Showing papers on "Slip (materials science) published in 1992"

Dislocation nucleation from a crack tip : an analysis based on the Peierls concept

Abstract: Dislocation nucleation from a stressed crack tip is analyzed based on the Peierls concept. A periodic relation between shear stress and atomic shear displacement is assumed to hold along the most highly stressed slip plane emanating from a crack tip. This allows some small slip displacement to occur near the tip in response to small applied loading and, with increase in loading, the incipient dislocation configuration becomes unstable and leads to a fully formed dislocation which is driven away from the crack. An exact solution for the loading at that nucleation instability is developed via the J -integral for the case when the crack and slip planes coincide, and an approximate solution is given when they do not. Solutions are also given for emission of dissociated dislocations, especially partial dislocation pairs in fcc crystals. The level of applied stress intensity factors required for dislocation nucleation is shown to be proportional to √γ us , where γ us , the unstable stacking energy, is a new solid state parameter identified by the analysis. It is the maximum energy encountered in the block-like sliding along a slip plane, in the Burgers vector direction, of one half of a crystal relative to the other. Approximate estimates of γ us are summarized and the results are used to evaluate brittle vs ductile response in fcc and bcc metals in terms of the competition between dislocation nucleation and Griffith cleavage at a crack tip. The predictions seem compatible with known behavior and also show that in many cases solids which are predicted to first cleave under pure mode I loading should instead first emit dislocations when that loading includes very small amounts of mode II and III shear. The analysis in this paper also reveals a feature of the near-tip slip distribution corresponding to the saddle point energy configuration for cracks that are loaded below the nucleation threshold, as is of interest for thermal activation.

Overview no. 96: Evolution of F.C.C. deformation structures in polyslip

Abstract: The microstructural evolution during polyslip in f.c.c. metals in investigated by the examples of Al, Ni, NiCo alloys and an AlMg alloy, deformed at room temperature either by rolling or by torsion. The principles governing this evolution appears to be the following: (a) There are differences in the number and selection of simultaneously acting slip systems among neighboring volume elements of individual grains. In any one volume element (called a cell block), the number of slip systems falls short of that required for homogeneous (Taylor) deformation, but groups of neighboring cell blocks fulfil the Taylor criterion collectively. (b) The dislocations are trapped into low-energy dislocation structures in which neigboring dislocations mutually screen their stresses. The microstructural evolution at small strains progresses by the subdivision of grains into cell blocks delineated by dislocation boundaries. These boundaries accomodate the lattice misorientations, which result from glide on different slip system combinations in neighbouring cell blocks. The cell blocks are subdivided into ordinary cells and both cell blocks and cells shrink with increasing strain. All observations appear to be in good accord with the theoretical interpretation. However, some problems remain to be solved quantitatively.

