Showing papers on "Shear stress 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.

Drag and drag partition on rough surfaces

Abstract: An analytic treatment of drag and drag partition on rough surfaces is given. The aims are to provide simple predictive expressions for practical applications, and to rationalize existing laboratory and atmospheric data into a single framework. Using dimensional analysis and two physical hypotheses, theoretical predictions are developed for total stress (described by the square root of the canopy drag coefficient), stress partition (described by the ratio Τ S/Τ of the stress Τ s on the underlying ground surface to total stress Τ), zero-plane displacement and roughness length. The stress partition prediction is the simple equation τS/τ= 1/(1+βλ), where λ= CRCS the ratio of element and surface drag coefficients. This prediction agrees very well with data and is free of adjustable constants. Other predictions also agree well with a range of laboratory and atmospheric data.

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.

Comparison between rough- and smooth-wall turbulent boundary layers

Abstract: Measurements in a zero-pressure-gradient turbulent boundary layer over a mesh-screen rough wall indicate several differences, in both inner and outer regions, in comparison to a smooth-wall boundary layer. The mean velocity distribution indicates that, apart from the expected k-type roughness function shift in the inner region, the strength of the rough-wall outer region ‘wake’ is larger than on a smooth wall. Normalizing on the wall shear stress, there is a significant increase in the normal turbulence intensity and a moderate increase in the Reynolds shear stress over the rough wall. The longitudinal turbulence intensity distribution is essentially the same for both surfaces. Normalized contributions to the Reynolds shear stress from the second (Q2) and fourth (Q4) quadrants are greater over the rough wall. The data indicate that not only are Q2 and Q4 events stronger on the rough wall but their frequency of occurrence is nearly twice as large for the rough wall as for the smooth wall. Comparison between smooth- and rough-wall spectra of the normal velocity fluctuation suggests that the strength of the active motion may depend on the nature of the surface.

Vascular endothelium responds to fluid shear stress gradients.

Abstract: In vitro investigations of the responses of vascular endothelium to fluid shear stress have typically been conducted under conditions where the time-mean shear stress is uniform. In contrast, the in vitro experiments reported here have re-created the large gradients in surface fluid shear stress found near arterial branches in vivo; specifically, we have produced a disturbed-flow region that includes both flow separation and reattachment. Near reattachment regions, shear stress is small but its gradient is large. Cells migrate away from this region, predominantly in the downstream direction. Those that remain divide at a rate that is high compared with that of cells subjected to uniform shear. We speculate that large shear stress gradients can induce morphological and functional changes in the endothelium in regions of disturbed flow in vivo and thus may contribute to the formation of atherosclerotic lesions.

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.

Fluid shear stress modulates cytosolic free calcium in vascular endothelial cells

Abstract: Cytosolic free Ca2+ concentration ([Ca2+]i) was monitored in single and groups of fura-2-loaded bovine aortic endothelial cells (BAEC) during exposure to laminar fluid shear stress. Application of a step increase in shear stress from 0.08 to 8 dyn/cm2 to confluent BAEC monolayers resulted in a transient increase in [Ca2+]i, which attained a peak value in 15-40 s, followed by a decline to baseline within 40-80 s. The magnitude of the [Ca2+]i responses increased with applied shear stress over the range of 0.2-4 dyn/cm2 and reached a maximum at greater than 4 dyn/cm2. Transient oscillations in [Ca2+]i with gradually diminishing amplitude were observed in individual cells subjected to continuous high shear stress. Elimination of extracellular Ca2+ with ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, blockade of Ca2+ entry with lanthanum, depolarization of the cell membrane with high K+, and preconditioning of BAEC in steady laminar flow had little effect on the [Ca2+]i response. In the presence of ATP or ADP, application of shear stress caused repetitive oscillations in [Ca2+]i in single BAEC, whose frequency was dependent on both agonist concentration and the magnitude of applied shear stress. However, apyrase, an ATPase and ADPase, did not inhibit the shear-induced [Ca2+]i responses in standard medium (no added ATP or ADP), suggesting that the shear-induced [Ca2+]i response is not due to ATP released by endothelial cells.

