress and shear stress are considered. Buckling load is obtained through the frequency analysis of two traveling waves for a circumferential wave number, i.e., it is defined as the least value of preloads that make two frequencies of both traveling waves for all circumferential wave numbers be coincide. Frequency equation is derived on the basis of the first-order shear deformation theory of shells and numerical calculation method is formulated for four kinds of boundary conditions. Numerical results showed that for the cases of the initial external pressure and axial compression stress, the buckling load increases monotonously as the rotational speed increases, but for the case of the initial shear stress, buckling load shows the up-and-down inclination with rotational speed and it proved that there exists a rotational speed region in which buckling load becomes lower than that for non-rotational shells.

Methods for Evaluating Residual Post-Liquefaction Ground Settlement and Horizontal Displacement

ABSTRACT During the 1995 Hyogoken-Nambu earthquake, large vertical residual ground settlements and horizontal displacements extensively occurred in liquefied soils behind caisson-type quay walls where lateral movements of the walls towards sea were led by strong seismic shaking. Case studies further show that both significant vertical residual ground settlements and horizontal displacements were also induced by this earthquake in liquefied level sandy ground with a sufficiently large lateral extent where no significant initial shear stresses were present. The mechanism regarding the occurrence of the above phenomena is revealed through new post-liquefaction stress-strain constitutive analysis. Consequently, the residual post-liquefaction ground settlement and horizontal displacement are found to be interdependent of each other, so that neither cannot be determined separately in principle. A new methodology with pragmatic charts is developed for concurrently evaluating the residual post-liquefaction ground settlement and horizontal displacement in liquefiable ground. The prediction results using the proposed method agree with actual observations during the 1995 Hyogoken-Nambu earthquake.

“Fracture” phenomena in shearing flow of viscous liquids

In start-up of steady shearing flow of two viscous unentangled liquids, namely low-molecular-weight polystyrene and α-D-glucose, the shear stress catastrophically collapses if the shear rate is raised above a value corresponding to a critical initial shear stress of around 0.1–0.3 MPa. The time dependence of the shear stress during this process is similar for the two liquids, but visualization of samples in situ and after quenching reveals significant differences. For α-D-glucose, the stress collapse evidently results from debonding of the sample from the rheometer tool, while in polystyrene, bubbles open up within the sample, as occurs in cavitation. Some similarities are pointed out between these phenomena and that of “lubrication failure” reported in the tribology literature.