Oblique plate convergence, slip vectors, and forearc deformation

Abstract: Slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis, suggesting that oblique convergence is taken up by partial decoupling. Decoupling means that a component of arc-parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench. Partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike-slip faults that are parallel to the trench and to each other and, starting with a force equilibrium condition, a relationship between the obliquity and the earthquake slip vector orientation is derived. Assuming that either fault slips when shear stress on it reaches a yield stress, oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle. For obliquity at or greater than this angle the stress on the strike-slip fault is large enough to start it slipping, and when both faults are active, the arc-parallel motion of the forearc deflects the slip vector back toward the trench-normal. If we assume that continued slip on either fault occurs at constant stress (but the two faults can be at different stresses), the slip vector will maintain a constant angle relative to the trench-normal even when obliquity is larger than the critical angle. This limiting angle of the slip vector, called ψmax (measured relative to the trench-normal), is simply the arcsine of the ratio of the shear forces resisting slip on the strike-slip and thrust faults. A consequence is that when the obliquity exceeds ψmax the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates. Slip vectors at the Java trench southwest of Sumatra show the relationship clearly with ψmax =20°±5°, while slip vectors at the Aleutian trench show the relationship less clearly with ψmax=25° to 45°. The greater angle at the Aleutian trench suggests that the upper plate is stronger in the Aleutian arc (relative to the thrust fault) than in the Sumatran arc, consistent with the Sumatran arc being continental and having a well-developed strike-slip fault while the Aleutian arc is oceanic and without a clear transcurrent fault. Slip vectors at the Philippine trench which, like Sumatra, has a large strike-slip fault inboard of it, tend to stay within 25° of the trench-normal when obliquity is as large as 50°. If obliquity exceeds ψmax and continues to increase along a subduction zone, the rate of motion of the forearc relative to the upper plate will vary with obliquity, in which case the forearc sliver should extend or contract parallel to the arc. From the geometry of modern island arcs, arc-parallel extension should be the more common and has been hypothesized for both Sumatra and the Aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations. From estimates of ψmax and the arc-parallel gradients in obliquity, arc-parallel strain rates are estimated to be 1 to 3×10−8/yr for the Sumatran forearc, 2 to 6×10−8/yr for the Aleutian forearc, and 0.3 to 3×10−8/yr for the Philippine forearc. Oblique convergence and subsequent arc-parallel extension, if accompanied by crustal thinning, may provide an important yet little appreciated mechanism for bringing high-grade metamorphic rocks to the surface of subduction complexes.

Effects of variable normal stress on rock friction: Observations and constitutive equations

Abstract: We investigate the effects of variable normal stress on frictional resistance by performing quasi-static sliding experiments with 5 × 5 cm blocks of Westerly granite in a double-direct shear apparatus under servo-control. The observed response to a change in normal stress mimics that which occurs in response to a change in slip velocity. In particular, a sudden change in normal stress results in a sudden change followed by a transient change in the resistance to sliding. We interpret these changes within the previously established constitutive framework in which frictional resistance is determined by the current slip speed V, the current normal stress, and the state of the sliding surface (Dieterich, 1979a, 1981; Ruina, 1980, 1983). Earlier work demonstrated that the state of the sliding surface depends on prior slip speed. Our observations indicate that the state of the sliding surface also depends on prior normal stress. In our model the functional dependence of state on normal stress is expressed in terms of the same state variable, θ, used previously to represent slip rate history effects. We assume that the steady state value of θ is independent of normal stress and that θSS = Dc/V, where Dc is a characteristic slip distance. We interpret the variable θ as a measure of effective contact time. At constant slip speed and from an initial steady state, a sudden change in normal stress results in a sudden change in θ followed by a gradual change in θ back toward the initial θSS, as sliding proceeds. The magnitude of the sudden change in θ is determined by a newly identified parameter that we call α. Earlier workers have established that stability is influenced by stiffness, dτSS/dV, Dc, and slip rate history (Rice and Ruina, 1983). We conclude that stability will also be influenced by normal stress history and by α.

Geodetic data inversion using a Bayesian information criterion for spatial distribution of fault slip

Abstract: SUMMARY We developed a new inversion method to reconstruct static images of seismic sources from geodetic data, using Akaike’s Bayesian Information Criterion (ABIC). Coseismic surface displacements are generally related with a slip distribution on a fault surface by linear integral equations. Parametric expansion of the fault slip distribution by a finite number of known basis functions yields a set of observation equations expressed in a simple vector form. Incorporating prior constraints on the smoothness of slip distribution with the observation equations, we construct a Bayesian model with unknown hyperparameters. The optimal values of the hyperparameters, which control the structure of the Bayesian model, are objectively determined from observed data by using ABIC. Once the values of hyperparameters are determined, we can use the maximum likelihood method to find the optimal distribution of fault slip. We examined the validity of this method through a numerical experiment using theoretical data with random noise. We analysed geodetic data associated with the 1946 Nankaido earthquake (Ms = 8.2) by using this method. The result shows that the fault slip distribution of this earthquake has two main peaks of 4 and 6 m, located off Kii Peninsula and Muroto Promontory. These two high-slip areas are clearly separated by a low-slip zone extending along Kii Strait. Such a slip distribution corresponds with the fact that the rupture process of this earthquake in the western part is notably different from that in the eastern part.