In situ stress measurements to 3.5 km depth in the Cajon Pass Scientific Research Borehole: Implications for the mechanics of crustal faulting

Abstract: Measurements of in situ stress orientation and magnitude at the site of the Cajon Pass research borehole have been made from depths of 0.9-3.5 km using the hydraulic fracturing technique and analysis of stress-induced well bore breakouts. The results of these measurements support two important conclusions about the mechanics of crustal faulting. First, the magnitudes of measured in situ stresses indicate ratios of shear to normal stres. s on favorably oriented fault planes that are consistent with predictions based on Mohr-Coulomb theory and laboratory-determined coefficients of friction in the range of 0.6-1.0 assuming hydrostatic pore pressure (this is commonly known as Byerlee's law). Thus the stress measurements indicate that the frictional strength of the crust adjacent to the San Andreas fault is high (i.e., consistent with laboratory-derived friction values) and that the level of shear stress in the crust adjacent to the San Andreas is principally controlled by its frictional strength. However, data on the orientation of maximum horizontal compression in the borehole from 1.75 to 3.5 km (N57oE + 19 o) indicate that the San Andreas must be quite weak as a complete absence of right-lateral shear stress resolved on planes parallel to the --N60oW striking San Andreas fault is observed. The lack of right-lateral shear stress on planes parallel to the San Andreas fault at this site is especially surprising as Cajon Pass is located along a section of the San Andreas which has not had a major earthquake since 1812 and is thus presumably quite "late" in the earthquake cycle. Nevertheless, both the orientation and magnitudes of stresses measured in the well are consistent with the style of active faulting in the area surrounding the drill site, most notably normal faulting and Quaternary age left-lateral slip on the Cleghorn fault that parallels the San Andreas in the vicinity of the drill site (Meisling and Weldon, 1982; Weldon, 1986; R. J. Weldon et al., unpublished report, 1981). We argue that the stress state (and Quaternary fault offsets) observed in the Cajon Pass area could exist only if the San Andreas moved at low shear stresses comparable to seismic stress drops rather than the much higher values predicted by Byerlee's law, a conclusion consistent with the lack of frictionally generated heat flow along the San Andreas system (e.g., Brune et al., 1969; Henyey and Wasserburg, 1971; Lachenbruch and Sass, 1973, 1980). Taken together, the Cajon Pass in situ stress and heat flow measurements (Lachenbruch and Sass, this issue) support a conceptual model of the San Andreas system in which the San Andreas is extremely weak with respect to the surrounding crust.

Vascular endothelium responds to fluid shear stress gradients.

Abstract: In vitro investigations of the responses of vascular endothelium to fluid shear stress have typically been conducted under conditions where the time-mean shear stress is uniform In contrast, the in vitro experiments reported here have re-created the large gradients in surface fluid shear stress found near arterial branches in vivo; specifically, we have produced a disturbed-flow region that includes both flow separation and reattachment Near reattachment regions, shear stress is small but its gradient is large Cells migrate away from this region, predominantly in the downstream direction Those that remain divide at a rate that is high compared with that of cells subjected to uniform shear We speculate that large shear stress gradients can induce morphological and functional changes in the endothelium in regions of disturbed flow in vivo and thus may contribute to the formation of atherosclerotic lesions

Transition to shear-driven turbulence in Couette-Taylor flow.

Abstract: Turbulent flow between concentric cylinders is studied in experiments for Reynolds numbers 800R1.23\ifmmode\times\else\texttimes\fi{}${10}^{6}$ for a system with radius ratio \ensuremath{\eta}=0.7246. Despite predictions for the torque scaling as a power law of the Reynolds number, high-precision torque measurements reveal no Reynolds-number range with a fixed power law. A well-defined nonhysteretic transition at R=1.3\ifmmode\times\else\texttimes\fi{}${10}^{4}$ is marked by a change in the Reynolds-number dependence of the torque. Flow quantities such as the axial turbulent diffusivity, the time scales asociated with the fluctuations of the wall shear stress, and the root-mean-square fluctuations of the wall shear stress and its time derivative are all shown to be simply related to the global torque measurements. Above the transition, the torque measurements and observed time scales indicate a close correspondence between this closed-flow system and open-wall--bounded-shear flows such as pipe flow, duct flow, and flow over a flat plate.

Scour Around Vertical Pile in Waves

Abstract: This paper presents the results of an experimental investigation on scour around piles exposed to waves. In addition to the actual scour tests, bed shear‐stress measurements and a flow visualization study are carried out. The effects of lee wake and horseshoe vortex are demonstrated to be the two key elements in the scour process. The development of these flow structures mainly depends on the Keulegan‐Carpenter (KC) number that hereby becomes the main parameter that governs the equilibrium scour depth on a live bed. Based on the present data, a design equation is established, relating the scour depth to the Keulegan‐Carpenter number. For the values of the Keulegan‐Carpenter number below six, the scour around the pile practically ceases to exist. The scour depth normalized by the pile diameter is found to increase with increasing Keulegan‐Carpenter number and approaches its steady‐current value for Keulegan‐Carpenter numbers above approximately 100.