Wall slip of molten high density polyethylenes. II. Capillary rheometer studies

Abstract: Above the critical stress for slip, the procedures normally used to analyze the results of capillary flow data give anomalous results. In particular, the Bagley plots are curved, even when a variation of viscosity with pressure is not anticipated, and the Mooney technique used to calculate the slip velocity gives results that indicate that the slip velocity depends on the L/D ratio. It is proposed that these phenomena arise from the dependence of the slip velocity on the wall normal stress, which implies a dependence on pressure. Based on this hypothesis, an approximate method is developed for interpreting the results of capillary flow experiments to determine the slip velocity as a function of both the wall shear stress and the pressure. The large available data set was used to incorporate into the model the effects of molecular weight parameters and temperature on the slip velocity. Finally, a detailed model for slip flow in a capillary was formulated that takes into account that the slip velocity and wall shear stress vary along the flow direction due to the pressure gradient. This model was used to evaluate the validity of the approximations used in the approximate data analysis technique for determining the slip velocity.

Stress field constraints on intraplate seismicity in eastern North America

Abstract: Focal mechanisms of 32 North American midplate earthquakes (mo = 3.8-6.5) were evaluated to determine if slip is compatible with a broad-scale regional stress field derived from plate-driving forces and, if so, under what conditions (stress regime, pore pressure, and frictional coefficient). Using independent information on in situ stress orientations from well bore breakout and hydraulic fracturing data and assuming that the regional principal stresses are in approximately horizontal and vertical planes (_ 10o), the constraint that the slip vector represents the direction of maximum resolved shear stress on the fault plane was used to calculate relative stress magnitudes defined by the parameterb = (S2 - S3)/(S - S3) from the fault/stress geometry. As long as the focal mechanism has a component of oblique slip (i.e., the B axis does not coincide with the intermediate principal stress direction), this calculation identifies which of the two nodal planes is a geometrically possible slip plane (Gephart, 1985). Slip in a majority of the earthquakes (25 of 32) was found to be geometrically compatible with reactivation of favorably oriented preexisting fault planes in response to the broad-scale uniform regional stress field. Slip in five events was clearly inconsistent with the regional stress field and appears to require a localized stress anomaly to explain the seismicity. Significantly, all five of these events occurred prior to 1970 (when many regional networks were installed), and their focal mechanisms are inconsistent with more recent solutions of nearby smaller events. The frictional likelihood of the geometrically possible slip on the selected fault planes was evaluated in the context of conventional frictional faulting theory. The ratio of shear to normal stress on the fault planes at hypocentral depth was calculated relative to an assumed regional stress field. Regional stress magnitudes were determined from (1) S/S3 ratios based on the frictional strength of optimally oriented faults (the basis for the linear brittle portion of lithospheric strength profiles), (2) the computed relative stress magnitude (b) values, and (3) a vertical principal stress assumed equal to the lithostat. Two end-member possibilities were examined to explain the observed slip in these less than optimally oriented fault planes. First, the frictional coefficient was held constant on all faults, hydrostatic pore pressure was assumed regionally, and the fault zone pore pressure was determined. Since pore pressure is a measurable quantity with real limits in the crust (P0 < S3), this end-member case was used to determine which of the geometrically possible slip planes were frictionally likely slip planes. Alternately, pore pressure was fixed at hydrostatic everywhere, and the required relative lowered frictional coefficient of the fault zone was computed. Slip in 23 of the 25 geometrically compatible earthquakes was determined to also be frictionally likely in response to an approximately horizontal and vertical regional stress field derived from plate-driving forces whose magnitudes are constrained by the frictional strength of optimally oriented faults (assuming hydrostatic pore pressure regionally). The conditions for slip on these 23 relatively "well-oriented" earthquake faults were determined relative to this regional crustal strength model and require only moderate increases in pore pressure (between about 0.4-0.8 of lithostatic, hydrostatic is about 0.37 of lithostatic) or, alternately, moderate lowering (<50%) of the frictional coefficient on the faults which slipped. Superlithostatic pore pressures are not required. Focal mechanisms for the two other earthquakes with slip vectors geometrically consistent with the regional stress field, however, did require pore pressures far exceeding the least principal stress (or extremely low coefficients of friction). These events may reflect either local stress rotations undetected with current sampling or poorly constrained focal mechanisms. The analysis also confirmed a roughly north to south contrast in stress regime between the central eastern United States and southeastern Canada previously inferred from a contrast in focal mecha- nisms between the two areas: most central eastern United States earthquakes occur in response to a strike-slip stress regime, whereas the southeastern Canadian events require a thrust faulting stress regime. This contrast in stress regime, with a constant maximum horizontal stress orientation determined by far-field plate-driving forces, requires a systematic lateral variation in relative stress magnitudes. Superposition of stresses due to simple flexural models of glacial rebound stresses are of the correct sense to explain the observed lateral variation, but maximum computed rebound-related stress magnitude changes are quite small (about 10 MPa) and do not appear large enough to account for the stress regime change if commonly assumed stress magnitudes determined from frictional strength apply to the crust at seismogenic depths.

The role of slip additives in tape-casting technology. I: Solvents and dispersants

Abstract: On examine la formulation des barbotines et son influence sur la microstructure, les proprietes des solvants et le role des dispersants : stabilisation electrostatique, stabilisation en milieu non-aqueux, stabilisation polymere.

Origin of rollover

Abstract: Rollover is the folding of hanging-wall fault blocks by bending or collapse in response to slip on nonplanar--commonly listric--normal faults. The shapes of rollover folds are controlled by a number of variables, including (1) the shape of the fault, (2) the total fault slip after a bed is deposited, (3) the direction of relative particle motion in hanging-wall collapse, (4) the history of sedimentation rate relative to fault slip rate, and (5) compaction. The importance and role of each of these variables is illustrated by a two-dimensional balanced structural modeling technique that treats continuously curved faults as though composed of a large number of straight fault segments. In this modeling, an active axial surface, oriented parallel to the direction of relative p rticle motion in hanging-wall collapse, emanates from each fault bend and is the instantaneous locus of folding. The quantitative correctness of this theory of rollover is tested by modeling natural structures from the Gulf of Mexico for which both fault shape and fold shape are known from high-quality seismic and well sections. The direction of hanging-wall collapse commonly is in the antithetic or synthetic normal-fault or Coulomb-shear orientations, although sliding along weak bedding planes also is an important collapse mechanism in some regions. Collapse is in the antithetic-shear direction for concave fault bends and in the synthetic-shear direction for convex bends. These collapse directions can be observed directly in some high-quality seismic images as axial surfaces emanating from fault bends. The shapes of rollovers within growth strata depend strongly on the sedimentation rate relative to fault slip rate, as well as the total slip after a bed is deposited. The crests of classic Gulf Coast rollovers are growth axial surfaces, along which are abrupt changes in sedimentation rate within the growth st atigraphic interval. These changes are produced by deformation of the sediment-water interface along active axial surfaces. Compaction can substantially modify the relationship between fault shape and rollover shape; however, under certain common conditions, the history of compaction can be neglected if the folding is modeled in the compacted state.