An earthquake mechanism based on rapid sealing of faults

Abstract: RECENT seismological, heat flow and stress measurements in active fault zones such as the San Andreas have led to the suggestion1,2 that such zones can be relatively weak. One explanation for this may be the presence of overpressured fluids along the fault3–5, which would reduce the shear stress required for sliding by partially 'floating' the rock. Although several mechanisms have been proposed for overpressurizing fault fluids3,4,6,7, we recall that 'pressure seals' are known to form in both sedimentary8 and igneous9 rocks by the redistribution of materials in solution; the formation of such a seal along the boundaries of a fault will prevent the communication of fluids between the porous, deforming fault zone and the surrounding country rock. Compaction of fault gouge, under hydrostatic loading and/or during shear, elevates pore pressure in the sealed fault and allows sliding at low shear stress. We report the results of laboratory sliding experiments on granite, which demonstrate that the sliding resistance of faults can be significantly decreased by sealing and compaction. The weakening that results from shear-induced compaction can be rapid, and may provide an instability mechanism for earthquakes.

Heat flow from Cajon Pass, fault strength, and tectonic implications

Abstract: Measured heat flow at Cajon Pass is consistent with predictions based on local site conditions and regional heat flow. With observations now ranging to a depth of 3½ km, there is still no evidence for significant frictional heating anywhere on the San Andreas fault. The result supports the view, long suggested from heat flow studies, that the fault is weak in spite of estimates based on Byerlee's law, isotropic strength, and hydrostatic fluid pressure that suggest a strength several times larger. Recent evidence (Zoback et al., 1987; Mount and Suppe, 1987) that the maximum principal stress might be almost normal to the San Andreas fault would support the weak-fault model and add constraints over and above those imposed by heat flow; e.g., local friction coefficients μ ≲ 0.1 or fluid pressures along the fault greater than lithostatic (λ > 1), compared to μ ≲ 0.2 or fluid pressure greater than twice hydrostatic (λ > 0.74) for the heat flow constraint alone. These constraints are a challenge to existing models of faulting, and they are stimulating promising new points of view. The balance of plate boundary forces around a weak fault depends on the basal traction coupling the seismic layer to the rest of the system; heat flow limits the coupling force across the fault to an insignificant ∼ 1011 N/m. The weak fault also precludes significant near-field basal driving tractions, but it permits a large basal drag force which could result in a highly stressed seismic layer offering appreciable resistance to plate motion through its base. Such tractions could develop progressively if the fault surface weakens as it evolves; if they exist, they should cause an observable reduction in shear stress resolved in the fault direction and a rotation of principal axes as the fault is approached; if they do not exist, the seismic layer rides passively on the lower crust. Heat flow measurements should detect whether such basal tractions might be associated with basal decoupling and flow. Coupling at the base of the seismic layer is controlled by the rheological profile, the usual representation of which raises three questions in applications to the San Andreas fault zone. First, the linear frictional portion through the seismic layer implies a resisting force on the fault much greater than the heat flow limit permits. Second, the large stresses implied for the temperature-sensitive ductile layer might be unsustainable; they could lead to shear heating and weakening at plate boundary strain rates. Third, in the ductile layer the stress is sensitive to whether deformation is concentrated in narrow vertical mylonite zones, as sometimes assumed in models of the earthquake cycle, or more broadly distributed by bulk flow in a deep-crustal “asthenosphere.” Horizontal basal shear stresses are of the same order as vertical strike-slip stresses near the base of the seismic layer; they could result in bulk flow or horizontal detachment leading to a different pattern of long-term stress, strain rate, and dissipation and a requirement for decoupling and basal drag on the seismic layer in the near field. Results from the San Andreas fault taken with long-standing speculation about the orthogonal relation between oceanic transform faults and extensional spreading centers suggest that strike-slip transform faults might be anomalously weak in both continental and oceanic settings.

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.

Friction angle measurements on a naturally formed gravel streambed: Implications for critical boundary shear stress

Abstract: We report the first measurements of friction angles for a naturally formed gravel streambed. For a given test grain size placed on a bed surface, friction angles varied from 10o to over 100o; friction angle distributions can be expressed as a function of test grain size, median bed grain size, and bed sorting parameter. Friction angles decrease with increasing grain size relative to the median bed grain size, and are a systematic function of sorting, with lower friction angles associated with poorer sorting. The probability distributions of critical shear stress for different grain sizes on a given bed surface, as calculated from our friction angle data, show a common origin, but otherwise diverge with larger grains having narrower and lower ranges of critical shear stresses. The potential mobility of a grain, as defined by its probability distribution of critical shear stress, may be overestimated for larger grains in this analysis, because our calculations do not take into account the effects of grain burial and altered near-bed flow fields.