Multimechanism friction constitutive model for ultrafine quartz gouge at hypocentral conditions

Abstract: Occurrence of instability in crustal faults depends in part on the small-magnitude dependence of frictional strength on slip rate and slip history. Rate dependence of friction reflects the operation of thermally activated mechanisms at points of contact along fault surfaces and is expected to change in space and time owing to variations in environmental conditions and slip rates during the seismic cycle. Several lines of evidence suggest solution-precipitation processes in fault zones may be activated during interseismic periods when slip rates are small and may contribute to fault healing. We develop a constitutive model for faulting at hypocentral conditions that is capable of describing the variation in frictional properties as different slip mechanisms are activated in response to changes in temperature or slip rate. This model is based on the assumption that slip mechanisms are thermally activated and follow an Arrhenius relationship between temperature and slip rate, which allows the addition of temperature dependence to existing rate- and state-dependent friction constitutive laws. Multiple slip mechanisms are treated as operating independently and concurrently, where each mechanism is described by the rate-, state-, and temperature-dependent friction constitutive relation. The constitutive model is used to analyze triaxial friction experiments on ultrafine-grained quartz gouge at temperatures to 600°C, effective confining pressure of 150 MPa, and water-saturated or room-dry conditions. These experiments investigated the stress relaxation response and slip history effects during slide-hold-slide tests with hold times up to 105s. The microstructure of the deformed quartz gouge and the transient friction behavior define at least two distinct frictional slip regimes: a low-temperature regime characterized by cataclastic mechanisms with significant slip history effects, and a high-temperature regime characterized by solution-precipitation-aided cataclastic flow with large-magnitude rate dependence and insignificant slip history effects. In the model the parameters of the friction constitutive relation (e.g., a, b, and L) are treated as constants for each slip mechanism but are different for the different mechanisms. This model accurately describes the frictional behavior within each regime and across the transition between regimes. The analysis suggests that the greatest-magnitude rate weakening behavior occurs at 100° to 300°C under wet conditions at laboratory slip rates. Significant solution-precipitation is activated at temperatures above 300°C at laboratory slip rates or at lower slip-rates and lower temperatures. The high-temperature solution-precipitation regime is described by a large-magnitude rate strengthening (a − b = 0.03) and an apparent activation energy of approximately 44 kJ mol−1. The constitutive analysis suggests that the solution-precipitation-aided flow mechanism could be important during interseismic periods at hypocentral conditions and low shear stress but apparently is not characterized by significant slip history effects.

Finite Plastic Deformation of Crystalline Solids

Abstract: Preface 1. A historical introduction 2. The kinematics of double slip 3. A general theory of elastoplastic crystals 4. Axial-load experiments and latent hardening in single crystals 5. Analysis of crystals in channel die compression 6. Theoretical connections between crystal and aggregate behaviour 7. Approximate polycrystal models Appendix: the general theory of work-conjugate stress and strain References Index.

Chapter 2 Fabrics of Experimental Fault Zones: Their Development and Relationship to Mechanical Behavior

Abstract: The fracture array of simulated fault zones is shown to evolve in a predictable and reproducible manner, from a stepwise fashion to a steady-state condition. At low confining pressures and increasing shear strain the sequence is: (1) Homogeneous shearing by grain-to-grain movements. (2) R 2 - and R 1 -fractures initiate at about the same time but propagate only a few grain diameters. They are at relatively high angles to the gouge-forcing block interface and widely spaced. These first two stages are one primarily of gouge compaction characterized by strain hardening. (3) Extension of R 1 S and coincident reorientation to lower angles closely paralleling the interface with the forcing blocks. P-fractures initiate. These occur from the ultimate strength through a strain softening stage. (4) Y-fractures form along which most of the displacement is accommodated, with the fracture array now close to steady state. Y's initially are close to one or both interfaces with the forcing blocks, but with increasing shear strain shift to the interior of the gouge. At this stage, sliding may change from stable slip to periodic oscillations, characteristic of stick-slip sliding. The development of the fracture array is interpreted to be the result of a reorientation of the stress field across and within the gouge zone. Riedel shears form in response to Coulomb failure, but Y-fractures appear as a result of the kinematic constraint produced by the more rigid bounding blocks. Modeling of the weak gouge zone within a stronger medium shows that the stress field may rotate to higher angles at the gouge boundaries. This is consistent with recent field observations. A significant implication is that without this recognition, laboratory values of frictional coefficients may be overestimated.