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.

Present-day stress orientations adjacent to active strike-slip faults: California and Sumatra

Abstract: Present-day stress directions interpreted from well bore breakouts adjacent to two crastal-scale, active, strike-slip faults (the San Andreas fault in California and the Great Sumatran fault in Sumatra) indicate that the maximum horizontal stress direction (SH) is oriented at a high angle (70°–90°) to both faults. The regionally defined stress fields spanning these faults show that adjacent young or actively growing folds have formed orthogonal to SH and are therefore in the thrust-fault orientation. These observations indicate a decoupling of the strike-slip and compressional components of the deformation within these broadly transpressive zones. Borehole breakouts in 118 wells in western California indicate a regionally consistent stress pattern with SH generally oriented NE-SW, nearly perpendicular (80°–90°) to the strike of the San Andreas fault. The orientation of SH nearly perpendicular to the San Andreas fault implies low shear stress on the fault and is consistent with geological interpretations of the Coast and Transverse Ranges indicating active compressional deformation, fault plane solutions for recent dip-slip-style earthquakes, principal stress directions determined from inversion of earthquake focal mechanisms, and induced hydraulic fracture orientations. A stress trajectory map for western California is computed using an iterative statistical algorithm in which observed directional data, such as breakout directions, are used to obtain a model regional stress field. Analysis of well bore breakouts in 25 wells within the central and southern oil districts of Sumatra indicates that the regionally defined SH adjacent to the active Great Sumatran strike-slip fault is oriented at a high angle (70°–80°) to the fault This orientation of SH is consistent regionally with geologic stress indicators and focal mechanisms of dip-slip earthquakes. Preliminary analysis of the stress field in the vicinity of the Philippine and Alpine faults suggests SH is also oriented at a high angle to these active strike-slip faults. Similarly, the minimum horizontal stress Sh is oriented at a high angle to the the Kane and Dead Sea transforms. The observation of SH and Sh in the vicinity of active strike-slip faults being oriented nearly perpendicular and parallel to the faults suggests that large, crustal-scale strike-slip faults may, in general, be inherently weak surfaces.

Estimation of flow resistance in gravel‐bedded rivers: A physical explanation of the multiplier of roughness length

Abstract: The need to estimate velocity and discharge indirectly in gravel-bedded rivers is a commonly-encountered problem. Semilogarithmic friction equations are used to estimate mean velocity using a friction factor obtained from depth and grain size information. Although such equations have a semi-theoretical basis, in natural gravel-bed channels, an empirical constant (6.8 or 3.5) has to be introduced to scale-up the characteristic grain size (D50 or D84) to represent the effective roughness length. In this paper, two contrasting approaches are used to suggest that the multiplier of characteristic grain size is attributable to the effect of small-scale form resistance, reflecting the occurrence of microtopographic bedforms in gravel-bedded environments. First, spatial elevation dependence in short, detailed bed profiles from a single gravel-bedded river is investigated using semivariogram and zero-crossing analyses. This leads to objective identification of two discrete scales of bed roughness, associated with grain and microtopographic roughness elements. Second, the autocorrelation structure of the three-dimensional near-bed velocity field is examined to identify regularities associated with eddy shedding and energy losses from larger grains and microtopographic bedforms. Apart from improving the capacity to determine friction factors for velocity and discharge estimation, the findings have implications in general for the initial motion of gravelly bed material.

Induced stresses due to fluid extraction from axisymmetric reservoirs

Abstract: Earthquakes can be induced by fluid extraction, as well as by fluid injection.Segall (1989) proposed that poroelastic stresses are responsible for inducing earthquakes associated with fluid extraction. Here, I present methods for computing poroelastic stress changes due to fluid extraction for general axisymmetric reservoir geometries. The results ofGeertsma (1973) for a thin disk reservoir with uniform pressure drop are recovered as a special case. Predicted surface subsidence agrees very well with measured leveling changes over the deep Lacq gas field in southwestern France. The induced stresses are finite if the reservoir pressure changes are continuous. Computed stress changes are on the order of several bars, suggesting that the preexisting stress states in regions of extraction induced seismicity are very close to frictional instability prior to production.

Micro-architecture and composition of artery walls: relationship to location, diameter and the distribution of mechanical stress.

Abstract: Purpose We reviewed the structural basis of the mechanical properties of the arterial wall, in order to establish a coherent micro-anatomical basis for the differences in compliance among different arteries and a framework for assessing changes in the mechanical properties of specific individual arteries in relation to changing physical stresses. Data identification The data and concepts presented here were derived from both earlier and ongoing work. Features that assure stability and integrity in relation to blood flow (wall shear stress) and pressure (mural tensile stress) were examined. Particular attention was paid to the morphogenetic and biosynthetic means by which arteries adapt to normal or abnormal modifications of these forces, particularly in relation to growth, location in the arterial tree and geometric configuration. Results and conclusions Thickness, composition and architecture of the artery wall, including thickness and composition of the intima, are normally determined by the stresses imposed by pressure and flow. Vessel radius is closely associated with flow, so that a normal baseline level of mean shear stress of about 15 dyn/cm2 is maintained or restored. Wall thickness and composition are determined by wall tension in relation to pressure and radius. Baseline levels of tensile stress differ with location but appear to be similar for homologous vessels. Changes in flow that modify the radius also modify wall tension. Changes in wall thickness and composition are likely to cause changes in compliance, due to altered flow and/or pressure patterns; these changes in compliance may be adaptive rather than destructive. Changes in the compliance of specific arteries over time may be used to evaluate the progression and severity of the conditions underlying these changes.

Chapter 3 Frictional Strength and the Effective Pressure Law of Montmorillonite and lllite Clays

Abstract: Low-strength clay minerals are a common constituent of fault gouges, and are often cited as a possible explanation for the low ambient shear stresses along the San Andreas fault inferred from heat flow constraints and in situ stress measurements. Montmorillonite, the weakest of the clay minerals, undergoes a gradual phase transition to illite with depth. In order to compare the shear stresses supported by these two minerals with those thought to exist along the San Andreas, we have measured the frictional sliding behavior of pure montmorillonite, mixed montmorillonite/illite and pure illite as a function of effective pressure, simulating burial to seismogenic depths. Strength measurements verify that the effective pressure law for friction holds for these minerals under all conditions. That is, the measured stresses were a function of the effective pressure, P c - P p , independent of the choice of confining and pore pressure. This relation, common for many other rock types, was previously untested for these clays under most conditions. Results show that dry samples were consistently stronger than saturated samples, and that strength increased with increasing illite content. In addition, the coefficient of friction increased as a function of pressure for the montmorillonite gouge, but was independent of pressure for the illite gouge. This behavior may be explained by the presence of loosely bonded interlayer water in the montmorillonite, which is squeezed out at higher pressures, changing the frictional characteristics of the clay. The nonexpanding illite was not affected in this way. For the montmorillonite-to-illite compositional profile, an average shear stress of 60 MPa was determined for crustal conditions to 15 km, assuming a normal hydrostatic gradient. If montmorillonite remains stable at depth, the resulting average shear stress is reduced to 30 MPa. In either case, these values are above the 10-20 MPa shear stress limit along the San Andreas inferred from heat flow constraints. Strength may be reduced to in-situ levels if fluid pressures become greater than hydrostatic within the gouge zone.

A finite element analysis of the push‐out test: Influence of test conditions

Abstract: The commonly used method for quantitative evaluation of the strength of a bone-implant interface is the push-out test. In order to give an impulse to standardization and to gain more insight in the biomechanics of the push-out test, a finite element analysis of this test was performed. This study focused on the influence of test conditions on the push-out results. The influence of the following four parameters on the interface stress distribution was tested: (a) clearance of the hole in the support jig, (b) Young's modulus of the implant; (c) cortical thickness; and (d) implant diameter. The distance between the implant and the support jig turned out to be very critical for the occurrence of peak stresses in the interface. Variations of the Young's modulus of the implants resulted in a wide range of interface shear stresses. Variation of the cortical thickness showed a reciprocal relationship between cortical thickness and interface shear stress. However, the interface stress distribution remained uniform under the specific test circumstances. These findings also hold for variations in implant diameter. The present investigation shows that the clearance of the hole in the support jig, and the Young's modulus of the implant are parameters which most strongly influence the interface stress distribution. The clearance of the hole in the support jig is the most critical parameter, but also the parameter that can be controlled most easily. Lack of standardization with regard to these parameters can lead to uninterpretable test results. It is recommended that the clearance of the hole in the support jig is at least 0.7 mm and that push-out results are only compared with each other when materials with similar Young's modulus are concerned.