Rupture geometry from high-precision relative hypocentre locations of microearthquake clusters

Abstract: SUMMARY In 1987, two microearthquake sequences, with a duration of about a week each and consisting of 37 and 46 events respectively, occurred in the upper crust below the Jura Mountains of northern Switzerland. The seismograms within each sequence exhibit a high degree of similarity, indicating tight clustering of hypocentres and similar focal mechanisms. Using a cross-correlation technique applied to the seismograms in the time domain, together with a new least-squares adjustment procedure, it was possible to determine relative hypocentre locations of the earthquakes in each cluster with a precision of a few tens of metres. The results show that hypocentres of events with the same focal mechanism lie on a plane which coincides exactly with one of the nodal planes of the fault-plane solution, and that consequently swarm-like sequences of similar earthquakes are due to repeated slip on the same fault. In one case, slip occurred as right-lateral motion on a steeply dipping plane striking in WNW-ESE direction, whereas the other case corresponds to left-lateral slip on two almost vertical planes striking roughly N-S. In 1988, an additional earthquake triplet occurred on a WNW-ESE oriented normal fault nearby. All three mechanisms are consistent with a general NNW-SSE oriented direction of maximum crustal shortening and a corresponding WSW-ENE oriented extension. Repeated slip on the same fault is indicative of a large degree of heterogeneity and of short-term temporal variability of both frictional resistance and stress distribution on the fault. An explanation for the occurrence of such swarm-like seismic activity in terms of barriers or asperities would require that shear stress on the unbroken patches increase from one event to the next. Since, however, the overall shear stress would be expected to decrease as a consequence of the stress released in each event, a more plausible mechanism involves pore-pressure fluctuations, caused by fluids under suprahydrostatic pressures migrating upward through pre-existing zones of weakness in the crust.

Role of slip and fracture in the oscillating flow of HDPE in a capillary

Abstract: Certain polymers exhibit two distinct branches in their capillary flow curves (wall shear stress versus apparent wall shear rate). This gives rise to oscillatory flow in constant‐piston‐speed rheometers and to flow curve hysteresis in controlled‐pressure rheometers. These curious phenomena have attracted considerable interest over a period of many years, but their basic mechanisms are still the subject of debate. Building on previous work we have developed a model that predicts all the essential features of the curves of pressure and flow rate versus time in the oscillatory flow regime. Fluid compressibility and the second branch of the flow curve are necessary features of the model, but fluid elasticity is found not to be an essential element. While our macroscopic measurements do not prove it conclusively, our data lead us to believe that on the high‐flow‐rate branch of the flow curve there is slip along a cylindrical fracture surface near the wall. The jump to the high‐flow branch occurs when this fracture occurs, at an upper critical value of the shear stress, while the jump back to the low‐flow branch occurs when adhesion is established at the fracture surface at a lower critical shear stress.

Deformation mechanisms and plastic resistance in single-crystal-textured high-density polyethylene

Abstract: The plastic resistances for the three most important mechanisms actig in HDPE, i.e. the (100)[001] and (010)[001] chain slips and (100)[010] transverse slip was investigated and the ranges of crystal odrientation in which a single one of these mechanisms dominares the observed deviation behavior were explored. An attempt was also made to detect the (110)[001] chain slip, which according to theoretical predictions should also be an active mechanism in the deformation of polyethylene crystals

Strike‐slip earthquakes on quasi‐vertical transcurrent faults: Inferences for general scaling relations

Abstract: The recent occurrence of several large strike-slip earthquakes provides the opportunity to review and complement available data on the scaling of seismic moment (MO) with length of rupture (L) for large earthquakes, depending on their tectonic setting and mechanism. For strike-slip earthquakes on quasi-vertical transcurrent faults, the MO versus L relation has a significant change of slope around MO ∼(0.6–0.8)*1020 N-m, and for larger earthquakes, MO scales linearly with L. This is compatible with models where slip is controlled by the width of the fault. Also, it appears to be easier to categorize large earthquakes by their mechanism (strike-slip on vertical transcurrent fault, versus pure thrust/normal) than their tectonic setting (interplate/intraplate).

Non-Schmid yield behavior in single crystals

Abstract: A parently , certain single crystals do not obey Schmid's law for slip on individual systems. For example, many intermetallic compounds such as Ni 3Al with the LI 2 structure display yield behaviors that are called “anomalous” in the sense that the critical resolved shear stress on the primary slip system at yield is a function of the orientation of the loading axis and the sense of load. To accommodate such non-Schmid behaviors, a generalization of Schmid's law is proposed where stress components other than the Schmid stress also enter the criterion. The general effects of “cross slip” in FCC single crystals as well as the detailed shield behavior of Ni 3Al are well described. Finally, we demonstrate that non-Schmid effects can make it more difficult to activate many slip systems simultaneously.

Pore pressure, stress increase, and fault weakening in low‐angle normal faulting

Abstract: Low-angle (dip < 30°) normal faults accommodate much extension of the continental crust They apparently move under low resolved shear stress and are anomalously weak, characteristics that they share with the San Andreas fault Structural, textural, and geochemical arguments suggest that low-angle normal faults are weak in both the ductile and brittle regimes, partly or totally due to elevated pore fluid pressure High pore pressure in detachment zones may be contained by upper-plate strata, mineral precipitation in their hanging walls, formation of low-permeability microbreccia layers, threshold pressure gradients, and low-permeability mylonites below chlorite breccia Mechanical analysis shows that fault weakening may preclude equality of the regional and fault-zone stress tensors, and predicts reorientation and increase of principal stresses in weak fault zones These changes suppress hydraulic fracturing in the brittle detachment zone and allow slip under frictional sliding conditions typical of upper crustal rocks Fault weakening focuses extension in the upper crust onto low-angle normal ductile-brittle shear zones in the midcrust, promoting propagation of low-angle brittle normal faults into the upper crust

Evolution of the crystalline texture of high-density polyethylene during uniaxial compression

Abstract: The formation of an axially aymmetrical texture in HDPE was studied through unixial compression at room temperature. The orientation of crystallographic axes was probed at various stages of the deformation process, up to an equivalent strain of 1.86, by means of WAXS pole figures. Additionally the lamellar orientation was studied using SAXS and TEM. The results of these structural and morphological studies demonstrated that the major deformation mechanism involved in plastic deformation of the crystalline phase were (100)[001] chain slip

Bond of reinforcement under controlled confinement

Abstract: Twelve specimens were tested to determine the local bond stress-slip characteristics of a #6 reinforcing bar embedded in a 3-in. diameter concrete cylinder. Radial confining stress around the concrete specimen and radial deformation, together with bond stress and slip, were assumed to be fundamental variables needed to describe the interface behavior properly. Configuration-independent bond stress-slip relationships for a short five-lug embedded length were obtained for various degrees of confining pressure. Maximum bond stresses could be increased almost threefold by increasing the confinement stress from 500 to 4500 psi at the bar level. Two types of #6 bars with different deformations were investigated.

Boundary Conditions for Rapid Granular Flow: Flat, Frictional Walls

Abstract: We employ Coulomb friction and both tangential and normal restitution in a model for a collision between a homogeneous sphere and a flat wall. We calculate the impulse and change in kinetic energy in typical collisions and use a particularly simple velocity distribution function to obtain the rates at which momenta and energy are supplied to the flow over a unit area of the wall. From these, we determine boundary conditions that relate the shear stress and energy flux in the flow at the wall to the normal stress, slip velocity, and fluctuation energy and to the parameters that characterize a collision.